[Transcriber’s Note:

DO NOT TRY THIS AT HOME. 

This e-text comes in three different forms: unicode (UTF-8), Latin-1 andascii-7. Use the one that works best on your text reader. 

 --If “œ” displays as a single character, and apostrophes and quotation marks are “curly” or angled, you have the utf-8 version (best). If any part of this paragraph displays as garbage, try changing your text reader’s “character set” or “file encoding”. If that doesn’t work, proceed to: --In the Latin-1 version, “œ” is two letters, but the word “aëriform” is usually written with dieresis (dots) over the “e”, and “æ” is a single letter. Apostrophes and quotation marks will be straight (“typewriter” form). Again, if you see any garbage in this paragraph and can’t get it to display properly, use: --The ASCII-7 or rock-bottom version. All necessary text will still be there; it just won’t be as pretty. 

The full caption of each Plate is given after its first mention in thetext--generally a few pages before the Plate’s physical appearance, asspecified in the caption. 

Many terms used in this book are different from today’s standardterminology. Note in particular:

 oxy-muriatic acid = the element chlorine phosphat of lime = calcium diphosphate _or_ the element calcium glucium = the element beryllium

 muriatic acid = hydrochloric acid muriat of lime = calcium chloride oxymuriate of potash = potassium chlorate carbonic acid = carbon dioxide

Further details and more examples are at the end of the e-text. 

Each Volume had its own table of contents. They have been merged forthis e-text, but the Vol. II title page was retained. Some Conversationswere renumbered between the 4th and 5th edition, resulting in theapparent disappearance of Conversations XI and XII. 

Typographical errors are listed at the end of the text. ]

 * * * * * * * * * * * * * *

 CONVERSATIONS ON CHEMISTRY;

 In Which The Elements Of That Science Are _Familiarly Explained_ And Illustrated By Experiments. 

 IN TWO VOLUMES. 

 _The Fifth Edition, revised, corrected, _ _and considerably enlarged. _

 VOL. I. ON SIMPLE BODIES. 

 _London:_ Printed For Longman, Hurst, Rees, Orme, and Brown, Paternoster-Row. 1817. 

Printed by A. Strahan, Printers-Street, London. 

ADVERTISEMENT. 

_The Author, in this fifth edition, has endeavoured to give an accountof the principal discoveries which have been made within the last fouryears in Chemical Science, and of the various important applications, such as the gas-lights, and the miner’s-lamp, to which they have givenrise. But in regard to doctrines or principles, the work has undergoneno material alteration. _

_London_, _July_, 1817. 

PREFACE. 

In venturing to offer to the public, and more particularly to the femalesex, an Introduction to Chemistry, the author, herself a woman, conceives that some explanation may be required; and she feels it themore necessary to apologise for the present undertaking, as herknowledge of the subject is but recent, and as she can have no realclaims to the title of chemist. 

On attending for the first time experimental lectures, the author foundit almost impossible to derive any clear or satisfactory informationfrom the rapid demonstrations which are usually, and perhapsnecessarily, crowded into popular courses of this kind. But frequentopportunities having afterwards occurred of conversing with a friend onthe subject of chemistry, and of repeating a variety of experiments, shebecame better acquainted with the principles of that science, and beganto feel highly interested in its pursuit. It was then that sheperceived, in attending the excellent lectures delivered at the RoyalInstitution, by the present Professor of Chemistry, the great advantagewhich her previous knowledge of the subject, slight as it was, gave herover others who had not enjoyed the same means of private instruction. Every fact or experiment attracted her attention, and served to explainsome theory to which she was not a total stranger; and she had thegratification to find that the numerous and elegant illustrations, forwhich that school is so much distinguished, seldom failed to produce onher mind the effect for which they were intended. 

Hence it was natural to infer, that familiar conversation was, instudies of this kind, a most useful auxiliary source of information; andmore especially to the female sex, whose education is seldom calculatedto prepare their minds for abstract ideas, or scientific language. 

As, however, there are but few women who have access to this mode ofinstruction; and as the author was not acquainted with any book thatcould prove a substitute for it, she thought that it might be useful forbeginners, as well as satisfactory to herself, to trace the steps bywhich she had acquired her little stock of chemical knowledge, and torecord, in the form of dialogue, those ideas which she had first derivedfrom conversation. 

But to do this with sufficient method, and to fix upon a mode ofarrangement, was an object of some difficulty. After much hesitation, and a degree of embarrassment, which, probably, the most competentchemical writers have often felt in common with the most superficial, a mode of division was adopted, which, though the most natural, does notalways admit of being strictly pursued--it is that of treating first ofthe simplest bodies, and then gradually rising to the most intricatecompounds. 

It is not the author’s intention to enter into a minute vindication ofthis plan. But whatever may be its advantages or inconveniences, themethod adopted in this work is such, that a young pupil, who shouldoccasionally recur to it, with a view to procure information onparticular subjects, might often find it obscure or unintelligible; forits various parts are so connected with each other as to form anuninterrupted chain of facts and reasonings, which will appearsufficiently clear and consistent to those only who may have patience togo through the whole work, or have previously devoted some attention tothe subject. 

It will, no doubt, be observed, that in the course of theseConversations, remarks are often introduced, which appear much too acutefor the young pupils, by whom they are supposed to be made. Of thisfault the author is fully aware. But, in order to avoid it, it wouldhave been necessary either to omit a variety of useful illustrations, orto submit to such minute explanations and frequent repetitions, as wouldhave rendered the work tedious, and therefore less suited to itsintended purpose. 

In writing these pages, the author was more than once checked in herprogress by the apprehension that such an attempt might be considered bysome, either as unsuited to the ordinary pursuits of her sex, orill-justified by her own recent and imperfect knowledge of the subject. But, on the one hand, she felt encouraged by the establishment of thosepublic institutions, open to both sexes, for the dissemination ofphilosophical knowledge, which clearly prove that the general opinion nolonger excludes women from an acquaintance with the elements of science;and, on the other, she flattered herself that whilst the impressionsmade upon her mind, by the wonders of Nature, studied in this new pointof view, were still fresh and strong, she might perhaps succeed thebetter in communicating to others the sentiments she herselfexperienced. 

The reader will soon perceive, in perusing this work, that he is oftensupposed to have previously acquired some slight knowledge of naturalphilosophy, a circumstance, indeed, which appears very desirable. Theauthor’s original intention was to commence this work by a small tract, explaining, on a plan analogous to this, the most essential rudiments ofthat science. This idea she has since abandoned; but the manuscript wasready, and might, perhaps, have been printed at some future period, hadnot an elementary work of a similar description, under the tide of“Scientific Dialogues, ” been pointed out to her, which, on a rapidperusal, she thought very ingenious, and well calculated to answer itsintended object. 

 Contents Of _The First Volume_. 

 ON SIMPLE BODIES. 

CONVERSATION I. Page

 ON THE GENERAL PRINCIPLES OF CHEMISTRY. 1

Connexion between Chemistry and Natural Philosophy. --Improved Stateof modern Chemistry. --Its use in the Arts. --The general Objects ofChemistry. --Definition of Elementary Bodies. --Definition ofDecomposition. --Integrant and Constituent Particles. --Distinctionbetween Simple and Compound Bodies. --Classification of SimpleBodies. --Of Chemical Affinity, or Attraction of Composition. --Examples of Composition and Decomposition. 

CONVERSATION II. 

 ON LIGHT AND HEAT. 26

Light and Heat capable of being separated. --Dr.  Herschel’sExperiments. --Phosphorescence. --Of Caloric. --Its twoModifications. --Free Caloric. --Of the three different States ofBodies, solid, fluid, and aeriform. --Dilatation of solid Bodies. --Pyrometer. --Dilatation of Fluids. --Thermometer. --Dilatation ofElastic Fluids. --Air Thermometer. --Equal Diffusion of Caloric. --Cold a Negative Quality. --Professor Prevost’s Theory of theRadiation of Heat. --Professor Pictet’s Experiments on the Reflexionof Heat. --Mr.  Leslie’s Experiments on the Radiation of Heat. 

CONVERSATION III. 

 CONTINUATION OF THE SUBJECT. 70

Of the different Power of Bodies to conduct Heat. --Attempt toaccount for this Power. --Count Rumford’s Theory of thenon-conducting Power of Fluids. --Phenomena of Boiling. --OfSolution in general. --Solvent Power of Water. --Difference betweenSolution and Mixture. --Solvent Power of Caloric. --Of Clouds, Rain, Dr.  Wells’ theory of Dew, Evaporation, &c. --Influence ofAtmospherical Pressure on Evaporation. --Ignition. 

CONVERSATION IV. 

 ON COMBINED CALORIC, COMPREHENDING SPECIFIC HEAT AND LATENT HEAT. 122

Of Specific Heat. --Of the different Capacities of Bodies for Heat. --Specific Heat not perceptible by the Senses. --How to beascertained. --Of Latent Heat. --Distinction between Latent andSpecific Heat. --Phenomena attending the Melting of Ice and theFormation of Vapour. --Phenomena attending the Formation of Ice, andthe Condensation of Elastic Fluids. --Instances of Condensation, andconsequent Disengagement of Heat, produced by Mixtures, by theSlaking of Lime. --General Remarks on Latent Heat. --Explanation ofthe Phenomena of Ether boiling, and Water freezing, at the sameTemperature. --Of the Production of Cold by Evaporation. --Calorimeter. --Meteorological Remarks. 

CONVERSATION V. 

 ON THE CHEMICAL AGENCIES OF ELECTRICITY. 160

Of Positive and Negative Electricity. --Galvani’s Discoveries. --Voltaic Battery. --Electrical Machine. --Theory of VoltaicExcitement. 

CONVERSATION VI. 

 ON OXYGEN AND NITROGEN. 181

The Atmosphere composed of Oxygen and Nitrogen in the State of Gas. --Definition of Gas. --Distinction between Gas and Vapour. --Oxygenessential to Combustion and Respiration. --Decomposition of theAtmosphere by Combustion. --Nitrogen Gas obtained by this Process. --Of Oxygenation in general. --Of the Oxydation of Metals. --OxygenGas obtained from Oxyd of Manganese. --Description of a Water-Bathfor collecting and preserving Gases. --Combustion of Iron Wire inOxygen Gas. --Fixed and volatile Products of Combustion. --PatentLamps. --Decomposition of the Atmosphere by Respiration. --Recomposition of the Atmosphere. 

CONVERSATION VII. 

 ON HYDROGEN. 214

Of Hydrogen. --Of the Formation of Water by the Combustion ofHydrogen. --Of the Decomposition of Water. --Detonation of HydrogenGas. --Description of Lavoisier’s Apparatus for the formation ofWater. --Hydrogen Gas essential to the Production of Flame. --Musical Tones produced by the Combustion of Hydrogen Gas within aGlass Tube. --Combustion of Candles explained. --Gas lights. --Detonation of Hydrogen Gas in Soap Bubbles. --Air Balloons. --Meteorological Phenomena ascribed to Hydrogen Gas. --Miner’s Lamp. 

 [Transcriber’s Note: The final two pages of the Table of Contents for Volume I were missing; everything after “Decomposition of Water” was supplied from earlier and later editions, compared against the body text. The section marked “Diamond” (Conv. IX) was called “Diamond is Carbon(e) in a state of perfect purity” in the 4th edn. , “Diamond” alone in later editions. ]

CONVERSATION VIII. 

 ON SULPHUR AND PHOSPHORUS. 256

Natural History of Sulphur. --Sublimation. --Alembic. --Combustionof Sulphur in Atmospheric Air. --Of Acidification in general. --Nomenclature of the Acids. --Combustion of Sulphur in Oxygen Gas. --Sulphuric Acid. --Sulphurous Acid. --Decomposition of Sulphur. --Sulphurated Hydrogen Gas. --Harrogate, or Hydro-sulphuratedWaters. --Phosphorus. --History of its Discovery. --Its Combustionin Oxygen Gas. --Phosphoric Acid. --Phosphorus Acid. --Eudiometer. --Combination of Phosphorus with Sulphur. --Phosphorated HydrogenGas. --Nomenclature of Binary Compounds. --Phosphoret of Limeburning under Water. 

CONVERSATION IX. 

 ON CARBON. 282

Method of obtaining pure Charcoal. --Method of making commonCharcoal. --Pure Carbon not to be obtained by Art. --Diamond. --Properties of Carbon. --Combustion of Carbon. --Production ofCarbonic Acid Gas. --Carbon susceptible of only one Degree ofAcidification. --Gaseous Oxyd of Carbon. --Of Seltzer Water andother Mineral Waters. --Effervescence. --Decomposition of Water byCarbon. --Of Fixed and Essential Oils. --Of the Combustion of Lampsand Candles. --Vegetable Acids. --Of the Power of Carbon to reviveMetals. 

CONVERSATION X. 

 ON METALS. 314

Natural History of Metals. --Of Roasting, Smelting, &c. --Oxydationof metals by the Atmosphere. --Change of Colours produced bydifferent degrees of Oxydation. --Combustion of Metals. --PerfectMetals burnt by Electricity only. --Some Metals revived by Carbonand other Combustibles. --Perfect Metals revived by Heat alone. --Ofthe Oxydation of certain Metals by the Decomposition of Water. Powerof Acids to promote this Effect. --Oxydation of Metals by Acids. --Metallic Neutral Salts. --Previous oxydation of the Metalrequisite. --Crystallisation. --Solution distinguished fromDissolution. --Five metals susceptible of acidification. --MeteoricStones. --Alloys, Soldering, Plating, &c. --Of Arsenic, and of thecaustic Effects of Oxygen. --Of Verdigris, Sympathetic Ink, &c. --Ofthe new Metals discovered by Sir H. Davy. 

 Contents Of _The Second Volume_. 

 ON COMPOUND BODIES. 

CONVERSATION XIII. Page

 ON THE ATTRACTION OF COMPOSITION. 1

Of the laws which regulate the Phenomena of the Attraction ofComposition. --1.  It takes place only between Bodies of a differentNature. --2.  Between the most minute Particles only. --3.  Between 2, 3, 4, or more Bodies. --Of Compound or Neutral Salts. --4.  Producesa Change of Temperature. --5.  The Properties which characteriseBodies in their separate State, destroyed by Combination. --6.  TheForce of Attraction estimated by that which is required by theSeparation of the Constituents. --7.  Bodies have amongst themselvesdifferent Degrees of Attraction. --Of simple elective and doubleelective Attractions. --Of quiescent and divellent Forces. --Law ofdefinite Proportions. --Decomposition of Salts by VoltaicElectricity. 

CONVERSATION XIV. 

 ON ALKALIES. 19

Of the Composition and general Properties of the Alkalies. --OfPotash. --Manner of preparing it. --Pearlash. --Soap. --Carbonat ofPotash. --Chemical Nomenclature. --Solution of Potash. --Of Glass. --Of Nitrat of Potash or Saltpetre. --Effect of Alkalies onVegetable Colours. --Of Soda. --Of Ammonia or Volatile Alkali. --Muriat of Ammonia. --Ammoniacal Gas. --Composition of Ammonia. --Hartshorn and Sal Volatile. --Combustion of Ammoniacal Gas. 

CONVERSATION XV. 

 ON EARTHS. 44

Composition of the Earths. --Of their Incombustibility. --Form theBasis of all Minerals. --Their Alkaline Properties. --Silex; itsProperties and Uses in the Arts. --Alumine; its Uses in Pottery, &c. --Alkaline Earths. --Barytes. --Lime; its extensive chemicalProperties and Uses in the Arts. --Magnesia. --Strontian. 

CONVERSATION XVI. 

 ON ACIDS. 69

Nomenclature of the Acids. --Of the Classification of Acids. --1stClass --Acids of simple and known Radicals, or Mineral Acids. --2d Class --Acids of double Radicals, or Vegetable Acids. --3d Class --Acids of triple Radicals or Animal Acids. --Of theDecomposition of Acids of the 1st Class by Combustible bodies. 

CONVERSATION XVII. 

 OF THE SULPHURIC AND PHOSPHORIC ACIDS: OR, THE COMBINATIONS OF OXYGEN WITH SULPHUR AND WITH PHOSPHORUS; AND OF THE SULPHATS AND PHOSPHATS. 80

Of the Sulphuric Acid. --Combustion of Animal or Vegetable Bodies bythis Acid. --Method of preparing it. -- The Sulphurous Acid obtainedin the Form of Gas. --May be obtained from Sulphuric Acid. --May bereduced to Sulphur. --Is absorbable by Water. --Destroys VegetableColours. --Oxyd of Sulphur. --Of Salts in general. --Sulphats. --Sulphat of Potash, or Sal Polychrest. --Cold produced by themelting of Salts. --Sulphat of Soda, or Glauber’s Salt. --Heatevolved during the Formation of Salts. --Crystallisation of Salts. --Water of Crystallisation. --Efflorescence and Deliquescence ofSalts. --Sulphat of Lime, Gypsum or Plaister of Paris. --Sulphat ofMagnesia. --Sulphat of Alumine, or Alum. --Sulphat of Iron. --OfInk. --Of the Phosphoric and Phosphorous Acids. --Phosphorusobtained from Bones. --Phosphat of Lime. 

CONVERSATION XVIII. 

 OF THE NITRIC AND CARBONIC ACIDS: OR THE COMBINATION OF OXYGEN WITH NITROGEN AND WITH CARBON; AND OF THE NITRATS AND CARBONATS. 100

Nitrogen susceptible of various Degrees of Acidification. --Of theNitric Acid. --Its Nature and Composition discovered byMr.  Cavendish. --Obtained from Nitrat of Potash. --Aqua Fortis. --Nitric Acid may be converted into Nitrous Acid. --Nitric Oxyd Gas. --Its Conversion into Nitrous Acid Gas. --Used as an EudiometricalTest. --Gaseous Oxyd of Nitrogen, or exhilarating Gas, obtained fromNitrat of Ammonia. --Its singular Effects on being respired. --Nitrats. --Of Nitrat of Potash, Nitre or Saltpetre. --OfGunpowder. --Causes of Detonation. --Decomposition of Nitre. --Deflagration. --Nitrat of Ammonia. --Nitrat of Silver. --Of theCarbonic Acid. --Formed by the Combustion of Carbon. --Constitutes acomponent Part of the Atmosphere. --Exhaled in some Caverns. --Grotto del Cane. --Great Weight of this Gas. --Produced fromcalcareous Stones by Sulphuric Acid. --Deleterious Effects of thisGas when respired. --Sources which keep up a Supply of this Gas inthe Atmosphere. --Its Effects on Vegetation. --Of the Carbonats ofLime; Marble, Chalk, Shells, Spars, and calcareous Stones. 

CONVERSATION XIX. 

 ON THE BORACIC, FLUORIC, MURIATIC, AND OXYGENATED MURIATIC ACIDS; AND ON MURIATS. 131

On the Boracic Acid. --Its Decomposition by Sir H. Davy. --Its BasisBoracium. --Its Recomposition. --Its Uses in the Arts. --Borax orBorat of Soda. --Of the Fluoric Acid. --Obtained from Fluor;corrodes Siliceous Earth; its supposed Composition. --Fluorine; itssupposed Basis. --Of the Muriatic Acid. --Obtained from Muriats. --Its gaseous Form. --Is absorbable by Water. --Its Decomposition. --Is susceptible of a stronger Degree of Oxygenation. --OxygenatedMuriatic Acid. --Its gaseous Form and other Properties. --Combustionof Bodies in this Gas. --It dissolves Gold. --Composition of AquaRegia. --Oxygenated Muriatic Acid destroys all Colours. --Sir H. Davy’s Theory of the Nature of Muriatic and Oxymuriatic Acid. --Chlorine. --Used for Bleaching and for Fumigations. --Itsoffensive Smell, &c. --Muriats. --Muriat of Soda, or common Salt. --Muriat of Ammonia. --Oxygenated Muriat of Potash. --Detonates withSulphur, Phosphorus, &c. --Experiment of burning Phosphorus underWater by means of this Salt and of Sulphuric Acid. 

CONVERSATION XX. 

 ON THE NATURE AND COMPOSITION OF VEGETABLES. 162

Of organised Bodies. --Of the Functions of Vegetables. --Of theElements of Vegetables. --Of the Materials of Vegetables. --Analysisof Vegetables. --Of Sap. --Mucilage, or Gum. --Sugar. --Manna, andHoney. --Gluten. --Vegetable Oils. --Fixed Oils, Linseed, Nut, andOlive Oils. --Volatile Oils, forming Essences and Perfumes. --Camphor. --Resins and Varnishes. --Pitch, Tar, Copal, Mastic, &c. --Gum Resins. --Myrrh, Assafœtida, &c. --Caoutchouc, or Gum Elastic. --Extractive colouring Matter; its Use in the Arts of Dyeing andPainting. --Tannin; its Use in the Art of preparing Leather. --WoodyFibre. --Vegetable Acids. --The Alkalies and Salts contained inVegetables. 

CONVERSATION XXI. 

 ON THE DECOMPOSITION OF VEGETABLES. 202

Of Fermentation in general. --Of the Saccharine Fermentation, theProduct of which is Sugar. --Of the Vinous Fermentation, the Productof which is Wine. --Alcohol, or Spirit of Wine. --Analysis of Wineby Distillation. --Of Brandy, Rum, Arrack, Gin, &c. --Tartrit ofPotash, or Cream of Tartar. --Liqueurs. --Chemical Properties ofAlcohol. --Its Combustion. --Of Ether. --Of the AcetousFermentation, the Product of which is Vinegar. --Fermentation ofBread. --Of the Putrid Fermentation, which reduces Vegetables totheir Elements. --Spontaneous Succession of these Fermentations. --Of Vegetables said to be petrified. --Of Bitumens: Naphtha, Asphaltum, Jet, Coal, Succin, or Yellow Amber. --Of Fossil Wood, Peat, and Turf. 

CONVERSATION XXII. 

 HISTORY OF VEGETATION. 243

Connexion between the Vegetable and Animal Kingdoms. --Of Manures. --Of Agriculture. --Inexhaustible Sources of Materials for thePurposes of Agriculture. --Of sowing Seed. --Germination of theSeed. --Function of the Leaves of Plants. --Effects of Light and Airon Vegetation. --Effects of Water on Vegetation. --Effects ofVegetation on the Atmosphere. --Formation of Vegetable Materials bythe Organs of Plants. --Vegetable Heat. --Of the Organs of Plants. --Of the Bark, consisting of Epidermis, Parenchyma, and CorticalLayers. --Of Alburnum, or Wood. --Leaves, Flowers, and Seeds. --Effects of the Season on Vegetation. --Vegetation of Evergreens inWinter. 

CONVERSATION XXIII. 

 ON THE COMPOSITION OF ANIMALS. 276

Elements of Animals. --Of the principal Materials of Animals, viz. --Gelatine, Albumen, Fibrine, Mucus. --Of Animal Acids. --Of AnimalColours, Prussian Blue, Carmine, and Ivory Black. 

CONVERSATION XXIV. 

 ON THE ANIMAL ECONOMY. 297

Of the principal Animal Organs. --Of Bones, Teeth, Horns, Ligaments, and Cartilage. --Of the Muscles, constituting the Organs of Motion. --Of the Vascular System, for the Conveyance of Fluids. --Of theGlands, for the Secretion of Fluids. --Of the Nerves, constitutingthe Organs of Sensation. --Of the Cellular Substance which connectsthe several Organs. --Of the Skin. 

CONVERSATION XXV. 

 ON ANIMALISATION, NUTRITION, AND RESPIRATION. 314

Digestion. --Solvent Power of the Gastric Juice. --Formation of aChyle. --Its Assimilation, or Conversion into Blood. --OfRespiration. --Mechanical Process of Respiration. --Chemical Processof Respiration. --Of the Circulation of the Blood. --Of theFunctions of the Arteries, the Veins, and the Heart. --Of the Lungs. --Effects of Respiration on the Blood. 

CONVERSATION XXVI. 

 ON ANIMAL HEAT; AND OF VARIOUS ANIMAL PRODUCTS. 336

Of the Analogy of Combustion and Respiration. --Animal Heat evolvedin the Lungs. --Animal Heat evolved in the Circulation. --Heatproduced by Fever. --Perspiration. --Heat produced by Exercise. --Equal Temperature of Animals at all Seasons. --Power of the AnimalBody to resist the Effects of Heat. --Cold produced by Perspiration. --Respiration of Fish and of Birds. --Effects of Respiration onMuscular Strength. --Of several Animal Products, viz. Milk, Butter, and Cheese; Spermaceti; Ambergris; Wax; Lac; Silk; Musk; Civet;Castor. --Of the putrid Fermentation. --Conclusion. 

CONVERSATIONS

ON

CHEMISTRY. 

CONVERSATION I. 

ON THE GENERAL PRINCIPLES OF CHEMISTRY. 

MRS. B. 

As you have now acquired some elementary notions of NATURAL PHILOSOPHY, I am going to propose to you another branch of science, to which I amparticularly anxious that you should devote a share of your attention. This is CHEMISTRY, which is so closely connected with NaturalPhilosophy, that the study of the one must be incomplete without someknowledge of the other; for, it is obvious that we can derive but a veryimperfect idea of bodies from the study of the general laws by whichthey are governed, if we remain totally ignorant of their intimatenature. 

CAROLINE. 

To confess the truth, Mrs.  B. , I am not disposed to form a veryfavourable idea of chemistry, nor do I expect to derive muchentertainment from it. I prefer the sciences which exhibit nature on agrand scale, to those that are confined to the minutiæ of petty details. Can the studies which we have lately pursued, the general properties ofmatter, or the revolutions of the heavenly bodies, be compared to themixing up of a few insignificant drugs? I grant, however, there may beentertaining experiments in chemistry, and should not dislike to trysome of them: the distilling, for instance, of lavender, or rosewater .  .  .  .  .  . 

MRS. B. 

I rather imagine, my dear Caroline, that your want of taste forchemistry proceeds from the very limited idea you entertain of itsobject. You confine the chemist’s laboratory to the narrow precincts ofthe apothecary’s and perfumer’s shops, whilst it is subservient to animmense variety of other useful purposes. Besides, my dear, chemistry isby no means confined to works of art. Nature also has her laboratory, which is the universe, and there she is incessantly employed in chemicaloperations. You are surprised, Caroline, but I assure you that the mostwonderful and the most interesting phenomena of nature are almost all ofthem produced by chemical powers. What Bergman, in the introduction tohis history of chemistry, has said of this science, will give you a morejust and enlarged idea of it. The knowledge of nature may be divided, heobserves, into three periods. The first was that in which the attentionof men was occupied in learning the external forms and characters ofobjects, and this is called _Natural History_. In the second, theyconsidered the effects of bodies acting on each other by theirmechanical power, as their weight and motion, and this constitutes thescience of _Natural Philosophy_. The third period is that in which theproperties and mutual action of the elementary parts of bodies wasinvestigated. This last is the science of CHEMISTRY, and I have no doubtyou will soon agree with me in thinking it the most interesting. 

You may easily conceive, therefore, that without entering into theminute details of practical chemistry, a woman may obtain such aknowledge of the science as will not only throw an interest on thecommon occurrences of life, but will enlarge the sphere of her ideas, and render the contemplation of nature a source of delightfulinstruction. 

CAROLINE. 

If this is the case, I have certainly been much mistaken in the notion Ihad formed of chemistry. I own that I thought it was chiefly confined tothe knowledge and preparation of medicines. 

MRS. B. 

That is only a branch of chemistry which is called Pharmacy; and, thoughthe study of it is certainly of great importance to the world at large, it belongs exclusively to professional men, and is therefore the lastthat I should advise you to pursue. 

EMILY. 

But, did not the chemists formerly employ themselves in search of thephilosopher’s stone, or the secret of making gold?

MRS. B. 

These were a particular set of misguided philosophers, who dignifiedthemselves with the name of Alchemists, to distinguish their pursuitsfrom those of the common chemists, whose studies were confined to theknowledge of medicines. 

But, since that period, chemistry has undergone so complete arevolution, that, from an obscure and mysterious art, it is now become aregular and beautiful science, to which art is entirely subservient. Itis true, however, that we are indebted to the alchemists for many veryuseful discoveries, which sprung from their fruitless attempts to makegold, and which, undoubtedly, have proved of infinitely greateradvantage to mankind than all their chimerical pursuits. 

The modern chemists, instead of directing their ambition to the vainattempt of producing any of the original substances in nature, ratheraim at analysing and imitating her operations, and have sometimessucceeded in forming combinations, or effecting decompositions, noinstances of which occur in the chemistry of Nature. They have littlereason to regret their inability to make gold, whilst, by theirinnumerable inventions and discoveries, they have so greatly stimulatedindustry and facilitated labour, as prodigiously to increase theluxuries as well as the necessaries of life. 

EMILY. 

But, I do not understand by what means chemistry can facilitate labour;is not that rather the province of the mechanic?

MRS. B. 

There are many ways by which labour may be rendered more easy, independently of mechanics; but even the machine, the most wonderful inits effects, the Steam-engine, cannot be understood without theassistance of chemistry. In agriculture, a chemical knowledge of thenature of soils, and of vegetation, is highly useful; and, in those artswhich relate to the comforts and conveniences of life, it would beendless to enumerate the advantages which result from the study of thisscience. 

CAROLINE. 

But, pray, tell us more precisely in what manner the discoveries ofchemists have proved so beneficial to society?

MRS. B. 

That would be an injudicious anticipation; for you would not comprehendthe nature of such discoveries and useful applications, as well as youwill do hereafter. Without a due regard to method, we cannot expect tomake any progress in chemistry. I wish to direct your observationschiefly to the chemical operations of Nature; but those of Art arecertainly of too high importance to pass unnoticed. We shall thereforeallow them also some share of our attention. 

EMILY. 

Well, then, let us now set to work regularly. I am very anxious tobegin. 

MRS. B. 

The object of chemistry is to obtain a knowledge of the intimate natureof bodies, and of their mutual action on each other. You find therefore, Caroline, that this is no narrow or confined science, which comprehendsevery thing material within our sphere. 

CAROLINE. 

On the contrary, it must be inexhaustible; and I am a loss to conceivehow any proficiency can be made in a science whose objects are sonumerous. 

MRS. B. 

If every individual substance were formed of different materials, thestudy of chemistry would, indeed, be endless; but you must observe thatthe various bodies in nature are composed of certain elementaryprinciples, which are not very numerous. 

CAROLINE. 

Yes; I know that all bodies are composed of fire, air, earth, and water;I learnt that many years ago. 

MRS. B. 

But you must now endeavour to forget it. I have already informed youwhat a great change chemistry has undergone since it has become aregular science. Within these thirty years especially, it hasexperienced an entire revolution, and it is now proved, that neitherfire, air, earth, nor water, can be called elementary bodies. For anelementary body is one that has never been decomposed, that is to say, separated into other substances; and fire, air, earth, and water, areall of them susceptible of decomposition. 

EMILY. 

I thought that decomposing a body was dividing it into its minutestparts. And if so, I do not understand why an elementary substance is notcapable of being decomposed, as well as any other. 

MRS. B. 

You have misconceived the idea of _decomposition_; it is very differentfrom mere _division_. The latter simply reduces a body into parts, butthe former separates it into the various ingredients, or materials, ofwhich it is composed. If we were to take a loaf of bread, and separatethe several ingredients of which it is made, the flour, the yeast, thesalt, and the water, it would be very different from cutting orcrumbling the loaf into pieces. 

EMILY. 

I understand you now very well. To decompose a body is to separate fromeach other the various elementary substances of which it consists. 

CAROLINE. 

But flour, water, and other materials of bread, according to ourdefinition, are not elementary substances?

MRS. B. 

No, my dear; I mentioned bread rather as a familiar comparison, toillustrate the idea, than as an example. 

The elementary substances of which a body is composed are called the_constituent_ parts of that body; in decomposing it, therefore, weseparate its constituent parts. If, on the contrary, we divide a body bychopping it to pieces, or even by grinding or pounding it to the finestpowder, each of these small particles will still consist of a portion ofthe several constituent parts of the whole body: these are called the_integrant_ parts; do you understand the difference?

EMILY. 

Yes, I think, perfectly. We _decompose_ a body into its _constituent_parts; and _divide_ it into its _integrant_ parts. 

MRS. B. 

Exactly so. If therefore a body consists of only one kind of substance, though it may be divided into its integrant parts, it is not possible todecompose it. Such bodies are therefore called _simple_ or _elementary_, as they are the elements of which all other bodies are composed. _Compound bodies_ are such as consist of more than one of theseelementary principles. 

CAROLINE. 

But do not fire, air, earth, and water, consist, each of them, but ofone kind of substance?

MRS. B. 

No, my dear; they are every one of them susceptible of being separatedinto various simple bodies. Instead of four, chemists now reckon upwardsof forty elementary substances. The existence of most of these isestablished by the clearest experiments; but, in regard to a few ofthem, particularly the most subtle agents of nature, _heat_, _light_, and _electricity_, there is yet much uncertainty, and I can only giveyou the opinion which seems most probably deduced from the latestdiscoveries. After I have given you a list of the elementary bodies, classed according to their properties, we shall proceed to examine eachof them separately, and then consider them in their combinations witheach other. 

Excepting the more general agents of nature, heat, light, andelectricity, it would seem that the simple form of bodies is that of ametal. 

CAROLINE. 

You astonish me! I thought the metals were only one class of minerals, and that there were besides, earths, stones, rocks, acids, alkalies, vapours, fluids, and the whole of the animal and vegetable kingdoms. 

MRS. B. 

You have made a tolerably good enumeration, though I fear not arrangedin the most scientific order. All these bodies, however, it is nowstrongly believed, may be ultimately resolved into metallic substances. Your surprise at this circumstance is not singular, as the decompositionof some of them, which has been but lately accomplished, has excited thewonder of the whole philosophical world. 

But to return to the list of simple bodies--these being usually found incombination with oxygen, I shall class them according to theirproperties when so combined. This will, I think, facilitate their futureinvestigation. 

EMILY. 

Pray what is oxygen?

MRS. B. 

A simple body; at least one that is supposed to be so, as it has neverbeen decomposed. It is always found united with the negativeelectricity. It will be one of the first of the elementary bodies whoseproperties I shall explain to you, and, as you will soon perceive, it isone of the most important in nature; but it would be irrelevant to enterupon this subject at present. We must now confine our attention to theenumeration and classification of the simple bodies in general. They maybe arranged as follows:

CLASS I. 

_Comprehending the imponderable agents, viz. _

 HEAT or CALORIC, LIGHT, ELECTRICITY. 

CLASS II. 

_Comprehending agents capable of uniting with inflammable bodies, and inmost instances of effecting their combustion. _

 OXYGEN, CHLORINE, IODINE. *

 [Footnote *: It has been questioned by some eminent chemists, whether these two last agents should not be classed among the inflammable bodies, as they are capable of combining with oxygen, as well as with inflammable bodies. But they seem to be more distinctly characterised by their property of supporting combustion than by any other quality. ]

CLASS III. 

_Comprehending bodies capable of uniting with oxygen, and, forming withit various compounds. This class may be divided as follows:_

DIVISION 1. 

 HYDROGEN, _forming_ water. 

DIVISION 2. 

_Bodies forming acids. _

 NITROGEN, _forming_ nitric acid. SULPHUR, _forming_ sulphuric acid. PHOSPHORUS, _forming_ phosphoric acid. CARBON, _forming_ carbonic acid. BORACIUM, _forming_ boracic acid. FLUORIUM, _forming_ fluoric acid. MURIATIUM, _forming_ muriatic acid. 

DIVISION 3. 

_Metallic bodies forming alkalies. _

 POTASSIUM, _forming_ potash. SODIUM, _forming_ soda. AMMONIUM, _forming_ ammonia. 

DIVISION 4. 

_Metallic bodies forming earths. _

 CALCIUM, _or metal forming_ lime. MAGNIUM, _forming_ magnesia. BARIUM, _forming_ barytes. STRONTIUM, _forming_ strontites. SILICIUM, _forming_ silex. ALUMIUM, _forming_ alumine. YTTRIUM, _forming_ yttria. GLUCIUM, _forming_ glucina. ZIRCONIUM, _forming_ zirconi. *

 [Footnote *: Of all these earths, three or four only have as yet been distinctly decomposed. ]

DIVISION 5. 

_Metals, either naturally metallic, or yielding their oxygen to carbonor to heat alone. _

_Subdivision 1. _

_Malleable Metals. _

 GOLD, PLATINA, PALLADIUM, SILVER* MERCURY† TIN, COPPER, IRON, LEAD, NICKEL, ZINC. 

 [Footnote *: These first four metals have commonly been distinguished by the appellation of perfect or noble metals, on account of their possessing the characteristic properties of ductility, malleability, inalterability, and great specific gravity, in an eminent degree. ]

 [Footnote †: Mercury, in its liquid state, cannot, of course, be called a malleable metal. But when frozen, it possesses a considerable degree of malleability. ]

_Subdiv. 2. _

_Brittle Metals. _

 ARSENIC, BISMUTH, ANTIMONY, MANGANESE, TELLURIUM, COBALT, TUNGSTEN, MOLYBDENUM, TITANIUM, CHROME, URANIUM, COLUMBIUM _or_ TANTALIUM, IRIDIUM, OSMIUM, RHODIUM. *

 [Footnote *: These last four or five metallic bodies are placed under this class for the sake of arrangement, though some of their properties have not been yet fully investigated. ]

CAROLINE. 

Oh, what a formidable list! You will have much to do to explain it, Mrs.  B. ; for I assure you it is perfectly unintelligible to me, and Ithink rather perplexes than assists me. 

MRS. B. 

Do not let that alarm you, my dear; I hope that hereafter thisclassification will appear quite clear, and, so far from perplexing you, will assist you in arranging your ideas. It would be in vain to attemptforming a division that would appear perfectly clear to a beginner: foryou may easily conceive that a chemical division being necessarilyfounded on properties with which you are almost wholly unacquainted, itis impossible that you should at once be able to understand its meaningor appreciate its utility. 

But, before we proceed further, it will be necessary to give you someidea of chemical attraction, a power on which the whole science depends. 

_Chemical Attraction_, or the _Attraction of Composition_, consists inthe peculiar tendency which bodies of a different nature have to unitewith each other. It is by this force that all the compositions, anddecompositions, are effected. 

EMILY. 

What is the difference between chemical attraction, and the attractionof cohesion, or of aggregation, which you often mentioned to us, informer conversations?

MRS. B. 

The attraction of cohesion exists only between particles of the _same_nature, whether simple or compound; thus it unites the particles of apiece of metal which is a simple substance, and likewise the particlesof a loaf of bread which is a compound. The attraction of composition, on the contrary, unites and maintains, in a state of combination, particles of a _dissimilar_ nature; it is this power that forms each ofthe compound particles of which bread consists; and it is by theattraction of cohesion that all these particles are connected into asingle mass. 

EMILY. 

The attraction of cohesion, then, is the power which unites theintegrant particles of a body: the attraction of composition that whichcombines the constituent particles. Is it not so?

MRS. B. 

Precisely: and observe that the attraction of cohesion unites particlesof a similar nature, without changing their original properties; theresult of such an union, therefore, is a body of the same kind as theparticles of which it is formed; whilst the attraction of composition, by combining particles of a dissimilar nature, produces compound bodies, quite different from any of their constituents. If, for instance, I pouron the piece of copper, contained in this glass, some of this liquid(which is called nitric acid), for which it has a strong attraction, every particle of the copper will combine with a particle of acid, andtogether they will form a new body, totally different from either thecopper or the acid. 

Do you observe the internal commotion that already begins to take place?It is produced by the combination of these two substances; and yet theacid has in this case to overcome not only the resistance which thestrong cohesion of the particles of copper opposes to their combinationwith it, but also to overcome the weight of the copper, which makes itsink to the bottom of the glass, and prevents the acid from having suchfree access to it as it would if the metal were suspended in the liquid. 

EMILY. 

The acid seems, however, to overcome both these obstacles withoutdifficulty, and appears to be very rapidly dissolving the copper. 

MRS. B. 

By this means it reduces the copper into more minute parts than couldpossibly be done by any mechanical power. But as the acid can act onlyon the surface of the metal, it will be some time before the union ofthese two bodies will be completed. 

You may, however, already see how totally different this compound isfrom either of its ingredients. It is neither colourless, like the acid, nor hard, heavy, and yellow like the copper. If you tasted it, you wouldno longer perceive the sourness of the acid. It has at present theappearance of a blue liquid; but when the union is completed, and thewater with which the acid is diluted is evaporated, the compound willassume the form of regular crystals, of a fine blue colour, andperfectly transparent*. Of these I can shew you a specimen, as I haveprepared some for that purpose. 

 [Footnote *: These crystals are more easily obtained from a mixture of sulphuric with a little nitric acid. ]

CAROLINE. 

How very beautiful they are, in colour, form, and transparency!

EMILY. 

Nothing can be more striking than this example of chemical attraction. 

MRS. B. 

The term _attraction_ has been lately introduced into chemistry as asubstitute for the word _affinity_, to which some chemists haveobjected, because it originated in the vague notion that chemicalcombinations depended upon a certain resemblance, or relationship, between particles that are disposed to unite; and this idea is not onlyimperfect, but erroneous, as it is generally particles of the mostdissimilar nature, that have the greatest tendency to combine. 

CAROLINE. 

Besides, there seems to be no advantage in using a variety of terms toexpress the same meaning; on the contrary it creates confusion; and aswe are well acquainted with the term Attraction in natural philosophy, we had better adopt it in chemistry likewise. 

MRS. B. 

If you have a clear idea of the meaning, I shall leave you at liberty toexpress it in the terms you prefer. For myself, I confess that I thinkthe word Attraction best suited to the general law that unites theintegrant particles of bodies; and Affinity better adapted to that whichcombines the constituent particles, as it may convey an idea of thepreference which some bodies have for others, which the term _attractionof composition_ does not so well express. 

EMILY. 

So I think; for though that preference may not result from anyrelationship, or similitude, between the particles (as you say was oncesupposed), yet, as it really exists, it ought to be expressed. 

MRS. B. 

Well, let it be agreed that you may use the terms _affinity_, _chemicalattraction_ and _attraction of composition_, indifferently, provided yourecollect that they have all the same meaning. 

EMILY. 

I do not conceive how bodies can be decomposed by chemical attraction. That this power should be the means of composing them, is very obvious;but that it should, at the same time, produce exactly the contraryeffect, appears to me very singular. 

MRS. B. 

To decompose a body is, you know, to separate its constituent parts, which, as we have just observed, cannot be done by mechanical means. 

EMILY. 

No: because mechanical means separate only the integrant particles; theyact merely against the attraction of cohesion, and only divide acompound into smaller parts. 

MRS. B. 

The decomposition of a body is performed by chemical powers. If youpresent to a body composed of two principles, a third, which has agreater affinity for one of them than the two first have for each other, it will be decomposed, that is, its two principles will be separated bymeans of the third body. Let us call two ingredients, of which the bodyis composed, A and B. If we present to it another ingredient C, whichhas a greater affinity for B than that which unites A and B, itnecessarily follows that B will quit A to combine with C. The newingredient, therefore, has effected a decomposition of the original bodyA B; A has been left alone, and a new compound, B C, has been formed. 

EMILY. 

We might, I think, use the comparison of two friends, who were veryhappy in each other’s society, till a third disunited them by thepreference which one of them gave to the new-comer. 

MRS. B. 

Very well. I shall now show you how this takes place in chemistry. 

Let us suppose that we wish to decompose the compound we have justformed by the combination of the two ingredients, copper and nitricacid; we may do this by presenting to it a piece of iron, for which theacid has a stronger attraction than for copper; the acid will, consequently, quit the copper to combine with the iron, and the copperwill be what the chemists call _precipitated_, that is to say, it willbe thrown down in its separate state, and reappear in its simple form. 

In order to produce this effect, I shall dip the blade of this knifeinto the fluid, and, when I take it out, you will observe, that, insteadof being wetted with a bluish liquid, like that contained in the glass, it will be covered with a thin coat of copper. 

CAROLINE. 

So it is really! but then is it not the copper, instead of the acid, that has combined with the iron blade?

MRS. B. 

No; you are deceived by appearances: it is the acid which combines withthe iron, and, in so doing, deposits or precipitates the copper on thesurface of the blade. 

EMILY. 

But, cannot three or more substances combine together, without any ofthem being precipitated?

MRS. B. 

That is sometimes the case; but, in general, the stronger affinitydestroys the weaker; and it seldom happens that the attraction ofseveral substances for each other is so equally balanced as to producesuch complicated compounds. 

CAROLINE. 

But, pray, Mrs. B. , what is the cause of the chemical attraction ofbodies for each other? It appears to me more extraordinary or unnatural, if I may use the expression, than the attraction of cohesion, whichunites particles of a similar nature. 

MRS. B. 

Chemical attraction may, like that of cohesion or gravitation, be one ofthe powers inherent in matter which, in our present state of knowledge, admits of no other satisfactory explanation than an immediate referenceto a divine cause. Sir H. Davy, however, whose important discoverieshave opened such improved views in chemistry, has suggested anhypothesis which may throw great light upon that science. He supposesthat there are two kinds of electricity, with one or other of which allbodies are united. These we distinguish by the names of _positive_ and_negative_ electricity; those bodies are disposed to combine, whichpossess opposite electricities, as they are brought together by theattraction which these electricities have for each other. But, whetherthis hypothesis be altogether founded on truth or not, it is impossibleto question the great influence of electricity in chemical combinations. 

EMILY. 

So, that we must suppose that the two electricities always attract eachother, and thus compel the bodies in which they exist to combine?

CAROLINE. 

And may not this be also the cause of the attraction of cohesion?

MRS. B. 

No, for in particles of the same nature the same electricities mustprevail, and it is only the different or opposite electric fluids thatattract each other. 

CAROLINE. 

These electricities seem to me to be a kind of chemical spirit, whichanimates the particles of bodies, and draws them together. 

EMILY. 

If it is known, then, with which of the electricities bodies are united, it can be inferred which will, and which will not, combine together?

MRS. B. 

Certainly. --I should not omit to mention, that some doubts have beenentertained whether electricity be really a material agent, or whetherit might not be a power inherent in bodies, similar to, or, perhapsidentical with, attraction. 

EMILY. 

But what then would be the electric spark which is visible, and musttherefore be really material?

MRS. B. 

What we call the electric spark, may, Sir H. Davy says, be merely theheat and light, or fire produced by the chemical combinations with whichthese phenomena are always connected. We will not, however, enter morefully on this important subject at present, but reserve the principalfacts which relate to it to a future conversation. 

Before we part, however, I must recommend you to fix in your memory thenames of the simple bodies, against our next interview. 

CONVERSATION II. 

ON LIGHT AND HEAT OR CALORIC. 

CAROLINE. 

We have learned by heart the names of all the simple bodies which youhave enumerated, and we are now ready to enter on the examination ofeach of them successively. You will begin, I suppose, with LIGHT?

MRS. B. 

Respecting the nature of light we have little more than conjectures. Itis considered by most philosophers as a real substance, immediatelyemanating from the sun, and from all luminous bodies, from which it isprojected in right lines with prodigious velocity. Light, however, beingimponderable, it cannot be confined and examined by itself; andtherefore it is to the effects it produces on other bodies, rather thanto its immediate nature, that we must direct our attention. 

The connection between light and heat is very obvious; indeed, it issuch, that it is extremely difficult to examine the one independently ofthe other. 

EMILY. 

But, is it possible to separate light from heat; I thought they wereonly different degrees of the same thing, fire?

MRS. B. 

I told you that fire was not now considered as a simple element. Whetherlight and heat be altogether different agents, or not, I cannot pretendto decide; but, in many cases, light may be separated from heat. Thefirst discovery of this was made by a celebrated Swedish chemist, Scheele. Another very striking illustration of the separation of heatand light was long after pointed out by Dr. Herschell. This philosopherdiscovered that these two agents were emitted in the rays of the sun, and that heat was less refrangible than light; for, in separating thedifferent coloured rays of light by a prism (as we did some time ago), he found that the greatest heat was beyond the spectrum, at a littledistance from the red rays, which, you may recollect, are the leastrefrangible. 

EMILY. 

I should like to try that experiment. 

MRS. B. 

It is by no means an easy one: the heat of a ray of light, refracted bya prism, is so small, that it requires a very delicate thermometer todistinguish the difference of the degree of heat within and without thespectrum. For in this experiment the heat is not totally separated fromthe light, each coloured ray retaining a certain portion of it, thoughthe greatest part is not sufficiently refracted to fall within thespectrum. 

EMILY. 

I suppose, then, that those coloured rays which are the leastrefrangible, retain the greatest quantity of heat?

MRS. B. 

They do so. 

EMILY. 

Though I no longer doubt that light and heat can be separated, Dr. Herschell’s experiment does not appear to me to afford sufficient proofthat they are essentially different; for light, which you call a simplebody, may likewise be divided into the various coloured rays. 

MRS. B. 

No doubt there must be some difference in the various coloured rays. Even their chemical powers are different. The blue rays, for instance, have the greatest effect in separating oxygen from bodies, as was foundby Scheele; and there exist also, as Dr. Wollaston has shown, rays morerefrangible than the blue, which produce the same chemical effect, and, what is very remarkable, are invisible. 

EMILY. 

Do you think it possible that heat may be merely a modification oflight?

MRS. B. 

That is a supposition which, in the present state of natural philosophy, can neither be positively affirmed nor denied. Let us, therefore, instead of discussing theoretical points, be contented with examiningwhat is known respecting the chemical effects of light. 

Light is capable of entering into a kind of transitory union withcertain substances, and this is what has been called phosphorescence. Bodies that are possessed of this property, after being exposed to thesun’s rays, appear luminous in the dark. The shells of fish, the bonesof land animals, marble, limestone, and a variety of combinations ofearths, are more or less powerfully phosphorescent. 

CAROLINE. 

I remember being much surprised last summer with the phosphorescentappearance of some pieces of rotten wood, which had just been dug out ofthe ground; they shone so bright that I at first supposed them to beglow-worms. 

EMILY. 

And is not the light of a glow-worm of a phosphorescent nature?

MRS. B. 

It is a very remarkable instance of phosphorescence in living animals;this property, however, is not exclusively possessed by the glow-worm. The insect called the lanthorn-fly, which is peculiar to warm climates, emits light as it flies, producing in the dark a remarkably sparklingappearance. But it is more common to see animal matter in a dead statepossessed of a phosphorescent quality; sea fish is often eminently so. 

EMILY. 

I have heard that the sea has sometimes had the appearance of beingilluminated, and that the light is supposed to proceed from the spawn offishes floating on its surface. 

MRS. B. 

This light is probably owing to that or some other animal matter. Seawater has been observed to become luminous from the substance of a freshherring having been immersed in it; and certain insects, of the Medusakind, are known to produce similar effects. 

But the strongest phosphorescence is produced by chemical compositionsprepared for the purpose, the most common of which consists of oystershells and sulphur, and is known by the name of Canton’s Phosphorus. 

EMILY. 

I am rather surprised, Mrs. B. , that you should have said so much of thelight emitted by phosphorescent bodies without taking any notice of thatwhich is produced by burning bodies. 

MRS. B. 

The light emitted by the latter is so intimately connected with thechemical history of combustion, that I must defer all explanation of ittill we come to the examination of that process, which is one of themost interesting in chemical science. 

Light is an agent capable of producing various chemical changes. It isessential to the welfare both of the animal and vegetable kingdoms; formen and plants grow pale and sickly if deprived of its salutaryinfluence. It is likewise remarkable for its property of destroyingcolour, which renders it of great consequence in the process ofbleaching. 

EMILY. 

Is it not singular that light, which in studying optics we were taughtto consider as the source and origin of colours, should have also thepower of destroying them?

CAROLINE. 

It is a fact, however, that we every day experience; you know how itfades the colours of linens and silks. 

EMILY. 

Certainly. And I recollect that endive is made to grow white instead ofgreen, by being covered up so as to exclude the light. But by what meansdoes light produce these effects?

MRS. B. 

This I cannot attempt to explain to you until you have obtained afurther knowledge of chemistry. As the chemical properties of light canbe accounted for only in their reference to compound bodies, it would beuseless to detain you any longer on this subject; we may therefore passon to the examination of heat, or caloric, with which we are somewhatbetter acquainted. 

HEAT and LIGHT may be always distinguished by the different sensationsthey produce, _Light_ affects the sense of sight; _Caloric_ that offeeling; the one produces _Vision_, the other the sensation of _Heat_. 

Caloric is found to exist in a variety of forms or modifications, and Ithink it will be best to consider it under the two following heads, viz. 

1. FREE OR RADIANT CALORIC. 

2. COMBINED CALORIC. 

The first, FREE or RADIANT CALORIC, is also called HEAT OF TEMPERATURE;it comprehends all heat which is perceptible to the senses, and affectsthe thermometer. 

EMILY. 

You mean such as the heat of the sun, of fire, of candles, of stoves; inshort, of every thing that burns?

MRS. B. 

And likewise of things that do not burn, as, for instance, the warmth ofthe body; in a word, all heat that is _sensible_, whatever may be itsdegree, or the source from which it is derived. 

CAROLINE. 

What then are the other modifications of caloric? It must be a strangekind of heat that cannot be perceived by our senses. 

MRS. B. 

None of the modifications of caloric should properly be called _heat_;for heat, strictly speaking, is the sensation produced by caloric, onanimated bodies; this word, therefore, in the accurate language ofscience, should be confined to express the sensation. But custom hasadapted it likewise to inanimate matter, and we say _the heat of anoven_, _the heat of the sun_, without any reference to the sensationwhich they are capable of exciting. 

It was in order to avoid the confusion which arose from thus confoundingthe cause and effect, that modern chemists adopted the new word_caloric_, to denote the principle which produces heat; yet they do notalways, in compliance with their own language, limit the word _heat_ tothe expression of the sensation, since they still frequently employ itin reference to the other modifications of caloric which are quiteindependent of sensation. 

CAROLINE. 

But you have not yet explained to us what these other modifications ofcaloric are. 

MRS. B. 

Because you are not acquainted with the properties of free caloric, andyou know that we have agreed to proceed with regularity. 

One of the most remarkable properties of free caloric is its power of_dilating_ bodies. This fluid is so extremely subtle, that it enters andpervades all bodies whatever, forces itself between their particles, andnot only separates them, but frequently drives them asunder to aconsiderable distance from each other. It is thus that caloric dilatesor expands a body so as to make it occupy a greater space than it didbefore. 

EMILY. 

The effect it has on bodies, therefore, is directly contrary to that ofthe attraction of cohesion; the one draws the particles together, theother drives them asunder. 

MRS. B. 

Precisely. There is a continual struggle between the attraction ofaggregation, and the expansive power of caloric; and from the action ofthese two opposite forces, result all the various forms of matter, ordegrees of consistence, from the solid, to the liquid and aëriformstate. And accordingly we find that most bodies are capable of passingfrom one of these forms to the other, merely in consequence of theirreceiving different quantities of caloric. 

CAROLINE. 

That is very curious; but I think I understand the reason of it. If agreat quantity of caloric is added to a solid body, it introduces itselfbetween the particles in such a manner as to overcome, in a considerabledegree, the attraction of cohesion; and the body, from a solid, is thenconverted into a fluid. 

MRS. B. 

This is the case whenever a body is fused or melted; but if you addcaloric to a liquid, can you tell me what is the consequence?

CAROLINE. 

The caloric forces itself in greater abundance between the particles ofthe fluid, and drives them to such a distance from each other, thattheir attraction of aggregation is wholly destroyed: the liquid is thentransformed into vapour. 

MRS. B. 

Very well; and this is precisely the case with boiling water, when it isconverted into steam or vapour, and with all bodies that assume anaëriform state. 

EMILY. 

I do not well understand the word aëriform?

MRS. B. 

Any elastic fluid whatever, whether it be merely vapour or permanentair, is called aëriform. 

But each of these various states, solid, liquid, and aëriform, admit ofmany different degrees of density, or consistence, still arising(chiefly at least) from the different quantities of caloric the bodiescontain. Solids are of various degrees of density, from that of gold, tothat of a thin jelly. Liquids, from the consistence of melted glue, ormelted metals, to that of ether, which is the lightest of all liquids. The different elastic fluids (with which you are not yet acquainted) aresusceptible of no less variety in their degrees of density. 

EMILY. 

But does not every individual body also admit of different degrees ofconsistence, without changing its state?

MRS. B. 

Undoubtedly; and this I can immediately show you by a very simpleexperiment. This piece of iron now exactly fits the frame, or ring, madeto receive it; but if heated red hot, it will no longer do so, for itsdimensions will be so much increased by the caloric that has penetratedinto it, that it will be much too large for the frame. 

The iron is now red hot; by applying it to the frame, we shall see howmuch it is dilated. 

EMILY. 

Considerably so indeed! I knew that heat had this effect on bodies, butI did not imagine that it could be made so conspicuous. 

MRS. B. 

By means of this instrument (called a Pyrometer) we may estimate, in themost exact manner, the various dilatations of any solid body by heat. The body we are now going to submit to trial is this small iron bar;I fix it to this apparatus, (PLATE I. Fig.  1. ) and then heat it bylighting the three lamps beneath it: when the bar expands, it increasesin length as well as thickness; and, as one end communicates with thiswheel-work, whilst the other end is fixed and immoveable, no sooner doesit begin to dilate than it presses against the wheel-work, and sets inmotion the index, which points out the degrees of dilatation on thedial-plate. 

 [Illustration: Plate I. Vol. I. P. 38. 

 Fig. 1. Pyrometer. A. A Bar of Metal. 1. 2. 3 Lamps burning. B. B Wheel work. C Index. 

 Fig. 2 A. A Glass tubes with bulbs. B. B Glasses of water in which they are immersed. ]

EMILY. 

This is, indeed, a very curious instrument; but I do not understand theuse of the wheels: would it not be more simple, and answer the purposeequally well, if the bar, in dilating, pressed against the index, andput it in motion without the intervention of the wheels?

MRS. B. 

The use of the wheels is merely to multiply the motion, and thereforerender the effect of the caloric more obvious; for if the index moved nomore than the bar increased in length, its motion would scarcely beperceptible; but by means of the wheels it moves in a much greaterproportion, which therefore renders the variations far more conspicuous. 

By submitting different bodies to the test of the pyrometer, it is foundthat they are far from dilating in the same proportion. Different metalsexpand in different degrees, and other kinds of solid bodies vary stillmore in this respect. But this different susceptibility of dilatation isstill more remarkable in fluids than in solid bodies, as I shall showyou. I have here two glass tubes, terminated at one end by large bulbs. We shall fill the bulbs, the one with spirit of wine, the other withwater. I have coloured both liquids, in order that the effect may bemore conspicuous. The spirit of wine, you see, dilates by the warmth ofmy hand as I hold the bulb. 

EMILY. 

It certainly does, for I see it is rising into the tube. But water, itseems, is not so easily affected by heat; for scarcely any change isproduced on it by the warmth of the hand. 

MRS. B. 

True; we shall now plunge the bulbs into hot water, (PLATE I. Fig.  2. )and you will see both liquids rise in the tubes; but the spirit of winewill ascend highest. 

CAROLINE. 

How rapidly it expands! Now it has nearly reached the top of the tube, though the water has hardly begun to rise. 

EMILY. 

The water now begins to dilate. Are not these glass tubes, with liquidsrising within them, very like thermometers?

MRS. B. 

A thermometer is constructed exactly on the same principle, and thesetubes require only a scale to answer the purpose of thermometers: butthey would be rather awkward in their dimensions. The tubes and bulbs ofthermometers, though of various sizes, are in general much smaller thanthese; the tube too is hermetically closed, and the air excluded fromit. The fluid most generally used in thermometers is mercury, commonlycalled quicksilver, the dilatations and contractions of which correspondmore exactly to the additions, and subtractions, of caloric, than thoseof any other fluid. 

CAROLINE. 

Yet I have often seen coloured spirit of wine used in thermometers. 

MRS. B. 

The expansions and contractions of that liquid are not quite so uniformas those of mercury; but in cases in which it is not requisite toascertain the temperature with great precision, spirit of wine willanswer the purpose equally well, and indeed in some respects better, asthe expansion of the latter is greater, and therefore more conspicuous. This fluid is used likewise in situations and experiments in whichmercury would be frozen; for mercury becomes a solid body, like a pieceof lead or any other metal, at a certain degree of cold: but no degreeof cold has ever been known to freeze spirit of wine. 

A thermometer, therefore, consists of a tube with a bulb, such as yousee here, containing a fluid whose degrees of dilatation and contractionare indicated by a scale to which the tube is fixed. The degree whichindicates the boiling point, simply means that, when the fluid issufficiently dilated to rise to this point, the heat is such that waterexposed to the same temperature will boil. When, on the other hand, thefluid is so much condensed as to sink to the freezing point, we knowthat water will freeze at that temperature. The extreme points of thescales are not the same in all thermometers, nor are the degrees alwaysdivided in the same manner. In different countries philosophers havechosen to adopt different scales and divisions. The two thermometersmost used are those of Fahrenheit, and of Reaumur; the first isgenerally preferred by the English, the latter by the French. 

EMILY. 

The variety of scale must be very inconvenient, and I should thinkliable to occasion confusion, when French and English experiments arecompared. 

MRS. B. 

The inconvenience is but very trifling, because the different gradationsof the scales do not affect the principle upon which thermometers areconstructed. When we know, for instance, that Fahrenheit’s scale isdivided into 212 degrees, in which 32° corresponds with the freezingpoint, and 212° with the point of boiling water: and that Reaumur’s isdivided only into 80 degrees, in which 0° denotes the freezing point, and 80° that of boiling water, it is easy to compare the two scalestogether, and reduce the one into the other. But, for greaterconvenience, thermometers are sometimes constructed with both thesescales, one on either side of the tube; so that the correspondence ofthe different degrees of the two scales is thus instantly seen. Hereis one of these scales, (PLATE II. Fig.  1. ) by which you can atonce perceive that each degree of Reaumur’s corresponds to 2¼ ofFahrenheit’s division. But I believe the French have, of late, giventhe preference to what they call the centigrade scale, in which thespace between the freezing and the boiling point is divided into 100degrees. 

 [Illustration: Plate II. Vol. I. P. 42. 

 Fig. 1. Thermometer. Fahrenheit’s Scale. Reaumur’s Scale. Boiling point of Water Freezing point of Water

 Fig. 2. Differential Thermometer. ]

CAROLINE. 

That seems to me the most reasonable division, and I cannot guesswhy the freezing point is called 32°, or what advantage is derivedfrom it. 

MRS. B. 

There really is no advantage in it; and it originated in a mistakenopinion of the instrument-maker, Fahrenheit, who first constructed thesethermometers. He mixed snow and salt together, and produced by thatmeans a degree of cold which he concluded was the greatest possible, andtherefore made his scale begin from that point. Between that and boilingwater he made 212 degrees, and the freezing point was found to be at32°. 

EMILY. 

Are spirit of wine, and mercury, the only liquids used in theconstruction of thermometers?

MRS. B. 

I believe they are the only liquids now in use, though some others, suchas linseed oil, would make tolerable thermometers: but for experimentsin which a very quick and delicate test of the changes of temperature isrequired, air is the fluid sometimes employed. The bulb of airthermometers is filled with common air only, and its expansion andcontraction are indicated by a small drop of any coloured liquor, whichis suspended within the tube, and moves up and down, according as theair within the bulb and tube expands or contracts. But in general, airthermometers, however sensible to changes of temperature, are by nomeans accurate in their indications. 

I can, however, show you an air thermometer of a very peculiarconstruction, which is remarkably well adapted for some chemicalexperiments, as it is equally delicate and accurate in its indications. 

CAROLINE. 

It looks like a double thermometer reversed, the tube being bent, andhaving a large bulb at each of its extremities. (PLATE II. Fig.  2. )

EMILY. 

Why do you call it an air thermometer; the tube contains a colouredliquid?

MRS. B. 

But observe that the bulbs are filled with air, the liquid beingconfined to a portion of the tube, and answering only the purpose ofshowing, by its motion in the tube, the comparative dilatation orcontraction of the air within the bulbs, which afford an indication oftheir relative temperature. Thus if you heat the bulb A, by the warmthof your hand, the fluid will rise towards the bulb B, and the contrarywill happen if you reverse the experiment. 

But if, on the contrary, both tubes are of the same temperature, as isthe case now, the coloured liquid, suffering an equal pressure on eachside, no change of level takes place. 

CAROLINE. 

This instrument appears, indeed, uncommonly delicate. The fluid is setin motion by the mere approach of my hand. 

MRS. B. 

You must observe, however, that this thermometer cannot indicate thetemperature of any particular body, or of the medium in which it isimmersed; it serves only to point out the _difference_ of temperaturebetween the two bulbs, when placed under different circumstances. Forthis reason it has been called _differential_ thermometer. You will seeby-and-bye to what particular purposes this instrument applies. 

EMILY. 

But do common thermometers indicate the exact quantity of caloriccontained either in the atmosphere, or in any body with which they arein contact?

MRS. B. 

No: first, because there are other modifications of caloric which do notaffect the thermometer; and, secondly, because the temperature of abody, as indicated by the thermometer, is only relative. When, forinstance, the thermometer remains stationary at the freezing point, weknow that the atmosphere (or medium in which it is placed, whatever itmay be) is as cold as freezing water; and when it stands at the boilingpoint, we know that this medium is as hot as boiling water; but we donot know the positive quantity of heat contained either in freezing orboiling water, any more than we know the real extremes of heat and cold;and consequently we cannot determine that of the body in which thethermometer is placed. 

CAROLINE. 

I do not quite understand this explanation. 

MRS. B. 

Let us compare a thermometer to a well, in which the water rises todifferent heights, according as it is more or less supplied by thespring which feeds it: if the depth of the well is unfathomable, it mustbe impossible to know the absolute quantity of water it contains; yet wecan with the greatest accuracy measure the number of feet the water hasrisen or fallen in the well at any time, and consequently know theprecise quantity of its increase or diminution, without having the leastknowledge of the whole quantity of water it contains. 

CAROLINE. 

Now I comprehend it very well; nothing appears to me to explain a thingso clearly as a comparison. 

EMILY. 

But will thermometers bear any degree of heat?

MRS. B. 

No; for if the temperature were much above the highest degree marked onthe scale of the thermometer, the mercury would burst the tube in anattempt to ascend. And at any rate, no thermometer can be applied totemperatures higher than the boiling point of the liquid used in itsconstruction, for the steam, on the liquid beginning to boil, wouldburst the tube. In furnaces, or whenever any very high temperature is tobe measured, a pyrometer, invented by Wedgwood, is used for thatpurpose. It is made of a certain composition of baked clay, which hasthe peculiar property of contracting by heat, so that the degree ofcontraction of this substance indicates the temperature to which it hasbeen exposed. 

EMILY. 

But is it possible for a body to contract by heat? I thought that heatdilated all bodies whatever. 

MRS. B. 

This is not an exception to the rule. You must recollect that the bulkof the clay is not compared, whilst hot, with that which it has whencold; but it is from the change which the clay has undergone by _havingbeen_ heated that the indications of this instrument are derived. Thischange consists in a beginning fusion which tends to unite the particlesof clay more closely, thus rendering it less pervious or spongy. 

Clay is to be considered as a spongy body, having many interstices orpores, from its having contained water when soft. These interstices areby heat lessened, and would by extreme heat be entirely obliterated. 

CAROLINE. 

And how do you ascertain the degrees of contraction of Wedgwood’spyrometer?

MRS. B. 

The dimensions of a piece of clay are measured by a scale graduated onthe side of a tapered groove, formed in a brass ruler; the more the clayis contracted by the heat, the further it will descend into the narrowpart of the tube. 

Before we quit the subject of expansion, I must observe to you that, asliquids expand more readily than solids, so elastic fluids, whether airor vapour, are the most expansible of all bodies. 

It may appear extraordinary that all elastic fluids whatever, undergothe same degree of expansion from equal augmentations of temperature. 

EMILY. 

I suppose, then, that all elastic fluids are of the same density?

MRS. B. 

Very far from it; they vary in density, more than either liquids orsolids. The uniformity of their expansibility, which at first may appearsingular, is, however, readily accounted for. For if the differentsusceptibilities of expansion of bodies arise from their various degreesof attraction of cohesion, no such difference can be expected in elasticfluids, since in these the attraction of cohesion does not exist, theirparticles being on the contrary possessed of an elastic or repulsivepower; they will therefore all be equally expanded by equal degrees ofcaloric. 

EMILY. 

True; as there is no power opposed to the expansive force of caloric inelastic bodies, its effect must be the same in all of them. 

MRS. B. 

Let us now proceed to examine the other properties of free caloric. 

Free caloric always tends to diffuse itself equally, that is to say, when two bodies are of different temperatures, the warmer graduallyparts with its heat to the colder, till they are both brought to thesame temperature. Thus, when a thermometer is applied to a hot body, itreceives caloric; when to a cold one, it communicates part of its owncaloric, and this communication continues until the thermometer and thebody arrive at the same temperature. 

EMILY. 

Cold, then, is nothing but a negative quality, simply implying theabsence of heat. 

MRS. B. 

Not the total absence, but a diminution of heat; for we know of no bodyin which some caloric may not be discovered. 

CAROLINE. 

But when I lay my hand on this marble table I feel it _positively_ cold, and cannot conceive that there is any caloric in it. 

MRS. B. 

The cold you experience consists in the loss of caloric that your handsustains in an attempt to bring its temperature to an equilibrium withthe marble. If you lay a piece of ice upon it, you will find that thecontrary effect will take place; the ice will be melted by the heatwhich it abstracts from the marble. 

CAROLINE. 

Is it not in this case the air of the room, which being warmer than themarble, melts the ice?

MRS. B. 

The air certainly acts on the surface which is exposed to it, but thetable melts that part with which it is in contact. 

CAROLINE. 

But why does caloric tend to an equilibrium? It cannot be on the sameprinciple as other fluids, since it has no weight?

MRS. B. 

Very true, Caroline, that is an excellent objection. You might also, with some propriety, object to the term _equilibrium_ being applied to abody that is without weight; but I know of no expression that wouldexplain my meaning so well. You must consider it, however, in afigurative rather than a literal sense; its strict meaning is an _equaldiffusion_. We cannot, indeed, well say by what power it diffuses itselfequally, though it is not surprising that it should go from the partswhich have the most to those which have the least. This subject is bestexplained by a theory suggested by Professor Prevost of Geneva, which isnow, I believe, generally adopted. 

According to this theory, caloric is composed of particles perfectlyseparate from each other, every one of which moves with a rapid velocityin a certain direction. These directions vary as much as imagination canconceive, the result of which is, that there are rays or lines of theseparticles moving with immense velocity in every possible direction. Caloric is thus universally diffused, so that when any portion of spacehappens to be in the neighbourhood of another, which contains morecaloric, the colder portion receives a quantity of calorific rays fromthe latter, sufficient to restore an equilibrium of temperature. Thisradiation does not only take place in free space, but extends also tobodies of every kind. Thus you may suppose all bodies whateverconstantly radiating caloric: those that are of the same temperaturegive out and absorb equal quantities, so that no variation oftemperature is produced in them; but when one body contains more freecaloric than another, the exchange is always in favour of the colderbody, until an equilibrium is effected; this you found to be the casewhen the marble table cooled your hand, and again when it melted theice. 

CAROLINE. 

This reciprocal radiation surprises me extremely; I thought, from whatyou first said, that the hotter bodies alone emitted rays of caloricwhich were absorbed by the colder; for it seems unnatural that a hotbody should receive any caloric from a cold one, even though it shouldreturn a greater quantity. 

MRS. B. 

It may at first appear so, but it is no more extraordinary than that acandle should send forth rays of light to the sun, which, you know, mustnecessarily happen. 

CAROLINE. 

Well, Mrs. B--, I believe that I must give up the point. But I wish Icould _see_ these rays of caloric; I should then have greater faith inthem. 

MRS. B. 

Will you give no credit to any sense but that of sight? You may feel therays of caloric which you receive from any body of a temperature higherthan your own; the loss of the caloric you part with in return, it istrue, is not perceptible; for as you gain more than you lose, instead ofsuffering a diminution, you are really making an acquisition of caloric. It is, therefore, only when you are parting with it to a body of a lowertemperature, that you are sensible of the sensation of cold, because youthen sustain an absolute loss of caloric. 

EMILY. 

And in this case we cannot be sensible of the small quantity of heat wereceive in exchange from the colder body, because it serves only todiminish the loss. 

MRS. B. 

Very well, indeed, Emily. Professor Pictet, of Geneva, has made somevery interesting experiments, which prove not only that caloric radiatesfrom all bodies whatever, but that these rays may be reflected, according to the laws of optics, in the same manner as light. I shallrepeat these experiments before you, having procured mirrors fit for thepurpose; and it will afford us an opportunity of using the differentialthermometer, which is particularly well adapted for these experiments. --I place an iron bullet, (PLATE III. Fig.  1. ) about two inches indiameter, and heated to a degree not sufficient to render it luminous, in the focus of this large metallic concave mirror. The rays of heatwhich fall on this mirror are reflected, agreeably to the property ofconcave mirrors, in a parallel direction, so as to fall on a similarmirror, which, you see, is placed opposite to the first, at the distanceof about ten feet; thence the rays converge to the focus of the secondmirror, in which I place one of the bulbs of this thermometer. Now, observe in what manner it is affected by the caloric which is reflectedon it from the heated bullet. --The air is dilated in the bulb which weplaced in the focus of the mirror, and the liquor rises considerably inthe opposite leg. 

 [Illustration: Plate III. Vol. I. P. 54 Mr. Pictet’s Apparatus for the Reflection of Heat. 

 Fig. 1. A. A. & B. B Concave mirrors fixed on stands. C Heated Bullet placed in the focus of the mirror A. D Thermometer, with its bulb placed in the focus of the mirror B. 1. 2. 3. 4 Rays of Caloric radiating from the bullet & falling on the mirror A. 5. 6. 7. 8 The same rays reflected from the mirror A to the mirror B. 9. 10. 11. 12 The same rays reflected by the mirror B to the Thermometer. ]

EMILY. 

But would not the same effect take place, if the rays of caloric fromthe heated bullet fell directly on the thermometer, without theassistance of the mirrors?

MRS. B. 

The effect would in that case be so trifling, at the distance at whichthe bullet and the thermometer are from each other, that it would bealmost imperceptible. The mirrors, you know, greatly increase theeffect, by collecting a large quantity of rays into a focus; place yourhand in the focus of the mirror, and you will find it much hotter therethan when you remove it nearer to the bullet. 

EMILY. 

That is very true; it appears extremely singular to feel the heatdiminish in approaching the body from which it proceeds. 

CAROLINE. 

And the mirror which produces so much heat, by converging the rays, isitself quite cold. 

MRS. B. 

The same number of rays that are dispersed over the surface of themirror are collected by it into the focus; but, if you consider howlarge a surface the mirror presents to the rays, and, consequently, howmuch they are diffused in comparison to what they are at the focus, which is little more than a point, I think you can no longer wonder thatthe focus should be so much hotter than the mirror. 

The principal use of the mirrors in this experiment is, to prove thatthe calorific emanation is reflected in the same manner as light. 

CAROLINE. 

And the result, I think, is very conclusive. 

MRS. B. 

The experiment may be repeated with a wax taper instead of the bullet, with a view of separating the light from the caloric. For this purpose atransparent plate of glass must be interposed between the mirrors; forlight, you know, passes with great facility through glass, whilst thetransmission of caloric is almost wholly impeded by it. We shall find, however, in this experiment, that some few of the calorific rays passthrough the glass together with the light, as the thermometer rises alittle; but, as soon as the glass is removed, and a free passage left tothe caloric, it will rise considerably higher. 

EMILY. 

This experiment, as well as that of Dr. Herschell’s, proves that lightand heat may be separated; for in the latter experiment the separationwas not perfect, any more than in that of Mr. Pictet. 

CAROLINE. 

I should like to repeat this experiment, with the difference ofsubstituting a cold body instead of the hot one, to see whether coldwould not be reflected as well as heat. 

MRS. B. 

That experiment was proposed to Mr. Pictet by an incredulous philosopherlike yourself, and he immediately tried it by substituting a piece ofice in the place of the heated bullet. 

CAROLINE. 

Well, Mrs. B. , and what was the result?

MRS. B. 

That we shall see; I have procured some ice for the purpose. 

EMILY. 

The thermometer falls considerably!

CAROLINE. 

And does not that prove that cold is not merely a _negative_ quality, implying simply an inferior degree of heat? The cold must be _positive_, since it is capable of reflection. 

MRS. B. 

So it at first appeared to Mr. Pictet; but upon a little considerationhe found that it afforded only an additional proof of the reflection ofheat: this I shall endeavour to explain to you. 

According to Mr. Prevost’s theory, we suppose that all bodies whateverradiate caloric; the thermometer used in these experiments thereforeemits calorific rays in the same manner as any other substance. When itstemperature is in equilibrium with that of the surrounding bodies, itreceives as much caloric as it parts with, and no change of temperatureis produced. But when we introduce a body of a lower temperature, suchas a piece of ice, which parts with less caloric than it receives, theconsequence is, that its temperature is raised, whilst that of thesurrounding bodies is proportionally lowered. 

EMILY. 

If, for instance, I was to bring a large piece of ice into this room, the ice would in time be melted, by absorbing caloric from the generalradiation which is going on throughout the room; and as it wouldcontribute very little caloric in return for what is absorbed, the roomwould necessarily be cooled by it. 

MRS. B. 

Just so; and as in consequence of the mirrors, a more considerableexchange of rays takes place between the ice and the thermometer, thanbetween these and any of the surrounding bodies, the temperature of thethermometer must be more lowered than that of any other adjacent object. 

CAROLINE. 

I confess I do not perfectly understand your explanation. 

MRS. B. 

This experiment is exactly similar to that made with the heated bullet:for, if we consider the thermometer as the hot body (which it certainlyis in comparison to the ice), you may then easily understand that it isby the loss of the calorific rays which the thermometer sends to theice, and not by any cold rays received from it, that the fall of themercury is occasioned: for the ice, far from emitting rays of cold, sends forth rays of caloric, which diminish the loss sustained by thethermometer. 

Let us say, for instance, that the radiation of the thermometer towardsthe ice is equal to 20, and that of the ice towards the thermometer to10: the exchange in favour of the ice is as 20 is to 10, or thethermometer absolutely loses 10, whilst the ice gains 10. 

CAROLINE. 

But if the ice actually sends rays of caloric to the thermometer, mustnot the latter fall still lower when the ice is removed?

MRS. B. 

No; for the space that the ice occupied, admits rays from all thesurrounding bodies to pass through it; and those being of the sametemperature as the thermometer, will not affect it, because as much heatnow returns to the thermometer as radiates from it. 

CAROLINE. 

I must confess that you have explained this in so satisfactory a manner, that I cannot help being convinced now that cold has no real claim tothe rank of a positive being. 

MRS. B. 

Before I conclude the subject of radiation I must observe to you thatdifferent bodies, (or rather surfaces, ) possess the power of radiatingcaloric in very different degrees. 

Some very curious experiments have been made by Mr. Leslie on thissubject, and it was for this purpose that he invented the differentialthermometer; with its assistance he ascertained that black surfacesradiate most, glass next, and polished surfaces the least of all. 

EMILY. 

Supposing these surfaces, of course, to be all of the same temperature. 

MRS. B. 

Undoubtedly. I will now show you the very simple and ingeniousapparatus, by means of which he made these experiments. This cubical tinvessel or canister, has each of its sides externally covered withdifferent materials; the one is simply blackened; the next is coveredwith white paper; the third with a pane of glass, and in the fourth thepolished tin surface remains uncovered. We shall fill this vessel withhot water, so that there can be no doubt but that all its sides will beof the same temperature. Now let us place it in the focus of one of themirrors, making each of its sides front it in succession. We shall beginwith the black surface. 

CAROLINE. 

It makes the thermometer which is in the focus of the other mirror riseconsiderably. Let us turn the paper surface towards the mirror. Thethermometer falls a little, therefore of course this side cannot emit orradiate so much caloric as the blackened side. 

EMILY. 

This is very surprising; for the sides are exactly of the same size, andmust be of the same temperature. But let us try the glass surface. 

MRS. B. 

The thermometer continues falling, and with the plain surface it fallsstill lower; these two surfaces therefore radiate less and less. 

CAROLINE. 

I think I have found out the reason of this. 

MRS. B. 

I should be very happy to hear it, for it has not yet (to my knowledge)been accounted for. 

CAROLINE. 

The water within the vessel gradually cools, and the thermometer inconsequence gradually falls. 

MRS. B. 

It is true that the water cools, but certainly in much less proportionthan the thermometer descends, as you will perceive if you now changethe tin surface for the black one. 

CAROLINE. 

I was mistaken certainly, for the thermometer rises again now that theblack surface fronts the mirror. 

MRS. B. 

And yet the water in the vessel is still cooling, Caroline. 

EMILY. 

I am surprised that the tin surface should radiate the least caloric, for a metallic vessel filled with hot water, a silver teapot, forinstance, feels much hotter to the hand than one of black earthen ware. 

MRS. B. 

That is owing to the different power which various bodies possess for_conducting_ caloric, a property which we shall presently examine. Thus, although a metallic vessel feels warmer to the hand, a vessel of thiskind is known to preserve the heat of the liquid within, better than oneof any other materials; it is for this reason that silver teapots makebetter tea than those of earthen ware. 

EMILY. 

According to these experiments, light-coloured dresses, in cold weather, should keep us warmer than black clothes, since the latter radiate somuch more than the former. 

MRS. B. 

And that is actually the case. 

EMILY. 

This property, of different surfaces to radiate in different degrees, appears to me to be at variance with the equilibrium of caloric; sinceit would imply that those bodies which radiate most, must ultimatelybecome coldest. 

Suppose that we were to vary this experiment, by using two metallicvessels full of boiling water, the one blackened, the other not; wouldnot the black one cool the first?

CAROLINE. 

True; but when they were both brought down to the temperature of theroom, the interchange of caloric between the canisters and the otherbodies of the room being then equal, their temperatures would remain thesame. 

EMILY. 

I do not see why that should be the case; for if different surfaces ofthe same temperature radiate in different degrees when heated, whyshould they not continue to do so when cooled down to the temperature ofthe room?

MRS. B. 

You have started a difficulty, Emily, which certainly requiresexplanation. It is found by experiment that the power of absorptioncorresponds with and is proportional to that of radiation; so that underequal temperatures, bodies compensate for the greater loss they sustainin consequence of their greater radiation by their greater absorption;so that if you were to make your experiment in an atmosphere heated likethe canisters, to the temperature of boiling water, though it is truethat the canisters would radiate in different degrees, no change oftemperature would be produced in them, because they would each absorbcaloric in proportion to their respective radiation. 

EMILY. 

But would not the canisters of boiling water also absorb caloric indifferent degrees in a room of the common temperature?

MRS. B. 

Undoubtedly they would. But the various bodies in the room would not, ata lower temperature, furnish either of the canisters with a sufficiencyof caloric to compensate for the loss they undergo; for, suppose theblack canister to absorb 400 rays of caloric, whilst the metallic oneabsorbed only 200; yet if the former radiate 800, whilst the latterradiates only 400, the black canister will be the first cooled down tothe temperature of the room. But from the moment the equilibrium oftemperature has taken place, the black canister, both receiving andgiving out 400 rays, and the metallic one 200, no change of temperaturewill take place. 

EMILY. 

I now understand it extremely well. But what becomes of the surplus ofcalorific rays, which good radiators emit and bad radiators refuse toreceive; they must wander about in search of a resting-place?

MRS. B. 

They really do so; for they are rejected and sent back, or, in otherwords, _reflected_ by the bodies which are bad radiators of caloric; andthey are thus transmitted to other bodies which happen to lie in theirway, by which they are either absorbed or again reflected, according asthe property of reflection, or that of absorption, predominates in thesebodies. 

CAROLINE. 

I do not well understand the difference between radiating and reflectingcaloric, for the caloric that is reflected from a body proceeds from itin straight lines, and may surely be said to radiate from it?

MRS. B. 

It is true that there at first appears to be a great analogy between_radiation_ and _reflection_, as they equally convey the idea of thetransmission of caloric. 

But if you consider a little, you will perceive that when a body_radiates_ caloric, the heat which it emits not only proceeds from, buthas its origin in the body itself. Whilst when a body _reflects_caloric, it parts with none of its own caloric, but only reflects thatwhich it receives from other bodies. 

EMILY. 

Of this difference we have very striking examples before us, in the tinvessel of water, and the concave mirrors; the first radiates its ownheat, the latter reflect the heat which they receive from other bodies. 

CAROLINE. 

Now, that I understand the difference, it no longer surprises me thatbodies which radiate, or part with their own caloric freely, should nothave the power of transmitting with equal facility that which theyreceive from other bodies. 

EMILY. 

Yet no body can be said to possess caloric of its own, if all caloric isoriginally derived from the sun. 

MRS. B. 

When I speak of a body radiating its own caloric, I mean that which ithas absorbed and incorporated either immediately from the sun’s rays, orthrough the medium of any other substance. 

CAROLINE. 

It seems natural enough that the power of absorption should be inopposition to that of reflection, for the more caloric a body receives, the less it will reject. 

EMILY. 

And equally so that the power of radiation should correspond with thatof absorption. It is, in fact, cause and effect; for a body cannotradiate heat without having previously absorbed it; just as a springthat is well fed flows abundantly. 

MRS. B. 

Fluids are in general very bad radiators of caloric; and air neitherradiates nor absorbs caloric in any sensible degree. 

We have not yet concluded our observations on free caloric. But I shalldefer, till our next meeting, what I have further to say on thissubject. I believe it will afford us ample conversation for anotherinterview. 

CONVERSATION III. 

CONTINUATION OF THE SUBJECT. 

MRS. B. 

In our last conversation, we began to examine the tendency of caloric torestore an equilibrium of temperature. This property, when once wellunderstood, affords the explanation of a great variety of facts whichappeared formerly unaccountable. You must observe, in the first place, that the effect of this tendency is gradually to bring all bodies thatare in contact to the same temperature. Thus, the fire which burns inthe grate, communicates its heat from one object to another, till everypart of the room has an equal proportion of it. 

EMILY. 

And yet this book is not so cold as the table on which it lies, thoughboth are at an equal distance from the fire, and actually in contactwith each other, so that, according to your theory, they should beexactly of the same temperature. 

CAROLINE. 

And the hearth, which is much nearer the fire than the carpet, iscertainly the colder of the two. 

MRS. B. 

If you ascertain the temperature of these several bodies by athermometer (which is a much more accurate test than your feeling), youwill find that it is exactly the same. 

CAROLINE. 

But if they are of the same temperature, why should the one feel colderthan the other?

MRS. B. 

The hearth and the table feel colder than the carpet or the book, because the latter are not such good _conductors of heat_ as the former. Caloric finds a more easy passage through marble and wood, than throughleather and worsted; the two former will therefore absorb heat morerapidly from your hand, and consequently give it a stronger sensation ofcold than the two latter, although they are all of them really of thesame temperature. 

CAROLINE. 

So, then, the sensation I feel on touching a cold body, is in proportionto the rapidity with which my hand yields its heat to that body?

MRS. B. 

Precisely; and, if you lay your hand successively on every object in theroom, you will discover which are good, and which are bad conductors ofheat, by the different degrees of cold you feel. But, in order toascertain this point, it is necessary that the several substances shouldbe of the same temperature, which will not be the case with those thatare very near the fire, or those that are exposed to a current of coldair from a window or door. 

EMILY. 

But what is the reason that some bodies are better conductors of heatthan others?

MRS. B. 

This is a point not well ascertained. It has been conjectured that acertain union or adherence takes place between the caloric and theparticles of the body through which it passes. If this adherence bestrong, the body detains the heat, and parts with it slowly andreluctantly; if slight, it propagates it freely and rapidly. Theconducting power of a body is therefore, inversely, as its tendency tounite with caloric. 

EMILY. 

That is to say, that the best conductors are those that have the leastaffinity for caloric. 

MRS. B. 

Yes; but the term affinity is objectionable in this case, because, asthat word is used to express a chemical attraction (which can bedestroyed only by decomposition), it cannot be applicable to the slightand transient union that takes place between free caloric and the bodiesthrough which it passes; an union which is so weak, that it constantlyyields to the tendency which caloric has to an equilibrium. Now youclearly understand, that the passage of caloric, through bodies that aregood conductors, is much more rapid than through those that are badconductors, and that the former both give and receive it more quickly, and therefore, in a given time, more abundantly, than bad conductors, which makes them feel either hotter or colder, though they may be, infact, both of the same temperature. 

CAROLINE. 

Yes, I understand it now; the table, and the book lying upon it, beingreally of the same temperature, would each receive, in the same space oftime, the same quantity of heat from my hand, were their conductingpowers equal; but as the table is the best conductor of the two, it willabsorb the heat from my hand more rapidly, and consequently produce astronger sensation of cold than the book. 

MRS. B. 

Very well, my dear; and observe, likewise, that if you were to heat thetable and the book an equal number of degrees above the temperature ofyour body, the table, which before felt the colder, would now feel thehotter of the two; for, as in the first case it took the heat mostrapidly from your hand, so it will now impart heat most rapidly to it. Thus the marble table, which seems to us colder than the mahogany one, will prove the hotter of the two to the ice; for, if it takes heat morerapidly from our hands, which are warmer, it will give out heat morerapidly to the ice, which is colder. Do you understand the reason ofthese apparently opposite effects?

EMILY. 

Perfectly. A body which is a good conductor of caloric, affords it afree passage; so that it penetrates through that body more rapidly thanthrough one which is a bad conductor; and consequently, if it is colderthan your hand, you lose more caloric, and if it is hotter, you gainmore than with a bad conductor of the same temperature. 

MRS. B. 

But you must observe that this is the case only when the conductors areeither hotter or colder than your hand; for, if you heat differentconductors to the temperature of your body, they will all feel equallywarm, since the exchange of caloric between bodies of the sametemperature is equal. Now, can you tell me why flannel clothing, whichis a very bad conductor of heat, prevents our feeling cold?

CAROLINE. 

It prevents the cold from penetrating .  .  .  .  .  .  .  . 

MRS. B. 

But you forget that cold is only a negative quality. 

CAROLINE. 

True; it only prevents the heat of our bodies from escaping so rapidlyas it would otherwise do. 

MRS. B. 

Now you have explained it right; the flannel rather keeps in the heat, than keeps out the cold. Were the atmosphere of a higher temperaturethan our bodies, it would be equally efficacious in keeping theirtemperature at the same degree, as it would prevent the free access ofthe external heat, by the difficulty with which it conducts it. 

EMILY. 

This, I think, is very clear. Heat, whether external or internal, cannoteasily penetrate flannel; therefore in cold weather it keeps us warm;and if the weather was hotter than our bodies, it would keep us cool. 

MRS. B. 

The most dense bodies are, generally speaking, the best conductors ofheat; probably because the denser the body the greater are the number ofpoints or particles that come in contact with caloric. At the commontemperature of the atmosphere a piece of metal will feel much colderthan a piece of wood, and the latter than a piece of woollen cloth; thisagain will feel colder than flannel; and down, which is one of thelightest, is at the same time one of the warmest bodies. 

CAROLINE. 

This is, I suppose, the reason that the plumage of birds preserves themso effectually from the influence of cold in winter?

MRS. B. 

Yes; but though feathers in general are an excellent preservativeagainst cold, down is a kind of plumage peculiar to aquatic birds, andcovers their chest, which is the part most exposed to the water; forthough the surface of the water is not of a lower temperature than theatmosphere, yet, as it is a better conductor of heat, it feels muchcolder, consequently the chest of the bird requires a warmer coveringthan any other part of its body. Besides, the breasts of aquatic birdsare exposed to cold not only from the temperature of the water, but alsofrom the velocity with which the breast of the bird strikes against it;and likewise from the rapid evaporation occasioned in that part by theair against which it strikes, after it has been moistened by dippingfrom time to time into the water. 

If you hold a finger of one hand motionless in a glass of water, and atthe same time move a finger of the other hand swiftly through water ofthe same temperature, a different sensation will be soon perceived inthe different fingers. 

Most animal substances, especially those which Providence has assignedas a covering for animals, such as fur, wool, hair, skin, &c. Are badconductors of heat, and are, on that account, such excellentpreservatives against the inclemency of winter, that our warmest apparelis made of these materials. 

EMILY. 

Wood is, I dare say, not so good a conductor as metal, and it is forthat reason, no doubt, that silver teapots have always wooden handles. 

MRS. B. 

Yes; and it is the facility with which metals conduct caloric that madeyou suppose that a silver pot radiated more caloric than an earthen one. The silver pot is in fact hotter to the hand when in contact with it;but it is because its conducting power more than counterbalances itsdeficiency in regard to radiation. 

We have observed that the most dense bodies are in general the bestconductors; and metals, you know, are of that class. Porous bodies, suchas the earths and wood, are worse conductors, chiefly, I believe, onaccount of their pores being filled with air; for air is a remarkablybad conductor. 

CAROLINE. 

It is a very fortunate circumstance that air should be a bad conductor, as it tends to preserve the heat of the body when exposed to coldweather. 

MRS. B. 

It is one of the many benevolent dispensations of Providence, in orderto soften the inclemency of the seasons, and to render almost allclimates habitable to man. 

In fluids of different densities, the power of conducting heat varies noless remarkably; if you dip your hand into this vessel full of mercury, you will scarcely conceive that its temperature is not lower than thatof the atmosphere. 

CAROLINE. 

Indeed I know not how to believe it, it feels so extremely cold. --Butwe may easily ascertain its true temperature by the thermometer. --It isreally not colder than the air;--the apparent difference then isproduced merely by the difference of the conducting power in mercury andin air. 

MRS. B. 

Yes; hence you may judge how little the sense of feeling is to be reliedon as a test of the temperature of bodies, and how necessary athermometer is for that purpose. 

It has indeed been doubted whether fluids have the power of conductingcaloric in the same manner as solid bodies. Count Rumford, a very fewyears since, attempted to prove, by a variety of experiments, thatfluids, when at rest, were not at all endowed with this property. 

CAROLINE. 

How is that possible, since they are capable of imparting cold or heatto us; for if they did not conduct heat, they would neither take itfrom, nor give it to us?

MRS. B. 

Count Rumford did not mean to say that fluids would not communicatetheir heat to solid bodies; but only that heat does not pervade fluids, that is to say, is not transmitted from one particle of a fluid toanother, in the same manner as in solid bodies. 

EMILY. 

But when you heat a vessel of water over the fire, if the particles ofwater do not communicate heat to each other, how does the water becomehot throughout?

MRS. B. 

By constant agitation. Water, as you have seen, expands by heat in thesame manner as solid bodies; the heated particles of water, therefore, at the bottom of the vessel, become specifically lighter than the restof the liquid, and consequently ascend to the surface, where, partingwith some of their heat to the colder atmosphere, they are condensed, and give way to a fresh succession of heated particles ascending fromthe bottom, which having thrown off their heat at the surface, are intheir turn displaced. Thus every particle is successively heated at thebottom, and cooled at the surface of the liquid; but as the firecommunicates heat more rapidly than the atmosphere cools the successionof surfaces, the whole of the liquid in time becomes heated. 

CAROLINE. 

This accounts most ingeniously for the propagation of heat upwards. Butsuppose you were to heat the upper surface of a liquid, the particlesbeing specifically lighter than those below, could not descend: howtherefore would the heat be communicated downwards?

MRS. B. 

If there were no agitation to force the heated surface downwards, CountRumford assures us that the heat would not descend. In proof of this hesucceeded in making the upper surface of a vessel of water boil andevaporate, while a cake of ice remained frozen at the bottom. 

CAROLINE. 

That is very extraordinary indeed!

MRS. B. 

It appears so, because we are not accustomed to heat liquids by theirupper surface; but you will understand this theory better if I show youthe internal motion that takes place in liquids when they experience achange of temperature. The motion of the liquid itself is indeedinvisible from the extreme minuteness of its particles; but if you mixwith it any coloured dust, or powder, of nearly the same specificgravity as the liquid, you may judge of the internal motion of thelatter by that of the coloured dust it contains. --Do you see the smallpieces of amber moving about in the liquid contained in this phial?

CAROLINE. 

Yes, perfectly. 

MRS. B. 

We shall now immerse the phial in a glass of hot water, and the motionof the liquid will be shown, by that which it communicates to the amber. 

EMILY. 

I see two currents, the one rising along the sides of the phial, theother descending in the centre: but I do not understand the reason ofthis. 

MRS. B. 

The hot water communicates its caloric, through the medium of the phial, to the particles of the fluid nearest to the glass; these dilate andascend laterally to the surface, where, in parting with their heat, theyare condensed, and in descending, form the central current. 

CAROLINE. 

This is indeed a very clear and satisfactory experiment; but how muchslower the currents now move than they did at first?

MRS. B. 

It is because the circulation of particles has nearly produced anequilibrium of temperature between the liquid in the glass and that inthe phial. 

CAROLINE. 

But these communicate laterally, and I thought that heat in liquidscould be propagated only upwards. 

MRS. B. 

You do not take notice that the heat is imparted from one liquid to theother, through the medium of the phial itself, the external surface ofwhich receives the heat from the water in the glass, whilst its internalsurface transmits it to the liquid it contains. Now take the phial outof the hot water, and observe the effect of its cooling. 

EMILY. 

The currents are reversed; the external current now descends, and theinternal one rises. --I guess the reason of this change:-- the phialbeing in contact with cold air instead of hot water, the externalparticles are cooled instead of being heated; they therefore descend andforce up the central particles, which, being warmer, are consequentlylighter. 

MRS. B. 

It is just so. Count Rumford hence infers that no alteration oftemperature can take place in a fluid, without an internal motion of itsparticles, and as this motion is produced only by the comparative levityof the heated particles, heat cannot be propagated downwards. 

But though I believe that Count Rumford’s theory as to heat beingincapable of pervading fluids is not strictly correct, yet there is, nodoubt, much truth in his observation, that the communication ismaterially promoted by a motion of the parts; and this accounts for thecold that is found to prevail at the bottom of the lakes in Switzerland, which are fed by rivers issuing from the snowy Alps. The water of theserivers being colder, and therefore more dense than that of the lakes, subsides to the bottom, where it cannot be affected by the warmertemperature of the surface; the motion of the waves may communicate thistemperature to some little depth, but it can descend no further than theagitation extends. 

EMILY. 

But when the atmosphere is colder than the lake, the colder surface ofthe water will descend, for the very reason that the warmer will not. 

MRS. B. 

Certainly: and it is on this account that neither a lake, nor any bodyof water whatever, can be frozen until every particle of the water hasrisen to the surface to give off its caloric to the colder atmosphere;therefore the deeper a body of water is, the longer will be the time itrequires to be frozen. 

EMILY. 

But if the temperature of the whole body of water be brought down to thefreezing point, why is only the surface frozen?

MRS. B. 

The temperature of the whole body is lowered, but not to the freezingpoint. The diminution of heat, as you know, produces a contraction inthe bulk of fluids, as well as of solids. This effect, however, does nottake place in water below the temperature of 40 degrees, which is 8degrees above the freezing point. At that temperature, therefore, theinternal motion, occasioned by the increased specific gravity of thecondensed particles, ceases; for when the water at the surface no longercondenses, it will no longer descend, and leave a fresh surface exposedto the atmosphere: this surface alone, therefore, will be furtherexposed to its severity, and will soon be brought down to the freezingpoint, when it becomes ice, which being a bad conductor of heat, preserves the water beneath a long time from being affected by theexternal cold. 

CAROLINE. 

And the sea does not freeze, I suppose, because its depth is so great, that a frost never lasts long enough to bring down the temperature ofsuch a great body of water to 40 degrees?

MRS. B. 

That is one reason why the sea, as a large mass of water, does notfreeze. But, independently of this, salt water does not freeze till itis cooled much below 32 degrees, and with respect to the law ofcondensation, salt water is an exception, as it condenses even manydegrees below the freezing point. When the caloric of fresh water, therefore, is imprisoned by the ice on its surface, the ocean stillcontinues throwing off heat into the atmosphere, which is a most signaldispensation of Providence to moderate the intensity of the cold inwinter. 

CAROLINE. 

This theory of the non-conducting power of liquids, does not, I suppose, hold good with respect to air, otherwise the atmosphere would not beheated by the rays of the sun passing through it?

MRS. B. 

Nor is it heated in that way. The pure atmosphere is a perfectlytransparent medium, which neither radiates, absorbs, nor conductscaloric, but transmits the rays of the sun to us without in any waydiminishing their intensity. The air is therefore not more heated, bythe sun’s rays passing through it, than diamond, glass, water, or anyother transparent medium. 

CAROLINE. 

That is very extraordinary! Are glass windows not heated then by the sunshining on them?

MRS. B. 

No; not if the glass be perfectly transparent. A most convincing proofthat glass transmits the rays of the sun without being heated by them isafforded by the burning lens, which by converging the rays to a focuswill set combustible bodies on fire, without its own temperature beingraised. 

EMILY. 

Yet, Mrs. B. , if I hold a piece of glass near the fire it is almostimmediately warmed by it; the glass therefore must retain some of thecaloric radiated by the fire? Is it that the solar rays alone passfreely through glass without paying tribute? It seems unaccountable thatthe radiation of a common fire should have power to do what the sun’srays cannot accomplish. 

MRS. B. 

It is not because the rays from the fire have more power, but ratherbecause they have less, that they heat glass and other transparentbodies. It is true, however, that as you approach the source of heat therays being nearer each other, the heat is more condensed, and canproduce effects of which the solar rays, from the great distance oftheir source, are incapable. Thus we should find it impossible to roasta joint of meat by the sun’s rays, though it is so easily done byculinary heat. Yet caloric emanated from burning bodies, which iscommonly called _culinary heat_, has neither the intensity nor thevelocity of solar rays. All caloric, we have said, is supposed toproceed originally from the sun; but after having been incorporated withterrestrial bodies, and again given out by them, though its nature isnot essentially altered, it retains neither the intensity nor thevelocity with which it first emanated from that luminary; it hastherefore not the power of passing through transparent mediums, such asglass and water, without being partially retained by those bodies. 

EMILY. 

I recollect that in the experiment on the reflection of heat, the glassskreen which you interposed between the burning taper and the mirror, arrested the rays of caloric, and suffered only those of light to passthrough it. 

CAROLINE. 

Glass windows, then, though they cannot be heated by the sun shining onthem, may be heated internally by a fire in the room? But, Mrs.  B. , since the atmosphere is not warmed by the solar rays passing through it, how does it obtain heat; for all the fires that are burning on thesurface of the earth would contribute very little towards warming it?

EMILY. 

The radiation of heat is not confined to burning bodies: for all bodies, you know, have that property; therefore, not only every thing upon thesurface of the earth, but the earth itself, must radiate heat; and thisterrestrial caloric, not having, I suppose, sufficient power to traversethe atmosphere, communicates heat to it. 

MRS. B. 

Your inference is extremely well drawn, Emily; but the foundation onwhich it rests is not sound; for the fact is, that terrestrial orculinary heat, though it cannot pass through the denser transparentmediums, such as glass or water, without loss, traverses the atmospherecompletely: so that all the heat which the earth radiates, unless itmeet with clouds or any foreign body to intercept its passage, passesinto the distant regions of the universe. 

CAROLINE. 

What a pity that so much heat should be wasted!

MRS. B. 

Before you are tempted to object to any law of nature, reflect whetherit may not prove to be one of the numberless dispensations of Providencefor our good. If all the heat which the earth has received from the sun, since the creation had been accumulated in it, its temperature by thistime would, no doubt, have been more elevated than any human being couldhave borne. 

CAROLINE. 

I spoke indeed very inconsiderately. But, Mrs.  B. , though the earth, atsuch a high temperature, might have scorched our feet, we should alwayshave had a cool refreshing air to breathe, since the radiation of theearth does not heat the atmosphere. 

EMILY. 

The cool air would have afforded but very insufficient refreshment, whilst our bodies were exposed to the burning radiation of the earth. 

MRS. B. 

Nor should we have breathed a cool air; for though it is true that heatis not communicated to the atmosphere by radiation, yet the air iswarmed by contact with heated bodies, in the same manner as solids orliquids. The stratum of air which is immediately in contact with theearth is heated by it; it becomes specifically lighter and rises, makingway for another stratum of air which is in its turn heated and carriedupwards; and thus each successive stratum of air is warmed by coming incontact with the earth. You may perceive this effect in a sultry day, ifyou attentively observe the strata of air near the surface of the earth;they appear in constant agitation, for though it is true the air isitself invisible, yet the sun shining on the vapours floating in it, render them visible, like the amber dust in the water. The temperatureof the surface of the earth is therefore the source from whence theatmosphere derives its heat, though it is communicated neither byradiation, nor transmitted from one particle of it to another by theconducting power; but every particle of air must come in contact withthe earth in order to receive heat from it. 

EMILY. 

Wind then by agitating the air should contribute to cool the earth andwarm the atmosphere, by bringing a more rapid succession of fresh strataof air in contact with the earth, and yet in general wind feels coolerthan still air?

MRS. B. 

Because the agitation of the air carries off heat from the surface ofour bodies more rapidly than still air, by occasioning a greater numberof points of contact in a given time. 

EMILY. 

Since it is from the earth and not the sun that the atmosphere receivesits heat, I no longer wonder that elevated regions should be colder thanplains and valleys; it was always a subject of astonishment to me, thatin ascending a mountain and approaching the sun, the air became colderinstead of being more heated. 

MRS. B. 

At the distance of about a hundred million of miles, which we are fromthe sun, the approach of a few thousand feet makes no sensibledifference, whilst it produces a very considerable effect with regard tothe warming the atmosphere at the surface of the earth. 

CAROLINE. 

Yet as the warm air rises from the earth and the cold air descends toit, I should have supposed that heat would have accumulated in the upperregions of the atmosphere, and that we should have felt the air warmeras we ascended?

MRS. B. 

The atmosphere, you know, diminishes in density, and consequently inweight, as it is more distant from the earth; the warm air, therefore, rises only till it meets with a stratum of air of its own density; andit will not ascend into the upper regions of the atmosphere until allthe parts beneath have been previously heated. The length of summer evenin warm climates does not heat the air sufficiently to melt the snowwhich has accumulated during the winter on very high mountains, althoughthey are almost constantly exposed to the heat of the sun’s rays, beingtoo much elevated to be often enveloped in clouds. 

EMILY. 

These explanations are very satisfactory; but allow me to ask you onemore question respecting the increased levity of heated liquids. Yousaid that when water was heated over the fire, the particles at thebottom of the vessel ascended as soon as heated, in consequence of theirspecific levity: why does not the same effect continue when the waterboils, and is converted into steam? and why does the steam rise from thesurface, instead of the bottom of the liquid?

MRS. B. 

The steam or vapour does ascend from the bottom, though it seems toarise from the surface of the liquid. We shall boil some water in thisFlorence flask, (PLATE IV. Fig.  1. ) in order that you may be wellacquainted with the process of ebullition;--you will then see, throughthe glass, that the vapour rises in bubbles from the bottom. We shallmake it boil by means of a lamp, which is more convenient for thispurpose than the chimney fire. 

 [Illustration: Plate IV. Vol. I. P. 84. 

 Fig. 1. Pneumatic Pump. Ether evaporated & water frozen in the air pump. A Phial of Ether. B Glass vessel containing water. C. C Thermometers one in the Ether, the other in the water. 

 Fig. 2. Boiling water in a flask over a Patent lamp. ]

EMILY. 

I see some small bubbles ascend, and a great many appear all over theinside of the flask; does the water begin to boil already?

MRS. B. 

No; what you now see are bubbles of air, which were either dissolved inthe water, or attached to the inner surface of the flask, and which, being rarefied by the heat, ascend in the water. 

EMILY. 

But the heat which rarefies the air inclosed in the water must rarefythe water at the same time; therefore, if it could remain stationary inthe water when both were cold, I do not understand why it should notwhen both are equally heated?

MRS. B. 

Air being much less dense than water, is more easily rarefied; theformer, therefore, expands to a great extent, whilst the lattercontinues to occupy nearly the same space; for water dilatescomparatively but very little without changing its state and becomingvapour. Now that the water in the flask begins to boil, observe whatlarge bubbles rise from the bottom of it. 

EMILY. 

I see them perfectly; but I wonder that they have sufficient power toforce themselves through the water. 

CAROLINE. 

They _must_ rise, you know, from their specific levity. 

MRS. B. 

You are right, Caroline; but vapour has not in all liquids (when broughtto the degree of vaporization) the power of overcoming the pressure ofthe less heated surface. Metals, for instance, mercury excepted, evaporate only from the surface; therefore no vapour will ascend fromthem till the degree of heat which is necessary to form it has reachedthe surface; that is to say, till the whole of the liquid is brought toa state of ebullition. 

EMILY. 

I have observed that steam, immediately issuing from the spout of ateakettle, is less visible than at a further distance from it; yet itmust be more dense when it first evaporates, than when it begins todiffuse itself in the air. 

MRS. B. 

When the steam is first formed, it is so perfectly dissolved by caloric, as to be invisible. In order however to understand this, it will benecessary for me to enter into some explanation respecting the nature ofSOLUTION. Solution takes place whenever a body is melted in a fluid. Inthis operation the body is reduced to such a minute state of division bythe fluid, as to become invisible in it, and to partake of its fluidity;but in common solutions this happens without any decomposition, the bodybeing only divided into its integrant particles by the fluid in which itis melted. 

CAROLINE. 

It is then a mode of destroying the attraction of aggregation. 

MRS. B. 

Undoubtedly. --The two principal solvent fluids are _water_, and_caloric_. You may have observed that if you melt salt in water, ittotally disappears, and the water remains clear, and transparent asbefore; yet though the union of these two bodies appears so perfect, itis not produced by any chemical combination; both the salt and the waterremain unchanged; and if you were to separate them by evaporating thelatter, you would find the salt in the same state as before. 

EMILY. 

I suppose that water is a solvent for solid bodies, and caloric forliquids?

MRS. B. 

Liquids of course can only be converted into vapour by caloric. But thesolvent power of this agent is not at all confined to that class ofbodies; a great variety of solid substances are dissolved by heat: thusmetals, which are insoluble in water, can be dissolved by intense heat, being first fused or converted into a liquid, and then rarefied into aninvisible vapour. Many other bodies, such as salt, gums, &c. Yield toeither of these solvents. 

CAROLINE. 

And that, no doubt, is the reason why hot water will melt them so muchbetter than cold water?

MRS. B. 

It is so. Caloric may, indeed, be considered as having, in everyinstance, some share in the solution of a body by water, since water, however low its temperature may be, always contains more or lesscaloric. 

EMILY. 

Then, perhaps, water owes its solvent power merely to the caloriccontained in it?

MRS. B. 

That, probably, would be carrying the speculation too far; I shouldrather think that water and caloric unite their efforts to dissolve abody, and that the difficulty or facility of effecting this, depend bothon the degree of attraction of aggregation to be overcome, and on thearrangement of the particles which are more or less disposed to bedivided and penetrated by the solvent. 

EMILY. 

But have not all liquids the same solvent power as water?

MRS. B. 

The solvent power of other liquids varies according to their nature, andthat of the substances submitted to their action. Most of thesesolvents, indeed, differ essentially from water, as they do not merelyseparate the integrant particles of the bodies which they dissolve, butattack their constituent principles by the power of chemical attraction, thus producing a true decomposition. These more complicated operationswe must consider in another place, and confine our attention at presentto the solutions by water and caloric. 

CAROLINE. 

But there are a variety of substances which, when dissolved in water, make it thick and muddy, and destroy its transparency. 

MRS. B. 

In this case it is not a solution, but simply a mixture. I shall showyou the difference between a solution and a mixture, by putting somecommon salt into one glass of water, and some powder of chalk intoanother; both these substances are white, but their effect on the waterwill be very different. 

CAROLINE. 

Very different indeed! The salt entirely disappears and leaves the watertransparent, whilst the chalk changes it into an opaque liquid likemilk. 

EMILY. 

And would lumps of chalk and salt produce similar effects on water?

MRS. B. 

Yes, but not so rapidly; salt is, indeed, soon melted though in a lump;but chalk, which does not mix so readily with water, would require amuch greater length of time; I therefore preferred showing you theexperiment with both substances reduced to powder, which does not in anyrespect alter their nature, but facilitates the operation merely bypresenting a greater quantity of surface to the water. 

I must not forget to mention a very curious circumstance respectingsolutions, which is, that a fluid is not nearly so much increased inbulk by holding a body in solution, as it would by mere mixture with thebody. 

CAROLINE. 

That seems impossible; for two bodies cannot exist together in the samespace. 

MRS. B. 

Two bodies may, by condensation, occupy less space when in union thanwhen separate, and this I can show you by an easy experiment. 

This phial, which contains some salt, I shall fill with water, pouringit in quickly, so as not to dissolve much of the salt; and when it isquite full I cork it. --If I now shake the phial till the salt isdissolved, you will observe that it is no longer full. 

CAROLINE. 

I shall try to add a little more salt. --But now, you see, Mrs.  B. , thewater runs over. 

MRS. B. 

Yes; but observe that the last quantity of salt you put in remains solidat the bottom, and displaces the water; for it has already melted allthe salt it is capable of holding in solution. This is called the pointof _saturation_; and the water in this case is said to be _saturated_with salt. 

EMILY. 

I think I now understand the solution of a solid body by waterperfectly: but I have not so clear an idea of the solution of a liquidby caloric. 

MRS. B. 

It is probably of a similar nature; but as caloric is an invisiblefluid, its action as a solvent is not so obvious as that of water. Caloric, we may conceive, dissolves water, and converts it into vapourby the same process as water dissolves salt; that is to say, theparticles of water are so minutely divided by the caloric as to becomeinvisible. Thus, you are now enabled to understand why the vapour ofboiling water, when it first issues from the spout of a kettle, isinvisible; it is so, because it is then completely dissolved by caloric. But the air with which it comes in contact, being much colder than thevapour, the latter yields to it a quantity of its caloric. The particlesof vapour being thus in a great measure deprived of their solvent, gradually collect, and become visible in the form of steam, which iswater in a state of imperfect solution; and if you were further todeprive it of its caloric, it would return to its original liquid state. 

CAROLINE. 

That I understand very well. If you hold a cold plate over a tea-urn, the steam issuing from it will be immediately converted into drops ofwater by parting with its caloric to the plate; but in what state is thesteam, when it becomes invisible by being diffused in the air?

MRS. B. 

It is not merely diffused, but is again dissolved by the air. 

EMILY. 

The air, then, has a solvent power, like water and caloric?

MRS. B. 

This was formerly believed to be the case. But it appears from morerecent enquiries that the solvent power of the atmosphere depends solelyupon the caloric contained in it. Sometimes the watery vapour diffusedin the atmosphere is but imperfectly dissolved, as is the case in theformation of clouds and fogs; but if it gets into a region sufficientlywarm, it becomes perfectly invisible. 

EMILY. 

Can any water dissolve in the atmosphere without its being previouslyconverted into vapour by boiling?

MRS. B. 

Unquestionably; and this constitutes the difference between_vaporization_ and _evaporation_. Water, when heated to the boilingpoint, can no longer exist in the form of water, and must necessarily beconverted into vapour or steam, whatever may be the state andtemperature of the surrounding medium; this is called vaporization. Butthe atmosphere, by means of the caloric it contains, can take up acertain portion of water at any temperature, and hold it in a state ofsolution. This is simply evaporation. Thus the atmosphere is continuallycarrying off moisture from the surface of the earth, until it issaturated with it. 

CAROLINE. 

That is the case, no doubt, when we feel the atmosphere damp. 

MRS. B. 

On the contrary, when the moisture is well dissolved it occasions nohumidity: it is only when in a state of imperfect solution and floatingin the atmosphere, in the form of watery vapour, that it producesdampness. This happens more frequently in winter than in summer; for thelower the temperature of the atmosphere, the less water it can dissolve;and in reality it never contains so much moisture as in a dry hotsummer’s day. 

CAROLINE. 

You astonish me! But why, then, is the air so dry in frosty weather, when its temperature is at the lowest?

EMILY. 

This, I conjecture, proceeds not so much from the moisture beingdissolved, as from its being frozen; is not that the case?

MRS. B. 

It is; and the freezing of the watery vapour which the atmospheric heatcould not dissolve, produces what is called a hoar frost; for theparticles descend in freezing, and attach themselves to whatever theymeet with on the surface of the earth. 

The tendency of free caloric to an equilibrium, together with itssolvent power, are likewise connected with the phenomena of rain, ofdew,  &c. When moist air of a certain temperature happens to pass througha colder region of the atmosphere, it parts with a portion of its heatto the surrounding air; the quantity of caloric, therefore, which servedto keep the water in a state of vapour, being diminished, the wateryparticles approach each other, and form themselves into drops of water, which being heavier than the atmosphere, descend to the earth. There arealso other circumstances, and particularly the variation in the weightof the atmosphere, which may contribute to the formation of rain. This, however, is an intricate subject, into which we cannot more fully enterat present. 

EMILY. 

In what manner do you account for the formation of dew?

MRS. B. 

Dew is a deposition of watery particles or minute drops from theatmosphere, precipitated by the coolness of the evening. 

CAROLINE. 

This precipitation is owing, I suppose, to the cooling of theatmosphere, which prevents its retaining so great a quantity of wateryvapour in solution as during the heat of the day. 

MRS. B. 

Such was, from time immemorial, the generally received opinionrespecting the cause of dew; but it has been very recently proved by acourse of ingenious experiments of Dr. Wells, that the deposition of dewis produced by the cooling of the surface of the earth, which he hasshown to take place previously to the cooling of the atmosphere; for onexamining the temperature of a plot of grass just before the dew-fall, he found that it was considerably colder than the air a few feet aboveit, from which the dew was shortly after precipitated. 

EMILY. 

But why should the earth cool in the evening sooner than the atmosphere?

MRS. B. 

Because it parts with its heat more readily than the air; the earth isan excellent radiator of caloric, whilst the atmosphere does not possessthat property, at least in any sensible degree. Towards evening, therefore, when the solar heat declines, and when after sunset itentirely ceases, the earth rapidly cools by radiating heat towards theskies; whilst the air has no means of parting with its heat but bycoming into contact with the cooled surface of the earth, to which itcommunicates its caloric. Its solvent power being thus reduced, it isunable to retain so large a portion of watery vapour, and deposits thosepearly drops which we call dew. 

EMILY. 

If this be the cause of dew, we need not be apprehensive of receivingany injury from it; for it can be deposited only on surfaces that arecolder than the atmosphere, which is never the case with our bodies. 

MRS. B. 

Very true; yet I would not advise you for this reason to be tooconfident of escaping all the ill effects which may arise from exposureto the dew; for it may be deposited on your clothes, and chill youafterwards by its evaporation from them. Besides, whenever the dew iscopious, there is a chill in the atmosphere which it is not always safeto encounter. 

CAROLINE. 

Wind, then, must promote the deposition of dew, by bringing a more rapidsuccession of particles of air in contact with the earth, just as itpromotes the cooling of the earth and warming of the atmosphere duringthe heat of the day?

MRS. B. 

Yes; provided the wind be unattended with clouds, for theseaccumulations of moisture not only prevent the free radiation of theearth towards the upper regions, but themselves radiate towards theearth; under these circumstances much less dew is formed than on fineclear nights, when the radiation of the earth passes without obstaclethrough the atmosphere to the distant regions of space, whence itreceives no caloric in exchange. The dew continues to be depositedduring the night, and is generally most abundant towards morning, whenthe contrast between the temperature of the earth and that of the air isgreatest. After sunrise the equilibrium of temperature between these twobodies is gradually restored by the solar rays passing freely throughthe atmosphere to the earth; and later in the morning the temperature ofthe earth gains the ascendency, and gives out caloric to the air bycontact, in the same manner as it receives it from the air during thenight. --Can you tell me, now, why a bottle of wine taken fresh from thecellar (in summer particularly), will soon be covered with dew; and eventhe glasses into which the wine is poured will be moistened with asimilar vapour?

EMILY. 

The bottle being colder than the surrounding air, must absorb caloricfrom it; the moisture therefore which that air contained becomesvisible, and forms the dew which is deposited on the bottle. 

MRS. B. 

Very well, Emily. Now, Caroline, can you inform me why, in a warm room, or close carriage, the contrary effect takes place; that is to say, thatthe inside of the windows is covered with vapour?

CAROLINE. 

I have heard that it proceeds from the breath of those within the roomor the carriage; and I suppose it is occasioned by the windows which, being colder than the breath, deprive it of part of its caloric, and bythis means convert it into watery vapour. 

MRS. B. 

You have both explained it extremely well. Bodies attract dew inproportion as they are good radiators of caloric, as it is this qualitywhich reduces their temperature below that of the atmosphere; hence wefind that little or no dew is deposited on rocks, sand, water; whilegrass and living vegetables, to which it is so highly beneficial, attract it in abundance--another remarkable instance of the wise andbountiful dispensations of Providence. 

EMILY. 

And we may again observe it in the abundance of dew in summer, and inhot climates, when its cooling effects are so much required; but I donot understand what natural cause increases the dew in hot weather?

MRS. B. 

The more caloric the earth receives during the day, the more it willradiate afterwards, and consequently the more rapidly its temperaturewill be reduced in the evening, in comparison to that of the atmosphere. In the West-Indies especially, where the intense heat of the day isstrongly contrasted with the coolness of the evening, the dew isprodigiously abundant. During a drought, the dew is less plentiful, asthe earth is not sufficiently supplied with moisture to be able tosaturate the atmosphere. 

CAROLINE. 

I have often observed, Mrs. B. , that when I walk out in frosty weather, with a veil over my face, my breath freezes upon it. Pray what is thereason of that?

MRS. B. 

It is because the cold air immediately seizes on the caloric of yourbreath, and, by robbing it of its solvent, reduces it to a denser fluid, which is the watery vapour that settles on your veil, and there itcontinues parting with its caloric till it is brought down to thetemperature of the atmosphere, and assumes the form of ice. 

You may, perhaps, have observed that the breath of animals, or ratherthe moisture contained in it, is visible in damp weather, or during afrost. In the former case, the atmosphere being over-saturated withmoisture, can dissolve no more. In the latter, the cold condenses itinto visible vapour; and for the same reason, the steam arising fromwater that is warmer than the atmosphere, becomes visible. Have younever taken notice of the vapour rising from your hands after havingdipped them into warm water?

CAROLINE. 

Frequently, especially in frosty weather. 

MRS. B. 

We have already observed that pressure is an obstacle to evaporation:there are liquids that contain so great a quantity of caloric, and whoseparticles consequently adhere so slightly together, that they may berapidly converted into vapour without any elevation of temperature, merely by taking off the weight of the atmosphere. In such liquids, youperceive, it is the pressure of the atmosphere alone that connects theirparticles, and keeps them in a liquid state. 

CAROLINE. 

I do not well understand why the particles of such fluids should bedisunited and converted into vapour, without any elevation oftemperature, in spite of the attraction of cohesion. 

MRS. B. 

It is because the degree of heat at which we usually observe thesefluids is sufficient to overcome their attraction of cohesion. Ether isof this description; it will boil and be converted into vapour, at thecommon temperature of the air, if the pressure of the atmosphere betaken off. 

EMILY. 

I thought that ether would evaporate without either the pressure of theatmosphere being taken away, or heat applied; and that it was for thatreason so necessary to keep it carefully corked up?

MRS. B. 

It is true it will evaporate, but without ebullition; what I am nowspeaking of is the vaporization of ether, or its conversion into vapourby boiling. I am going to show you how suddenly the ether in this phialwill be converted into vapour, by means of the air-pump. --Observe withwhat rapidity the bubbles ascend, as I take off the pressure of theatmosphere. 

CAROLINE. 

It positively boils: how singular to see a liquid boil without heat!

MRS. B. 

Now I shall place the phial of ether in this glass, which it nearlyfits, so as to leave only a small space, which I fill with water; and inthis state I put it again under the receiver. (PLATE IV. Fig.  1. )* Youwill observe, as I exhaust the air from it, that whilst the ether boils, the water freezes. 

 [Footnote *: Two pieces of thin glass tubes, sealed at one end, might answer this purpose better. The experiment, however, as here described, is difficult, and requires a very nice apparatus. But if, instead of phials or tubes, two watch-glasses be used, water may be frozen almost instantly in the same manner. The two glasses are placed over one another, with a few drops of water interposed between them, and the uppermost glass is filled with ether. After working the pump for a minute or two, the glasses are found to adhere strongly together, and a thin layer of ice is seen between them. ]

CAROLINE. 

It is indeed wonderful to see water freeze in contact with a boilingfluid!

EMILY. 

I am at a loss to conceive how the ether can pass to the state of vapourwithout an addition of caloric. Does it not contain more caloric in astate of vapour, than in a state of liquidity?

MRS. B. 

It certainly does; for though it is the pressure of the atmosphere whichcondenses it into a liquid, it is by forcing out the caloric thatbelongs to it when in an aëriform state. 

EMILY. 

You have, therefore, two difficulties to explain, Mrs.  B. --First, fromwhence the ether obtains the caloric necessary to convert it into vapourwhen it is relieved from the pressure of the atmosphere; and, secondly, what is the reason that the water, in which the bottle of ether stands, is frozen?

CAROLINE. 

Now, I think, I can answer both these questions. The ether obtains theaddition of caloric required, from the water in the glass; and the lossof caloric, which the latter sustains, is the occasion of its freezing. 

MRS. B. 

You are perfectly right; and if you look at the thermometer which I haveplaced in the water, whilst I am working the pump, you will see thatevery time bubbles of vapour are produced, the mercury descends; whichproves that the heat of the water diminishes in proportion as the etherboils. 

EMILY. 

This I understand now very well; but if the water freezes in consequenceof yielding its caloric to the ether, the equilibrium of heat must, inthis case, be totally destroyed. Yet you have told us, that the exchangeof caloric between two bodies of equal temperature, was always equal;how, then, is it that the water, which was originally of the sametemperature as the ether, gives out caloric to it, till the water isfrozen, and the ether made to boil?

MRS. B. 

I suspected that you would make these objections; and, in order toremove them, I enclosed two thermometers in the air-pump; one whichstands in the glass of water, the other in the phial of ether; and youmay see that the equilibrium of temperature is not destroyed; for as thethermometer descends in the water, that in the ether sinks in the samemanner; so that both thermometers indicate the same temperature, thoughone of them is in a boiling, the other in a freezing liquid. 

EMILY. 

The ether, then, becomes colder as it boils? This is so contrary tocommon experience, that I confess it astonishes me exceedingly. 

CAROLINE. 

It is, indeed, a most extraordinary circumstance. But pray, how do youaccount for it?

MRS. B. 

I cannot satisfy your curiosity at present; for before we can attempt toexplain this apparent paradox, it is necessary to become acquainted withthe subject of LATENT HEAT: and that, I think, we must defer till ournext interview. 

CAROLINE. 

I believe, Mrs. B. , that you are glad to put off the explanation; for itmust be a very difficult point to account for. 

MRS. B. 

I hope, however, that I shall do it to your complete satisfaction. 

EMILY. 

But before we part, give me leave to ask you one question. Would notwater, as well as ether, boil with less heat, if deprived of thepressure of the atmosphere?

MRS. B. 

Undoubtedly. You must always recollect that there are two forces toovercome, in order to make a liquid boil or evaporate; the attraction ofaggregation, and the weight of the atmosphere. On the summit of a highmountain (as Mr. De Saussure ascertained on Mount Blanc) much less heatis required to make water boil, than in the plain, where the weight ofthe atmosphere is greater. * Indeed if the weight of the atmosphere beentirely removed by means of a good air-pump, and if water be placed inthe exhausted receiver, it will evaporate so fast, however cold itmaybe, as to give it the appearance of boiling from the surface. Butwithout the assistance of the air-pump, I can show you a very prettyexperiment, which proves the effect of the pressure of the atmosphere inthis respect. 

Observe, that this Florence flask is about half full of water, and theupper half of invisible vapour, the water being in the act of boiling. --I take it from the lamp, and cork it carefully--the water, you see, immediately ceases boiling. --I shall now dip the flask into a bason ofcold water. †

 [Footnote *: On the top of Mount Blanc, water boiled when heated only to 187 degrees, instead of 212 degrees. ]

 [Footnote †: The same effect may be produced by wrapping a cold wet linen cloth round the upper part of the flask. In order to show how much the water cools whilst it is boiling, a thermometer, graduated on the tube itself, may be introduced into the bottle through the cork. ]

CAROLINE. 

But look, Mrs. B. , the hot water begins to boil again, although the coldwater must rob it more and more of its caloric! What can be the reasonof that?

MRS. B. 

Let us examine its temperature. You see the thermometer immersed in itremains stationary at 180 degrees, which is about 30 degrees below theboiling point. When I took the flask from the lamp, I observed to youthat the upper part of it was filled with vapour; this being compelledto yield its caloric to the cold water, was again condensed into water--What, then, filled the upper part of the flask?

EMILY. 

Nothing; for it was too well corked for the air to gain admittance, andtherefore the upper part of the flask must be a vacuum. 

MRS. B. 

The water below, therefore, no longer sustains the pressure of theatmosphere, and will consequently boil at a much lower temperature. Thus, you see, though it had lost many degrees of heat, it began boilingagain the instant the vacuum was formed above it. The boiling has nowceased, the temperature of the water being still farther reduced; if ithad been ether, instead of water, it would have continued boiling muchlonger, for ether boils, under the usual atmospheric pressure, at atemperature as low as 100 degrees; and in a vacuum it boils at almostany temperature; but water being a more dense fluid, requires a moreconsiderable quantity of caloric to make it evaporate quickly, even whenthe pressure of the atmosphere is removed. 

EMILY. 

What proportion of vapour can the atmosphere contain in a state ofsolution?

MRS. B. 

I do not know whether it has been exactly ascertained by experiment; butat any rate this proportion must vary, both according to the temperatureand the weight of the atmosphere; for the lower the temperature, and thegreater the pressure, the smaller must be the proportion of vapour thatthe atmosphere can contain. 

To conclude the subject of free caloric, I should mention _Ignition_, bywhich is meant that emission of light which is produced in bodies at avery high temperature, and which is the effect of accumulated caloric. 

EMILY. 

You mean, I suppose, that light which is produced by a burning body?

MRS. B. 

No: ignition is quite independent of combustion. Clay, chalk, and indeedall incombustible substances, may be made red hot. When a body burns, the light emitted is the effect of a chemical change which takes place, whilst ignition is the effect of caloric alone, and no other change thanthat of temperature is produced in the ignited body. 

All solid bodies, and most liquids, are susceptible of ignition, or, inother words, of being heated so as to become luminous; and it isremarkable that this takes place pretty nearly at the same temperaturein all bodies, that is, at about 800 degrees of Fahrenheit’s scale. 

EMILY. 

But how can liquids attain so high a temperature, without beingconverted into vapour?

MRS. B. 

By means of confinement and pressure. Water confined in a strong ironvessel (called Papin’s digester) can have its temperature raised toupwards of 400 degrees. Sir James Hall has made some very curiousexperiments on the effects of heat assisted by pressure; by means ofstrong gun-barrels, he succeeded in melting a variety of substanceswhich were considered as infusible: and it is not unlikely that, bysimilar methods, water itself might be heated to redness. 

EMILY. 

I am surprised at that: for I thought that the force of steam was suchas to destroy almost all mechanical resistance. 

MRS. B. 

The expansive force of steam is prodigious; but in order to subjectwater to such high temperatures, it is prevented by confinement frombeing converted into steam, and the expansion of heated water iscomparatively trifling. --But we have dwelt so long on the subject offree caloric, that we must reserve the other modifications of that agentto our next meeting, when we shall endeavour to proceed more rapidly. 

CONVERSATION IV. 

ON COMBINED CALORIC, COMPREHENDING SPECIFIC AND LATENT HEAT. 

MRS. B. 

We are now to examine the other modifications of caloric. 

CAROLINE. 

I am very curious to know of what nature they can be; for I have nonotion of any kind of heat that is not perceptible to the senses. 

MRS. B. 

In order to enable you to understand them, it will be necessary to enterinto some previous explanations. 

It has been discovered by modern chemists, that bodies of a differentnature, heated to the same temperature, do not contain the same quantityof caloric. 

CAROLINE. 

How could that be ascertained? Have you not told us that it isimpossible to discover the absolute quantity of caloric which bodiescontain?

MRS. B. 

True; but at the same time I said that we were enabled to form ajudgment of the proportions which bodies bore to each other in thisrespect. Thus it is found that, in order to raise the temperature ofdifferent bodies the same number of degrees, different quantities ofcaloric are required for each of them. If, for instance, you place apound of lead, a pound of chalk, and a pound of milk, in a hot oven, they will be gradually heated to the temperature of the oven; but thelead will attain it first, the chalk next, and the milk last. 

CAROLINE. 

That is a natural consequence of their different bulks; the lead beingthe smallest body, will be heated soonest, and the milk, which is thelargest, will require the longest time. 

MRS. B. 

That explanation will not do, for if the lead be the least in bulk, itoffers also the least surface to the caloric, the quantity of heattherefore which can enter into it in the same space of time isproportionally smaller. 

EMILY. 

Why, then, do not the three bodies attain the temperature of the oven atthe same time?

MRS. B. 

It is supposed to be on account of the different capacity of thesebodies for caloric. 

CAROLINE. 

What do you mean by the capacity of a body for caloric?

MRS. B. 

I mean a certain disposition of bodies to require more or less caloricfor raising their temperature to any degree of heat. Perhaps the factmay be thus explained:

Let us put as many marbles into this glass as it will contain, and poursome sand over them--observe how the sand penetrates and lodges betweenthem. We shall now fill another glass with pebbles of various forms--yousee that they arrange themselves in a more compact manner than themarbles, which, being globular, can touch each other by a single pointonly. The pebbles, therefore, will not admit so much sand between them;and consequently one of these glasses will necessarily contain more sandthan the other, though both of them be equally full. 

CAROLINE. 

This I understand perfectly. The marbles and the pebbles represent twobodies of different kinds, and the sand the caloric contained in them;it appears very plain, from this comparison, that one body may admit ofmore caloric between its particles than another. 

MRS. B. 

You can no longer be surprised, therefore, that bodies of a differentcapacity for caloric should require different proportions of that fluidto raise their temperatures equally. 

EMILY. 

But I do not conceive why the body that contains the most caloric shouldnot be of the highest temperature; that is to say, feel hot inproportion to the quantity of caloric it contains?

MRS. B. 

The caloric that is employed in filling the capacity of a body, is notfree caloric; but is imprisoned as it were in the body, and is thereforeimperceptible: for we can feel only the caloric which the body partswith, and not that which it retains. 

CAROLINE. 

It appears to me very extraordinary that heat should be confined in abody in such a manner as to be imperceptible. 

MRS. B. 

If you lay your hand on a hot body, you feel only the caloric whichleaves it, and enters your hand; for it is impossible that you should besensible of that which remains in the body. The thermometer, in the samemanner, is affected only by the free caloric which a body transmits toit, and not at all by that which it does not part with. 

CAROLINE. 

I begin to understand it: but I confess that the idea of insensible heatis so new and strange to me, that it requires some time to render itfamiliar. 

MRS. B. 

Call it insensible caloric, and the difficulty will appear much lessformidable. It is indeed a sort of contradiction to call it heat, whenit is so situated as to be incapable of producing that sensation. Yetthis modification of caloric is commonly called SPECIFIC HEAT. 

CAROLINE. 

But it certainly would have been more correct to have called it_specific caloric_. 

EMILY. 

I do not understand how the term _specific_ applies to this modificationof caloric?

MRS. B. 

It expresses the relative quantity of caloric which different _species_of bodies of the same weight and temperature are capable of containing. This modification is also frequently called _heat of capacity_, a termperhaps preferable, as it explains better its own meaning. 

You now understand, I suppose, why the milk and chalk required a longerportion of time than the lead to raise their temperature to that of theoven?

EMILY. 

Yes: the milk and chalk having a greater capacity for caloric than thelead, a greater proportion of that fluid became insensible in thosebodies: and the more slowly, therefore, their temperature was raised. 

CAROLINE. 

But might not this difference proceed from the different conductingpowers of heat in these three bodies, since that which is the bestconductor must necessarily attain the temperature of the oven first?

MRS. B. 

Very well observed, Caroline. This objection would be insurmountable, ifwe could not, by reversing the experiment, prove that the milk, thechalk, and the lead, actually absorbed different quantities of caloric, and we know that if the different time they took in heating, proceededmerely from their different conducting powers, they would each haveacquired an equal quantity of caloric. 

CAROLINE. 

Certainly. But how can you reverse this experiment?

MRS. B. 

It may be done by cooling the several bodies to the same degree in anapparatus adapted to receive and measure the caloric which they giveout. Thus, if you plunge them into three equal quantities of water, eachat the same temperature, you will be able to judge of the relativequantity of caloric which the three bodies contained, by that, which, incooling, they communicated to their respective portions of water: forthe same quantity of caloric which they each absorbed to raise theirtemperature, will abandon them in lowering it; and on examining thethree vessels of water, you will find the one in which you immersed thelead to be the least heated; that which held the chalk will be the next;and that which contained the milk will be heated the most of all. Thecelebrated Lavoisier has invented a machine to estimate, upon thisprinciple, the specific heat of bodies in a more perfect manner; but Icannot explain it to you, till you are acquainted with the nextmodification of caloric. 

EMILY. 

The more dense a body is, I suppose, the less is its capacity forcaloric?

MRS. B. 

This is not always the case with bodies of different nature; iron, forinstance, contains more specific heat than tin, though it is more dense. This seems to show that specific heat does hot merely depend upon theinterstices between the particles; but, probably, also upon somepeculiar constitution of the bodies which we do not comprehend. 

EMILY. 

But, Mrs. B. , it would appear to me more proper to compare bodies by_measure_, rather than by _weight_, in order to estimate their specificheat. Why, for instance, should we not compare _pints_ of milk, ofchalk, and of lead, rather than _pounds_ of those substances; for equalweights may be composed of very different quantities?

MRS. B. 

You are mistaken, my dear; equal weight must contain equal quantities ofmatter; and when we wish to know what is the relative quantity ofcaloric, which substances of various kinds are capable of containingunder the same temperature, we must compare equal weights, and not equalbulks of those substances. Bodies of the same weight may undoubtedly beof very different dimensions; but that does not change their realquantity of matter. A pound of feathers does not contain one atom morethan a pound of lead. 

CAROLINE. 

I have another difficulty to propose. It appears to me, that if thetemperature of the three bodies in the oven did not rise equally, theywould never reach the same degree; the lead would always keep itsadvantage over the chalk and milk, and would perhaps be boiling beforethe others had attained the temperature of the oven. I think you mightas well say that, in the course of time, you and I should be of the sameage?

MRS. B. 

Your comparison is not correct, Caroline. As soon as the lead reachedthe temperature of the oven, it would remain stationary; for it wouldthen give out as much heat as it would receive. You should recollectthat the exchange of radiating heat, between two bodies of equaltemperature, is equal: it would be impossible, therefore, for the leadto accumulate heat after having attained the temperature of the oven;and that of the chalk and milk therefore would ultimately arrive at thesame standard. Now I fear that this will not hold good with respect toour ages, and that, as long as I live, I shall never cease to keep myadvantage over you. 

EMILY. 

I think that I have found a comparison for specific heat, which is veryapplicable. Suppose that two men of equal weight and bulk, but whorequired different quantities of food to satisfy their appetites, sitdown to dinner, both equally hungry; the one would consume a muchgreater quantity of provisions than the other, in order to be equallysatisfied. 

MRS. B. 

Yes, that is very fair; for the quantity of food necessary to satisfytheir respective appetites, varies in the same manner as the quantity ofcaloric requisite to raise equally the temperature of different bodies. 

EMILY. 

The thermometer, then, affords no indication of the specific heat ofbodies?

MRS. B. 

None at all: no more than satiety is a test of the quantity of foodeaten. The thermometer, as I have repeatedly said, can be affected onlyby free caloric, which alone raises the temperature of bodies. 

But there is another mode of proving the existence of specific heat, which affords a very satisfactory illustration of that modification. This, however, I did not enlarge upon before, as I thought it mightappear to you rather complicated. --If you mix two fluids of differenttemperatures, let us say the one at 50 degrees, and the other at 100degrees, of what temperature do you suppose the mixture will be?

CAROLINE. 

It will be no doubt the medium between the two, that is to say, 75degrees. 

MRS. B. 

That will be the case if the two bodies happen to have the same capacityfor caloric; but if not, a different result will be obtained. Thus, forinstance, if you mix together a pound of mercury, heated at 50 degrees, and a pound of water heated at 100 degrees, the temperature of themixture, instead of being 75 degrees, will be 80 degrees; so that thewater will have lost only 12 degrees, whilst the mercury will havegained 38 degrees; from which you will conclude that the capacity ofmercury for heat is less than that of water. 

CAROLINE. 

I wonder that mercury should have so little specific heat. Did we notsee it was a much better conductor of heat than water?

MRS. B. 

And it is precisely on that account that its specific heat is less. Forsince the conductive power of bodies depends, as we have observedbefore, on their readiness to receive heat and part with it, it isnatural to expect that those bodies which are the worst conductorsshould absorb the most caloric before they are disposed to part with itto other bodies. But let us now proceed to LATENT HEAT. 

CAROLINE. 

And pray what kind of heat is that?

MRS. B. 

It is another modification of combined caloric, which is so analogous tospecific heat, that most chemists make no distinction between them; butMr. Pictet, in his Essay on Fire, has so clearly discriminated them, that I am induced to adopt his view of the subject. We therefore call_latent heat_ that portion of insensible caloric which is employed inchanging the state of bodies; that is to say, in converting solids intoliquids, or liquids; into vapour. When a body changes its state fromsolid to liquid, or from liquid to vapour, its expansion occasions asudden and considerable increase of capacity for heat, in consequence ofwhich it immediately absorbs a quantity of caloric, which becomes fixedin the body which it has transformed; and, as it is perfectly concealedfrom our senses, it has obtained the name of _latent_ heat. 

CAROLINE. 

I think it would be much more correct to call this modification latentcaloric instead of latent heat, since it does not excite the sensationof heat. 

MRS. B. 

This modification of heat was discovered and named by Dr. Black longbefore the French chemists introduced the term caloric, and we must notpresume to alter it, as it is still used by much better chemists thanourselves. And, besides, you are not to suppose that the nature of heatis altered by being variously modified: for if latent heat and specificheat do not excite the same sensations as free caloric, it is owing totheir being in a state of confinement, which prevents them from actingupon our organs; and consequently, as soon as they are extricated fromthe body in which they are imprisoned, they return to their state offree caloric. 

EMILY. 

But I do not yet clearly see in what respect latent heat differs fromspecific heat; for they are both of them imprisoned and concealed inbodies. 

MRS. B. 

Specific heat is that which is employed in filling the capacity of abody for caloric, in the state in which this body actually exists; whilelatent heat is that which is employed only in effecting a change ofstate, that is, in converting bodies from a solid to a liquid, or from aliquid to an aëriform state. But I think that, in a general point ofview, both these modifications might be comprehended under the name of_heat of capacity_, as in both cases the caloric is equally engaged infilling the capacities of bodies. 

I shall now show you an experiment, which I hope will give you a clearidea of what is understood by latent heat. 

The snow which you see in this phial has been cooled by certain chemicalmeans (which I cannot well explain to you at present), to 5 or 6 degreesbelow the freezing point, as you will find indicated by the thermometerwhich is placed in it. We shall expose it to the heat of a lamp, and youwill see the thermometer gradually rise, till it reaches the freezingpoint----

EMILY. 

But there it stops, Mrs. B. , and yet the lamp burns just as well asbefore. Why is not its heat communicated to the thermometer?

CAROLINE. 

And the snow begins to melt, therefore it must be rising above thefreezing point?

MRS. B. 

The heat no longer affects the thermometer, because it is whollyemployed in converting the ice into water. As the ice melts, the caloricbecomes _latent_ in the new-formed liquid, and therefore cannot raiseits temperature; and the thermometer will consequently remainstationary, till the whole of the ice be melted. 

CAROLINE. 

Now it is all melted, and the thermometer begins to rise again. 

MRS. B. 

Because the conversion of the ice into water being completed, thecaloric no longer becomes latent; and therefore the heat which the waternow receives raises its temperature, as you find the thermometerindicates. 

EMILY. 

But I do not think that the thermometer rises so quickly in the water asit did in the ice, previous to its beginning to melt, though the lampburns equally well?

MRS. B. 

That is owing to the different specific heat of ice and water. Thecapacity of water for caloric being greater than that of ice, more heatis required to raise its temperature, and therefore the thermometerrises slower in the water than in the ice. 

EMILY. 

True; you said that a solid body always increased its capacity for heatby becoming fluid; and this is an instance of it. 

MRS. B. 

Yes, and the latent heat is that which is absorbed in consequence of thegreater capacity which the water has for heat, in comparison to ice. 

I must now tell you a curious calculation founded on that consideration. I have before observed to you that though the thermometer shows us thecomparative warmth of bodies, and enables us to determine the same pointat different times and places, it gives us no idea of the absolutequantity of heat in any body. We cannot tell how low it ought to fall bythe privation of all heat, but an attempt has been made to infer it inthe following manner. It has been found by experiment, that the capacityof water for heat, when compared with that of ice, is as 10 to 9, sothat, at the same temperature, ice contains one tenth of caloric lessthan water. By experiment also it is observed, that in order to meltice, there must be added to it as much heat, as would, if it did notmelt it, raise its temperature 140 degrees. This quantity of heat istherefore absorbed when the ice, by being converted into water, is madeto contain one-ninth more caloric than it did before. Therefore 140degrees is a ninth part of the heat contained in ice at 30 degrees; andthe point of zero, or the absolute privation of heat, must consequentlybe 1260 degrees below 32 degrees. 

This mode of investigating so curious a question is ingenious, but itscorrectness is not yet established by similar calculations for otherbodies. The points of absolute cold, indicated by this method in variousbodies, are very remote from each other; it is however possible, thatthis may arise from some imperfection in the experiments. 

CAROLINE. 

It is indeed very ingenious--but we must now attend to our presentexperiment. The water begins to boil, and the thermometer is againstationary. 

MRS. B. 

Well, Caroline, it is your turn to explain the phenomenon. 

CAROLINE. 

It is wonderfully curious! The caloric is now busy in changing the waterinto steam, in which it hides itself, and becomes insensible. This isanother example of latent heat, producing a change of form. At first itconverted a solid body into a liquid, and now it turns the liquid intovapour!

MRS. B. 

You see, my dear, how easily you have become acquainted with thesemodifications of insensible heat, which at first appeared sounintelligible. If, now, we were to reverse these changes, and condensethe vapour into water, and the water into ice, the latent heat wouldre-appear entirely, in the form of free caloric. 

EMILY. 

Pray do let us see the effect of latent heat returning to its freestate. 

MRS. B. 

For the purpose of showing this, we need simply conduct the vapourthrough this tube into this vessel of cold water, where it will partwith its latent heat and return to its liquid form. 

EMILY. 

How rapidly the steam heats the water!

MRS. B. 

That is because it does not merely impart its free caloric to the water, but likewise its latent heat. This method of heating liquids, has beenturned to advantage, in several economical establishments. Thesteam-kitchens, which are getting into such general use, are upon thesame principle. The steam is conveyed through a pipe in a similarmanner, into the several vessels which contain the provisions to bedressed, where it communicates to them its latent caloric, and returnsto the state of water. Count Rumford makes great use of this principlein many of his fire-places: his grand maxim is to avoid all unnecessarywaste of caloric, for which purpose he confines the heat in such amanner, that not a particle of it shall unnecessarily escape; and whilehe economises the free caloric, he takes care also to turn the latentheat to advantage. It is thus that he is enabled to produce a degree ofheat superior to that which is obtained in common fire-places, though heemploys less fuel. 

EMILY. 

When the advantages of such contrivances are so clear and plain, I cannot understand why they are not universally used. 

MRS. B. 

A long time is always required before innovations, however useful, canbe reconciled with the prejudices of the vulgar. 

EMILY. 

What a pity it is that there should be a prejudice against newinventions; how much more rapidly the world would improve, if suchuseful discoveries were immediately and universally adopted!

MRS. B. 

I believe, my dear, that there are as many novelties attempted to beintroduced, the adoption of which would be prejudicial to society, asthere are of those which would be beneficial to it. The well-informed, though by no means exempt from error, have an unquestionable advantageover the illiterate, in judging what is likely or not to proveserviceable; and therefore we find the former more ready to adopt suchdiscoveries as promise to be really advantageous, than the latter, whohaving no other test of the value of a novelty but time and experience, at first oppose its introduction. The well-informed, however, arefrequently disappointed in their most sanguine expectations, and theprejudices of the vulgar, though they often retard the progress ofknowledge, yet sometimes, it must be admitted, prevent the propagationof error. --But we are deviating from our subject. 

We have converted steam into water, and are now to change water intoice, in order to render the latent heat sensible, as it escapes from thewater on its becoming solid. For this purpose we must produce a degreeof cold that will make water freeze. 

CAROLINE. 

That must be very difficult to accomplish in this warm room. 

MRS. B. 

Not so much as you think. There are certain chemical mixtures whichproduce a rapid change from the solid to the fluid state, or thereverse, in the substances combined, in consequence of which changelatent heat is either extricated or absorbed. 

EMILY. 

I do not quite understand you. 

MRS. B. 

This snow and salt, which you see me mix together, are melting rapidly;heat, therefore, must be absorbed by the mixture, and cold produced. 

CAROLINE. 

It feels even colder than ice, and yet the snow is melted. This is veryextraordinary. 

MRS. B. 

The cause of the intense cold of the mixture is to be attributed to thechange from a solid to a fluid state. The union of the snow and saltproduces a new arrangement of their particles, in consequence of whichthey become liquid; and the quantity of caloric, required to effect thischange, is seized upon by the mixture wherever it can be obtained. Thiseagerness of the mixture for caloric, during its liquefaction, is such, that it converts part of its own free caloric into latent heat, and itis thus that its temperature is lowered. 

EMILY. 

Whatever you put in this mixture, therefore, would freeze?

MRS. B. 

Yes; at least any fluid that is susceptible of freezing at thattemperature. I have prepared this mixture of salt and snow for thepurpose of freezing the water from which you are desirous of seeing thelatent heat escape. I have put a thermometer in the glass of water thatis to be frozen, in order that you may see how it cools. 

CAROLINE. 

The thermometer descends, but the heat which the water is now losing, isits _free_, not its _latent_ heat. 

MRS. B. 

Certainly; it does not part with its latent heat till it changes itsstate and is converted into ice. 

EMILY. 

But here is a very extraordinary circumstance! The thermometer is fallenbelow the freezing point, and yet the water is not frozen. 

MRS. B. 

That is always the case previous to the freezing of water when it is ina state of rest. Now it begins to congeal, and you may observe that thethermometer again rises to the freezing point. 

CAROLINE. 

It appears to me very strange that the thermometer should rise the verymoment that the water freezes; for it seems to imply that the water wascolder before it froze than when in the act of freezing. 

MRS. B. 

It is so; and after our long dissertation on this circumstance, I didnot think it would appear so surprising to you. Reflect a little, and Ithink you will discover the reason of it. 

CAROLINE. 

It must be, no doubt, the extrications of latent heat, at the instantthe water freezes, that raises the temperature. 

MRS. B. 

Certainly; and if you now examine the thermometer, you will find thatits rise was but temporary, and lasted only during the disengagement ofthe latent heat--now that all the water is frozen it falls again, andwill continue to fall till the ice and mixture are of an equaltemperature. 

EMILY. 

And can you show us any experiments in which liquids, by being mixed, become solid, and disengage latent heat?

MRS. B. 

I could show you several; but you are not yet sufficiently advanced tounderstand them well. I shall, however, try one, which will afford you astriking instance of the fact. The fluid which you see in this phialconsists of a quantity of a certain salt called _muriat of lime_, dissolved in water. Now, if I pour into it a few drops of this otherfluid, called _sulphuric acid_, the whole, or very nearly the whole, will be instantaneously converted into a solid mass. 

EMILY. 

How white it turns! I feel the latent heat escaping, for the bottle iswarm, and the fluid is changed to a solid white substance like chalk!

CAROLINE. 

This is, indeed, the most curious experiment we have seen yet. But praywhat is that white vapour that ascends from the mixture?

MRS. B. 

You are not yet enough of a chemist to understand that. --But take care, Caroline, do not approach too near it, for it has a very pungent smell. 

I shall show you another instance similar to that of the water, whichyou observed to become warmer as it froze. I have in this phial asolution of a salt called sulphat of soda or Glauber’s salt, made verystrong, and corked up when it was hot, and kept without agitation tillit became cold, as you may feel the phial is. Now when I take out thecork and let the air fall upon it, (for being closed when boiling, therewas a vacuum in the upper part) observe that the salt will suddenlycrystallize.  .  .  . 

CAROLINE. 

Surprising! how beautifully the needles of salt have shot through thewhole phial!

MRS. B. 

Yes, it is very striking--but pray do not forget the object of theexperiment. Feel how warm the phial has become by the conversion of partof the liquid into a solid. 

EMILY. 

Quite warm I declare! this is a most curious experiment of thedisengagement of latent heat. 

MRS. B. 

The slakeing of lime is another remarkable instance of the extricationof latent heat. Have you never observed how quick-lime smokes when wateris poured upon it, and how much heat it produces?

CAROLINE. 

Yes; but I do not understand what change of state takes place in thelime that occasions its giving out latent heat; for the quick-lime, which is solid, is (if I recollect right) reduced to powder, by thisoperation, and is, therefore, rather expanded than condensed. 

MRS. B. 

It is from the water, not the lime, that the latent heat is set free. The water incorporates with, and becomes solid in the lime; inconsequence of which, the heat, which kept it in a liquid state, isdisengaged, and escapes in a sensible form. 

CAROLINE. 

I always thought that the heat originated in the lime. It seems verystrange that water, and cold water too, should contain so much heat. 

EMILY. 

After this extrication of caloric, the water must exist in a state ofice in the lime, since it parts with the heat which kept it liquid. 

MRS. B. 

It cannot properly be called ice, since ice implies a degree of cold, atleast equal to the freezing point. Yet as water, in combining with lime, gives out more heat than in freezing, it must be in a state of stillgreater solidity in the lime, than it is in the form of ice; and you mayhave observed that it does not moisten or liquefy the lime in thesmallest degree. 

EMILY. 

But, Mrs. B. , the smoke that rises is white; if it was only pure caloricwhich escaped, we might feel, but could not see it. 

MRS. B. 

This white vapour is formed by some of the particles of lime, in a stateof fine dust, which are carried off by the caloric. 

EMILY. 

In all changes of state, then, a body either absorbs or disengageslatent heat?

MRS. B. 

You cannot exactly say _absorbs latent heat_, as the heat becomes latentonly on being confined in the body; but you may say, generally, thatbodies, in passing from a solid to a liquid form, or from the liquidstate to that of vapour, absorb heat; and that when the reverse takesplace, heat is disengaged. *

 [Footnote *: This rule, if not universal, admits of very few exceptions. ]

EMILY. 

We can now, I think, account for the ether boiling, and the waterfreezing in vacuo, at the same temperature. †

 [Footnote †: See page 102. ]

MRS. B. 

Let me hear how you explain it. 

EMILY. 

The latent heat, which the water gave out in freezing, was immediatelyabsorbed by the ether, during its conversion into vapour; and therefore, from a latent state in one liquid, it passed into a latent state in theother. 

MRS. B. 

But this only partly accounts for the result of the experiment; itremains to be explained why the temperature of the ether, while in astate of ebullition, is brought down to the freezing temperature of thewater. --It is because the ether, during its evaporation, reduces itsown temperature, in the same proportion as that of the water, byconverting its free caloric into latent heat: so that, though one liquidboils, and the other freezes, their temperatures remain in a state ofequilibrium. 

EMILY. 

But why does not water, as well as ether, reduce its own temperature byevaporating?

MRS. B. 

The fact is that it does, though much less rapidly than ether. Thus, forinstance, you may often have observed, in the heat of summer, how muchany particular spot may be cooled by watering, though the water used forthat purpose be as warm as the air itself. Indeed so much cold may beproduced by the mere evaporation of water, that the inhabitants ofIndia, by availing themselves of the most favourable circumstances forthis process which their warm climate can afford, namely, the cool ofthe night, and situations most exposed to the night breeze, succeed incausing water to freeze, though the temperature of the air be as high as60 degrees. The water is put into shallow earthen trays, so as to exposean extensive surface to the process of evaporation, and in the morning, the water is found covered with a thin cake of ice, which is collectedin sufficient quantity to be used for purposes of luxury. 

CAROLINE. 

How delicious it must be to drink liquids so cold in those tropicalclimates! But, Mrs.  B. , could we not try that experiment?

MRS. B. 

If we were in the country, I have no doubt but that we should be able tofreeze water, by the same means, and under similar circumstances. But wecan do it immediately, upon a small scale, in this very room, in whichthe thermometer stands at 70 degrees. For this purpose we need onlyplace some water in a little cup under the receiver of the air-pump(PLATE V. Fig. 1. ), and exhaust the air from it. What will be theconsequence, Caroline?

 [Illustration: Plate V. Vol. I. Page 138. 

 Fig. 1. The air-pump & receiver for Mr. Leslie’s experiment. C a saucer with sulphuric Acid. B a glass or earthen cup containing Water. D a stand for the cup with its legs made of Glass. A a Thermometer. 

 Fig. 2. Dr. Wollaston’s Cryophorus. 

 Fig. 5. Dr. Marcet’s mode of using the Cryophorus. 

 Fig. 3. & 4. The different parts of Fig. 5. Seen separate. ]

CAROLINE. 

Of course the water will evaporate more quickly, since there will nolonger be any atmospheric pressure on its surface: but will this besufficient to make the water freeze?

MRS. B. 

Probably not, because the vapour will not be carried off fast enough;but this will be accomplished without difficulty if we introduce intothe receiver (fig.  1. ), in a saucer, or other large shallow vessel, somestrong sulphuric acid, a substance which has a great attraction forwater, whether in the form of vapour, or in the liquid state. Thisattraction is such that the acid will instantly absorb the moisture asit rises from the water, so as to make room for the formation of freshvapour; this will of course hasten the process, and the cold producedfrom the rapid evaporation of the water, will, in a few minutes, besufficient to freeze its surface. * We shall now exhaust the air from thereceiver. 

 [Footnote *: This experiment was first devised by Mr. Leslie, and has since been modified in a variety of forms. ]

EMILY. 

Thousands of small bubbles already rise through the water from theinternal surface of the cup; what is the reason of this?

MRS. B. 

These are bubbles of air which were partly attached to the vessel, andpartly diffused in the water itself; and they expand and rise inconsequence of the atmospheric pressure being removed. 

CAROLINE. 

See, Mrs. B. ; the thermometer in the cup is sinking fast; it has alreadydescended to 40 degrees!

EMILY. 

The water seems now and then violently agitated on the surface, as if itwas boiling; and yet the thermometer is descending fast!

MRS. B. 

You may call it _boiling_, if you please, for this appearance is, aswell as boiling, owing to the rapid formation of vapour; but here, asyou have just observed, it takes place from the surface, for it is onlywhen heat is applied to the bottom of the vessel that the vapour isformed there. --Now crystals of ice are actually shooting all over thesurface of the water. 

CAROLINE. 

How beautiful it is! The surface is now entirely frozen--but thethermometer remains at 32 degrees. 

MRS. B. 

And so it will, conformably with our doctrine of latent heat, until thewhole of the water is frozen; but it will then again begin to descendlower and lower, in consequence of the evaporation which goes on fromthe surface of the ice. 

EMILY. 

This is a most interesting experiment; but it would be still morestriking if no sulphuric acid were required. 

MRS. B. 

I will show you a freezing instrument, contrived by Dr. Wollaston, uponthe same principle as Mr. Leslie’s experiment, by which water may befrozen by its own evaporation alone, without the assistance of sulphuricacid. 

This tube, which, as you see (PLATE V. Fig. 2. ), is terminated at eachextremity by a bulb, one of which is half full of water, is internallyperfectly exhausted of air; the consequence of this is, that the waterin the bulb is always much disposed to evaporate. This evaporation, however, does not proceed sufficiently fast to freeze the water; but ifthe empty ball be cooled by some artificial means, so as to condensequickly the vapour which rises from the water, the process may be thusso much promoted as to cause the water to freeze in the other ball. Dr. Wollaston has called this instrument _Cryophorus_. 

CAROLINE. 

So that cold seems to perform here the same part which the sulphuricacid acted in Mr. Leslie’s experiment?

MRS. B. 

Exactly so; but let us try the experiment. 

EMILY. 

How will you cool the instrument? You have neither ice nor snow. 

MRS. B. 

True: but we have other means of effecting this. * You recollect what anintense cold can be produced by the evaporation of ether in an exhaustedreceiver. We shall inclose the bulb in this little bag of fine flannel(fig. 3. ), then soke it in ether, and introduce it into the receiver ofthe air-pump. (Fig.  5. ) For this purpose we shall find it moreconvenient to use a cryophorus of this shape (fig. 4. ), as its elongatedbulb passes easily through a brass plate which closes the top of thereceiver. If we now exhaust the receiver quickly, you will see, in lessthan a minute, the water freeze in the other bulb, out of the receiver. 

 [Footnote *: This mode of making the experiment was proposed, and the particulars detailed, by Dr. Marcet, in the 34th vol. Of Nicholson’s Journal, page 119. ]

EMILY. 

The bulb already looks quite dim, and small drops of water arecondensing on its surface. 

CAROLINE. 

And now crystals of ice shoot all over the water. This is, indeed, a very curious experiment!

MRS. B. 

You will see, some other day, that, by a similar method, evenquicksilver may be frozen. --But we cannot at present indulge in anyfurther digression. 

Having advanced so far on the subject of heat, I may now give you anaccount of the calorimeter, an instrument invented by Lavoisier, uponthe principles just explained, for the purpose of estimating thespecific heat of bodies. It consists of a vessel, the inner surface ofwhich is lined with ice, so as to form a sort of hollow globe of ice, inthe midst of which the body, whose specific heat is to be ascertained, is placed. The ice absorbs caloric from this body, till it has broughtit down to the freezing point; this caloric converts into water acertain portion of the ice which runs out through an aperture at thebottom of the machine; and the quantity of ice changed to water is atest of the quantity of caloric which the body has given out indescending from a certain temperature to the freezing point. 

CAROLINE. 

In this apparatus, I suppose, the milk, chalk, and lead, would meltdifferent quantities of ice, in proportion to their different capacitiesfor caloric?

MRS. B. 

Certainly: and thence we are able to ascertain, with precision, theirrespective capacities for heat. But the calorimeter affords us no moreidea of the absolute quantity of heat contained in a body, than thethermometer; for though by means of it we extricate both the free andcombined caloric, yet we extricate them only to a certain degree, whichis the freezing point; and we know not how much they contain of eitherbelow that point. 

EMILY. 

According to the theory of latent heat, it appears to me that theweather should be warm when it freezes, and cold in a thaw: for latentheat is liberated from every substance that it freezes, and such a largesupply of heat must warm the atmosphere; whilst, during a thaw, thatvery quantity of free heat must be taken from the atmosphere, and returnto a latent state in the bodies which it thaws. 

MRS. B. 

Your observation is very natural; but consider that in a frost theatmosphere is so much colder than the earth, that all the caloric whichit takes from the freezing bodies is insufficient to raise itstemperature above the freezing point; otherwise the frost must cease. But if the quantity of latent heat extricated does not destroy thefrost, it serves to moderate the suddenness of the change of temperatureof the atmosphere, at the commencement both of frost, and of a thaw. Inthe first instance, its extrication diminishes the severity of the cold;and, in the latter, its absorption moderates the warmth occasioned by athaw: it even sometimes produces a discernible chill, at the breaking upof a frost. 

CAROLINE. 

But what are the general causes that produce those sudden changes in theweather, especially from hot to cold, which we often experience?

MRS. B. 

This question would lead us into meteorological discussions, to which Iam by no means competent. One circumstance, however, we can easilyunderstand. When the air has passed over cold countries, it willprobably arrive here at a temperature much below our own, and then itmust absorb heat from every object it meets with, which will produce ageneral fall of temperature. 

CAROLINE. 

But pray, now that we know so much of the effects of heat, will youinform us whether it is really a distinct body, or, as I have heard, a peculiar kind of motion produced in bodies?

MRS. B. 

As I before told you, there is yet much uncertainty as to the nature ofthese subtle agents. But I am inclined to consider heat not as meremotion, but as a separate substance. Late experiments too appear to makeit a compound body, consisting of the two electricities, and in our nextconversation I shall inform you of the principal facts on which thatopinion is founded. 

CONVERSATION V. 

ON THE CHEMICAL AGENCIES OF ELECTRICITY. 

MRS. B. 

Before we proceed further it will be necessary to give you some accountof certain properties of electricity, which have of late years beendiscovered to have an essential connection with the phenomena ofchemistry. 

CAROLINE. 

It is ELECTRICITY, if I recollect right, which comes next in our list ofsimple substances?

MRS. B. 

I have placed electricity in that list, rather from the necessity ofclassing it somewhere, than from any conviction that it has a right tothat situation, for we are as yet so ignorant of its intimate nature, that we are unable to determine, not only whether it is simple orcompound, but whether it is in fact a material agent; or, as Sir H. Davyhas hinted, whether it may not be merely a property inherent in matter. As, however, it is necessary to adopt some hypothesis for theexplanation of the discoveries which this agent has enabled us to make, I have chosen the opinion, at present most prevalent, which supposes theexistence of two kinds of electricity, distinguished by the names of_positive_ and _negative_ electricity. 

CAROLINE. 

Well, I must confess, I do not feel nearly so interested in a science inwhich so much uncertainty prevails, as in those which rest uponestablished principles; I never was fond of electricity, because, however beautiful and curious the phenomena it exhibits may be, thetheories, by which they were explained, appeared to me so various, soobscure and inadequate, that I always remained dissatisfied. I was inhopes that the new discoveries in electricity had thrown so great alight on the subject, that every thing respecting it would now have beenclearly explained. 

MRS. B. 

That is a point which we are yet far from having attained. But, in spiteof the imperfection of our theories, you will be amply repaid by theimportance and novelty of the subject. The number of new facts whichhave already been ascertained, and the immense prospect of discoverywhich has lately been opened to us, will, I hope, ultimately lead to aperfect elucidation of this branch of natural science; but at presentyou must be contented with studying the effects, and in some degreeexplaining the phenomena, without aspiring to a precise knowledge of theremote cause of electricity. 

You have already obtained some notions of electricity: in our presentconversation, therefore, I shall confine myself to that part of thescience which is of late discovery, and is more particularly connectedwith chemistry. 

It was a trifling and accidental circumstance which first gave rise tothis new branch of physical science. Galvani, a professor of naturalphilosophy at Bologna, being engaged (about twenty years ago) in someexperiments on muscular irritability, observed, that when a piece ofmetal was laid on the nerve of a frog, recently dead, whilst the limbsupplied by that nerve rested upon some other metal, the limb suddenlymoved, on a communication being made between the two pieces of metal. 

EMILY. 

How is this communication made?

MRS. B. 

Either by bringing the two metals into contact, or by connecting them bymeans of a metallic conductor. But without subjecting a frog to anycruel experiments, I can easily make you sensible of this kind ofelectric action. Here is a piece of zinc, (one of the metals I mentionedin the list of elementary bodies)--put it _under_ your tongue, and thispiece of silver _upon_ your tongue, and let both the metals project alittle beyond the tip of the tongue--very well--now make the projectingparts of the metals touch each other, and you will instantly perceive apeculiar sensation. 

EMILY. 

Indeed I did, a singular taste, and I think a degree of heat: but I canhardly describe it. 

MRS. B. 

The action of these two pieces of metal on the tongue is, I believe, precisely similar to that made on the nerve of a frog. I shall notdetain you by a detailed account of the theory by which Galvaniattempted to account for this fact, as his explanation was soonoverturned by subsequent experiments, which proved that _Galvanism_ (thename this new power had obtained) was nothing more than electricity. Galvani supposed that the virtue of this new agent resided in the nervesof the frog, but Volta, who prosecuted this subject with much greatersuccess, shewed that the phenomena did not depend on the organs of thefrog, but upon the electrical agency of the metals, which is excited bythe moisture of the animal, the organs of the frog being only a delicatetest of the presence of electric influence. 

CAROLINE. 

I suppose, then, the saliva of the mouth answers the same purpose as themoisture of the frog, in exciting the electricity of the pieces ofsilver and zinc with which Emily tried the experiment on her tongue. 

MRS. B. 

Precisely. It does not appear, however, necessary that the fluid usedfor this purpose should be of an animal nature. Water, and acids verymuch diluted by water, are found to be the most effectual in promotingthe developement of electricity in metals; and, accordingly, theoriginal apparatus which Volta first constructed for this purpose, consisted of a pile or succession of plates of zinc and copper, eachpair of which was connected by pieces of cloth or paper impregnated withwater; and this instrument, from its original inconvenient structure andlimited strength, has gradually arrived at its present state of powerand improvement, such as is exhibited in the Voltaic battery. In thisapparatus, a specimen of which you see before you (PLATE VI. Fig. 1. ), the plates of zinc and copper are soldered together in pairs, each pairbeing placed at regular distances in wooden troughs and the intersticesbeing filled with fluid. 

 [Illustration: Plate VI. P. 151. 

 Fig. 1. Voltaic Battery. Fig. 2. Fig. 4. Fig. 1. 2. & 4. Voltaic Batteries

 Fig. 3. Electrical Machine. A the Cylinder. B the Conductor. R the Rubber. C the Chain. ]

CAROLINE. 

Though you will not allow us to enquire into the precise cause ofelectricity, may we not ask in what manner the fluid acts on the metalsso as to produce it?

MRS. B. 

The action of the fluid on the metals, whether water or acid be used, isentirely of a chemical nature. But whether electricity is excited bythis chemical action, or whether it is produced by the contact of thetwo metals, is a point upon which philosophers do not yet perfectlyagree. 

EMILY. 

But can the mere contact of two metals, without any intervening fluid, produce electricity?

MRS. B. 

Yes, if they are afterwards separated. It is an established fact, thatwhen two metals are put in contact, and afterwards separated, that whichhas the strongest attraction for oxygen exhibits signs of positive, theother of negative electricity. 

CAROLINE. 

It seems then but reasonable to infer that the power of the Voltaicbattery should arise from the contact of the plates of zinc and copper. 

MRS. B. 

It is upon this principle that Volta and Sir H. Davy explain thephenomena of the pile; but notwithstanding these two great authorities, many philosophers entertain doubts on the truth of this theory. Theprincipal difficulty which occurs in explaining the phenomena of theVoltaic battery on this principle, is, that two such plates show nosigns of different states of electricity whilst in contact, but only onbeing separated after contact. Now in the Voltaic battery, those platesthat are in contact always continue so, being soldered together: andthey cannot therefore receive a succession of charges. Besides, if weconsider the mere disturbance of the balance of electricity by thecontact of the plates, as the sole cause of the production of Voltaicelectricity, it remains to be explained how this disturbed balancebecomes an inexhaustible source of electrical energy, capable of pouringforth a constant and copious supply of electrical fluid, though withoutany means of replenishing itself from other sources. This subject, itmust be owned, is involved in too much obscurity to enable us to speakvery decidedly in favour of any theory. But, in order to avoidperplexing you with different explanations, I shall confine myself toone which appears to me to be least encumbered with difficulties, andmost likely to accord with truth. *

This theory supposes the electricity to be excited by the chemicalaction of the acid on the zinc; but you are yet such novices inchemistry, that I think it will be necessary to give you some previousexplanation of the nature of this action. 

All metals have a strong attraction for oxygen, and this element isfound in great abundance both in water and in acids. The action of thediluted acid on the zinc consists therefore in its oxygen combining withit, and dissolving its surface. 

 [Footnote *: This mode of explaining the phenomena of the Voltaic pile is called the _chemical theory_ of electricity, because it ascribes the cause of these phenomena to certain chemical changes which take place during their appearance. In the preceding edition of this work, the same theory was presented in a more elaborate, but less easy form than it is in this. The mode of viewing the subject which is here sketched was long since suggested by Dr. Bostock, of whose theory, however, this is by no means to be considered as a complete statement. ]

CAROLINE. 

In the same manner I suppose as we saw an acid dissolve copper?

MRS. B. 

Yes; but in the Voltaic battery the diluted acid is not strong enough toproduce so complete an effect; it acts only on the surface of the zinc, to which it yields its oxygen, forming upon it a film or crust, which isa compound of the oxygen and the metal. 

EMILY. 

Since there is so strong a chemical attraction between oxygen andmetals, I suppose they are naturally in different states of electricity?

MRS. B. 

Yes; it appears that all metals are united with the positive, and thatoxygen is the grand source of the negative electricity. 

CAROLINE. 

Does not then the acid act on the plates of copper, as well as on thoseof zinc?

MRS. B. 

No; for though copper has an affinity for oxygen, it is less strong thanthat of zinc; and therefore the energy of the acid is only exerted uponthe zinc. 

It will be best, I believe, in order to render the action of the Voltaicbattery more intelligible, to confine our attention at first to theeffect produced on two plates only. (PLATE VI. Fig.  2. )

If a plate of zinc be placed opposite to one of copper, or any othermetal less attractive of oxygen, and the space between them (suppose ofhalf an inch in thickness), be filled with an acid or any fluid capableof oxydating the zinc, the oxydated surface will have its capacity forelectricity diminished, so that a quantity of electricity will beevolved from that surface. This electricity will be received by thecontiguous fluid, by which it will be transmitted to the oppositemetallic surface, the copper, which is not oxydated, and is thereforedisposed to receive it; so that the copper plate will thus becomepositive, whilst the zinc plate will be in the negative state. 

This evolution of electrical fluid however will be very limited; for asthese two plates admit of but very little accumulation of electricity, and are supposed to have no communication with other bodies, the actionof the acid, and further developement of electricity, will beimmediately stopped. 

EMILY. 

This action, I suppose, can no more continue to go on, than that of acommon electrical machine, which is not allowed to communicate withother bodies?

MRS. B. 

Precisely; the common electrical machine, when excited by the frictionof the rubber, gives out both the positive and negative electricities. --(PLATE VI. Fig.  3. ) The positive, by the rotation of the glass cylinder, is conveyed into the conductor, whilst the negative goes into therubber. But unless there is a communication made between the rubber andthe ground, but a very inconsiderable quantity of electricity can beexcited; for the rubber, like the plates of the battery, has too small acapacity to admit of an accumulation of electricity. Unless thereforethe electricity can pass out of the rubber, it will not continue to gointo it, and consequently no additional accumulation will take place. Now as one kind of electricity cannot be given out without the other, the developement of the positive electricity is stopped as well as thatof the negative, and the conductor therefore cannot receive a successionof charges. 

CAROLINE. 

But does not the conductor, as well as the rubber, require acommunication with the earth, in order to get rid of its electricity?

MRS. B. 

No; for it is susceptible of receiving and containing a considerablequantity of electricity, as it is much larger than the rubber, andtherefore has a greater capacity; and this continued accumulation ofelectricity in the conductor is what is called a charge. 

EMILY. 

But when an electrical machine is furnished with two conductors toreceive the two electricities, I suppose no communication with the earthis required?

MRS. B. 

Certainly not, until the two are fully charged; for the two conductorswill receive equal quantities of electricity. 

CAROLINE. 

I thought the use of the chain had been to convey the electricity _from_the ground into the machine?

MRS. B. 

That was the idea of Dr. Franklin, who supposed that there was but onekind of electricity, and who, by the terms positive and negative (whichhe first introduced), meant only different quantities of the same kindof electricity. The chain was in that case supposed to conveyelectricity _from_ the ground through the rubber into the conductor. Butas we have adopted the hypothesis of two electricities, we must considerthe chain as a vehicle to conduct the negative electricity into theearth. 

EMILY. 

And are both kinds of electricity produced whenever electricity isexcited?

MRS. B. 

Yes, invariably. If you rub a tube of glass with a woollen cloth, theglass becomes positive, and the cloth negative. If, on the contrary, youexcite a stick of sealing-wax by the same means, it is the rubber whichbecomes positive, and the wax negative. 

But with regard to the Voltaic battery, in order that the acid may actfreely on the zinc, and the two electricities be given out withoutinterruption, some method must be devised, by which the plates may partwith their electricities as fast as they receive them. --Can you thinkof any means by which this might be effected?

EMILY. 

Would not two chains or wires, suspended from either plate to theground, conduct the electricities into the earth, and thus answer thepurpose?

MRS. B. 

It would answer the purpose of carrying off the electricity, I admit;but recollect, that though it is necessary to find a vent for theelectricity, yet we must not lose it, since it is the power which we areendeavouring to obtain. Instead, therefore, of conducting it into theground, let us make the wires, from either plate, meet: the twoelectricities will thus be brought together, and will combine andneutralize each other; and as long as this communication continues, thetwo plates having a vent for their respective electricities, the actionof the acid will go on freely and uninterruptedly. 

EMILY. 

That is very clear, so far as two plates only are concerned; but Icannot say I understand how the energy of the succession of plates, orrather pairs of plates, of which the Galvanic trough is composed, ispropagated and accumulated throughout a battery?

MRS. B. 

In order to shew you how the intensity of the electricity is increasedby increasing the number of plates, we will examine the action of fourplates; if you understand these, you will readily comprehend that of anynumber whatever. In this figure (PLATE VI. Fig.  4. ), you will observethat the two central plates are united; they are soldered together, (aswe observed in describing the Voltaic trough, ) so as to form but oneplate which offers two different surfaces, the one of copper, the otherof zinc. 

Now you recollect that, in explaining the action of two plates, wesupposed that a quantity of electricity was evolved from the surface ofthe first zinc plate, in consequence of the action of the acid, and wasconveyed by the interposed fluid to the copper plate, No.  2, which thusbecame positive. This copper plate communicates its electricity to thecontiguous zinc plate, No.  3, in which, consequently, some accumulationof electricity takes place. When, therefore, the fluid in the next cellacts upon the zinc plate, electricity is extricated from it in largerquantity, and in a more concentrated form, than before. Thisconcentrated electricity is again conveyed by the fluid to the next pairof plates, No. 4 and 5, when it is farther increased by the action ofthe fluid in the third cell, and so on, to any number of plates of whichthe battery may consist; so that the electrical energy will continue toaccumulate in proportion to the number of double plates, the first zincplate of the series being the most negative, and the last copper platethe most positive. 

CAROLINE. 

But does the battery become more and more strongly charged, merely bybeing allowed to stand undisturbed?

MRS. B. 

No, for the action will soon stop, as was explained before, unless avent be given to the accumulated electricities. This is easily done, however, by establishing a communication by means of the wires(Fig.  1. ), between the two ends of the battery: these being brought intocontact, the two electricities meet and neutralize each other, producingthe shock and other effects of electricity; and the action goes on withrenewed energy, being no longer obstructed by the accumulation of thetwo electricities which impeded its progress. 

EMILY. 

Is it the union of the two electricities which produces the electricspark?

MRS. B. 

Yes; and it is, I believe, this circumstance which gave rise to Sir H. Davy’s opinion that caloric may be a compound of the two electricities. 

CAROLINE. 

Yet surely caloric is very different from the electrical spark?

MRS. B. 

The difference may consist probably only in intensity: for the heat ofthe electric spark is considerably more intense, though confined to avery minute spot, than any heat we can produce by other means. 

EMILY. 

Is it quite certain that the electricity of the Voltaic battery isprecisely of the same nature as that of the common electrical machine?

MRS. B. 

Undoubtedly; the shock given to the human body, the spark, thecircumstance of the same substances which are conductors of the onebeing also conductors of the other, and of those bodies, such as glassand sealing-wax, which are non-conductors of the one, being alsonon-conductors of the other, are striking proofs of it. Besides, Sir H. Davy has shewn in his Lectures, that a Leyden jar, and a common electricbattery, can be charged with electricity obtained from a Voltaicbattery, the effect produced being perfectly similar to that obtained bya common machine. 

Dr. Wollaston has likewise proved that similar chemical decompositionsare effected by the electric machine and by the Voltaic battery; and hasmade other experiments which render it highly probable, that the originof both electricities is essentially the same, as they show that therubber of the common electrical machine, like the zinc in the Voltaicbattery, produces the two electricities by combining with oxygen. 

CAROLINE. 

But I do not see whence the rubber obtains oxygen, for there is neitheracid nor water used in the common machine, and I always understood thatthe electricity was excited by the friction. 

MRS. B. 

It appears that by friction the rubber obtains oxygen from theatmosphere, which is partly composed of that element. The oxygencombines with the amalgam of the rubber, which is of a metallic nature, much in the same way as the oxygen of the acid combines with the zinc inthe Voltaic battery, and it is thus that the two electricities aredisengaged. 

CAROLINE. 

But, if the electricities of both machines are similar, why not use thecommon machine for chemical decompositions?

MRS. B. 

Though its effects are similar to those of the Voltaic battery, they areincomparably weaker. Indeed Dr. Wollaston, in using it for chemicaldecompositions, was obliged to act upon the most minute quantities ofmatter, and though the result was satisfactory in proving the similarityof its effects to those of the Voltaic battery, these effects were toosmall in extent to be in any considerable degree applicable to chemicaldecomposition. 

CAROLINE. 

How terrible, then, the shock must be from a Voltaic battery, since itis so much more powerful than an electrical machine!

MRS. B. 

It is not nearly so formidable as you think; at least it is by no meansproportional to the chemical effect. The great superiority of theVoltaic battery consists in the large _quantity_ of electricity thatpasses; but in regard to the _rapidity_ or _intensity_ of the charge, itis greatly surpassed by the common electrical machine. It would seemthat the shock or sensation depends chiefly upon the intensity; whilst, on the contrary, for chemical purposes, it is quantity which isrequired. In the Voltaic battery, the electricity, though copious, is soweak as not to be able to force its way through the fluid whichseparates the plates, whilst that of a common machine will pass throughany space of water. 

CAROLINE. 

Would not it be possible to increase the intensity of the Voltaicbattery till it should equal that of the common machine?

MRS. B. 

It can actually be increased till it imitates a weak electrical machine, so as to produce a visible spark when accumulated in a Leyden jar. Butit can never be raised sufficiently to pass through any considerableextent of air, because of the ready communication through the fluidsemployed. 

By increasing the number of plates of a battery, you increase its_intensity_, whilst, by enlarging the dimensions of the plates, youaugment its _quantity_; and, as the superiority of the battery over thecommon machine consists entirely in the quantity of electricityproduced, it was at first supposed that it was the size, rather than thenumber of plates that was essential to the augmentation of power. Itwas, however, found upon trial, that the quantity of electricityproduced by the Voltaic battery, even when of a very moderate size, wassufficiently copious, and that the chief advantage in this apparatus wasobtained by increasing the intensity, which, however, still falls veryshort of that of the common machine. 

I should not omit to mention, that a very splendid, and, at the sametime, most powerful battery, was, a few years ago, constructed under thedirection of Sir H. Davy, which he repeatedly exhibited in his course ofelectro-chemical lectures. It consists of two thousand double plates ofzinc and copper, of six square inches in dimensions, arranged in troughsof Wedgwood-ware, each of which contains twenty of these plates. Thetroughs are furnished with a contrivance for lifting the plates out ofthem in a very convenient and expeditious manner. *

 [Footnote *: A model of this mode of construction is exhibited in PLATE XIII. Fig.  1. ]

CAROLINE. 

Well, now that we understand the nature of the action of the Voltaicbattery, I long to hear an account of the discoveries to which it hasgiven rise. 

MRS. B. 

You must restrain your impatience, my dear, for I cannot with anypropriety introduce the subject of these discoveries till we come tothem in the regular course of our studies. But, as almost everysubstance in nature has already been exposed to the influence of theVoltaic battery, we shall very soon have occasion to notice its effects. 

CONVERSATION VI. 

ON OXYGEN AND NITROGEN. 

MRS. B. 

To-day we shall examine the chemical properties of the ATMOSPHERE. 

CAROLINE. 

I thought that we were first to learn the nature of OXYGEN, which comenext in our table of simple bodies?

MRS. B. 

And so you shall; the atmosphere being composed of two principles, OXYGEN and NITROGEN, we shall proceed to analyse it, and consider itscomponent parts separately. 

EMILY. 

I always thought that the atmosphere had been a very complicated fluid, composed of all the variety of exhalations from the earth. 

MRS. B. 

Such substances may be considered rather as heterogeneous andaccidental, than as forming any of its component parts; and theproportion they bear to the whole mass is quite inconsiderable. 

ATMOSPHERICAL AIR is composed of two gasses, known by the names ofOXYGEN GAS and NITROGEN or AZOTIC GAS. 

EMILY. 

Pray what is a gas?

MRS. B. 

The name of gas is given to any fluid capable of existing constantly inan aeriform state, under the pressure and at the temperature of theatmosphere. 

CAROLINE. 

Is not water, or any other substance, when evaporated by heat, calledgas?

MRS. B. 

No, my dear; vapour is, indeed, an elastic fluid, and bears a strongresemblance to a gas; there are, however, several points in which theyessentially differ, and by which you may always distinguish them. Steam, or vapour, owes its elasticity merely to a high temperature, which isequal to that of boiling water. And it differs from boiling water onlyby being united with more caloric, which, as we before explained, is ina latent state. When steam is cooled, it instantly returns to the formof water; but air, or gas, has never yet been rendered liquid or solidby any degree of cold. 

EMILY. 

But does not gas, as well as vapour, owe its elasticity to caloric?

MRS. B. 

It was the prevailing opinion; and the difference of gas or vapour wasthought to depend on the different manner in which caloric was unitedwith the basis of these two kinds of elastic fluids. In vapour, it wasconsidered as in a latent state; in gas, it was said to be chemicallycombined. But the late researches of Sir H. Davy have given rise to anew theory respecting gasses; and there is now reason to believe thatthese bodies owe their permanently elastic state, not solely to caloric, but likewise to the prevalence of either the one or the other of the twoelectricities. 

EMILY. 

When you speak, then, of the simple bodies oxygen and nitrogen, you meanto express those substances which are the basis of the two gasses?

MRS. B. 

Yes, in strict propriety, for they can properly be called gasses onlywhen brought to an aeriform state. 

CAROLINE. 

In what proportions are they combined in the atmosphere?

MRS. B. 

The oxygen gas constitutes a little more than one-fifth, and thenitrogen gas a little less than four-fifths. When separated, they arefound to possess qualities totally different from each other. For oxygengas is essential both to respiration and combustion, while neither ofthese processes can be performed in nitrogen gas. 

CAROLINE. 

But if nitrogen gas is unfit for respiration, how does it happen thatthe large proportion of it which enters into the composition of theatmosphere is not a great impediment to breathing?

MRS. B. 

We should breathe more freely than our lungs could bear, if we respiredoxygen gas alone. The nitrogen is no impediment to respiration, andprobably, on the contrary, answers some useful purpose, though we do notknow in what manner it acts in that process. 

EMILY. 

And by what means can the two gasses, which compose the atmospheric air, be separated?

MRS. B. 

There are many ways of analysing the atmosphere: the two gasses may beseparated first by combustion. 

EMILY. 

You surprise me! how is it possible that combustion should separatethem?

MRS. B. 

I should previously remind you that oxygen is supposed to be the onlysimple body naturally combined with negative electricity. In all theother elements the positive electricity prevails, and they haveconsequently, all of them, an attraction for oxygen. *

 [Footnote *: If chlorine or oxymuriatic gas be a simple body, according to Sir H. Davy’s view of the subject, it must be considered as an exception to this statement; but this subject cannot be discussed till the properties and nature of chlorine come under examination. ]

CAROLINE. 

Oxygen the only negatively electrified body! that surprises meextremely; how then are the combinations of the other bodies performed, if, according to your explanation of chemical attraction, bodies aresupposed only to combine in virtue of their opposite states ofelectricity?

MRS. B. 

Observe that I said, that oxygen was the only _simple_ body, naturallynegative. Compound bodies, in which oxygen prevails over the othercomponent parts, are also negative, but their negative energy is greateror less in proportion as the oxygen predominates. Those compounds intowhich oxygen enters in less proportion than the other constituents, arepositive, but their positive energy is diminished in proportion to thequantity of oxygen which enters into their composition. 

All bodies, therefore, that are not already combined with oxygen, willattract it, and, under certain circumstances, will absorb it from theatmosphere, in which case the nitrogen gas will remain alone, and maythus be obtained in its separate state. 

CAROLINE. 

I do not understand how a gas can be absorbed?

MRS. B. 

It is only the oxygen, or basis of the gas, which is absorbed; and thetwo electricities escaping, that is to say, the negative from theoxygen, the positive from the burning body, unite and produce caloric. 

EMILY. 

And what becomes of this caloric?

MRS. B. 

We shall make this piece of dry wood attract oxygen from the atmosphere, and you will see what becomes of the caloric. 

CAROLINE. 

You are joking, Mrs. B--; you do not mean to decompose the atmospherewith a piece of dry stick?

MRS. B. 

Not the whole body of the atmosphere, certainly; but if we can make thispiece of wood attract any quantity of oxygen from it, a proportionalquantity of atmospherical air will be decomposed. 

CAROLINE. 

If wood has so strong an attraction for oxygen, why does it notdecompose the atmosphere spontaneously?

MRS. B. 

It is found by experience, that an elevation of temperature is requiredfor the commencement of the union of the oxygen and the wood. 

This elevation of temperature was formerly thought to be necessary, inorder to diminish the cohesive attraction of the wood, and enable theoxygen to penetrate and combine with it more readily. But since theintroduction of the new theory of chemical combination, another causehas been assigned, and it is now supposed that the high temperature, byexalting the electrical energies of bodies, and consequently their forceof attraction, facilitates their combination. 

EMILY. 

If it is true, that caloric is composed of the two electricities, anelevation of temperature must necessarily augment the electric energiesof bodies. 

MRS. B. 

I doubt whether that would be a necessary consequence; for, admittingthis composition of caloric, it is only by its being decomposed thatelectricity can be produced. Sir H. Davy, however, in his numerousexperiments, has found it to be an almost invariable rule that theelectrical energies of bodies are increased by elevation of temperature. 

What means then shall we employ to raise the temperature of the wood, soas to enable it to attract oxygen from the atmosphere?

CAROLINE. 

Holding it near the fire, I should think, would answer the purpose. 

MRS. B. 

It may, provided you hold it sufficiently close to the fire; for a veryconsiderable elevation of temperature is required. 

CAROLINE. 

It has actually taken fire, and yet I did not let it touch the coals, but I held it so very close that I suppose it caught fire merely fromthe intensity of the heat. 

MRS. B. 

Or you might say, in other words, that the caloric which the woodimbibed, so much elevated its temperature, and exalted its electricenergy, as to enable it to attract oxygen very rapidly from theatmosphere. 

EMILY. 

Does the wood absorb oxygen while it is burning?

MRS. B. 

Yes, and the heat and light are produced by the union of the twoelectricities which are set at liberty, in consequence of the oxygencombining with the wood. 

CAROLINE. 

You astonish me! the heat of a burning body proceeds then as much fromthe atmosphere as from the body itself?

MRS. B. 

It was supposed that the caloric, given out during combustion, proceededentirely, or nearly so, from the decomposition of the oxygen gas; but, according to Sir H. Davy’s new view of the subject, both the oxygen gas, and the combustible body, concur in supplying the heat and light, by theunion of their opposite electricities. 

EMILY. 

I have not yet met with any thing in chemistry that has surprised ordelighted me so much as this explanation of combustion. I was at firstwondering what connection there could be between the affinity of a bodyfor oxygen and its combustibility; but I think I understand it nowperfectly. 

MRS. B. 

Combustion then, you see, is nothing more than the rapid combination ofa body with oxygen, attended by the disengagement of light and heat. 

EMILY. 

But are there no combustible bodies whose attraction for oxygen is sostrong, that they will combine with it, without the application of heat?

CAROLINE. 

That cannot be; otherwise we should see bodies burning spontaneously. 

MRS. B. 

But there are some instances of this kind, such as phosphorus, potassium, and some compound bodies, which I shall hereafter make youacquainted with. These bodies, however, are prepared by art, for ingeneral, all the combustions that could occur spontaneously, at thetemperature of the atmosphere, have already taken place; therefore newcombustions cannot happen without the temperature of the body beingraised. Some bodies, however, will burn at a much lower temperature thanothers. 

CAROLINE. 

But the common way of burning a body is not merely to approach it to onealready on fire, but rather to put the one in actual contact with theother, as when I burn this piece of paper by holding it in the flame ofthe fire. 

MRS. B. 

The closer it is in contact with the source of caloric, the sooner willits temperature be raised to the degree necessary for it to burn. If youhold it near the fire, the same effect will be produced; but more timewill be required, as you found to be the case with the piece of stick. 

EMILY. 

But why is it not necessary to continue applying caloric throughout theprocess of combustion, in order to keep up the electric energy of thewood, which is required to enable it to combine with the oxygen?

MRS. B. 

The caloric which is gradually produced by the two electricities duringcombustion, keeps up the temperature of the burning body; so that whenonce combustion has begun, no further application of caloric isrequired. 

CAROLINE. 

Since I have learnt this wonderful theory of combustion, I cannot takemy eyes from the fire; and I can scarcely conceive that the heat andlight, which I always supposed to proceed entirely from the coals, arereally produced as much by the atmosphere. 

EMILY. 

When you blow the fire, you increase the combustion, I suppose, bysupplying the coals with a greater quantity of oxygen gas?

MRS. B. 

Certainly; but of course no blowing will produce combustion, unless thetemperature of the coals be first raised. A single spark, however, issometimes sufficient to produce that effect; for, as I said before, whenonce combustion has commenced, the caloric disengaged is sufficient toelevate the temperature of the rest of the body, provided that there bea free access of oxygen. It however sometimes happens that if a fire beill made, it will be extinguished before all the fuel is consumed, fromthe very circumstance of the combustion being so slow that the caloricdisengaged is insufficient to keep up the temperature of the fuel. Youmust recollect that there are three things required in order to producecombustion; a combustible body, oxygen, and a temperature at which theone will combine with the other. 

EMILY. 

You said that combustion was one method of decomposing the atmosphere, and obtaining the nitrogen gas in its simple state; but how do yousecure this gas, and prevent it from mixing with the rest of theatmosphere?

MRS. B. 

It is necessary for this purpose to burn the body within a close vessel, which is easily done. --We shall introduce a small lighted taper (PLATEVII. Fig.  1. ) under this glass receiver, which stands in a bason overwater, to prevent all communication with the external air. 

 [Illustration: Plate VII. Vol. I. P. 181. 

 Fig. 1. Combustion of a taper under a receiver. 

 Fig. 2. A Retort on a stand. 

 Fig. 3. Preparation of oxygen gas. A Furnace. B Earthen Retort in the furnace. C Water bath. D Receiver. E. E Tube conveying the gas from the Retort through the water into the Receiver. F. F. F Shelf perforated on which the Receiver stands. 

 Fig. 4. Combustion of iron wire in oxygen gas. ]

CAROLINE. 

How dim the light burns already! --It is now extinguished. 

MRS. B. 

Can you tell us why it is extinguished?

CAROLINE. 

Let me consider. --The receiver was full of atmospherical air; thetaper, in burning within it, must have combined with the oxygencontained in that air, and the caloric that was disengaged produced thelight of the taper. But when the whole of the oxygen was absorbed, thewhole of its electricity was disengaged; consequently no more caloriccould be produced, the taper ceased to burn, and the flame wasextinguished. 

MRS. B. 

Your explanation is perfectly correct. 

EMILY. 

The two constituents of the oxygen gas being thus disposed of, whatremains under the receiver must be pure nitrogen gas?

MRS. B. 

There are some circumstances which prevent the nitrogen gas, thusobtained, from being perfectly pure; but we may easily try whether theoxygen has disappeared, by putting another lighted taper under it. --Yousee how instantaneously the flame is extinguished, for want of oxygen tosupply the negative electricity required for the formation of caloric;and were you to put an animal under the receiver, it would immediatelybe suffocated. But that is an experiment which I do not think yourcuriosity will tempt you to try. 

EMILY. 

Certainly not. --But look, Mrs. B. , the receiver is full of a thickwhite smoke. Is that nitrogen gas?

MRS. B. 

No, my dear; nitrogen gas is perfectly transparent and invisible, likecommon air. This cloudiness proceeds from a variety of exhalations, which arise from the burning taper, and the nature of which you cannotyet understand. 

CAROLINE. 

The water within the receiver has now risen a little above its level inthe bason. What is the reason of this?

MRS. B. 

With a moment’s reflection, I dare say, you would have explained ityourself. The water rises in consequence of the oxygen gas within ithaving been destroyed, or rather decomposed, by the combustion of thetaper. 

CAROLINE. 

Then why did not the water rise immediately when the oxygen gas wasdestroyed?

MRS. B. 

Because the heat of the taper, whilst burning, produced a dilatation ofthe air in the vessel, which at first counteracted this effect. 

Another means of decomposing the atmosphere is the _oxygenation_ ofcertain metals. This process is very analogous to combustion; it is, indeed, only a more general term to express the combination of a bodywith oxygen. 

CAROLINE. 

In what respect, then, does it differ from combustion?

MRS. B. 

The combination of oxygen in combustion is always accompanied by adisengagement of light and heat; whilst this circumstance is not anecessary consequence of simple oxygenation. 

CAROLINE. 

But how can a body absorb oxygen without the combination of the twoelectricities which produce caloric?

MRS. B. 

Oxygen does not always present itself in a gaseous state; it is aconstituent part of a vast number of bodies, both solid and liquid, inwhich it exists in a much denser state than in the atmosphere; and fromthese bodies it may be obtained without much disengagement of caloric. It may likewise, in some cases, be absorbed from the atmosphere withoutany sensible production of light and heat; for, if the process be slow, the caloric is disengaged in such small quantities, and so gradually, that it is not capable of producing either light or heat. In this casethe absorption of oxygen is called _oxygenation_ or _oxydation_, insteadof _combustion_, as the production of sensible light and heat isessential to the latter. 

EMILY. 

I wonder that metals can unite with oxygen; for, as they are so dense, their attraction of aggregation must be very great; and I should havethought that oxygen could never have penetrated such bodies. 

MRS. B. 

Their strong attraction for oxygen counterbalances this obstacle. Mostmetals, however, require to be made red-hot before they are capable ofattracting oxygen in any considerable quantity. By this combination theylose most of their metallic properties, and fall into a kind of powder, formerly called _calx_, but now much more properly termed an _oxyd_;thus we have _oxyd of lead_, _oxyd of iron_,  &c. 

EMILY. 

And in the Voltaic battery, it is, I suppose, an oxyd of zinc, that isformed by the union of the oxygen with that metal?

MRS. B. 

Yes, it is. 

CAROLINE. 

The word oxyd, then, simply means a metal combined with oxygen?

MRS. B. 

Yes; but the term is not confined to metals, though chiefly applied tothem. Any body whatever, that has combined with a certain quantity ofoxygen, either by means of oxydation or combustion, is called an _oxyd_, and is said to be _oxydated_ or _oxygenated_. 

EMILY. 

Metals, when converted into oxyds, become, I suppose, negative?

MRS. B. 

Not in general; because in most oxyds the positive energy of the metalmore than counterbalances the native energy of the oxygen with which itcombines. 

This black powder is an oxyd of manganese, a metal which has so strongan affinity for oxygen, that it attracts that substance from theatmosphere at any known temperature: it is therefore never found in itsmetallic form, but always in that of an oxyd, in which state, you see, it has very little of the appearance of a metal. It is now heavier thanit was before oxydation, in consequence of the additional weight of theoxygen with which it has combined. 

CAROLINE. 

I am very glad to hear that; for I confess I could not help having somedoubts whether oxygen was really a substance, as it is not to beobtained in a simple and palpable state; but its weight is, I think, a decisive proof of its being a real body. 

MRS. B. 

It is easy to estimate its weight, by separating it from the manganese, and finding how much the latter has lost. 

EMILY. 

But if you can take the oxygen from the metal, shall we not then have itin its palpable simple state?

MRS. B. 

No; for I can only separate the oxygen from the manganese, by presentingto it some other body, for which it has a greater affinity than for themanganese. Caloric affording the two electricities is decomposed, andone of them uniting with the oxygen, restores it to the aëriform state. 

EMILY. 

But you said just now, that manganese would attract oxygen from theatmosphere in which it is combined with the negative electricity; how, therefore, can the oxygen have a superior affinity for that electricity, since it abandons it to combine with the manganese?

MRS. B. 

I give you credit for this objection, Emily; and the only answer I canmake to it is, that the mutual affinities of metals for oxygen, and ofoxygen for electricity, vary at different temperatures; a certain degreeof heat will, therefore, dispose a metal to combine with oxygen, whilst, on the contrary, the former will be compelled to part with the latter, when the temperature is further increased. I have put some oxyd ofmanganese into a retort, which is an earthen vessel with a bent neck, such as you see here. (PLATE VII. Fig.  2. ) --The retort containing themanganese you cannot see, as I have enclosed it in this furnace, whereit is now red-hot. But, in order to make you sensible of the escape ofthe gas, which is itself invisible, I have connected the neck of theretort with this bent tube, the extremity of which is immersed in thisvessel of water. (PLATE VII. Fig.  3. ) --Do you see the bubbles of airrise through the water?

CAROLINE. 

Perfectly. This, then, is pure oxygen gas; what a pity it should belost! Could you not preserve it?

MRS. B. 

We shall collect it in this receiver. --For this purpose, you observe, I first fill it with water, in order to exclude the atmospherical air;and then place it over the bubbles that issue from the retort, so as tomake them rise through the water to the upper part of the receiver. 

EMILY. 

The bubbles of oxygen gas rise, I suppose, from their specific levity?

MRS. B. 

Yes; for though oxygen forms rather a heavy gas, it is light compared towater. You see how it gradually displaces the water from the receiver. It is now full of gas, and I may leave it inverted in water on thisshelf, where I can keep the gas as long as I choose, for futureexperiments. This apparatus (which is indispensable in all experimentsin which gases are concerned) is called a water-bath. 

CAROLINE. 

It is a very clever contrivance, indeed; equally simple and useful. Howconvenient the shelf is for the receiver to rest upon under water, andthe holes in it for the gas to pass into the receiver! I long to makesome experiments with this apparatus. 

MRS. B. 

I shall try your skill that way, when you have a little more experience. I am now going to show you an experiment, which proves, in a verystriking manner, how essential oxygen is to combustion. You will seethat iron itself will burn in this gas, in the most rapid and brilliantmanner. 

CAROLINE. 

Really! I did not know that it was possible to burn iron. 

EMILY. 

Iron is a simple body, and you know, Caroline, that all simple bodiesare naturally positive, and therefore must have an affinity for oxygen. 

MRS. B. 

Iron will, however, not burn in atmospherical air without a very greatelevation of temperature; but it is eminently combustible in pure oxygengas; and what will surprise you still more, it can be set on firewithout any considerable rise of temperature. You see this spiral ironwire--I fasten it at one end to this cork, which is made to fit anopening at the top of the glass-receiver. (PLATE VII. Fig.  4. )

EMILY. 

I see the opening in the receiver; but it is carefully closed by aground glass-stopper. 

MRS. B. 

That is in order to prevent the gas from escaping; but I shall take outthe stopper, and put in the cork, to which the wire hangs. --Now I meanto burn this wire in the oxygen gas, but I must fix a small piece oflighted tinder to the extremity of it, in order to give the firstimpulse to combustion; for, however powerful oxygen is in promotingcombustion, you must recollect that it cannot take place without someelevation of temperature. I shall now introduce the wire into thereceiver, by quickly changing the stoppers. 

CAROLINE. 

Is there no danger of the gas escaping while you change the stoppers?

MRS. B. 

Oxygen gas is a little heavier than atmospherical air, therefore it willnot mix with it very rapidly; and, if I do not leave the openinguncovered, we shall not lose any----

CAROLINE. 

Oh, what a brilliant and beautiful flame!

EMILY. 

It is as white and dazzling as the sun! --Now a piece of the melted wiredrops to the bottom: I fear it is extinguished; but no, it burns againas bright as ever. 

MRS. B. 

It will burn till the wire is entirely consumed, provided the oxygen isnot first expended: for you know it can burn only while there is oxygento combine with it. 

CAROLINE. 

I never saw a more beautiful light. My eyes can hardly bear it! Howastonishing to think that all this caloric was contained in the smallquantity of gas and iron that was enclosed in the receiver; and that, without producing any sensible heat!

CAROLINE. 

How wonderfully quick combustion goes on in pure oxygen gas! But pray, are these drops of burnt iron as heavy as the wire was before?

MRS. B. 

They are even heavier; for the iron, in burning, has acquired exactlythe weight of the oxygen which has disappeared, and is now combined withit. It has become an oxyd of iron. 

CAROLINE. 

I do not know what you mean by saying that the oxygen has _disappeared_, Mrs.  B. , for it was always invisible. 

MRS. B. 

True, my dear; the expression was incorrect. But though you could notsee the oxygen gas, I believe you had no doubt of its presence, as theeffect it produced on the wire was sufficiently evident. 

CAROLINE. 

Yes, indeed; yet you know it was the caloric, and not the oxygen gasitself, that dazzled us so much. 

MRS. B. 

You are not quite correct in your turn, in saying the caloric dazzledyou; for caloric is invisible; it affects only the sense of feeling; itwas the light which dazzled you. 

CAROLINE. 

True; but light and caloric are such constant companions, that it isdifficult to separate them, even in idea. 

MRS. B. 

The easier it is to confound them, the more careful you should be inmaking the distinction. 

CAROLINE. 

But why has the water now risen, and filled part of the receiver?

MRS. B. 

Indeed, Caroline, I did not suppose you would have asked such aquestion! I dare say, Emily, you can answer it. 

EMILY. 

Let me reflect . . . . . . The oxygen has combined with the wire; thecaloric has escaped; consequently nothing can remain in the receiver, and the water will rise to fill the vacuum. 

CAROLINE. 

I wonder that I did not think of that. I wish that we had weighed thewire and the oxygen gas before combustion; we might then have foundwhether the weight of the oxyd was equal to that of both. 

MRS. B. 

You might try the experiment if you particularly wished it; but I canassure you, that, if accurately performed, it never fails to show thatthe additional weight of the oxyd is precisely equal to that of theoxygen absorbed, whether the process has been a real combustion, or asimple oxygenation. 

CAROLINE. 

But this cannot be the case with combustions in general; for when anysubstance is burnt in the common air, so far from increasing in weight, it is evidently diminished, and sometimes entirely consumed. 

MRS. B. 

But what do you mean by the expression _consumed_? You cannot supposethat the smallest particle of any substance in nature can be actuallydestroyed. A compound body is decomposed by combustion; some of itsconstituent parts fly off in a gaseous form, while others remain in aconcrete state; the former are called the _volatile_, the latter the_fixed products_ of combustion. But if we collect the whole of them, weshall always find that they exceed the weight of the combustible body, by that of the oxygen which has combined with them during combustion. 

EMILY. 

In the combustion of a coal fire, then, I suppose that the ashes arewhat would be called the fixed product, and the smoke the volatileproduct?

MRS. B. 

Yet when the fire burns best, and the quantity of volatile productsshould be the greatest, there is no smoke; how can you account for that?

EMILY. 

Indeed I cannot; therefore I suppose that I was not right in myconjecture. 

MRS. B. 

Not quite: ashes, as you supposed, are a fixed product of combustion;but smoke, properly speaking, is not one of the volatile products, as itconsists of some minute undecomposed particles of the coals that arecarried off by the heated air without being burnt, and are eitherdeposited in the form of soot, or dispersed by the wind. Smoke, therefore, ultimately, becomes one of the _fixed_ products ofcombustion. And you may easily conceive that the stronger the fire is, the less smoke is produced, because the fewer particles escapecombustion. On this principle depends the invention of Argand’s PatentLamps; a current of air is made to pass through the cylindrical wick ofthe lamp, by which means it is so plentifully supplied with oxygen, thatscarcely a particle of oil escapes combustion, nor is there any smokeproduced. 

EMILY. 

But what then are the volatile products of combustion?

MRS. B. 

Various new compounds, with which you are not yet acquainted, and whichbeing converted by caloric either into vapour or gas, are invisible; butthey can be collected, and we shall examine them at some future period. 

CAROLINE. 

There are then other gases, besides the oxygen and nitrogen gases. 

MRS. B. 

Yes, several: any substance that can assume and maintain the form of anelastic fluid at the temperature of the atmosphere, is called a gas. Weshall examine the several gases in their respective places; but we mustnow confine our attention to those that compose the atmosphere. 

I shall show you another method of decomposing the atmosphere, which isvery simple. In breathing, we retain a portion of the oxygen, and expirethe nitrogen gas; so that if we breathe in a closed vessel, for acertain length of time, the air within it will be deprived of its oxygengas. Which of you will make the experiment?

CAROLINE. 

I should be very glad to try it. 

MRS. B. 

Very well; breathe several times through this glass tube into thereceiver with which it is connected, until you feel that your breath isexhausted. 

CAROLINE. 

I am quite out of breath already!

MRS. B. 

Now let us try the gas with a lighted taper. 

EMILY. 

It is very pure nitrogen gas, for the taper is immediately extinguished. 

MRS. B. 

That is not a proof of its being pure, but only of the absence ofoxygen, as it is that principle alone which can produce combustion, every other gas being absolutely incapable of it. 

EMILY. 

In the methods which you have shown us, for decomposing the atmosphere, the oxygen always abandons the nitrogen; but is there no way of takingthe nitrogen from the oxygen, so as to obtain the latter pure from theatmosphere?

MRS. B. 

You must observe, that whenever oxygen is taken from the atmosphere, itis by decomposing the oxygen gas; we cannot do the same with thenitrogen gas, because nitrogen has a stronger affinity for caloric thanfor any other known principle: it appears impossible therefore toseparate it from the atmosphere by the power of affinities. But if wecannot obtain the oxygen gas, by this means, in its separate state, wehave no difficulty (as you have seen) to procure it in its gaseous form, by taking it from those substances that have absorbed it from theatmosphere, as we did with the oxyd of manganese. 

EMILY. 

Can atmospherical air be recomposed, by mixing due proportions of oxygenand nitrogen gases?

MRS. B. 

Yes: if about one part of oxygen gas be mixed with about four parts ofnitrogen gas, atmospherical air is produced. *

 [Footnote *: The proportion of oxygen in the atmosphere varies from 21 to 22 per cent. ]

EMILY. 

The air, then, must be an oxyd of nitrogen?

MRS. B. 

No, my dear; for there must be a chemical combination between oxygen andnitrogen in order to produce an oxyd; whilst in the atmosphere these twosubstances are separately combined with caloric, forming two distinctgases, which are simply mixed in the formation of the atmosphere. 

I shall say nothing more of oxygen and nitrogen at present, as we shallcontinually have occasion to refer to them in our future conversations. They are both very abundant in nature; nitrogen is the most plentiful inthe atmosphere, and exists also in all animal substances; oxygen forms aconstituent part, both of the animal and vegetable kingdoms, from whichit may be obtained by a variety of chemical means. But it is now time toconclude our lesson. I am afraid you have learnt more to-day than youwill be able to remember. 

CAROLINE. 

I assure you that I have been too much interested in it, ever to forgetit. In regard to nitrogen there seems to be but little to remember; itmakes a very insignificant figure in comparison to oxygen, although itcomposes a much larger portion of the atmosphere. 

MRS. B. 

Perhaps this insignificance you complain of may arise from the compoundnature of nitrogen, for though I have hitherto considered it as a simplebody, because it is not known in any natural process to be decomposed, yet from some experiments of Sir H. Davy, there appears to be reason forsuspecting that nitrogen is a compound body, as we shall see afterwards. But even in its simple state, it will not appear so insignificant whenyou are better acquainted with it; for though it seems to perform but apassive part in the atmosphere, and has no very striking properties, when considered in its separate state, yet you will see by-and-bye whata very important agent it becomes, when combined with other bodies. Butno more of this at present; we must reserve it for its proper place. 

CONVERSATION VII. 

ON HYDROGEN. 

CAROLINE. 

The next simple bodies we come to are CHLORINE and IODINE. Pray whatkinds of substances are these; are they also invisible?

MRS. B. 

No; for chlorine, in the state of gas, has a distinct greenish colour, and is therefore visible; and iodine, in the same state, has a beautifulclaret-red colour. The knowledge of these two bodies, however, and theexplanation of their properties, imply various considerations, which youwould not yet be able to understand; we shall therefore defer theirexamination to some future conversation, and we shall pass on to thenext simple substance, HYDROGEN, which we cannot, any more than oxygen, obtain in a visible or palpable form. We are acquainted with it only inits gaseous state, as we are with oxygen and nitrogen. 

CAROLINE. 

But in its gaseous state it cannot be called a simple substance, sinceit is combined with heat and electricity?

MRS. B. 

True, my dear; but as we do not know in nature of any substance which isnot more or less combined with caloric and electricity, we are apt tosay that a substance is in its pure state when combined with thoseagents only. 

Hydrogen was formerly called _inflammable air_, as it is extremelycombustible, and burns with a great flame. Since the invention of thenew nomenclature, it has obtained the name of hydrogen, which is derivedfrom two Greek words, the meaning of which is, _to produce water_. 

EMILY. 

And how does hydrogen produce water?

MRS. B. 

By its combustion. Water is composed of eighty-five parts, by weight, ofoxygen, combined with fifteen parts of hydrogen; or of two parts, bybulk of hydrogen gas, to one part of oxygen gas. 

CAROLINE. 

Really! is it possible that water should be a combination of two gases, and that one of these should be inflammable air! Hydrogen must be a mostextraordinary gas that will produce both fire and water. 

EMILY. 

But I thought you said that combustion could take place in no gas butoxygen?

MRS. B. 

Do you recollect what the process of combustion consists in?

EMILY. 

In the combination of a body with oxygen, with disengagement of lightand heat. 

MRS. B. 

Therefore when I say that hydrogen is combustible, I mean that it has anaffinity for oxygen; but, like all other combustible substances, itcannot burn unless supplied with oxygen, and also heated to a propertemperature. 

CAROLINE. 

The simply mixing fifteen parts of hydrogen, with eighty-five parts ofoxygen gas, will not, therefore, produce water?

MRS. B. 

No; water being a much denser fluid than gases, in order to reduce thesegases to a liquid, it is necessary to diminish the quantity of caloricor electricity which maintains them in an elastic form. 

EMILY. 

That I should think might be done by combining the oxygen and hydrogentogether; for in combining they would give out their respectiveelectricities in the form of caloric, and by this means would becondensed. 

CAROLINE. 

But you forget, Emily, that in order to make the oxygen and hydrogencombine, you must begin by elevating their temperature, which increases, instead of diminishing, their electric energies. 

MRS. B. 

Emily is, however, right; for though it is necessary to raise theirtemperature, in order to make them combine, as that combination affordsthem the means of parting with their electricities, it is eventually thecause of the diminution of electric energy. 

CAROLINE. 

You love to deal in paradoxes to-day, Mrs.  B. --Fire, then, produceswater?

MRS. B. 

The combustion of hydrogen gas certainly does; but you do not seem tohave remembered the theory of combustion so well as you thought youwould. Can you tell me what happens in the combustion of hydrogen gas?

CAROLINE. 

The hydrogen combines with the oxygen, and their opposite electricitiesare disengaged in the form of caloric. --Yes, I think I understand itnow--by the loss of this caloric, the gases are condensed into a liquid. 

EMILY. 

Water, then, I suppose, when it evaporates and incorporates with theatmosphere, is decomposed and converted into hydrogen and oxygen gases?

MRS. B. 

No, my dear--there you are quite mistaken: the decomposition of water istotally different from its evaporation; for in the latter case (as youshould recollect) water is only in a state of very minute division; andis merely suspended in the atmosphere, without any chemical combination, and without any separation of its constituent parts. As long as theseremain combined, they form WATER, whether in a state of liquidity, or inthat of an elastic fluid, as vapour, or under the solid form of ice. 

In our experiments on latent heat, you may recollect that we causedwater successively to pass through these three forms, merely by anincrease or diminution of caloric, without employing any power ofattraction, or effecting any decomposition. 

CAROLINE. 

But are there no means of decomposing water?

MRS. B. 

Yes, several: charcoal, and metals, when heated red hot, will attractthe oxygen from water, in the same manner as they will from theatmosphere. 

CAROLINE. 

Hydrogen, I see, is like nitrogen, a poor dependant friend of oxygen, which is continually forsaken for greater favourites. 

MRS. B. 

The connection, or friendship, as you choose to call it, is much moreintimate between oxygen and hydrogen, in the state of water, thanbetween oxygen and nitrogen, in the atmosphere; for, in the first case, there is a chemical union and condensation of the two substances; in thelatter, they are simply mixed together in their gaseous state. You willfind, however, that, in some cases, nitrogen is quite as intimatelyconnected with oxygen, as hydrogen is. --But this is foreign to ourpresent subject. 

EMILY. 

Water, then, is an oxyd, though the atmospherical air is not?

MRS. B. 

It is not commonly called an oxyd, though, according to our definition, it may, no doubt, be referred to that class of bodies. 

CAROLINE. 

I should like extremely to see water decomposed. 

MRS. B. 

I can gratify your curiosity by a much more easy process than theoxydation of charcoal or metals: the decomposition of water by theselatter means takes up a great deal of time, and is attended with muchtrouble; for it is necessary that the charcoal or metal should be madered hot in a furnace, that the water should pass over them in a state ofvapour, that the gas formed should be collected over the water-bath, &c. In short, it is a very complicated affair. But the same effect may beproduced with the greatest facility, by the action of the Voltaicbattery, which this will give me an opportunity of exhibiting. 

CAROLINE. 

I am very glad of that, for I longed to see the power of this apparatusin decomposing bodies. 

MRS. B. 

For this purpose I fill this piece of glass-tube (PLATE VIII. Fig. 1. )with water, and cork it up at both ends; through one of the corks Iintroduce that wire of the battery which conveys the positiveelectricity; and the wire which conveys the negative electricity is madeto pass through the other cork, so that the two wires approach eachother sufficiently near to give out their respective electricities. 

 [Illustration: Plate VIII. Vol. I. P. 206

 Fig. 1. Apparatus for the decomposition of water by the Voltaic Battery. 

 Fig. 2. Apparatus for decomposing water by Voltaic Electricity & obtaining the gasses separate. 

 Fig. 3. Apparatus for preparing & collecting hydrogen gas. 

 Fig. 4. Receiver full of hydrogen gas inverted over water. 

 Fig. 5 Slow combustion of hydrogen gas. 

 Fig. 6. Apparatus for illustrating the formation of water by the combustion of hydrogen gas. 

 Fig. 7. Apparatus for producing harmonic sounds by the combustion of hydrogen gas. ]

CAROLINE. 

It does not appear to me that you approach the wires so near as you didwhen you made the battery act by itself. 

MRS. B. 

Water being a better conductor of electricity than air, the two wireswill act on each other at a greater distance in the former than in thelatter. 

EMILY. 

Now the electrical effect appears: I see small bubbles of air emittedfrom each wire. 

MRS. B. 

Each wire decomposes the water, the positive by combining with itsoxygen which is negative, the negative by combining with its hydrogenwhich is positive. 

CAROLINE. 

That is wonderfully curious! But what are the small bubbles of air?

MRS. B. 

Those that appear to proceed from the positive wire, are the result ofthe decomposition of the water by that wire. That is to say, thepositive electricity having combined with some of the oxygen of thewater, the particles of hydrogen which were combined with that portionof oxygen are set at liberty, and appear in the form of small bubbles ofgas or air. 

EMILY. 

And I suppose the negative fluid having in the same manner combined withsome of the hydrogen of the water, the particles of oxygen that werecombined with it, are set free, and emitted in a gaseous form. 

MRS. B. 

Precisely so. But I should not forget to observe, that the wires used inthis experiment are made of platina, a metal which is not capable ofcombining with oxygen; for otherwise the wire would combine with theoxygen, and the hydrogen alone would be disengaged. 

CAROLINE. 

But could not water be decomposed without the electric circle beingcompleted? If, for instance, you immersed only the positive wire in thewater, would it not combine with the oxygen, and the hydrogen gas begiven out?

MRS. B. 

No; for as you may recollect, the battery cannot act unless the circlebe completed; since the positive wire will not give out its electricity, unless attracted by that of the negative wire. 

CAROLINE. 

I understand it now. --But look, Mrs. B. , the decomposition of the waterwhich has now been going on for some time, does not sensibly diminishits quantity--what is the reason of that?

MRS. B. 

Because the quantity decomposed is so extremely small. If you comparethe density of water with that of the gases into which it is resolved, you must be aware that a single drop of water is sufficient to producethousands of such small bubbles as those you now perceive. 

CAROLINE. 

But in this experiment, we obtain the oxygen and hydrogen gases mixedtogether. Is there any means of procuring the two gases separately?

MRS. B. 

They can be collected separately with great ease, by modifying a littlethe experiment. Thus if instead of one tube, we employ two, as you seehere, (c,  d, PLATE VIII. Fig. 2. ) both tubes being closed at one end, and open at the other; and if after filling these tubes with water, weplace them standing in a glass of water (e), with their open enddownwards, you will see that the moment we connect the wires (a,  b)which proceed upwards from the interior of each tube, the one with oneend of the battery, and the other with the other end, the water in thetubes will be decomposed; hydrogen will be given out round the wire inthe tube connected with the positive end of the battery, and oxygen inthe other; and these gases will be evolved, exactly in the proportionswhich I have before mentioned, namely, two measures of hydrogen for oneof oxygen. We shall now begin the experiment, but it will be some timebefore any sensible quantity of the gases can be collected. 

EMILY. 

The decomposition of water in this way, slow as it is, is certainly verystriking; but I confess that I should be still more gratified, if youcould shew it us on a larger scale, and by a quicker process. I am sorrythat the decomposition of water by charcoal or metals is attended withso much inconvenience. 

MRS. B. 

Water may be decomposed by means of metals without any difficulty; butfor this purpose the intervention of an acid is required. Thus, if weadd some sulphuric acid (a substance with the nature of which you arenot yet acquainted) to the water which the metal is to decompose, theacid disposes the metal to combine with the oxygen of the water soreadily and abundantly, that no heat is required to hasten the process. Of this I am going to shew you an instance. I put into this bottle thewater that is to be decomposed, as also the metal that is to effect thatdecomposition by combining with the oxygen, and the acid which is tofacilitate the combination of the metal and the oxygen. You will seewith what violence these will act on each other. 

CAROLINE. 

But what metal is it that you employ for this purpose?

MRS. B. 

It is iron; and it is used in the state of filings, as these present agreater surface to the acid than a solid piece of metal. For as it isthe surface of the metal which is acted upon by the acid, and isdisposed to receive the oxygen produced by the decomposition of thewater, it necessarily follows that the greater is the surface, the moreconsiderable is the effect. The bubbles which are now rising arehydrogen gas----

CAROLINE. 

How disagreeably it smells!

MRS. B. 

It is indeed unpleasant, though, I believe, not particularly hurtful. Weshall not, however, suffer any more to escape, as it will be wanted forexperiments. I shall, therefore, collect it in a glass-receiver, bymaking it pass through this bent tube, which will conduct it into thewater-bath. (PLATE VIII. Fig.  3. )

EMILY. 

How very rapidly the gas escapes! it is perfectly transparent, andwithout any colour whatever. --Now the receiver is full----

MRS. B. 

We shall, therefore, remove it, and substitute another in its place. Butyou must observe, that when the receiver is full, it is necessary tokeep it inverted with the mouth under water, otherwise the gas wouldescape. And in order that it may not be in the way, I introduce withinthe bath, under the water, a saucer, into which I slide the receiver, sothat it can be taken out of the bath and conveyed any where, the waterin the saucer being equally effectual in preventing its escape as thatin the bath. (PLATE VIII. Fig.  4. )

EMILY. 

I am quite surprised to see what a large quantity of hydrogen gas can beproduced by such a small quantity of water, especially as oxygen is theprincipal constituent of water. 

MRS. B. 

In weight it is; but not in volume. For though the proportion, byweight, is nearly six parts of oxygen to one of hydrogen, yet theproportion of the volume of the gases, is about one part of oxygen totwo of hydrogen; so much heavier is the former than the latter. 

CAROLINE. 

But why is the vessel in which the water is decomposed so hot? As thewater changes from a liquid to a gaseous form, cold should be producedinstead of heat. 

MRS. B. 

No; for if one of the constituents of water is converted into a gas, theother becomes solid in combining with the metal. 

EMILY. 

In this case, then, neither heat nor cold should be produced?

MRS. B. 

True: but observe that the sensible heat which is disengaged in thisoperation, is not owing to the decomposition of the water, but to anextrication of heat produced by the mixture of water and sulphuric acid. I will mix some water and sulphuric acid together in this glass, thatyou may feel the surprising quantity of heat that is disengaged by theirunion--now take hold of the glass----

CAROLINE. 

Indeed I cannot; it feels as hot as boiling water. I should haveimagined there would have been heat enough disengaged to have renderedthe liquid solid. 

MRS. B. 

As, however, it does not produce that effect, we cannot refer this heatto the modification called latent heat. We may, however, I think, consider it as heat of capacity, as the liquid is condensed by its loss;and if you were to repeat the experiment, in a graduated tube, you wouldfind that the two liquids, when mixed, occupy considerably less spacethan they did separately. --But we will reserve this to anotheropportunity, and attend at present to the hydrogen gas which we havebeen producing. 

If I now set the hydrogen gas, which is contained in this receiver, atliberty all at once, and kindle it as soon as it comes in contact withthe atmosphere, by presenting it to a candle, it will so suddenly andrapidly decompose the oxygen gas, by combining with its basis, that anexplosion, or a _detonation_ (as chemists commonly call it), will beproduced. For this purpose, I need only take up the receiver, andquickly present its open mouth to the candle---- so .  .  .  . 

CAROLINE. 

It produced only a sort of hissing noise, with a vivid flash of light. I had expected a much greater report. 

MRS. B. 

And so it would have been, had the gases been closely confined at themoment they were made to explode. If, for instance, we were to put inthis bottle a mixture of hydrogen gas and atmospheric air; and if, aftercorking the bottle, we should kindle the mixture by a very smallorifice, from the sudden dilatation of the gases at the moment of theircombination, the bottle must either fly to pieces, or the cork be blownout with considerable violence. 

CAROLINE. 

But in the experiment which we have just seen, if you did not kindle thehydrogen gas, would it not equally combine with the oxygen?

MRS. B. 

Certainly not; for, as I have just explained to you, it is necessarythat the oxygen and hydrogen gases be burnt together, in order tocombine chemically and produce water. 

CAROLINE. 

That is true; but I thought this was a different combination, for I seeno water produced. 

MRS. B. 

The water resulting from this detonation was so small in quantity, andin such a state of minute division, as to be invisible. But watercertainly was produced; for oxygen is incapable of combining withhydrogen in any other proportions than those that form water; thereforewater must always be the result of their combination. 

If, instead of bringing the hydrogen gas into sudden contact with theatmosphere (as we did just now) so as to make the whole of it explodethe moment it is kindled, we allow but a very small surface of gas toburn in contact with the atmosphere, the combustion goes on quietly andgradually at the point of contact, without any detonation, because thesurfaces brought together are too small for the immediate union ofgases. The experiment is a very easy one. This phial, with a narrowneck, (PLATE VIII. Fig. 5. ) is full of hydrogen gas, and is carefullycorked. If I take out the cork without moving the phial, and quicklyapproach the candle to the orifice, you will see how different theresult will be----

EMILY. 

How prettily it burns, with a blue flame! The flame is gradually sinkingwithin the phial--now it has entirely disappeared. But does not thiscombustion likewise produce water?

MRS. B. 

Undoubtedly. In order to make the formation of the water sensible toyou, I shall procure a fresh supply of hydrogen gas, by putting intothis bottle (PLATE VIII. Fig. 6. ) iron filings, water, and sulphuricacid, materials similar to those which we have just used for the samepurpose. I shall then cork up the bottle, leaving only a small orificein the cork, with a piece of glass-tube fixed to it, through which thegas will issue in a continued rapid stream. 

CAROLINE. 

I hear already the hissing of the gas through the tube, and I can feel astrong current against my hand. 

MRS. B. 

This current I am going to kindle with the candle--see how vividly itburns----

EMILY. 

It burns like a candle with a long flame. But why does this combustionlast so much longer than in the former experiment?

MRS. B. 

The combustion goes on uninterruptedly as long as the new gas continuesto be produced. Now if I invert this receiver over the flame, you willsoon perceive its internal surface covered with a very fine dew, whichis pure water----

CAROLINE. 

Yes, indeed; the glass is now quite dim with moisture! How glad I amthat we can see the water produced by this combustion. 

EMILY. 

It is exactly what I was anxious to see; for I confess I was a littleincredulous. 

MRS. B. 

If I had not held the glass-bell over the flame, the water would haveescaped in the state of vapour, as it did in the former experiment. Wehave here, of course, obtained but a very small quantity of water; butthe difficulty of procuring a proper apparatus, with sufficientquantities of gases, prevents my showing it you on a larger scale. 

The composition of water was discovered about the same period, both byMr. Cavendish, in this country, and by the celebrated French chemistLavoisier. The latter invented a very perfect and ingenious apparatus toperform, with great accuracy, and upon a large scale, the formation ofwater by the combination of oxygen and hydrogen gases. Two tubes, conveying due proportions, the one of oxygen, the other of hydrogen gas, are inserted at opposite sides of a large globe of glass, previouslyexhausted of air; the two streams of gas are kindled within the globe, by the electrical spark, at the point where they come in contact; theyburn together, that is to say, the hydrogen combines with the oxygen, the caloric is set at liberty, and a quantity of water is producedexactly equal, in weight, to that of the two gases introduced into theglobe. 

CAROLINE. 

And what was the greatest quantity of water ever formed in thisapparatus?

MRS. B. 

Several ounces; indeed, very nearly a pound, if I recollect right; butthe operation lasted many days. 

EMILY. 

This experiment must have convinced all the world of the truth of thediscovery. Pray, if improper proportions of the gases were mixed and setfire to, what would be the result?

MRS. B. 

Water would equally be formed, but there would be a residue of eitherone or other of the gases, because, as I have already told you, hydrogenand oxygen will combine only in the proportions requisite for theformation of water. 

EMILY. 

Look, Mrs. B. , our experiment with the Voltaic battery (PLATE VIII. Fig. 2. ) has made great progress; a quantity of gas has been formed in eachtube, but in one of them there is twice as much gas as in the other. 

MRS. B. 

Yes; because, as I said before, water is composed of two volumes ofhydrogen to one of oxygen--and if we should now mix these gases togetherand set fire to them by an electrical spark, both gases would entirelydisappear, and a small quantity of water would be formed. 

There is another curious effect produced by the combustion of hydrogengas, which I shall show you, though I must acquaint you first, that Icannot well explain the cause of it. For this purpose, I must put somematerials into our apparatus, in order to obtain a stream of hydrogengas, just as we have done before. The process is already going on, andthe gas is rushing through the tube--I shall now kindle it with thetaper----

EMILY. 

It burns exactly as it did before---- What is the curious effect whichyou were mentioning?

MRS. B. 

Instead of the receiver, by means of which we have just seen the dropsof water form, we shall invert over the flame this piece of tube, whichis about two feet in length, and one inch in diameter (PLATE VIII. Fig.  7. ); but you must observe that it is open at both ends. 

EMILY. 

What a strange noise it makes! something like the Æolian harp, but notso sweet. 

CAROLINE. 

It is very singular, indeed; but I think rather too powerful to bepleasing. And is not this sound accounted for?

MRS. B. 

That the percussion of glass, by a rapid stream of gas, should produce asound, is not extraordinary: but the sound here is so peculiar, that noother gas has a similar effect. Perhaps it is owing to a brisk vibratorymotion of the glass, occasioned by the successive formation andcondensation of small drops of water on the sides of the glass tube, andthe air rushing in to replace the vacuum formed. *

 [Footnote *: This ingenious explanation was first suggested by Dr. Delarive. --See Journals of the Royal Institution, vol. I. P. 259. ]

CAROLINE. 

How very much this flame resembles the burning of a candle. 

MRS. B. 

The burning of a candle is produced by much the same means. A great dealof hydrogen is contained in candles, whether of tallow or wax. Thishydrogen being converted into gas by the heat of the candle, combineswith the oxygen of the atmosphere, and flame and water result from thiscombination. So that, in fact, the flame of a candle is owing to thecombustion of hydrogen gas. An elevation of temperature, such as isproduced by a lighted match or taper, is required to give the firstimpulse to the combustion; but afterwards it goes on of itself, becausethe candle finds a supply of caloric in the successive quantities ofheat which results from the union of the two electricities given out bythe gases during their combustion. But there are other circumstancesconnected with the combustion of candles and lamps, which I cannotexplain to you till you are acquainted with _carbon_, which is one oftheir constituent parts. In general, however, whenever you see flame, you may infer that it is owing to the formation and burning of hydrogengas*; for flame is the peculiar mode of burning hydrogen gas, which, with only one or two apparent exceptions, does not belong to any othercombustible. 

 [Footnote *: Or rather, _hydro-carbonat_, a gas composed of hydrogen and carbon, which will be noticed under the head _Carbon_. ]

EMILY. 

You astonish me! I understood that flame was the caloric produced by theunion of the two electricities, in all combustions whatever?

MRS. B. 

Your error proceeded from your vague and incorrect idea of flame; youhave confounded it with light and caloric in general. Flame alwaysimplies caloric, since it is produced by the combustion of hydrogen gas;but all caloric does not imply flame. Many bodies burn with intense heatwithout producing flame. Coals, for instance, burn with flame until allthe hydrogen which they contain is evaporated; but when they afterwardsbecome red hot, much more caloric is disengaged than when they produceflame. 

CAROLINE. 

But the iron wire, which you burnt in oxygen gas, appeared to me to emitflame; yet, as it was a simple metal, it could contain no hydrogen?

MRS. B. 

It produced a sparkling dazzling blaze of light, but no real flame. 

EMILY. 

And what is the cause of the regular shape of the flame of a candle?

MRS. B. 

The regular stream of hydrogen gas which exhales from its combustiblematter. 

CAROLINE. 

But the hydrogen gas must, from its great levity, ascend into the upperregions of the atmosphere; why therefore does not the flame continue toaccompany it?

MRS. B. 

The combustion of the hydrogen gas is completed at the point where theflame terminates; it then ceases to be hydrogen gas, as it is convertedby its combination with oxygen into watery vapour; but in a state ofsuch minute division as to be invisible. 

CAROLINE. 

I do not understand what is the use of the wick of a candle, since thehydrogen gas burns so well without it?

MRS. B. 

The combustible matter of the candle must be decomposed in order toemit the hydrogen gas, and the wick is instrumental in effecting thisdecomposition. Its combustion first melts the combustible matter, and .  .  .  . 

CAROLINE. 

But in lamps the combustible matter is already fluid, and yet they alsorequire wicks?

MRS. B. 

I am going to add that, afterwards, the burning wick (by the power ofcapillary attraction) gradually draws up the fluid to the point wherecombustion takes place; for you must have observed that the wick doesnot burn quite to the bottom. 

CAROLINE. 

Yes; but I do not understand why it does not. 

MRS. B. 

Because the air has not so free an access to that part of the wick whichis immediately in contact with the candle, as to the part just above, sothat the heat there is not sufficient to produce its decomposition; thecombustion therefore begins a little above this point. 

CAROLINE. 

But, Mrs. B. , in those beautiful lights, called _gas-lights_, which arenow seen in many streets, and will, I hope, be soon adopted every where, I can perceive no wick at all. How are these lights managed?

MRS. B. 

I am glad you have put me in mind of saying a few words on this veryuseful and interesting improvement. In this mode of lighting, the gas isconveyed to the extremity of a tube, where it is kindled, and burns aslong as the supply continues. There is, therefore, no occasion for awick, or any other fuel whatever. 

EMILY. 

But how is all this gas procured in such large quantities?

MRS. B. 

It is obtained from coal, by distillation. --Coal, when exposed to heatin a close vessel, is decomposed; and hydrogen, which is one of itsconstituents, rises in the state of gas, combined with another of itscomponent parts, carbon, forming a compound gas, called _Hydrocarbonat_, the nature of which we shall again have an opportunity of noticing whenwe treat of carbon. This gas, like hydrogen, is perfectly transparent, invisible, and highly inflammable; and in burning it emits that vividlight which you have so often observed. 

CAROLINE. 

And does the process for procuring it require nothing but heating thecoals, and conveying the gas through tubes?

MRS. B. 

Nothing else; except that the gas must be made to pass, immediately atits formation, through two or three large vessels of water, in which itdeposits some other ingredients, and especially water, tar, and oil, which also arise from the distillation of coals. The gas-lightapparatus, therefore, consists simply in a large iron vessel, in whichthe coals are exposed to the heat of a furnace, --some reservoirs ofwater, in which the gas deposits its impurities, --and tubes that conveyit to the desired spot, being propelled with uniform velocity throughthe tubes by means of a certain degree of pressure which is made uponthe reservoir. 

EMILY. 

What an admirable contrivance! Do you not think, Mrs.  B. , that it willsoon get into universal use?

MRS. B. 

Most probably, as to the lighting of streets, offices, and publicplaces, as it far surpasses any former invention for that purpose; butas to the interior of private houses, this mode of lighting has not yetbeen sufficiently tried to know whether it will be found generallydesirable, either in regard to economy or convenience. It may, however, be considered as one of the happiest applications of chemistry to thecomforts of life; and there is every reason to suppose that it willanswer the full extent of public, expectation. 

I have another experiment to show you with hydrogen gas, which I thinkwill entertain you. Have you ever blown bubbles with soap and water?

EMILY. 

Yes, often, when I was a child; and I used to make them float in the airby blowing them upwards. 

MRS. B. 

We shall fill some such bubbles with hydrogen gas, instead ofatmospheric air, and you will see with what ease and rapidity they willascend, without the assistance of blowing, from the lightness of thegas. --Will you mix some soap and water whilst I fill this bladder withthe gas contained in the receiver which stands on the shelf in thewater-bath?

CAROLINE. 

What is the use of the brass-stopper and turn-cock at the top of thereceiver?

MRS. B. 

It is to afford a passage to the gas when required. There is, you see, a similar stop-cock fastened to this bladder, which is made to fit thaton the receiver. I screw them one on the other, and now turn the twococks, to open a communication between the receiver and the bladder;then, by sliding the receiver off the shelf, and gently sinking it intothe bath, the water rises in the receiver and forces the gas into thebladder. (PLATE IX. Fig.  1. )

 [Illustration: Plate IX. Vol. I. P. 228

 Fig. 1. Apparatus for transferring gases from a Receiver into a bladder. Fig. 2. Apparatus for blowing Soap bubbles. ]

CAROLINE. 

Yes, I see the bladder swell as the water rises in the receiver. 

MRS. B. 

I think that we have already a sufficient quantity in the bladder forour purpose; we must be careful to stop both the cocks before weseparate the bladder from the receiver, lest the gas should escape. --Now I must fix a pipe to the stopper of the bladder, and by dippingits mouth into the soap and water, take up a few drops--then I againturn the cock, and squeeze the bladder in order to force the gas intothe soap and water at the mouth of the pipe. (PLATE IX. Fig.  2. )

EMILY. 

There is a bubble--but it bursts before it leaves the mouth of the pipe. 

MRS. B. 

We must have patience and try again; it is not so easy to blow bubblesby means of a bladder, as simply with the breath. 

CAROLINE. 

Perhaps there is not soap enough in the water; I should have had warmwater, it would have dissolved the soap better. 

EMILY. 

Does not some of the gas escape between the bladder and the pipe?

MRS. B. 

No, they are perfectly air tight; we shall succeed presently, I daresay. 

CAROLINE. 

Now a bubble ascends; it moves with the rapidity of a balloon. Howbeautifully it refracts the light!

EMILY. 

It has burst against the ceiling--you succeed now wonderfully; but whydo they all ascend and burst against the ceiling?

MRS. B. 

Hydrogen gas is so much lighter than atmospherical air, that it ascendsrapidly with its very light envelope, which is burst by the force withwhich it strikes the ceiling. 

Air-balloons are filled with this gas, and if they carried no otherweight than their covering, would ascend as rapidly as these bubbles. 

CAROLINE. 

Yet their covering must be much heavier than that of these bubbles?

MRS. B. 

Not in proportion to the quantity of gas they contain. I do not knowwhether you have ever been present at the filling of a large balloon. The apparatus for that purpose is very simple. It consists of a numberof vessels, either jars or barrels, in which the materials for theformation of the gas are mixed, each of these being furnished with atube, and communicating with a long flexible pipe, which conveys the gasinto the balloon. 

EMILY. 

But the fire-balloons which were first invented, and have been sinceabandoned, on account of their being so dangerous, were constructed, I suppose, on a different principle. 

MRS. B. 

They were filled simply with atmospherical air, considerably rarefied byheat; and the necessity of having a fire underneath the balloon, inorder to preserve the rarefaction of the air within it, was thecircumstance productive of so much danger. 

If you are not yet tired of experiments, I have another to show you. Itconsists in filling soap-bubbles with a mixture of hydrogen and oxygengases, in the proportions that form water; and afterwards setting fireto them. 

EMILY. 

They will detonate, I suppose?

MRS. B. 

Yes, they will. As you have seen the method of transferring the gas fromthe receiver into the bladder, it is not necessary to repeat it. I havetherefore provided a bladder which contains a due proportion of oxygenand hydrogen gases, and we have only to blow bubbles with it. 

CAROLINE. 

Here is a fine large bubble rising--shall I set fire to it with thecandle?

MRS. B. 

If you please . . . . 

CAROLINE. 

Heavens, what an explosion! --It was like the report of a gun: I confessit frightened me much. I never should have imagined it could be so loud. 

EMILY. 

And the flash was as vivid as lightning. 

MRS. B. 

The combination of the two gases takes place during that instant of timethat you see the flash, and hear the detonation. 

EMILY. 

This has a strong resemblance to thunder and lightning. 

MRS. B. 

These phenomena, however, are generally of an electrical nature. Yetvarious meteorological effects may be attributed to accidentaldetonations of hydrogen gas in the atmosphere; for nature abounds withhydrogen: it constitutes a very considerable portion of the whole massof water belonging to our globe, and from that source almost every otherbody obtains it. It enters into the composition of all animalsubstances, and of a great number of minerals; but it is most abundantin vegetables. From this immense variety of bodies, it is oftenspontaneously disengaged; its great levity makes it rise into thesuperior regions of the atmosphere; and when, either by an electricalspark, or any casual elevation of temperature, it takes fire, it mayproduce such meteors or luminous appearances as are occasionally seen inthe atmosphere. Of this kind are probably those broad flashes which weoften see on a summer-evening, without hearing any detonation. 

EMILY. 

Every flash, I suppose, must produce a quantity of water?

CAROLINE. 

And this water, naturally, descends in the form of rain?

MRS. B. 

That probably is often the case, though it is not a necessaryconsequence; for the water may be dissolved by the atmosphere, as itdescends towards the lower regions, and remain there in the form ofclouds. 

The application of electrical attraction to chemical phenomena is likelyto lead to many very interesting discoveries in meteorology; forelectricity evidently acts a most important part in the atmosphere. Thissubject however, is, as yet, not sufficiently developed for me toventure enlarging upon it. The phenomena of the atmosphere are far frombeing well understood; and even with the little that is known, I am butimperfectly acquainted. 

But before we take leave of hydrogen, I must not omit to mention to youa most interesting discovery of Sir H. Davy, which is connected withthis subject. 

CAROLINE. 

You allude, I suppose, to the new miner’s lamp, which has of late beenso much talked of? I have long been desirous of knowing what thatdiscovery was, and what purpose it was intended to answer. 

MRS. B. 

It often happens in coal-mines, that quantities of the gas, called bychemists _hydro-carbonat_, or by the miners _fire-damp_, (the same fromwhich the gas-lights are obtained, ) ooze out from fissures in the bedsof coal, and fill the cavities in which the men are at work; and thisgas being inflammable, the consequence is, that when the men approachthose places with a lighted candle, the gas takes fire, and explosionshappen which destroy the men and horses employed in that part of thecolliery, sometimes in great numbers. 

EMILY. 

What tremendous accidents these must be! But whence does that gasoriginate?

MRS. B. 

Being the chief product of the combustion of coal, no wonder thatinflammable gas should occasionally appear in situations in which thismineral abounds, since there can be no doubt that processes ofcombustion are frequently taking place at a great depth under thesurface of the earth; and therefore those accumulations of gas may ariseeither from combustions actually going on, or from former combustions, the gas having perhaps been confined there for ages. 

CAROLINE. 

And how does Sir H. Davy’s lamp prevent those dreadful explosions?

MRS. B. 

By a contrivance equally simple and ingenious; and one which does noless credit to the philosophical views from which it was deduced, thanto the philanthropic motives from which the enquiry sprung. Theprinciple of the lamp is shortly this: It was ascertained, two or threeyears ago, both by Mr. Tennant and by Sir Humphry himself, that thecombustion of inflammable gas could not be propagated through smalltubes; so that if a jet of an inflammable gaseous mixture, issuing froma bladder or any other vessel, through a small tube, be set fire to, itburns at the orifice of the tube, but the flame never penetrates intothe vessel. It is upon this fact that Sir Humphry’s safety-lamp isfounded. 

EMILY. 

But why does not the flame ever penetrate through the tube into thevessel from which the gas issues, so as to explode at once the whole ofthe gas?

MRS. B. 

Because, no doubt, the inflamed gas is so much cooled in its passagethrough a small tube as to cease to burn before the combustion reachesthe reservoir. 

CAROLINE. 

And how can this principle be applied to the construction of a lamp?

MRS. B. 

Nothing easier. You need only suppose a lamp enclosed all round in glassor horn, but having a number of small open tubes at the bottom, andothers at the top, to let the air in and out. Now, if such a lamp orlanthorn be carried into an atmosphere capable of exploding, anexplosion or combustion of the gas will take place within the lamp; andalthough the vent afforded by the tubes will save the lamp frombursting, yet, from the principle just explained, the combustion willnot be propagated to the external air through the tubes, so that nofarther consequence will ensue. 

EMILY. 

And is that all the mystery of that valuable lamp?

MRS. B. 

No; in the early part of the enquiry a lamp of this kind was actuallyproposed; but it was but a rude sketch compared to its present state ofimprovement. Sir H. Davy, after a succession of trials, by which hebrought his lamp nearer and nearer to perfection, at last conceived thehappy idea that if the lamp were surrounded with a wire-work orwire-gauze, of a close texture, instead of glass or horn, the tubularcontrivance I have just described would be entirely superseded, sinceeach of the interstices of the gauze would act as a tube in preventingthe propagation of explosions; so that this pervious metallic coveringwould answer the various purposes of transparency, of permeability toair, and of protection against explosion. This idea, Sir Humphryimmediately submitted to the test of experiment, and the result hasanswered his most sanguine expectations, both in his laboratory and inthe collieries, where it has already been extensively tried. And he hasnow the happiness of thinking that his invention will probably be themeans of saving every year a number of lives, which would have been lostin digging out of the bowels of the earth one of the most valuablenecessaries of life. Here is one of these lamps, every part of which youwill at once comprehend. (See PLATE X. Fig.  1. )

 [Illustration: Plate X. 

 Fig. 1. A. The cistern containing the Oil B. The rim or screw by which the gauze cage is fixed to the cistern. C. Apperture for supplying Oil. E. A wire for trimming the wick. D. F. The wire gauze cylinder. G. A double top. 

 Fig. 2. A. The reservoir of condensed air. B. The condensing Syringe. C. The bladder for Oxygen. D. The moveable jet. ]

CAROLINE. 

How very simple and ingenious! But I do not yet well see why anexplosion taking place within the lamp should not communicate to theexternal air around it, through the interstices of the wire?

MRS. B. 

This has been and is still a subject of wonder, even to philosophers;and the only mode they have of explaining it is, that flame or ignitioncannot pass through a fine wire-work, because the metallic wire coolsthe flame sufficiently to extinguish it in passing through the gauze. This property of the wire-gauze is quite similar to that of the tubeswhich I mentioned on introducing the subject; for you may consider eachinterstice of the gauze as an extremely short tube of a very smalldiameter. 

EMILY. 

But I should expect the wire would often become red-hot, by the burningof the gas within the lamp?

MRS. B. 

And this is actually the case, for the top of the lamp is very apt tobecome red-hot. But, fortunately, inflammable gaseous mixtures cannot beexploded by red-hot wire, the intervention of actual flame beingrequired for that purpose; so that the wire does not set fire to theexplosive gas around it. 

EMILY. 

I can understand that; but if the wire be red-hot, how can it cool theflame within, and prevent its passing through the gauze?

MRS. B. 

The gauze, though red-hot, is not so hot as the flame by which it hasbeen heated; and as metallic wire is a good conductor, the heat does notmuch accumulate in it, as it passes off quickly to the other parts ofthe lamp, as well as to any contiguous bodies. 

CAROLINE. 

This is indeed a most interesting discovery, and one which shows at oncethe immense utility with which science may be practically applied tosome of the most important purposes. 

CONVERSATION VIII. 

ON SULPHUR AND PHOSPHORUS. 

MRS. B. 

SULPHUR is the next substance that comes under our consideration. Itdiffers in one essential point from the preceding, as it exists in asolid form at the temperature of the atmosphere. 

CAROLINE. 

I am glad that we have at last a solid body to examine; one that we cansee and touch. Pray, is it not with sulphur that the points of matchesare covered, to make them easily kindle?

MRS. B. 

Yes, it is; and you therefore already know that sulphur is a verycombustible substance. It is seldom discovered in nature in a pureunmixed state; so great is its affinity for other substances, that it isalmost constantly found combined with some of them. It is most commonlyunited with metals, under various forms, and is separated from them by avery simple process. It exists likewise in many mineral waters, and somevegetables yield it in various proportions, especially those of thecruciform tribe. It is also found in animal matter; in short, it may bediscovered in greater or less quantity, in the mineral, vegetable, andanimal kingdoms. 

EMILY. 

I have heard of _flowers of sulphur_, are they the produce of any plant?

MRS. B. 

By no means: they consist of nothing more than common sulphur, reducedto a very fine powder by a process called _sublimation_. --You see someof it in this phial; it is exactly the same substance as this lump ofsulphur, only its colour is a paler yellow, owing to its state of veryminute division. 

EMILY. 

Pray what is sublimation?

MRS. B. 

It is the evaporation, or, more properly speaking, the volatilisation ofsolid substances, which, in cooling, condense again in a concrete form. The process, in this instance, must be performed in a closed vessel, both to prevent combustion, which would take place if the access of airwere not carefully precluded, and likewise in order to collect thesubstance after the operation. As it is rather a slow process, we shallnot try the experiment now; but you will understand it perfectly if Ishow you the apparatus used for the purpose. (PLATE XI. Fig. 1. ) Somelumps of sulphur are put into a receiver of this kind, which is called a_cucurbit_. Its shape, you see, somewhat resembles that of a pear, andis open at the top, so as to adapt itself exactly to a kind of conicalreceiver of this sort, called the head. The cucurbit, thus covered withits head, is placed over a sand-bath; this is nothing more than a vesselfull of sand, which is kept heated by a furnace, such as you see here, so as to preserve the apparatus in a moderate and uniform temperature. The sulphur then soon begins to melt, and immediately after this, a thick white smoke rises, which is gradually deposited within the head, or upper part of the apparatus, where it condenses against the sides, somewhat in the form of a vegetation, whence it has obtained the name offlowers of sulphur. This apparatus, which is called an _alembic_, ishighly useful in all kinds of distillations, as you will see when wecome to treat of those operations. Alembics are not commonly made ofglass, like this, which is applicable only to distillations upon a verysmall scale. Those used in manufactures are generally made of copper, and are, of course, considerably larger. The principal construction, however, is always the same, although their shape admits of somevariation. 

 [Illustration: Plate XI. Vol. I. P. 237. 

 Fig. 1. Sublimation of Sulphur. A Alembic. B Sand-bath. C Furnace. 

 Fig. 2. Eudiometer. 

 Fig. 3. Decomposition of water by Carbon. A Retort containing water. B Lamp to heat the water. C. C Porcelain tube containing Carbone. D Furnace through which the tube passes. E Receiver for the gas produced. F Water bath. ]

CAROLINE. 

What is the use of that neck, or tube, which bends down from the upperpiece of the apparatus?

MRS. B. 

It is of no use in sublimations; but in distillations (the generalobject of which is to evaporate, by heat, in closed vessels, thevolatile parts of a compound body, and to condense them again into aliquid, ) it serves to carry off the condensed fluid, which otherwisewould fall back into the cucurbit. But this is rather foreign to ourpresent subject. Let us return to the sulphur. You now perfectlyunderstand, I suppose, what is meant by sublimation?

EMILY. 

I believe I do. Sublimation appears to consist in destroying, by meansof heat, the attraction of aggregation of the particles of a solid body, which are thus volatilised; and as soon as they lose the caloric whichproduced that effect, they are deposited in the form of a fine powder. 

CAROLINE. 

It seems to me to be somewhat similar to the transformation of waterinto vapour, which returns to its liquid state when deprived of caloric. 

EMILY. 

There is this difference, however, that the sulphur does not return toits former state, since, instead of lumps, it changes to a fine powder. 

MRS. B. 

Chemically speaking, it is exactly the same substance, whether in theform of lump or powder. For if this powder be melted again by heat, itwill, in cooling, be restored to the same solid state in which it wasbefore its sublimation. 

CAROLINE. 

But if there be no real change, produced by the sublimation of thesulphur, what is the use of that operation?

MRS. B. 

It divides the sulphur into very minute parts, and thus disposes it toenter more readily into combination with other bodies. It is used alsoas a means of purification. 

CAROLINE. 

Sublimation appears to me like the beginning of combustion, for thecompletion of which one circumstance only is wanting, the absorption ofoxygen. 

MRS. B. 

But that circumstance is every thing. No essential alteration isproduced in sulphur by sublimation; whilst in combustion it combineswith the oxygen, and forms a new compound totally different in everyrespect from sulphur in its pure state. --We shall now _burn_ somesulphur, and you will see how very different the result will be. Forthis purpose I put a small quantity of flowers of sulphur into this cup, and place it in a dish, into which I have poured a little water: I nowset fire to the sulphur with the point of this hot wire; for itscombustion will not begin unless its temperature be considerably raised. --You see that it burns with a faint blueish flame; and as I invert overit this receiver, white fumes arise from the sulphur, and fill thevessel. --You will soon perceive that the water is rising within thereceiver, a little above its level in the plate. --Well, Emily, can youaccount for this?

EMILY. 

I suppose that the sulphur has absorbed the oxygen from theatmospherical air within the receiver, and that we shall find someoxygenated sulphur in the cup. As for the white smoke, I am quite at aloss to guess what it may be. 

MRS. B. 

Your first conjecture is very right: but you are mistaken in the last;for nothing will be left in the cup. The white vapour is the oxygenatedsulphur, which assumes the form of an elastic fluid of a pungent andoffensive smell, and is a powerful acid. Here you see a chemicalcombination of oxygen and sulphur, producing a true gas, which wouldcontinue such under the pressure and at the temperature of theatmosphere, if it did not unite with the water in the plate, to which itimparts its acid taste, and all its acid properties. --You see, now, with what curious effects the combustion of sulphur is attended. 

CAROLINE. 

This is something quite new; and I confess that I do not perfectlyunderstand why the sulphur turns acid. 

MRS. B. 

It is because it unites with oxygen, which is the acidifying principle. And, indeed, the word _oxygen_ is derived from two Greek wordssignifying _to produce an acid_. 

CAROLINE. 

Why, then, is not water, which contains such a quantity of oxygen, acid?

MRS. B. 

Because hydrogen, which is the other constituent of water, is notsusceptible of acidification. --I believe it will be necessary, beforewe proceed further, to say a few words of the general nature of acids, though it is rather a deviation from our plan of examining the simplebodies separately, before we consider them in a state of combination. 

Acids may be considered as a peculiar class of _burnt_ bodies, whichduring their combustion, or combination with oxygen, have acquired verycharacteristic properties. They are chiefly discernible by their sourtaste, and by turning red most of the blue vegetable colours. These twoproperties are common to the whole class of acids; but each of them isdistinguished by other peculiar qualities. Every acid consists of someparticular substance, (which constitutes its basis, and is different ineach, ) and of oxygen, which is common to them all. 

EMILY. 

But I do not clearly see the difference between acids and oxyds. 

MRS. B. 

Acids were, in fact, oxyds, which, by the addition of a sufficientquantity of oxygen, have been converted into acids. For acidification, you must observe, always implies previous oxydation, as a body must havecombined with the quantity of oxygen requisite to constitute it an oxyd, before it can combine with the greater quantity that is necessary torender it an acid. 

CAROLINE. 

Are all oxyds capable of being converted into acids?

MRS. B. 

Very far from it; it is only certain substances which will enter intothat peculiar kind of union with oxygen that produces acids, and thenumber of these is proportionally very small; but all burnt bodies maybe considered as belonging either to the class of oxyds, or to that ofacids. At a future period, we shall enter more at large into thissubject. At present, I have but one circumstance further to point out toyour observation respecting acids: it is, that most of them aresusceptible of two degrees of acidification, according to the differentquantities of oxygen with which their basis combines. 

EMILY. 

And how are these two degrees of acidification distinguished?

MRS. B. 

By the peculiar properties which result from them. The acid we have justmade is the first or weakest degree of acidification, and is called_sulphureous acid_; if it were fully saturated with oxygen, it would becalled _sulphuric acid_. You must therefore remember, that in this, asin all acids, the first degree of acidification is expressed by thetermination in _ous_; the stronger, by the termination in _ic_. 

CAROLINE. 

And how is the sulphuric acid made?

MRS. B. 

By burning sulphur in pure oxygen gas, and thus rendering its combustionmuch more complete. I have provided some oxygen gas for this purpose; itis in that bottle, but we must first decant the gas into the glassreceiver which stands on the shelf in the bath, and is full of water. 

CAROLINE. 

Pray, let me try to do it, Mrs. B. 

MRS. B. 

It requires some little dexterity--hold the bottle completely underwater, and do not turn the mouth upwards, till it is immediately underthe aperture in the shelf, through which the gas is to pass into thereceiver, and then turn it up gradually. --Very well, you have only leta few bubbles escape, and that must be expected at a first trial. --NowI shall put this piece of sulphur into the receiver, through the openingat the top, and introduce along with it a small piece of lighted tinderto set fire to it. --This requires being done very quickly, lest theatmospherical air should get in, and mix with the pure oxygen gas. 

EMILY. 

How beautifully it burns!

CAROLINE. 

But it is already buried in the thick vapour. This, I suppose, issulphuric acid?

EMILY. 

Are these acids always in a gaseous state?

MRS. B. 

Sulphureous acid, as we have already observed, is a permanent gas, andcan be obtained in a liquid form only by condensing it in water. In itspure state, the sulphureous acid is invisible, and it now appears in theform of a white smoke, from its combining with the moisture. But thevapour of sulphuric acid, which you have just seen to rise during thecombustion, is not a gas, but only a vapour, which condenses into liquidsulphuric acid, by losing its caloric. But it appears from Sir H. Davy’sexperiments, that this formation and condensation of sulphuric acidrequires the presence of water, for which purpose the vapour is receivedinto cold water, which may afterwards be separated from the acid byevaporation. 

Sulphur has hitherto been considered as a simple substance; but Sir H. Davy has suspected that it contains a small portion of hydrogen, andperhaps also of oxygen. 

On submitting sulphur to the action of the Voltaic battery, he observedthat the negative wire gave out hydrogen; and the existence of hydrogenin sulphur was rendered still more probable by his observing that asmall quantity of water was produced during the combustion of sulphur. 

EMILY. 

And pray of what nature is sulphur when perfectly pure?

MRS. B. 

Sulphur has probably never been obtained perfectly free fromcombination, so that its radical may possibly possess properties verydifferent from those of common sulphur. It has been suspected to be of ametallic nature; but this is mere conjecture. 

Before we quit the subject of sulphur, I must tell you that it issusceptible of combining with a great variety of substances, andespecially with hydrogen, with which you are already acquainted. Hydrogen gas can dissolve a small portion of it. 

EMILY. 

What! can a gas dissolve a solid substance?

MRS. B. 

Yes; a solid substance may be so minutely divided by heat, as to becomesoluble in a gas: and there are several instances of it. But you mustobserve, that, in this case, a chemical union or combination of thesulphur with the hydrogen gas is produced. In order to effect this, thesulphur must be strongly heated in contact with the gas; the heatreduces the sulphur to such a state of extreme division, and diffuses itso thoroughly through the gas, that they combine and incorporatetogether. And as a proof that there must be a chemical union between thesulphur and the gas, it is sufficient to remark that they are notseparated when the sulphur loses the caloric by which it wasvolatilized. Besides, it is evident, from the peculiar fetid smell ofthis gas, that it is a new compound totally different from either of itsconstituents; it is called _sulphuretted hydrogen gas_, and is containedin great abundance in sulphureous mineral waters. 

CAROLINE. 

Are not the Harrogate waters of this nature?

MRS. B. 

Yes; they are naturally impregnated with sulphuretted hydrogen gas, andthere are many other springs of the same kind, which shows that this gasmust often be formed in the bowels of the earth by spontaneous processesof nature. 

CAROLINE. 

And could not such waters be made artificially by impregnating commonwater with this gas?

MRS. B. 

Yes; they can be so well imitated, as perfectly to resemble theHarrogate waters. 

Sulphur combines likewise with phosphorus, and with the alkalies, andalkaline earths, substances with which you are yet unacquainted. Wecannot, therefore, enter into these combinations at present. In our nextlesson we shall treat of phosphorus. 

EMILY. 

May we not begin that subject to-day; this lesson has been so short?

MRS. B. 

I have no objection, if you are not tired. What do you say, Caroline?

CAROLINE. 

I am as desirous as Emily of prolonging the lesson to-day, especially aswe are to enter on a new subject; for I confess that sulphur has notappeared to me so interesting as the other simple bodies. 

MRS. B. 

Perhaps you may find phosphorus more entertaining. You must not, however, be discouraged when you meet with some parts of a study lessamusing than others; it would answer no good purpose to select the mostpleasing parts, since, if we did not proceed with some method, in orderto acquire a general idea of the whole, we could scarcely expect to takeinterest in any particular subjects. 

PHOSPHORUS. 

PHOSPHORUS is considered as a simple body; though, like sulphur, it hasbeen suspected of containing hydrogen. It was not known by the earlierchemists. It was first discovered by Brandt, a chemist of Hamburgh, whilst employed in researches after the philosopher’s stone; but themethod of obtaining it remained a secret till it was a second timediscovered both by Kunckel and Boyle, in the year 1680. You see aspecimen of phosphorus in this phial; it is generally moulded into smallsticks of a yellowish colour, as you find it here. 

CAROLINE. 

I do not understand in what the discovery consisted; there may be asecret method of making an artificial composition, but how can you talkof _making_ a substance which naturally exists?

MRS. B. 

A body may exist in nature so closely combined with other substances, asto elude the observation of chemists, or render it extremely difficultto obtain it in its separate state. This is the case with phosphorus, which is always so intimately combined with other substances, that itsexistence remained unnoticed till Brandt discovered the means ofobtaining it free from other combinations. It is found in all animalsubstances, and is now chiefly extracted from bones, by a chemicalprocess. It exists also in some plants, that bear a strong analogy toanimal matter in their chemical composition. 

EMILY. 

But is it never found in its pure separate state?

MRS. B. 

Never, and this is the reason that it has remained so long undiscovered. 

Phosphorus is eminently combustible; it melts and takes fire at thetemperature of one hundred degrees, and absorbs in its combustion nearlyonce and a half its own weight of oxygen. 

CAROLINE. 

What! will a pound of phosphorus consume a pound and half of oxygen?

MRS. B. 

So it appears from accurate experiments. I can show you with whatviolence it combines with oxygen, by burning some of it in that gas. Wemust manage the experiment in the same manner as we did the combustionof sulphur. You see I am obliged to cut this little bit of phosphorusunder water, otherwise there would be danger of its taking fire by theheat of my fingers. I now put into the receiver, and kindle it by meansof a hot wire. 

EMILY. 

What a blaze! I can hardly look at it. I never saw any thing sobrilliant. Does it not hurt your eyes, Caroline?

CAROLINE. 

Yes; but still I cannot help looking at it. A prodigious quantity ofoxygen must indeed be absorbed, when so much light and caloric aredisengaged!

MRS. B. 

In the combustion of a pound of phosphorus, a sufficient quantity ofcaloric is set free to melt upwards of a hundred pounds of ice; this hasbeen computed by direct experiments with the calorimeter. 

EMILY. 

And is the result of this combustion, like that of sulphur, an acid?

MRS. B. 

Yes; phosphoric acid. And had we duly proportioned the phosphorus andthe oxygen, they would have been completely converted into phosphoricacid, weighing together, in this new state, exactly the sum of theirweights separately. The water would have ascended into the receiver, onaccount of the vacuum formed, and would have filled it entirely. In thiscase, as in the combustion of sulphur, the acid vapour formed isabsorbed and condensed in the water of the receiver. But when thiscombustion is performed without any water or moisture being present, theacid then appears in the form of concrete whitish flakes, which are, however, extremely ready to melt upon the least admission of moisture. 

EMILY. 

Does phosphorus, in burning in atmospherical air, produce, like sulphur, a weaker sort of the same acid?

MRS. B. 

No: for it burns in atmospherical air, nearly at the same temperature asin pure oxygen gas; and it is in both cases so strongly disposed tocombine with the oxygen, that the combustion is perfect, and the productsimilar; only in atmospherical air, being less rapidly supplied withoxygen, the process is performed in a slower manner. 

CAROLINE. 

But is there no method of acidifying phosphorus in a slighter manner, soas to form _phosphorus_ acid?

MRS. B. 

Yes, there is. When simply exposed to the atmosphere, phosphorusundergoes a kind of slow combustion at any temperature above zero. 

EMILY. 

But is not the process in this case rather an oxydation than acombustion? For if the oxygen is too slowly absorbed for a sensiblequantity of light and heat to be disengaged, it is not a truecombustion. 

MRS. B. 

The case is not as you suppose: a faint light is emitted which is verydiscernible in the dark; but the heat evolved is not sufficiently strongto be sensible: a whitish vapour arises from this combustion, which, uniting with water, condenses into liquid phosphorus acid. 

CAROLINE. 

Is it not very singular that phosphorus should burn at so low atemperature in atmospherical air, whilst it does not burn in pure oxygenwithout the application of heat?

MRS. B. 

So it at first appears. But this circumstance seems to be owing to thenitrogen gas of the atmosphere. This gas dissolves small particles ofphosphorus, which being thus minutely divided and diffused in theatmospherical air, combines with the oxygen, and undergoes this slowcombustion. But the same effect does not take place in oxygen gas, because it is not capable of dissolving phosphorus; it is thereforenecessary, in this case, that heat should be applied to effect thatdivision of particles, which, in the former instance, is produced by thenitrogen. 

EMILY. 

I have seen letters written with phosphorus, which are invisible byday-light, but may be read in the dark by their own light. They look asif they were written with fire; yet they do not seem to burn. 

MRS. B. 

But they do really burn; for it is by their slow combustion that thelight is emitted; and phosphorus acid is the result of this combustion. 

Phosphorus is sometimes used as a test to estimate the purity ofatmospherical air. For this purpose, it is burnt in a graduated tube, called an _Eudiometer_ (PLATE XI. Fig. 2. ), and from the quantity of airwhich the phosphorus absorbs, the proportion of oxygen in the airexamined is deduced; for the phosphorus will absorb all the oxygen, andthe nitrogen alone will remain. 

EMILY. 

And the more oxygen is contained in the atmosphere, the purer, I suppose, it is esteemed?

MRS. B. 

Certainly. Phosphorus, when melted, combines with a great variety ofsubstances. With sulphur it forms a compound so extremely combustible, that it immediately takes fire on coming in contact with the air. It iswith this composition that phosphoric matches are prepared, which kindleas soon as they are taken out of their case and are exposed to the air. 

EMILY. 

I have a box of these curious matches; but I have observed, that in verycold weather, they will not take fire without being previously rubbed. 

MRS. B. 

By rubbing them you raise their temperature; for, you know, friction isone of the means of extricating heat. 

EMILY. 

Will phosphorus combine with hydrogen gas, as sulphur does?

MRS. B. 

Yes; and the compound gas which results from this combination has asmell still more fetid than the sulphuretted hydrogen; it resembles thatof garlic. 

The _phosphoretted hydrogen gas_ has this remarkable peculiarity, thatit takes fire spontaneously in the atmosphere, at any temperature. It isthus, probably, that are produced those transient flames, or flashes oflight, called by the vulgar _Will-of-the Whisp_, or more properly_Ignes-fatui_, which are often seen in church-yards, and places wherethe putrefactions of animal matter exhale phosphorus and hydrogen gas. 

CAROLINE. 

Country people, who are so much frightened by those appearances, wouldsoon be reconciled to them, if they knew from what a simple cause theyproceed. 

MRS. B. 

There are other combinations of phosphorus that have also very singularproperties, particularly that which results from its union with lime. 

EMILY. 

Is there any name to distinguish the combination of two substances, likephosphorus and lime, neither of which are oxygen, and which cannottherefore produce either an oxyd or an acid?

MRS. B. 

The names of such combinations are composed from those of theiringredients, merely by a slight change in their termination. Thus thecombination of sulphur with lime is called a _sulphuret_, and that ofphosphorus, a _phosphuret of lime_. This latter compound, I was going tosay, has the singular property of decomposing water, merely by beingthrown into it. It effects this by absorbing the oxygen of water, inconsequence of which bubbles of hydrogen gas ascend, holding in solutiona small quantity of phosphorus. 

EMILY. 

These bubbles then are _phosphoretted hydrogen gas_?

MRS. B. 

Yes; and they produce the singular appearance of a flash of fire issuingfrom water, as the bubbles kindle and detonate on the surface of thewater, at the instant that they come in contact with the atmosphere. 

CAROLINE. 

Is not this effect nearly similar to that produced by the combination ofphosphorus and sulphur, or, more properly speaking, the _phosphuret ofsulphur_?

MRS. B. 

Yes; but the phenomenon appears more extraordinary in this case, fromthe presence of water, and from the gaseous form of the combustiblecompound. Besides, the experiment surprises by its great simplicity. Youonly throw a piece of phosphoret of lime into a glass of water, andbubbles of fire will immediately issue from it. 

CAROLINE. 

Cannot we try the experiment?

MRS. B. 

Very easily: but we must do it in the open air; for the smell of thephosphorated hydrogen gas is so extremely fetid, that it would beintolerable in the house. But before we leave the room, we may produce, by another process, some bubbles of the same gas, which are much lessoffensive. 

There is in this little glass retort a solution of potash in water;I add to it a small piece of phosphorus. We must now heat the retortover the lamp, after having engaged its neck under water--you see itbegins to boil; in a few minutes bubbles will appear, which take fireand detonate as they issue from the water. 

CAROLINE. 

There is one--and another. How curious it is! --But I do not understandhow this is produced. 

MRS. B. 

It is the consequence of a display of affinities too complicated, I fear, to be made perfectly intelligible to you at present. 

In a few words, the reciprocal action of the potash, phosphorus, caloric, and water are such, that some of the water is decomposed, andthe hydrogen gas thereby formed carries off some minute particles ofphosphorus, with which it forms phosphoretted hydrogen gas, a compoundwhich spontaneously takes fire at almost any temperature. 

EMILY. 

What is that circular ring of smoke which slowly rises from each bubbleafter its detonation?

MRS. B. 

It consists of water and phosphoric acid in vapour, which are producedby the combustion of hydrogen and phosphorus. 

CONVERSATION IX. 

ON CARBON. 

CAROLINE. 

To-day, Mrs.  B. , I believe we are to learn the nature and properties ofCARBON. This substance is quite new to me; I never heard it mentionedbefore. 

MRS. B. 

Not so new as you imagine; for carbon is nothing more than charcoal in astate of purity, that is to say, unmixed with any foreign ingredients. 

CAROLINE. 

But charcoal is made by art, Mrs. B. , and a body consisting of onesimple substance cannot be fabricated?

MRS. B. 

You again confound the idea, of making a simple body, with that ofseparating it from a compound. The chemical processes by which a simplebody is obtained in a state of purity, consist in _unmaking_ thecompound in which it is contained, in order to separate from it thesimple substance in question. The method by which charcoal is usuallyobtained, is, indeed, commonly called _making_ it; but, uponexamination, you will find this process to consist simply in separatingit from other substances with which it is found combined in nature. 

Carbon forms a considerable part of the solid matter of all organisedbodies; but it is most abundant in the vegetable creation, and it ischiefly obtained from wood. When the oil and water (which are otherconstituents of vegetable matter) are evaporated, the black, porous, brittle substance that remains, is charcoal. 

CAROLINE. 

But if heat be applied to the wood in order to evaporate the oil andwater, will not the temperature of the charcoal be raised so as to makeit burn; and if it combines with oxygen, can we any longer call it pure?

MRS. B. 

I was going to say, that, in this operation, the air must be excluded. 

CAROLINE. 

How then can the vapour of the oil and water fly off?

MRS. B. 

In order to produce charcoal in its purest state (which is, even then, but a less imperfect sort of carbon), the operation should be performedin an earthen retort. Heat being applied to the body of the retort, theevaporable part of the wood will escape through its neck, into which noair can penetrate as long as the heated vapour continues to fill it. Andif it be wished to collect these volatile products of the wood, this caneasily be done by introducing the neck of the retort into the water-bathapparatus, with which you are acquainted. But the preparation of commoncharcoal, such as is used in kitchens and manufactures, is performed ona much larger scale, and by an easier and less expensive process. 

EMILY. 

I have seen the process of making common charcoal. The wood is ranged onthe ground in a pile of a pyramidical form, with a fire underneath; thewhole is then covered with clay, a few holes only being left for thecirculation of air. 

MRS. B. 

These holes are closed as soon as the wood is fairly lighted, so thatthe combustion is checked, or at least continues but in a very imperfectmanner; but the heat produced by it is sufficient to force out andvolatilize, through the earthy cover, most part of the oily and wateryprinciples of the wood, although it cannot reduce it to ashes. 

EMILY. 

Is pure carbon as black as charcoal?

MRS. B. 

The purest charcoal we can prepare is so; but chemists have never yetbeen able to separate it entirely from hydrogen. Sir H. Davy says, thatthe most perfect carbon that is prepared by art contains about five percent. Of hydrogen; he is of opinion, that if we could obtain it quitefree from foreign ingredients, it would be metallic, in common withother simple substances. 

But there is a form in which charcoal appears, that I dare say willsurprise you. --This ring, which I wear on my finger, owes itsbrilliancy to a small piece of carbon. 

CAROLINE. 

Surely, you are jesting, Mrs. B. ?

EMILY. 

I thought your ring was diamond?

MRS. B. 

It is so. But diamond is nothing more than carbon in a crystallizedstate. 

EMILY. 

That is astonishing! Is it possible to see two things apparently moredifferent than diamond and charcoal?

CAROLINE. 

It is, indeed, curious to think that we adorn ourselves with jewels ofcharcoal!

MRS. B. 

There are many other substances, consisting chiefly of carbon, that areremarkably white. Cotton, for instance, is almost wholly carbon. 

CAROLINE. 

That, I own, I could never have imagined! --But pray, Mrs.  B. , since itis known of what substance diamond and cotton are composed, why shouldthey not be manufactured, or imitated, by some chemical process, whichwould render them much cheaper, and more plentiful than the present modeof obtaining them?

MRS. B. 

You might as well, my dear, propose that we should make flowers andfruit, nay, perhaps even animals, by a chemical process; for it is knownof what these bodies consist, since every thing which we are acquaintedwith in nature is formed from the various simple substances that we haveenumerated. But you must not suppose that a knowledge of the componentparts of a body will in every case enable us to imitate it. It is muchless difficult to decompose bodies, and discover of what materials theyare made, than it is to recompose them. The first of these processes iscalled _analysis_, the last _synthesis_. When we are able to ascertainthe nature of a substance by both these methods, so that the result ofone confirms that of the other, we obtain the most complete knowledge ofit that we are capable of acquiring. This is the case with water, withthe atmosphere, with most of the oxyds, acids, and neutral salts, andwith many other compounds. But the more complicated combinations ofnature, even in the mineral kingdom, are in general beyond our reach, and any attempt to imitate organised bodies must ever prove fruitless;their formation is a secret that rests in the bosom of the Creator. Yousee, therefore, how vain it would be to attempt to make cotton bychemical means. But, surely, we have no reason to regret our inabilityin this instance, when nature has so clearly pointed out a method ofobtaining it in perfection and abundance. 

CAROLINE. 

I did not imagine that the principle of life could be imitated by theaid of chemistry; but it did not appear to me ridiculous to suppose thatchemists might attain a perfect imitation of inanimate nature. 

MRS. B. 

They have succeeded in this point in a variety of instances; but, as youjustly observe, the principle of life, or even the minute and intimateorganisation of the vegetable kingdom, are secrets that have almostentirely eluded the researches of philosophers; nor do I imagine thathuman art will ever be capable of investigating them with completesuccess. 

EMILY. 

But diamond, since it consists of one simple unorganised substance, might be, one would think, perfectly imitable by art?

MRS. B. 

It is sometimes as much beyond our power to obtain a simple body in astate of perfect purity, as it is to imitate a complicated combination;for the operations by which nature separates bodies are frequently asinimitable as those which she uses for their combination. This is thecase with carbon; all the efforts of chemists to separate it entirelyfrom other substances have been fruitless, and in the purest state inwhich it can be obtained by art, it still retains a portion of hydrogen, and probably of some other foreign ingredients. We are ignorant of themeans which nature employs to crystallize it. It may probably be thework of ages, to purify, arrange, and unite the particles of carbon inthe form of diamond. Here is some charcoal in the purest state we canprocure it: you see that it is a very black, brittle, light, poroussubstance, entirely destitute of either taste or smell. Heat, withoutair, produces no alteration in it, as it is not volatile; but, on thecontrary, it invariably remains at the bottom of the vessel after allthe other parts of the vegetable are evaporated. 

EMILY. 

Yet carbon is, no doubt, combustible, since you say that charcoal wouldabsorb oxygen if air were admitted during its preparation?

CAROLINE. 

Unquestionably. Besides, you know, Emily, how much it is used incooking. But pray what is the reason that charcoal burns without smoke, whilst a wood fire smokes so much?

MRS. B. 

Because, in the conversion of wood into charcoal, the volatile particlesof the former have been evaporated. 

CAROLINE. 

Yet I have frequently seen charcoal burn with flame; therefore it must, in that case, contain some hydrogen. 

MRS. B. 

Very true; but you must recollect that charcoal, especially that whichis used for common purposes, is not perfectly pure. It generally retainssome remains of the various other component parts of vegetables, andhydrogen particularly, which accounts for the flame in question. 

CAROLINE. 

But what becomes of the carbon itself during its combustion?

MRS. B. 

It gradually combines with the oxygen of the atmosphere, in the same wayas sulphur and phosphorus, and, like those substances, it is convertedinto a peculiar acid, which flies off in a gaseous form. There is thisdifference, however, that the acid is not, in this instance, as in thetwo cases just mentioned, a mere condensable vapour, but a permanentelastic fluid, which always remains in the state of gas, under anypressure and at any temperature. The nature of this acid was firstascertained by Dr. Black, of Edinburgh; and, before the introduction ofthe new nomenclature, it was called _fixed air_. It is now distinguishedby the more appropriate name of _carbonic acid gas_. 

EMILY. 

Carbon, then, can be volatilized by burning, though, by heat alone, nosuch effect is produced?

MRS. B. 

Yes; but then it is no longer simple carbon, but an acid of which carbonforms the basis. In this state, carbon retains no more appearance ofsolidity or corporeal form, than the basis of any other gas. And youmay, I think, from this instance, derive a more clear idea of the basisof the oxygen, hydrogen, and nitrogen gases, the existence of which, asreal bodies, you seemed to doubt, because they were not to be obtainedsimply in a solid form. 

EMILY. 

That is true; we may conceive the basis of the oxygen, and of the othergases, to be solid, heavy substances, like carbon; but so much expandedby caloric as to become invisible. 

CAROLINE. 

But does not the carbonic acid gas partake of the blackness of charcoal?

MRS. B. 

Not in the least. Blackness, you know, does not appear to be essentialto carbon, and it is pure carbon, and not charcoal, that we mustconsider as the basis of carbonic acid. We shall make some carbonicacid, and, in order to hasten the process, we shall burn the carbon inoxygen gas. 

EMILY. 

But do you mean then to burn diamond?

MRS. B. 

Charcoal will answer the purpose still better, being softer and moreeasy to inflame; besides the experiments on diamond are ratherexpensive. 

CAROLINE. 

But is it possible to burn diamond?

MRS. B. 

Yes, it is; and in order to effect this combustion, nothing more isrequired than to apply a sufficient degree of heat by means of theblow-pipe, and of a stream of oxygen gas. Indeed it is by burningdiamond that its chemical nature has been ascertained. It has long beenknown as a combustible substance, but it is within these few years onlythat the product of its combustion has been proved to be pure carbonicacid. This remarkable discovery is due to Mr. Tennant. 

Now let us try to make some carbonic acid. --Will you, Emily, decantsome oxygen gas from this large jar into the receiver in which we are toburn the carbon; and I shall introduce this small piece of charcoal, with a little lighted tinder, which will be necessary to give the firstimpulse to the combustion. 

EMILY. 

I cannot conceive how so small a piece of tinder, and that but justlighted, can raise the temperature of the carbon sufficiently to setfire to it; for it can produce scarcely any sensible heat, and it hardlytouches the carbon. 

MRS. B. 

The tinder thus kindled has only heat enough to begin its owncombustion, which, however, soon becomes so rapid in the oxygen gas, asto raise the temperature of the charcoal sufficiently for this to burnlikewise, as you see is now the case. 

EMILY. 

I am surprised that the combustion of carbon is not more brilliant; itdoes not give out near so much light or caloric as phosphorus, orsulphur. Yet since it combines with so much oxygen, why is not aproportional quantity of light and heat disengaged from thedecomposition of the oxygen gas, and the union of its electricity withthat of the charcoal?

MRS. B. 

It is not surprising that less light and heat should be liberated inthis than in almost any other combustion, since the oxygen, instead ofentering into a solid or liquid combination, as it does in thephosphoric and sulphuric acids, is employed in forming another elasticfluid; it therefore parts with less of its caloric. 

EMILY. 

True; and, on second consideration, it appears, on the contrary, surprising that the oxygen should, in its combination with carbon, retain a sufficient portion of caloric to maintain both substances in agaseous state. 

CAROLINE. 

We may then judge of the degree of solidity in which oxygen is combinedin a burnt body, by the quantity of caloric liberated during itscombustion?

MRS. B. 

Yes; provided that you take into the account the quantity of oxygenabsorbed by the combustible body, and observe the proportion which thecaloric bears to it. 

CAROLINE. 

But why should the water, after the combustion of carbon, rise in thereceiver, since the gas within it retains an aëriform state?

MRS. B. 

Because the carbonic acid gas is gradually absorbed by the water; andthis effect would be promoted by shaking the receiver. 

EMILY. 

The charcoal is now extinguished, though it is not nearly consumed; ithas such an extraordinary avidity for oxygen, I suppose, that thereceiver did not contain enough to satisfy the whole. 

MRS. B. 

That is certainly the case; for if the combustion were performed in theexact proportions of 28 parts of carbon to 72 of oxygen, both theseingredients would disappear, and 100 parts of carbonic acid would beproduced. 

CAROLINE. 

Carbonic acid must be a very strong acid, since it contains so great aproportion of oxygen?

MRS. B. 

That is a very natural inference; yet it is erroneous. For the carbonicis the weakest of all the acids. The strength of an acid seems to dependupon the nature of its basis, and its mode of combination, as well asupon the proportion of the acidifying principle. The same quantity ofoxygen that will convert some bodies into strong acids, will only besufficient simply to oxydate others. 

CAROLINE. 

Since this acid is so weak, I think chemists should have called it the_carbonous_, instead of the _carbonic_ acid. 

EMILY. 

But, I suppose, the carbonous acid is still weaker, and is formed byburning carbon in atmospherical air. 

MRS. B. 

It has been lately discovered, that carbon may be converted into a gas, by uniting with a smaller proportion of oxygen; but as this gas does notpossess any acid properties, it is no more than an oxyd; it is called_gaseous oxyd of carbon_. 

CAROLINE. 

Pray is not carbonic acid a very wholesome gas to breathe, as itcontains so much oxygen?

MRS. B. 

On the contrary, it is extremely pernicious. Oxygen, when in a state ofcombination with other substances, loses, in almost every instance, itsrespirable properties, and the salubrious effects which it has on theanimal economy when in its unconfined state. Carbonic acid is not onlyunfit for respiration, but extremely deleterious if taken into thelungs. 

EMILY. 

You know, Caroline, how very unwholesome the fumes of burning charcoalare reckoned. 

CAROLINE. 

Yes; but, to confess the truth, I did not consider that a charcoal fireproduced carbonic acid gas. --Can this gas be condensed into a liquid?

MRS. B. 

No: for, as I told you before, it is a permanent elastic fluid. Butwater can absorb a certain quantity of this gas, and can even beimpregnated with it, in a very strong degree, by the assistance ofagitation and pressure, as I am going to show you. I shall decant somecarbonic acid gas into this bottle, which I fill first with water, inorder to exclude the atmospherical air; the gas is then introducedthrough the water, which you see it displaces, for it will not mix withit in any quantity, unless strongly agitated, or allowed to stand overit for some time. The bottle is now about half full of carbonic acidgas, and the other half is still occupied by the water. By corking thebottle, and then violently shaking it, in this way, I can mix the gasand water together. --Now will you taste it?

EMILY. 

It has a distinct acid taste. 

CAROLINE. 

Yes, it is sensibly sour, and appears full of little bubbles. 

MRS. B. 

It possesses likewise all the other properties of acids, but, of course, in a less degree than the pure carbonic acid gas, as it is so muchdiluted by water. 

This is a kind of artificial Seltzer water. By analysing that which isproduced by nature, it was found to contain scarcely any thing more thancommon water impregnated with a certain proportion of carbonic acid gas. We are, therefore, able to imitate it, by mixing those proportions ofwater and carbonic acid. Here, my dear, is an instance, in which, by achemical process, we can exactly copy the operations of nature; for theartificial Seltzer waters can be made in every respect similar to thoseof nature; in one point, indeed, the former have an advantage, sincethey may be prepared stronger, or weaker, as occasion requires. 

CAROLINE. 

I thought I had tasted such water before. But what renders it so briskand sparkling?

MRS. B. 

This sparkling, or effervescence, as it is called, is always occasionedby the action of an elastic fluid escaping from a liquid; in theartifical Seltzer water, it is produced by the carbonic acid, whichbeing lighter than the water in which it was strongly condensed, fliesoff with great rapidity the instant the bottle is uncorked; this makesit necessary to drink it immediately. The bubbling that took place inthis bottle was but trifling, as the water was but very slightlyimpregnated with carbonic acid. It requires a particular apparatus toprepare the gaseous artificial mineral waters. 

EMILY. 

If, then, a bottle of Seltzer water remains for any length of timeuncorked, I suppose it returns to the state of common water?

MRS. B. 

The whole of the carbonic acid gas, or very nearly so, will soondisappear; but there is likewise in Seltzer water a very small quantityof soda, and of a few other saline or earthy ingredients, which willremain in the water, though it should be kept uncorked for any length oftime. 

CAROLINE. 

I have often heard of people drinking soda-water. Pray what sort ofwater is that?

MRS. B. 

It is a kind of artificial Seltzer water, holding in solution, besidesthe gaseous acid, a particular saline substance, called soda, whichimparts to the water certain medicinal qualities. 

CAROLINE. 

But how can these waters be so wholesome, since carbonic acid is sopernicious?

MRS. B. 

A gas, we may conceive, though very prejudicial to breathe, may bebeneficial to the stomach. --But it would be of no use to attemptexplaining this more fully at present. 

CAROLINE. 

Are waters never impregnated with other gases?

MRS. B. 

Yes; there are several kinds of gaseous waters. I forgot to tell youthat waters have, for some years past, been prepared, impregnated bothwith oxygen and hydrogen gases. These are not an imitation of nature, but are altogether obtained by artificial means. They have been latelyused medicinally, particularly on the continent, where, I understand, they have acquired some reputation. 

EMILY. 

If I recollect right, Mrs. B. , you told us that carbon was capable ofdecomposing water; the affinity between oxygen and carbon must, therefore, be greater than between oxygen and hydrogen?

MRS. B. 

Yes; but this is not the case unless their temperature be raised to acertain degree. It is only when carbon is red-hot, that it is capable ofseparating the oxygen from the hydrogen. Thus, if a small quantity ofwater be thrown on a red-hot fire, it will increase rather thanextinguish the combustion; for the coals or wood (both of which containa quantity of carbon) decompose the water, and thus supply the fire bothwith oxygen and hydrogen gases. If, on the contrary, a large mass ofwater be thrown over the fire, the diminution of heat thus produced issuch, that the combustible matter loses the power of decomposing thewater, and the fire is extinguished. 

EMILY. 

I have heard that fire-engines sometimes do more harm than good, andthat they actually increase the fire when they cannot throw water enoughto extinguish it. It must be owing, no doubt, to the decomposition ofthe water by the carbon during the conflagration. 

MRS. B. 

Certainly. --The apparatus which you see here (PLATE XI. Fig. 3. ), maybe used to exemplify what we have just said. It consists in a kind ofopen furnace, through which a porcelain tube, containing charcoal, passes. To one end of the tube is adapted a glass retort with water init; and the other end communicates with a receiver placed on thewater-bath. A lamp being applied to the retort, and the water made toboil, the vapour is gradually conveyed through the red-hot charcoal, bywhich it is decomposed; and the hydrogen gas which results from thisdecomposition is collected in the receiver. But the hydrogen thusobtained is far from being pure; it retains in solution a minute portionof carbon, and contains also a quantity of carbonic acid. This rendersit heavier than pure hydrogen gas, and gives it some peculiarproperties; it is distinguished by the name of _carbonated hydrogengas_. 

CAROLINE. 

And whence does it obtain the carbonic acid that is mixed with it?

EMILY. 

I believe I can answer that question, Caroline. --From the union of theoxygen (proceeding from the decomposed water) with the carbon, which, you know, makes carbonic acid. 

CAROLINE. 

True; I should have recollected that. --The product of the decompositionof water by red-hot charcoal, therefore, is carbonated hydrogen gas, andcarbonic acid gas. 

MRS. B. 

You are perfectly right now. 

Carbon is frequently found combined with hydrogen in a state ofsolidity, especially in coals, which owe their combustible nature tothese two principles. 

EMILY. 

Is it the hydrogen, then, that produces the flame of coals?

MRS. B. 

It is so; and when all the hydrogen is consumed, the carbon continues toburn without flame. But again, as I mentioned when speaking of thegas-lights, the hydrogen gas produced by the burning of coals is notpure; for, during the combustion, particles of carbon are successivelyvolatilized with the hydrogen, with which they form what is called a_hydro-carbonat_, which is the principal product of this combustion. 

Carbon is a very bad conductor of heat; for this reason, it is employed(in conjunction with other ingredients) for coating furnaces and otherchemical apparatus. 

EMILY. 

Pray what is the use of coating furnaces?

MRS. B. 

In most cases, in which a furnace is used, it is necessary to produceand preserve a great degree of heat, for which purpose every possiblemeans are used to prevent the heat from escaping by communicating withother bodies, and this object is attained by coating over the inside ofthe furnace with a kind of plaster, composed of materials that are badconductors of heat. 

Carbon, combined with a small quantity of iron, forms a compound calledplumbago, or black-lead, of which pencils are made. This substance, agreeably to the nomenclature, is _a carburet of iron_. 

EMILY. 

Why, then, is it called black-lead?

MRS. B. 

It is an ancient name given to it by ignorant people, from its shiningmetallic appearance; but it is certainly a most improper name for it, asthere is not a particle of lead in the composition. There is only onemine of this mineral, which is in Cumberland. It is supposed to approachas nearly to pure carbon as the best prepared charcoal does, as itcontains only five parts of iron, unadulterated by any other foreigningredients. There is another carburet of iron, in which the iron, though united only to an extremely small proportion of carbon, acquiresvery remarkable properties; this is steel. 

CAROLINE. 

Really; and yet steel is much harder than iron?

MRS. B. 

But carbon is not ductile like iron, and therefore may render the steelmore brittle, and prevent its bending so easily. Whether it is that thecarbon, by introducing itself into the pores of the iron, and, byfilling them, makes the metal both harder and heavier; or whether thischange depends upon some chemical cause, I cannot pretend to decide. Butthere is a subsequent operation, by which the hardness of steel is verymuch increased, which simply consists in heating the steel till it isred-hot, and then plunging it into cold water. 

Carbon, besides the combination just mentioned, enters into thecomposition of a vast number of natural productions, such, for instance, as all the various kinds of oils, which result from the combination ofcarbon, hydrogen, and caloric, in various proportions. 

EMILY. 

I thought that carbon, hydrogen, and caloric, formed carbonated hydrogengas?

MRS. B. 

That is the case when a small portion of carbonic acid gas is held insolution by hydrogen gas. Different proportions of the same principles, together with the circumstances of their union, produce very differentcombinations; of this you will see innumerable examples. Besides, we arenot now talking of gases, but of carbon and hydrogen, combined only witha quantity of caloric sufficient to bring them to the consistency of oilor fat. 

CAROLINE. 

But oil and fat are not of the same consistence?

MRS. B. 

Fat is only congealed oil; or oil, melted fat. The one requires a littlemore heat to maintain it in a fluid state than the other. Have you neverobserved the fat of meat turned to oil by the caloric it has imbibedfrom the fire?

EMILY. 

Yet oils in general, as salad-oil, and lamp-oil, do not turn to fat whencold?

MRS. B. 

Not at the common temperature of the atmosphere, because they retain toomuch caloric to congeal at that temperature; but if exposed to asufficient degree of cold, their latent heat is extricated, and theybecome solid fat substances. Have you never seen salad oil frozen inwinter?

EMILY. 

Yes; but it appears to me in that state very different from animal fat. 

MRS. B. 

The essential constituent parts of either vegetable or animal oils arethe same, carbon and hydrogen; their variety arises from the differentproportions of these substances, and from other accessory ingredientsthat may be mixed with them. The oil of a whale, and the oil of roses, are, in their essential constituent parts, the same; but the one isimpregnated with the offensive particles of animal matter, the otherwith the delicate perfume of a flower. 

The difference of _fixed oils_, and _volatile_ or _essential oils_, consists also in the various proportions of carbon and hydrogen. Fixedoils are those which will not evaporate without being decomposed; thisis the case with all common oils, which contain a greater proportion ofcarbon than the essential oils. The essential oils (which comprehend thewhole class of essences and perfumes) are lighter; they contain moreequal proportions of carbon and hydrogen, and are volatilized orevaporated without being decomposed. 

EMILY. 

When you say that one kind of oil will evaporate, and the other bedecomposed, you mean, I suppose, by the application of heat?

MRS. B. 

Not necessarily; for there are oils that will evaporate slowly at thecommon temperature of the atmosphere; but for a more rapidvolatilization, or for their decomposition, the assistance of heat isrequired. 

CAROLINE. 

I shall now remember, I think, that fat and oil are really the samesubstances, both consisting of carbon and hydrogen; that in fixed oilsthe carbon preponderates, and heat produces a decomposition; while, inessential oils, the proportion of hydrogen is greater, and heat producesa volatilization only. 

EMILY. 

I suppose the reason why oil burns so well in lamps is because its twoconstituents are so combustible?

MRS. B. 

Certainly; the combustion of oil is just the same as that of a candle;if tallow, it is only oil in a concrete state; if wax, or spermaceti, its chief chemical ingredients are still hydrogen and carbon. 

EMILY. 

I wonder, then, there should be so great a difference between tallow andwax?

MRS. B. 

I must again repeat, that the same substances, in different proportions, produce results that have sometimes scarcely any resemblance to eachother. But this is rather a general remark that I wish to impress uponyour minds, than one which is applicable to the present case; for tallowand wax are far from being very dissimilar; the chief differenceconsists in the wax being a purer compound of carbon and hydrogen thanthe tallow, which retains more of the gross particles of animal matter. The combustion of a candle, and that of a lamp, both produce water andcarbonic acid gas. Can you tell me how these are formed?

EMILY. 

Let me reflect . . . . Both the candle and lamp burn by means of fixedoil--this is decomposed as the combustion goes on; and the constituentparts of the oil being thus separated, the carbon unites to a portion ofoxygen from the atmosphere to form carbonic acid gas, whilst thehydrogen combines with another portion of oxygen, and forms with itwater. --The products, therefore, of the combustion of oils are waterand carbonic acid gas. 

CAROLINE. 

But we see neither water nor carbonic acid produced by the combustion ofa candle. 

MRS. B. 

The carbonic acid gas, you know, is invisible, and the water being in astate of vapour, is so likewise. Emily is perfectly correct in herexplanation, and I am very much pleased with it. 

All the vegetable acids consist of various proportions of carbon andhydrogen, acidified by oxygen. Gums, sugar, and starch, are likewisecomposed of these ingredients; but, as the oxygen which they contain isnot sufficient to convert them into acids, they are classed with theoxyds, and called vegetable oxyds. 

CAROLINE. 

I am very much delighted with all these new ideas; but, at the sametime, I cannot help being apprehensive that I may forget many of them. 

MRS. B. 

I would advise you to take notes, or, what would answer better still, towrite down, after every lesson, as much of it as you can recollect. And, in order to give you a little assistance, I shall lend you the heads orindex, which I occasionally consult for the sake of preserving somemethod and arrangement in these conversations. Unless you follow somesuch plan, you cannot expect to retain nearly all that you learn, howgreat soever be the impression it may make on you at first. 

EMILY. 

I will certainly follow your advice. --Hitherto I have found that Irecollected pretty well what you have taught us; but the history ofcarbon is a more extensive subject than any of the simple bodies we haveyet examined. 

MRS. B. 

I have little more to say on carbon at present; but hereafter you willsee that it performs a considerable part in most chemical operations. 

CAROLINE. 

That is, I suppose, owing to its entering into the composition of sogreat a variety of substances?

MRS. B. 

Certainly; it is the basis, you have seen, of all vegetable matter; andyou will find that it is very essential to the process of animalization. But in the mineral kingdom also, particularly in its form of carbonicacid, we shall often discover it combined with a great variety ofsubstances. 

In chemical operations, carbon is particularly useful, from its verygreat attraction for oxygen, as it will absorb this substance from manyoxygenated or burnt bodies, and thus deoxygenate, or _unburn_ them, andrestore them to their original combustible state. 

CAROLINE. 

I do not understand how a body can be _unburnt_, and restored to itsoriginal state. This piece of tinder, for instance, that has been burnt, if by any means the oxygen were extracted from it, would not be restoredto its former state of linen; for its texture is destroyed by burning, and that must be the case with all organized or manufactured substances, as you observed in a former conversation. 

MRS. B. 

A compound body is decomposed by combustion in a way which generallyprecludes the possibility of restoring it to its former state; theoxygen, for instance, does not become fixed in the tinder, but itcombines with its volatile parts, and flies off in the shape of gas, orwatery vapour. You see, therefore, how vain it would be to attempt therecomposition of such bodies. But, with regard to simple bodies, or atleast bodies whose component parts are not disturbed by the process ofoxygenation or deoxygenation, it is often possible to restore them, after combustion, to their original state. --The metals, for instance, undergo no other alteration by combustion than a combination withoxygen; therefore, when the oxygen is taken from them, they return totheir pure metallic state. But I shall say nothing further of this atpresent, as the metals will furnish ample subject for another morning;and they are the class of simple bodies that come next underconsideration. 

CONVERSATION X. 

ON METALS. 

MRS. B. 

The METALS, which we are now to examine, are bodies of a very differentnature from those which we have hitherto considered. They do not, likethe bases of gases, elude the immediate observation of our senses; forthey are the most brilliant, the most ponderous, and the most palpablesubstances in nature. 

CAROLINE. 

I doubt, however, whether the metals will appear to us so interesting, and give us so much entertainment as those mysterious elements whichconceal themselves from our view. Besides, they cannot afford so muchnovelty; they are bodies with which we are already so well acquainted. 

MRS. B. 

You are not aware, my dear, of the interesting discoveries which were afew years ago made by Sir H. Davy respecting this class of bodies. Bythe aid of the Voltaic battery, he has obtained from a variety ofsubstances, metals before unknown, the properties of which are equallynew and curious. We shall begin, however, by noticing those metals withwhich you profess to be so well acquainted. But the acquaintance, youwill soon perceive, is but very superficial; and I trust that you willfind both novelty and entertainment in considering the metals in achemical point of view. To treat of this subject fully, would require awhole course of lectures; for metals form of themselves a most importantbranch of practical chemistry. We must, therefore, confine ourselves toa general view of them. These bodies are seldom found naturally in theirmetallic form: they are generally more or less oxygenated or combinedwith sulphur, earths, or acids, and are often blended with each other. They are found buried in the bowels of the earth in most parts of theworld, but chiefly in mountainous districts, where the surface of theglobe has suffered from the earthquakes, volcanos, and other convulsionsof nature. They are spread in strata or beds, called veins, and theseveins are composed of a certain quantity of metal, combined with variousearthy substances, with which they form minerals of different nature andappearance, which are called _ores_. 

CAROLINE. 

I now feel quite at home, for my father has a lead-mine in Yorkshire, and I have heard a great deal about veins of ore, and of the _roasting_and _smelting_ of the lead; but, I confess, that I do not understand inwhat these operations consist. 

MRS. B. 

Roasting is the process by which the volatile parts of the ore areevaporated; smelting, that by which the pure metal is afterwardsseparated from the earthy remains of the ore. This is done by throwingthe whole into a furnace, and mixing with it certain substances thatwill combine with the earthy parts and other foreign ingredients of theore; the metal being the heaviest, falls to the bottom, and runs out byproper openings in its pure metallic state. 

EMILY. 

You told us in a preceding lesson that metals had a great affinity foroxygen. Do they not, therefore, combine with oxygen, when stronglyheated in the furnace, and run out in the state of oxyds?

MRS. B. 

No; for the scoriæ, or oxyd, which soon forms on the surface of thefused metal, when it is oxydable, prevents the air from having anyfurther influence on the mass; so that neither combustion noroxygenation can take place. 

CAROLINE. 

Are all the metals equally combustible?

MRS. B. 

No; their attraction for oxygen varies extremely. There are some thatwill combine with it only at a very high temperature, or by theassistance of acids; whilst there are others that oxydate spontaneouslyand with great rapidity, even at the lowest temperature; such is inparticular manganese, which scarcely ever exists in the metallic state, as it immediately absorbs oxygen on being exposed to the air, andcrumbles to an oxyd in the course of a few hours. 

EMILY. 

Is not that the oxyd from which you extracted the oxygen gas?

MRS. B. 

It is: so that, you see, this metal attracts oxygen at a lowtemperature, and parts with it when strongly heated. 

EMILY. 

Is there any other metal that oxydates at the temperature of theatmosphere?

MRS. B. 

They all do, more or less, excepting gold, silver, and platina. 

Copper, lead, and iron, oxydate slowly in the air, and cover themselveswith a sort of rust, a process which depends on the gradual conversionof the surface into an oxyd. This rusty surface preserves the interiormetal from oxydation, as it prevents the air from coming in contact withit. Strictly speaking, however, the word rust applies only to the oxyd, which forms on the surface of iron, when exposed to air and moisture, which oxyd appears to be united with a small portion of carbonic acid. 

EMILY. 

When metals oxydate from the atmosphere without an elevation oftemperature, some light and heat, I suppose, must be disengaged, thoughnot in sufficient quantities to be sensible. 

MRS. B. 

Undoubtedly; and, indeed, it is not surprising that in this case thelight and heat should not be sensible, when you consider how extremelyslow, and, indeed, how imperfectly, most metals oxydate by mere exposureto the atmosphere. For the quantity of oxygen with which metals arecapable of combining, generally depends upon their temperature; and theabsorption stops at various points of oxydation, according to the degreeto which their temperature is raised. 

EMILY. 

That seems very natural; for the greater the quantity of caloricintroduced into a metal, the more will its positive electricity beexalted, and consequently the stronger will be its affinity for oxygen. 

MRS. B. 

Certainly. When the metal oxygenates with sufficient rapidity for lightand heat to become sensible, combustion actually takes place. But thishappens only at very high temperatures, and the product is neverthelessan oxyd; for though, as I have just said, metals will combine withdifferent proportions of oxygen, yet with the exception of only five ofthem, they are not susceptible of acidification. 

Metals change colour during the different degrees of oxydation whichthey undergo. Lead, when heated in contact with the atmosphere, firstbecomes grey; if its temperature be then raised, it turns yellow, and astill stronger heat changes it to red. Iron becomes successively agreen, brown, and white oxyd. Copper changes from brown to blue, andlastly green. 

EMILY. 

Pray, is the white lead with which houses are painted prepared byoxydating lead?

MRS. B. 

Not merely by oxydating, but by being also united with carbonic acid. Itis a carbonat of lead. The mere oxyd of lead is called red lead. Litharge is another oxyd of lead, containing less oxygen. Almost all themetallic oxyds are used as paints. The various sorts of ochres consistchiefly of iron more or less oxydated. And it is a remarkablecircumstance, that if you burn metals rapidly, the light or flame theyemit during combustion partakes of the colours which the oxydsuccessively assumes. 

CAROLINE. 

How is that accounted for, Mrs. B. ? For light, you know, does notproceed from the burning body, but from the decomposition of the oxygengas?

MRS. B. 

The correspondence of the colour of the light with that of the oxydwhich emits it, is, in all probability, owing to some particles of themetal which are volatilised and carried off by the caloric. 

CAROLINE. 

It is then a sort of metallic gas. 

EMILY. 

Why is it reckoned so unwholesome to breathe the air of a place in whichmetals are melting?

MRS. B. 

Perhaps the notion is too generally entertained. But it is true withrespect to lead, and some other noxious metals, because, unless care betaken, the particles of the oxyd which are volatilised by the heat areinhaled in with the breath, and may produce dangerous effects. 

I must show you some instances of the combustion of metals; it wouldrequire the heat of a furnace to make them burn in the common air, butif we supply them with a stream of oxygen gas, we may easilyaccomplish it. 

CAROLINE. 

But it will still, I suppose, be necessary in some degree to raise theirtemperature?

MRS. B. 

This, as you shall see, is very easily done, particularly if theexperiment be tried upon a small scale. --I begin by lighting this pieceof charcoal with the candle, and then increase the rapidity of itscombustion by blowing upon it with a blow-pipe. (PLATE XII. Fig.  1. )

 [Illustration: Plate XII. Apparatus for the combustion of metals by means of oxygen gas. 

 Fig. 1. Igniting charcoal with a taper & blow-pipe. 

 Fig. 2. Combustion of metals by means of a blow-pipe conveying a stream of oxygen gas from a gas holder. ]

EMILY. 

That I do not understand; for it is not every kind of air, but merelyoxygen gas, that produces combustion. Now you said that in breathing weinspired, but did not expire oxygen gas. Why, therefore, should the airwhich you breathe through the blow-pipe promote the combustion of thecharcoal?

MRS. B. 

Because the air, which has but once passed through the lungs, is yet butlittle altered, a small portion only of its oxygen being destroyed; sothat a great deal more is gained by increasing the rapidity of thecurrent, by means of the blow-pipe, than is lost in consequence of theair passing once through the lungs, as you shall see--

EMILY. 

Yes, indeed, it makes the charcoal burn much brighter. 

MRS. B. 

Whilst it is red-hot, I shall drop some iron filings on it, and supplythem with a current of oxygen gas, by means of this apparatus, (PLATEXII. Fig 2. ) which consists simply of a closed tin cylindrical vessel, full of oxygen gas, with two apertures and stop-cocks, by one of which astream of water is thrown into the vessel through a long funnel, whilstby the other the gas is forced out through a blow-pipe adapted to it, asthe water gains admittance. --Now that I pour water into the funnel, youmay hear the gas issuing from the blow-pipe--I bring the charcoal closeto the current, and drop the filings upon it--

CAROLINE. 

They emit much the same vivid light as the combustion of the iron wirein oxygen gas. 

MRS. B. 

The process is, in fact, the same; there is only some difference in themode of conducting it. Let us burn some tin in the same manner--you seethat it is equally combustible. --Let us now try some copper--

CAROLINE. 

This burns with a greenish flame; it is, I suppose, owing to the colourof the oxyd?

EMILY. 

Pray, shall we not also burn some gold?

MRS. B. 

That is not in our power, at least in this way. Gold, silver, andplatina, are incapable of being oxydated by the greatest heat that wecan produce by the common method. It is from this circumstance, thatthey have been called perfect metals. Even these, however, have anaffinity for oxygen; but their oxydation or combustion can be performedonly by means of acids or by electricity. The spark given out by theVoltaic battery produces at the point of contact a greater degree ofheat than any other process; and it is at this very high temperatureonly that the affinity of these metals for oxygen will enable them toact on each other. 

I am sorry that I cannot show you the combustion of the perfect metalsby this process, but it requires a considerable Voltaic battery. Youwill see these experiments performed in the most perfect manner, whenyou attend the chemical lectures of the Royal Institution. But in themean time I can, without difficulty, show you an ingenious apparatuslately contrived for the purpose of producing intense heats, the powerof which nearly equals that of the largest Voltaic batteries. It simplyconsists, you see, in a strong box, made of iron or copper, (PLATE X. Fig. 2. ) to which may be adapted this air-syringe or condensing-pump, and a stop-cock terminating in a small orifice similar to that of ablow-pipe. By working the condensing syringe, up and down in thismanner, a quantity of air is accumulated in the vessel, which may beincreased to almost any extent; so that if we now turn the stop-cock, the condensed air will rush out, forming a jet of considerable force;and if we place the flame of a lamp in the current, you will see howviolently the flame is driven in that direction. 

CAROLINE. 

It seems to be exactly the same effect as that of a blow-pipe worked bythe mouth, only much stronger. 

EMILY. 

Yes; and this new instrument has this additional advantage, that it doesnot fatigue the mouth and lungs like the common blow-pipe, and requiresno art in blowing. 

MRS. B. 

Unquestionably; but yet this blow-pipe would be of very limited utility, if its energy and power could not be greatly increased by some othercontrivance. Can you imagine any mode of producing such an effect?

EMILY. 

Could not the reservoir be charged with pure oxygen, instead of commonair, as in the case of the gas-holder?

MRS. B. 

Undoubtedly; and this is precisely the contrivance I allude to. Thevessel need only be supplied with air from a bladder full of oxygen, instead of the air of the room, and this, you see, may be easily done byscrewing the bladder on the upper part of the syringe, so that inworking the syringe the oxygen gas is forced from the bladder into thecondensing vessel. 

CAROLINE. 

With the aid of this small apparatus, therefore, we could obtain thesame effects as those we have just produced with the gas-holder, bymeans of a column of water forcing the gas out of it?

MRS. B. 

Yes; and much more conveniently so. But there is a mode of using thisapparatus by which more powerful effects still may be obtained. Itconsists in condensing in the reservoir, not oxygen alone, but a mixtureof oxygen and hydrogen in the exact proportion in which they unite toproduce water; and then kindling the jet formed by the mixed gases. Theheat disengaged by this combustion, without the help of any lamp, isprobably the most intense known; and various effects are said to havebeen obtained from it which exceed all expectation. 

CAROLINE. 

But why should we not try this experiment?

MRS. B. 

Because it is not exempt from danger; the combustion (notwithstandingvarious contrivances which have been resorted to with a view to preventaccident) being apt to penetrate into the inside of the vessel, and toproduce a dangerous and violent explosion. --We shall, therefore, nowproceed in our subject. 

CAROLINE. 

I think you said the oxyds of metals could be restored to their metallicstate?

MRS. B. 

Yes; this is called _reviving_ a metal. Metals are in general capable ofbeing revived by charcoal, when heated red hot, charcoal having agreater attraction for oxygen than the metals. You need only, therefore, decompose, or unburn the oxyd, by depriving it of its oxygen, and themetal will be restored to its pure state. 

EMILY. 

But will the carbon, by this operation, be burnt, and be converted intocarbonic acid?

MRS. B. 

Certainly. There are other combustible substances to which metals at ahigh temperature will part with their oxygen. They will also yield it toeach other, according to their several degrees of attraction for it; andif the oxygen goes into a more dense state in the metal which it enters, than it existed in that which it quits, a proportional disengagement ofcaloric will take place. 

CAROLINE. 

And cannot the oxyds of gold, silver, and platina, which are formed bymeans of acids or of the electric fluid, be restored to their metallicstate?

MRS. B. 

Yes, they may; and the intervention of a combustible body is notrequired; heat alone will take the oxygen from them, convert it into agas, and revive the metal. 

EMILY. 

You said that rust was an oxyd of iron; how is it, then, that water, ormerely dampness, produces it, which, you know, it very frequently doeson steel grates, or any iron instruments?

MRS. B. 

In that case the metal decomposes the water, or dampness (which isnothing but water in a state of vapour), and obtains the oxygen from it. 

CAROLINE. 

I thought that it was necessary to bring metals to a very hightemperature to enable them to decompose water. 

MRS. B. 

It is so, if it is required that the process should be performedrapidly, and if any considerable quantity is to be decomposed. Rust, youknew, is sometimes months in forming, and then it is only the surface ofthe metal that is oxydated. 

EMILY. 

Metals, then, that do not rust, are incapable of spontaneous oxydation, either by air or water?

MRS. B. 

Yes; and this is the case with the perfect metals, which, on thataccount, preserve their metallic lustre so well. 

EMILY. 

Are all metals capable of decomposing water, provided their temperaturebe sufficiently raised?

MRS. B. 

No; a certain degree of attraction is requisite, besides the assistanceof heat. Water, you recollect, is composed of oxygen and hydrogen; and, unless the affinity of the metal for oxygen be stronger than that ofhydrogen, it is in vain that we raise its temperature, for it cannottake the oxygen from the hydrogen. Iron, zinc, tin, and antimony, have astronger affinity for oxygen than hydrogen has, therefore these fourmetals are capable of decomposing water. But hydrogen having anadvantage over all the other metals with respect to its affinity foroxygen, it not only withholds its oxygen from them, but is even capable, under certain circumstances, of taking the oxygen from the oxyds ofthese metals. 

EMILY. 

I confess that I do not quite understand why hydrogen can take oxygenfrom those metals that do not decompose water. 

CAROLINE. 

Now I think I do perfectly. Lead, for instance, will not decomposewater, because it has not so strong an attraction for oxygen as hydrogenhas. Well, then, suppose the lead to be in a state of oxyd; hydrogenwill take the oxygen from the lead, and unite with it to form water, because hydrogen has a stronger attraction for oxygen, than oxygen hasfor lead; and it is the same with all the other metals which do notdecompose water. 

EMILY. 

I understand your explanation, Caroline, very well; and I imagine thatit is because lead cannot decompose water that it is so much employedfor pipes for conveying that fluid. 

MRS. B. 

Certainly; lead is, on that account, particularly appropriate to suchpurposes; whilst, on the contrary, this metal, if it was oxydable bywater, would impart to it very noxious qualities, as all oxyds of leadare more or less pernicious. 

But, with regard to the oxydation of metals, the most powerful mode ofeffecting it is by means of acids. These, you know, contain a muchgreater proportion of oxygen than either air or water; and will, most ofthem, easily yield it to metals. Thus, you recollect, the zinc plates ofthe Voltaic battery are oxydated by the acid and water, much moreeffectually than by water alone. 

CAROLINE. 

And I have often observed that if I drop vinegar, lemon, or any acid onthe blade of a knife, or on a pair of scissars, it will immediatelyproduce a spot of rust. 

EMILY. 

Metals have, then, three ways of obtaining oxygen; from the atmosphere, from water, and from acids. 

MRS. B. 

The two first you have already witnessed, and I shall now show you howmetals take the oxygen from an acid. This bottle contains nitric acid;I shall pour some of it over this piece of copper-leaf .  .  .  .  .  .  . 

CAROLINE. 

Oh, what a disagreeable smell!

EMILY. 

And what is it that produces the effervescency and that thick yellowvapour?

MRS. B. 

It is the acid, which being abandoned by the greatest part of itsoxygen, is converted into a weaker acid, which escapes in the form ofgas. 

CAROLINE. 

And whence proceeds this heat?

MRS. B. 

Indeed, Caroline, I think you might now be able to answer that questionyourself. 

CAROLINE. 

Perhaps it is that the oxygen enters into the metal in a more solidstate than it existed in the acid, in consequence of which caloric isdisengaged. 

MRS. B. 

If the combination of the oxygen and the metal results from the union oftheir opposite electricities, of course caloric must be given out. 

EMILY. 

The effervescence is over; therefore I suppose that the metal is nowoxydated. 

MRS. B. 

Yes. But there is another important connection between metals and acids, with which I must now make you acquainted. Metals, when in the state ofoxyds, are capable of being dissolved by acids. In this operation theyenter into a chemical combination with the acid, and form an entirelynew compound. 

CAROLINE. 

But what difference is there between the _oxydation_ and the_dissolution_ of the metal by an acid?

MRS. B. 

In the first case, the metal merely combines with a portion of oxygentaken from the acid, which is thus partly deoxygenated, as in theinstance you have just seen; in the second case, the metal, after beingpreviously oxydated, is actually dissolved in the acid, and enters intoa chemical combination with it, without producing any furtherdecomposition or effervescence. --This complete combination of an oxydand an acid forms a peculiar and important class of compound salts. 

EMILY. 

The difference between an oxyd and a compound salt, therefore, is veryobvious: the one consists of a metal and oxygen; the other of an oxydand an acid. 

MRS. B. 

Very well: and you will be careful to remember that the metals areincapable of entering into this combination with acids, unless they arepreviously oxydated; therefore, whenever you bring a metal in contactwith an acid, it will be first oxydated and afterwards dissolved, provided that there be a sufficient quantity of acid for bothoperations. 

There are some metals, however, whose solution is more easilyaccomplished, by diluting the acid in water; and the metal will, in thiscase, be oxydated, not by the acid, but by the water, which it willdecompose. But in proportion as the oxygen of the water oxydates thesurface of the metal, the acid combines with it, washes it off, andleaves a fresh surface for the oxygen to act upon: then other coats ofoxyd are successively formed, and rapidly dissolved by the acid, whichcontinues combining with the new-formed surfaces of oxyd till the wholeof the metal is dissolved. During this process the hydrogen gas of thewater is disengaged, and flies off with effervescence. 

EMILY. 

Was not this the manner in which the sulphuric acid assisted the ironfilings in decomposing water?

MRS. B. 

Exactly; and it is thus that several metals, which are incapable aloneof decomposing water, are enabled to do it by the assistance of an acid, which, by continually washing off the covering of oxyd, as it is formed, prepares a fresh surface of metal to act upon the water. 

CAROLINE. 

The acid here seems to act a part not very different from that of ascrubbing-brush. --But pray would not this be a good method of cleaningmetallic utensils?

MRS. B. 

Yes; on some occasions a weak acid, as vinegar, is used for cleaningcopper. Iron plates, too, are freed from the rust on their surface bydiluted muriatic acid, previous to their being covered with tin. Youmust remember, however, that in this mode of cleaning metals the acidshould be quickly afterwards wiped off, otherwise it would produce freshoxyd. 

CAROLINE. 

Let us watch the dissolution of the copper in the nitric acid; for I amvery impatient to see the salt that is to result from it. The mixture isnow of a beautiful blue colour; but there is no appearance of theformation of a salt; it seems to be a tedious operation. 

MRS. B. 

The crystallisation of the salt requires some length of time to becompleted; if, however, you are so impatient, I can easily show you ametallic salt already formed. 

CAROLINE. 

But that would not satisfy my curiosity half so well as one of our ownmanufacturing. 

MRS. B. 

It is one of our own preparing that I mean to show you. When wedecomposed water a few days since, by the oxydation of iron filingsthrough the assistance of sulphuric acid, in what did the processconsist?

CAROLINE. 

In proportion as the water yielded its oxygen to the iron, the acidcombined with the new-formed oxyd, and the hydrogen escaped alone. 

MRS. B. 

Very well; the result, therefore, was a compound salt, formed by thecombination of sulphuric acid with oxyd of iron. It still remains in thevessel in which the experiment was performed. Fetch it, and we shallexamine it. 

EMILY. 

What a variety of processes the decomposition of water, by a metal andan acid, implies; 1st, the decomposition of the water; 2dly, theoxydation of the metal; and 3dly, the formation of a compound salt. 

CAROLINE. 

Here it is, Mrs. B. --What beautiful green crystals! But we do notperceive any crystals in the solution of copper in nitrous acid?

MRS. B. 

Because the salt is now suspended in the water which the nitrous acidcontains, and will remain so till it is deposited in consequence of restand cooling. 

EMILY. 

I am surprised that a body so opake as iron can be converted into suchtransparent crystals. 

MRS. B. 

It is the union with the acid that produces the transparency; for if thepure metal were melted, and afterwards permitted to cool andcrystallise, it would be found just as opake as before. 

EMILY. 

I do not understand the exact meaning of _crystallisation_?

MRS. B. 

You recollect that when a solid body is dissolved either by water orcaloric it is not decomposed; but that its integrant parts are onlysuspended in the solvent. When the solution is made in water, theintegrant particles of the body will, on the water being evaporated, again unite into a solid mass by the force of their mutual attraction. But when the body is dissolved by caloric alone, nothing more isnecessary, in order to make its particles reunite, than to reduce itstemperature. And, in general, if the solvent, whether water or caloric, be slowly separated by evaporation or by cooling, and care taken thatthe particles be not agitated during their reunion, they will arrangethemselves in regular masses, each individual substance assuming apeculiar form or arrangement; and this is what is calledcrystallisation. 

EMILY. 

Crystallisation, therefore, is simply the reunion of the particles of asolid body that has been dissolved in a fluid. 

MRS. B. 

That is a very good definition of it. But I must not forget to observe, that _heat_ and _water_ may unite their solvent powers; and, in thiscase, crystallisation may be hastened by cooling, as well as byevaporating the liquid. 

CAROLINE. 

But if the body dissolved is of a volatile nature, will it not evaporatewith the fluid?

MRS. B. 

A crystallised body held in solution only by water is scarcely ever sovolatile as the fluid itself, and care must be taken to manage the heatso that it may be sufficient to evaporate the water only. 

I should not omit also to mention that bodies, in crystallising fromtheir watery solution, always retain a small portion of water, whichremains confined in the crystal in a solid form, and does not reappearunless the body loses its crystalline state. This is called the _waterof crystallisation_. But you must observe, that whilst a body may beseparated from its solution in water or caloric simply by cooling or byevaporation, an acid can be taken from a metal with which it is combinedonly by stronger affinities, which produce a decomposition. 

EMILY. 

Are the perfect metals susceptible of being dissolved and converted intocompound salts by acids?

MRS. B. 

Gold is acted upon by only one acid, the _oxygenated muriatic_, a veryremarkable acid, which, when in its most concentrated state, dissolvesgold or any other metal, by burning them rapidly. 

Gold can, it is true, be dissolved likewise by a mixture of two acids, commonly called _aqua regia_; but this mixed solvent derives thatproperty from containing the peculiar acid which I have just mentioned. Platina is also acted upon by this acid only; silver is dissolved bynitric acid. 

CAROLINE. 

I think you said that some of the metals might be so strongly oxydatedas to become acid?

MRS. B. 

There are five metals, arsenic, molybdena, chrome, tungsten, andcolumbium, which are susceptible of combining with a sufficient quantityof oxygen to be converted into acids. 

CAROLINE. 

Acids are connected with metals in such a variety of ways, that I amafraid of some confusion in remembering them. --In the first place, acids will yield their oxygen to metals. Secondly, they will combinewith them in their state of oxyds, to form compound salts; and lastly, several of the metals are themselves susceptible of acidification. 

MRS. B. 

Very well; but though metals have so great an affinity for acids, it isnot with that class of bodies alone that they will combine. They aremost of them, in their simple state, capable of uniting with sulphur, with phosphorus, with carbon, and with each other; these combinations, according to the nomenclature which was explained to you on a formeroccasion, are called _sulphurets_, _phosphorets_, _carburets_,  &c. 

The metallic phosphorets offer nothing very remarkable. The sulphuretsform the peculiar kind of mineral called _pyrites_, from which certainkinds of mineral waters, as those of Harrogate, derive their chiefchemical properties. In this combination, the sulphur, together with theiron, have so strong an attraction for oxygen, that they obtain it bothfrom the air and from water, and by condensing it in a solid form, produce the heat which raises the temperature of the water in such aremarkable degree. 

EMILY. 

But if pyrites obtain oxygen from water, that water must suffer adecomposition, and hydrogen gas be evolved. 

MRS. B. 

That is actually the case in the hot springs alluded to, which give outan extremely fetid gas, composed of hydrogen impregnated with sulphur. 

CAROLINE. 

If I recollect right, steel and plumbago, which you mentioned in thelast lesson, are both carburets of iron?

MRS. B. 

Yes; and they are the only carburets of much consequence. 

A curious combination of metals has lately very much attracted theattention of the scientific world: I mean the meteoric stones that fallfrom the atmosphere. They consist principally of native or pure iron, which is never found in that state in the bowels of the earth; andcontain also a small quantity of nickel and chrome, a combinationlikewise new in the mineral kingdom. 

These circumstances have led many scientific persons to believe thatthose substances have fallen from the moon, or some other planet, whileothers are of opinion either that they are formed in the atmosphere, orare projected into it by some unknown volcano on the surface of ourglobe. 

CAROLINE. 

I have heard much of these stones, but I believe many people are ofopinion that they are formed on the surface of the earth, and laugh attheir pretended celestial origin. 

MRS. B. 

The fact of their falling is so well ascertained, that I think no personwho has at all investigated the subject, can now entertain any doubt ofit. Specimens of these stones have been discovered in all parts of theworld, and to each of them some tradition or story of its fall has beenfound connected. And as the analysis of all those specimens affordsprecisely the same results, there is strong reason to conjecture thatthey all proceed from the same source. It is to Mr. Howard thatphilosophers are indebted for having first analysed these stones, anddirected their attention to this interesting subject. 

CAROLINE. 

But pray, Mrs. B. , how can solid masses of iron and nickel be formedfrom the atmosphere, which consists of the two airs, nitrogen andoxygen?

MRS. B. 

I really do not see how they could, and think it much more probable thatthey fall from the moon. --But we must not suffer this digression totake up too much of our time. 

The combinations of metals with each other are called alloys; thus brassis an alloy of copper and zinc; bronze, of copper and tin,  &c. 

EMILY. 

And is not pewter also a combination of metal?

MRS. B. 

It is. The pewter made in this country is mostly composed of tin, with avery small proportion of zinc and lead. 

CAROLINE. 

Block-tin is a kind of pewter, I believe?

MRS. B. 

Properly speaking, block-tin means tin in blocks, or square massiveingots; but in the sense in which it is used by ignorant workmen, it isiron plated with tin, which renders it more durable, as tin will not soeasily rust. Tin alone, however, would be too soft a metal to be workedfor common use, and all tin-vessels and utensils are in fact made ofplates of iron, thinly coated with tin, which prevents the iron fromrusting. 

CAROLINE. 

Say rather _oxydating_, Mrs. B. --Rust is a word that should be explodedin chemistry. 

MRS. B. 

Take care, however, not to introduce the word oxydate, instead of rust, in general conversation; for you would probably not be understood, andyou might be suspected of affectation. 

Metals differ very much in their affinity for each other; some will notunite at all, others readily combine together, and on this property ofmetals the art of _soldering_ depends. 

EMILY. 

What is soldering?

MRS. B. 

It is joining two pieces of metal together, by a more fusible metalinterposed between them. Thus tin is a solder for lead; brass, gold, orsilver, are solder for iron,  &c. 

CAROLINE. 

And is not _plating_ metals something of the same nature?

MRS. B. 

In the operation of plating, two metals are united, one being coveredwith the other, but without the intervention of a third; iron or coppermay thus be covered with gold or silver. 

EMILY. 

Mercury appears to me of a very different nature from the other metals. 

MRS. B. 

One of its greatest peculiarities is, that it retains a fluid state atthe temperature of the atmosphere. All metals are fusible at differentdegrees of heat, and they have likewise each the property of freezing orbecoming solid at a certain fixed temperature. Mercury congeals only atseventy-two degrees below the freezing point. 

EMILY. 

That is to say, that in order to freeze, it requires a temperature ofseventy-two degrees colder than that at which water freezes. 

MRS. B. 

Exactly so. 

CAROLINE. 

But is the temperature of the atmosphere ever so low as that?

MRS. B. 

Yes, often in Siberia; but happily never in this part of the globe. Here, however, mercury may be congealed by artificial cold; I mean suchintense cold as can be produced by some chemical mixtures, or by therapid evaporation of ether under the air-pump. *

 [Footnote *: By a process analogous to that described, page 155. Of this volume. ]

CAROLINE. 

And can mercury be made to boil and evaporate?

MRS. B. 

Yes, like any other liquid; only it requires a much greater degree ofheat. At the temperature of six hundred degrees, it begins to boil andevaporate like water. 

Mercury combines with gold, silver, tin, and with several other metals;and, if mixed with any of them in a sufficient proportion, it penetratesthe solid metal, softens it, loses its own fluidity, and forms an_amalgam_, which is the name given to the combination of any metal withmercury, forming a substance more or less solid, according as themercury or the other metal predominates. 

EMILY. 

In the list of metals there are some whose names I have never beforeheard mentioned. 

MRS. B. 

Besides those which Sir H. Davy has obtained, there are several thathave been recently discovered, whose properties are yet but littleknown, as for instance, titanium, which was discovered by the Rev. Mr. Gregor, in the tin-mines of Cornwall; columbium or tantalium, which haslately been discovered by Mr. Hatchett; and osmium, iridium, palladium, and rhodium, all of which Dr. Wollaston and Mr. Tennant found mixed inminute quantities with crude platina, and the distinct existence ofwhich they proved by curious and delicate experiments. 

CAROLINE. 

Arsenic has been mentioned amongst the metals. I had no notion that itbelonged to that class of bodies, for I had never seen it but as apowder, and never thought of it but as a most deadly poison. 

MRS. B. 

In its pure metallic state, I believe, it is not so poisonous; but ithas such a great affinity for oxygen, that it absorbs it from theatmosphere at its natural temperature: you have seen it, therefore, onlyin its state of oxyd, when, from its combination with oxygen, it hasacquired its very poisonous properties. 

CAROLINE. 

Is it possible that oxygen can impart poisonous qualities? That valuablesubstance which produces light and fire, and which all bodies in natureare so eager to obtain?

MRS. B. 

Most of the metallic oxyds are poisonous, and derive this property fromtheir union with oxygen. The white lead, so much used in paint, owes itspernicious effects to oxygen. In general, oxygen, in a concrete state, appears to be particularly destructive in its effects on flesh or anyanimal matter; and those oxyds are most caustic that have an acridburning taste, which proceeds from the metal having but a slightaffinity for oxygen, and therefore easily yielding it to the flesh, which it corrodes and destroys. 

EMILY. 

What is the meaning of the word _caustic_, which you have just used?

MRS. B. 

It expresses that property which some bodies possess, of disorganizingand destroying animal matter, by operating a kind of combustion, or atleast a chemical decomposition. You must often have heard of causticused to burn warts, or other animal excrescences; most of these bodiesowe their destructive power to the oxygen with which they are combined. The common caustic, called _lunar caustic_, is a compound formed by theunion of nitric acid and silver; and it is supposed to owe its causticqualities to the oxygen contained in the nitric acid. 

CAROLINE. 

But, pray, are not acids still more caustic than oxyds, as they containa greater proportion of oxygen?

MRS. B. 

Some of the acids are; but the caustic property of a body depends notonly upon the quantity of oxygen which it contains, but also upon itsslight affinity for that principle, and the consequent facility withwhich it yields it. 

EMILY. 

Is not this destructive property of oxygen accounted for?

MRS. B. 

It proceeds probably from the strong attraction of oxygen for hydrogen;for if the one rapidly absorb the other from the animal fibre, a disorganisation of the substance must ensue. 

EMILY. 

Caustics are, then, very properly said to burn the flesh, since thecombination of oxygen and hydrogen is an actual combustion. 

CAROLINE. 

Now, I think, this effect would be more properly termed an oxydation, asthere is no disengagement of light and heat. 

MRS. B. 

But there really is a sensation of heat produced by the action ofcaustics. 

EMILY. 

If oxygen is so caustic, why does not that which is contained in theatmosphere burn us?

MRS. B. 

Because it is in a gaseous state, and has a greater attraction for itselectricity than for the hydrogen of our bodies. Besides, should the airbe slightly caustic, we are in a great measure sheltered from itseffects by the skin; you know how much a wound, however trifling, smartson being exposed to it. 

CAROLINE. 

It is a curious idea, however, that we should live in a slow fire. But, if the air was caustic, would it not have an acrid taste?

MRS. B. 

It possibly may have such a taste; though in so slight a degree, thatcustom has rendered it insensible. 

CAROLINE. 

And why is not water caustic? When I dip my hand into water, thoughcold, it ought to burn me from the caustic nature of its oxygen. 

MRS. B. 

Your hand does not decompose the water; the oxygen in that state is muchbetter supplied with hydrogen than it would be by animal matter, and ifits causticity depend on its affinity for that principle, it will bevery far from quitting its state of water to act upon your hand. Youmust not forget that oxyds are caustic in proportion as the oxygenadheres slightly to them. 

EMILY. 

Since the oxyd of arsenic is poisonous, its acid, I suppose, is fully asmuch so?

MRS. B. 

Yes; it is one of the strongest poisons in nature. 

EMILY. 

There is a poison called _verdigris_, which forms on brass and copperwhen not kept very clean; and this, I have heard, is an objection tothese metals being made into kitchen utensils. Is this poison likewiseoccasioned by oxygen?

MRS. B. 

It is produced by the intervention of oxygen; for verdigris is acompound salt formed by the union of vinegar and copper; it is of abeautiful green colour, and much used in painting. 

EMILY. 

But, I believe, verdigris is often formed on copper when no vinegar hasbeen in contact with it. 

MRS. B. 

Not real verdigris, but compound salts, somewhat resembling it, may beproduced by the action of any acid on copper. 

The solution of copper in nitric acid, if evaporated, affords a saltwhich produces an effect on tin that will surprise you, and I haveprepared some from the solution we made before, that I might show it toyou. I shall first sprinkle some water on this piece of tin-foil, andthen some of the salt. --Now observe that I fold it up suddenly, andpress it into one lump. 

CAROLINE. 

What a prodigious vapour issues from it--and sparks of fire I declare!

MRS. B. 

I thought it would surprise you. The effect, however, I dare say youcould account for, since it is merely the consequence of the oxygen ofthe salt rapidly entering into a closer combination with the tin. 

There is also a beautiful green salt too curious to be omitted; it isproduced by the combination of cobalt with muriatic acid, which has thesingular property of forming what is called _sympathetic ink_. Characters written with this solution are invisible when cold, but whena gentle heat is applied, they assume a fine bluish green colour. 

CAROLINE. 

I think one might draw very curious landscapes with the assistance ofthis ink; I would first make a water-colour drawing of a winter-scene, in which the trees should be leafless, and the grass scarcely green:I would then trace all the verdure with the invisible ink, and wheneverI chose to create spring, I should hold it before the fire, and itswarmth would cover the landscape with a rich verdure. 

MRS. B. 

That will be a very amusing experiment, and I advise you by all means totry it. 

 [Transcriber’s Note: Several cobalt compounds, including the cobalt chloride described here, are still in use as invisible (“sympathetic”) inks. They are safe if used appropriately. ]

Before we part, I must introduce to your acquaintance the curious metalswhich Sir H. Davy has recently discovered. The history of theseextraordinary bodies is yet so much in its infancy, that I shall confinemyself to a very short account of them; it is more important to pointout to you the vast, and apparently inexhaustible, field of researchwhich has been thrown open to our view by Sir H. Davy’s memorablediscoveries, than to enter into a minute account of particular bodies orexperiments. 

CAROLINE. 

But I have heard that these discoveries, however splendid andextraordinary, are not very likely to prove of any great benefit to theworld, as they are rather objects of curiosity than of use. 

MRS. B. 

Such may be the illiberal conclusions of the ignorant and narrow-minded;but those who can duly estimate the advantages of enlarging the sphereof science, must be convinced that the acquisition of every new fact, however unconnected it may at first appear with practical utility, mustultimately prove beneficial to mankind. But these remarks are scarcelyapplicable to the present subject; for some of the new metals havealready proved eminently useful as chemical agents, and are likely soonto be employed in the arts. For the enumeration of these metals, I mustrefer you to our list of simple bodies; they are derived from thealkalies, the earths, and three of the acids, all of which had beenhitherto considered as undecompoundable or simple bodies. 

When Sir H. Davy first turned his attention to the effects of theVoltaic battery, he tried its power on a variety of compound bodies, andgradually brought to light a number of new and interesting facts, whichled the way to more important discoveries. It would be highlyinteresting to trace his steps in this new department of science, but itwould lead us too far from our principal object. A general view of hismost remarkable discoveries is all that I can aim at, or that you could, at present, understand. 

The facility with which compound bodies yielded to the Voltaicelectricity, induced him to make trial of its effects on substanceshitherto considered as simple, but which he suspected of being compound, and his researches were soon crowned with the most complete success. 

The body which he first submitted to the Voltaic battery, and which hadnever yet been decomposed, was one of the fixed alkalies, called potash. This substance gave out an elastic fluid at the positive wire, which wasascertained to be oxygen, and at the negative wire, small globules of avery high metallic lustre, very similar in appearance to mercury; thusproving that potash, which had hitherto been considered as a simpleincombustible body, was in fact a metallic oxyd; and that itsincombustibility proceeded from its being already combined with oxygen. 

EMILY. 

I suppose the wires used in this experiment were of platina, as theywere when you decomposed water; for if of iron, the oxygen would havecombined with the wire, instead of appearing in the form of gas. 

MRS. B. 

Certainly: the metal, however, would equally have been disengaged. SirH. Davy has distinguished this new substance by the name of POTASSIUM, which is derived from that of the alkali, from which it is procured. I have some small pieces of it in this phial, but you have already seenit, as it is the metal which we burnt in contact with sulphur. 

EMILY. 

What is the liquid in which you keep it?

MRS. B. 

It is naptha, a bituminous liquid, with which I shall hereafter make youacquainted. It is almost the only fluid in which potassium can bepreserved, as it contains no oxygen, and this metal has so powerful anattraction for oxygen, that it will not only absorb it from the air, butlikewise from water, or any body whatever that contains it. 

EMILY. 

This, then, is one of the bodies that oxydates spontaneously without theapplication of heat?

MRS. B. 

Yes; and it has this remarkable peculiarity that it attracts oxygen muchmore rapidly from water than from air; so that when thrown into water, however cold, it actually bursts into flame. I shall now throw a smallpiece, about the size of a pin’s head, on this drop of water. 

CAROLINE. 

It instantaneously exploded, producing a little flash of light! this is, indeed, a most curious substance!

MRS. B. 

By its combustion it is reconverted into potash; and as potash is nowdecidedly a compound body, I shall not enter into any of its propertiestill we have completed our review of the simple bodies; but we may heremake a few observations on its basis, potassium. If this substance isleft in contact with air, it rapidly returns to the state of potash, with a disengagement of heat, but without any flash of light. 

EMILY. 

But is it not very singular that it should burn better in water than inair?

CAROLINE. 

I do not think so: for if the attraction of potassium for oxygen is sostrong that it finds no more difficulty in separating it from thehydrogen in water, than in absorbing it from the air, it will no doubtbe more amply and rapidly supplied by water than by air. 

MRS. B. 

That cannot, however, be precisely the reason, for when potassium isintroduced under water, without contact of air, the combustion is not sorapid, and indeed, in that case, there is no luminous appearance; but aviolent action takes place, much heat is excited, the potash isregenerated, and hydrogen gas is evolved. 

Potassium is so eminently combustible, that instead of requiring, likeother metals, an elevation of temperature, it will burn rapidly incontact with water, even below the freezing point. This you may witnessby throwing a piece on this lump of ice. 

CAROLINE. 

It again exploded with flame, and has made a deep hole in the ice. 

MRS. B. 

This hole contains a solution of potash; for the alkali being extremelysoluble, disappears in the water at the instant it is produced. Itspresence, however, may be easily ascertained, alkalies having theproperty of changing paper, stained with turmeric, to a red colour; ifyou dip one end of this slip of paper into the hole in the ice you willsee it change colour, and the same, if you wet it with the drop of waterin which the first piece of potassium was burnt. 

CAROLINE. 

It has indeed changed the paper from yellow to red. 

MRS. B. 

This metal will burn likewise in carbonic acid gas, a gas that hadalways been supposed incapable of supporting combustion, as we wereunacquainted with any substance that had a greater attraction for oxygenthan carbon. Potassium, however, readily decomposes this gas, byabsorbing its oxygen, as I shall show you. This retort is filled withcarbonic acid gas. --I will put a small piece of potassium in it; butfor this combustion a slight elevation of temperature is required, forwhich purpose I shall hold the retort over the lamp. 

CAROLINE. 

Now it has taken fire, and burns with violence! It has burst the retort. 

MRS. B. 

Here is the piece of regenerated potash; can you tell me why it isbecome so black?

EMILY. 

No doubt it is blackened by the carbon, which, when its oxygen enteredinto combination with the potassium, was deposited on its surface. 

MRS. B. 

You are right. This metal is perfectly fluid at the temperature of onehundred degrees; at fifty degrees it is solid, but soft and malleable;at thirty-two degrees it is hard and brittle, and its fracture exhibitsan appearance of confused crystallization. It is scarcely more than halfas heavy as water; its specific gravity being about six when water isreckoned at ten; so that this metal is actually lighter than any knownfluid, even than ether. 

Potassium combines with sulphur and phosphorus, forming sulphurets andphosphurets; it likewise forms alloys with several metals, andamalgamates with mercury. 

EMILY. 

But can a sufficient quantity of potassium be obtained, by means of theVoltaic battery, to admit of all its properties and relations to otherbodies being satisfactorily ascertained?

MRS. B. 

Not easily; but I must not neglect to inform you that a method ofobtaining this metal in considerable quantities has since beendiscovered. Two eminent French chemists, Thenard and Gay Lussac, stimulated by the triumph which Sir H. Davy had obtained, attempted toseparate potassium from its combination with oxygen, by common chemicalmeans, and without the aid of electricity. They caused red hot potash ina state of fusion to filter through iron turnings in an iron tube, heated to whiteness. Their experiment was crowned with the most completesuccess; more potassium was obtained by this single operation, thatcould have been collected in many weeks by the most diligent use of theVoltaic battery. 

EMILY. 

In this experiment, I suppose, the oxygen quitted its combination withthe potassium to unite with the iron turnings?

MRS. B. 

Exactly so; and the potassium was thus obtained in its simple state. From that time it has become a most convenient and powerful instrumentof deoxygenation in chemical experiments. This important improvement, engrafted on Sir H. Davy’s previous discoveries, served but to add tohis glory, since the facts which he had established, when possessed ofonly a few atoms of this curious substance, and the accuracy of hisanalytical statements, were all confirmed when an opportunity occurredof repeating his experiments upon this substance, which can now beobtained in unlimited quantities. 

CAROLINE. 

What a satisfaction Sir H. Davy must have felt, when by an effort ofgenius he succeeded in bringing to light and actually giving existence, to these curious bodies, which without him might perhaps have everremained concealed from our view!

MRS. B. 

The next substance which Sir H. Davy submitted to the influence of theVoltaic battery was _Soda_, the other fixed alkali, which yielded to thesame powers of decomposition; from this alkali too, a metallic substancewas obtained, very analogous in its properties to that which had beendiscovered in potash; Sir H. Davy has called it SODIUM. It is ratherheavier than potassium, though considerably lighter than water; it isnot so easily fusible as potassium. 

Encouraged by these extraordinary results, Sir H. Davy next performed aseries of beautiful experiments on _Ammonia_, or the volatile alkali, which, from analogy, he was led to suspect might also contain oxygen. This he soon ascertained to be the fact, but he has not yet succeeded inobtaining the basis of ammonia in a separate state; it is from analogy, and from the power which the volatile alkali has, in its gaseous form, to oxydate iron, and also from the amalgams which can be obtained fromammonia by various processes, that the proofs of that alkali being alsoa metallic oxyd are deduced. 

Thus, then, the three alkalies, two of which had always been consideredas simple bodies, have now lost all claim to that title, and I haveaccordingly classed the alkalies amongst the compounds, whose propertieswe shall treat of in a future conversation. 

EMILY. 

What are the other newly discovered metals which you have alluded to inyour list of simple bodies?

MRS. B. 

They are the metals of the earths which became next the object of Sir H. Davy’s researches; these bodies had never yet been decomposed, thoughthey were strongly suspected not only of being compounds, but of beingmetallic oxyds. From the circumstance of their incombustibility it wasconjectured, with some plausibility, that they might possibly be bodiesthat had been already burnt. 

CAROLINE. 

And metals, when oxydated, become, to all appearance, a kind of earthysubstance. 

MRS. B. 

They have, besides, several features of resemblance with metallic oxyds;Sir H. Davy had therefore great reason to be sanguine in hisexpectations of decomposing them, and he was not disappointed. He couldnot, however, succeed in obtaining the basis of the earths in a pureseparate state; but metallic alloys were formed with other metals, whichsufficiently proved the existence of the metallic basis of the earths. 

The last class of new metallic bodies which Sir H. Davy discovered wasobtained from the three undecompounded acids, the boracic, the fluoric, and the muriatic acids; but as you are entirely unacquainted with thesebodies, I shall reserve the account of their decomposition till we cometo treat of their properties as acids. 

Thus in the course of two years, by the unparalleled exertions of asingle individual, chemical science has assumed a new aspect. Bodieshave been brought to light which the human eye never before beheld, andwhich might have remained eternally concealed under their impenetrabledisguise. 

It is impossible at the present period to appreciate to their fullextent the consequences which science or the arts may derive from thesediscoveries; we may, however, anticipate the most important results. 

In chemical analysis we are now in possession of more energetic agentsof decomposition than were ever before known. 

In geology new views are opened, which will probably operate arevolution in that obscure and difficult science. It is already provedthat all the earths, and, in fact, the solid surface of this globe, aremetallic bodies mineralized by oxygen, and as our planet has beencalculated to be considerably more dense upon the whole than on thesurface, it is reasonable to suppose that the interior part is composedof a metallic mass, the surface of which only has been mineralized bythe atmosphere. 

The eruptions of volcanos, those stupendous problems of nature, admitnow of an easy explanation. For if the bowels of the earth are the grandrecess of these newly discovered inflammable bodies, whenever waterpenetrates into them, combustions and explosions must take place; and itis remarkable that the lava which is thrown out, is the very kind ofsubstance which might be expected to result from these combustions. 

I must now take my leave of you; we have had a very long conversationto-day, and I hope you will be able to recollect what you have learnt. At our next interview we shall enter on a new subject. 

END OF THE FIRST VOLUME. 

 Printed by A. Strahan, Printers-Street, London. 

 * * * * * * * * *

 CONVERSATIONS ON CHEMISTRY;

 In Which The Elements Of That Science Are _Familiarly Explained_ And Illustrated By Experiments. 

 IN TWO VOLUMES. 

 _The Fifth Edition, revised, corrected, _ _and considerably enlarged. _

 VOL. II. ON COMPOUND BODIES. 

 _London:_ Printed For Longman, Hurst, Rees, Orme, and Brown, Paternoster-Row. 1817. 

CONVERSATION XIII. 

ON THE ATTRACTION OF COMPOSITION. 

MRS. B. 

Having completed our examination of the simple or elementary bodies, weare now to proceed to those of a compound nature; but before we enter onthis extensive subject, it will be necessary to make you acquainted withthe principal laws by which chemical combinations are governed. 

You recollect, I hope, what we formerly said of the nature of theattraction of composition, or chemical attraction, or affinity, as it isalso called?

EMILY. 

Yes, I think perfectly; it is the attraction that subsists betweenbodies of a different nature, which occasions them to combine and form acompound, when they come in contact, and, according to Sir H. Davy’sopinion, this effect is produced by the attraction of the oppositeelectricities, which prevail in bodies of different kinds. 

MRS. B. 

Very well; your definition comprehends the first law of chemicalattraction, which is, that _it takes place only between bodies of adifferent nature_; as, for instance, between an acid and an alkali;between oxygen and a metal,  &c. 

CAROLINE. 

That we understand of course; for the attraction between particles of asimilar nature is that of aggregation, or cohesion, which is independentof any chemical power. 

MRS. B. 

The 2d law of chemical attraction is, that _it takes place only betweenthe most minute particles of bodies_; therefore, the more you divide theparticles of the bodies to be combined, the more readily they act uponeach other. 

CAROLINE. 

That is again a circumstance which we might have supposed, for the finerthe particles of the two substances are, the more easily and perfectlythey will come in contact with each other, which must greatly facilitatetheir union. It was for this purpose, you said, that you used ironfilings, in preference to wires or pieces of iron, for the decompositionof water. 

MRS. B. 

It was once supposed that no mechanical power could divide bodies intoparticles sufficiently minute for them to act on each other; and that, in order to produce the extreme division requisite for a chemicalaction, one, if not both of the bodies, should be in a fluid state. There are, however, a few instances in which two solid bodies, veryfinely pulverized, exert a chemical action on one another; but suchexceptions to the general rule are very rare indeed. 

EMILY. 

In all the combinations that we have hitherto seen, one of theconstituents has, I believe, been either liquid or aëriform. Incombustions, for instance, the oxygen is taken from the atmosphere, inwhich it existed in the state of gas; and whenever we have seen acidscombine with metals or with alkalies, they were either in a liquid or anaëriform state. 

MRS. B. 

The 3d law of chemical attraction is, that _it can take place betweentwo, three, four, or even a greater number of bodies_. 

CAROLINE. 

Oxyds and acids are bodies composed of two constituents; but I recollectno instance of the combination of a greater number of principles. 

MRS. B. 

The compound salts, formed by the union of the metals with acids, arecomposed of three principles. And there are salts formed by thecombination of the alkalies with the earths which are of a similardescription. 

CAROLINE. 

Are they of the same kind as the metallic salts?

MRS. B. 

Yes; they are very analogous in their nature, although different in manyof their properties. 

A methodical nomenclature, similar to that of the acids, has beenadopted for the compound salts. Each individual salt derives its namefrom its constituent parts, so that every name implies a knowledge ofthe composition of the salt. 

The three alkalies, the alkaline earths, and the metals, are called_salifiable bases_ or _radicals_; and the acids, _salifying principles_. The name of each salt is composed both of that of the acid and thesalifiable base; and it terminates in _at_ or _it_, according to thedegree of the oxygenation of the acid. Thus, for instance, all thosesalts which are formed by the combination of the sulphuric acid with anyof the salifiable bases are called _sulphats_, and the name of theradical is added for the specific distinction of the salt; if it bepotash, it will compose a _sulphat of potash_; if ammonia, _sulphat ofammonia_,  &c. 

EMILY. 

The crystals which we obtained from the combination of iron andsulphuric acid were therefore _sulphat of iron_?

MRS. B. 

Precisely; and those which we prepared by dissolving copper in nitricacid, _nitrat of copper_, and so on. --But this is not all; if the saltbe formed by that class of acids which ends in _ous_, (which you knowindicates a less degree of oxygenation, ) the termination of the name ofthe salt will be in _it_, as _sulphit of potash_, _sulphit ofammonia_,  &c. 

EMILY. 

There must be an immense number of compound salts, since there is sogreat a variety of salifiable radicals, as well as of salifyingprinciples. 

MRS. B. 

Their real number cannot be ascertained, since it increases every day. But we must not proceed further in the investigation of the compoundsalts, until we have completed the examination of the nature of theingredients of which they are composed. 

The 4th law of chemical attraction is, that _a change of temperaturealways takes place at the moment of combination_. This arises from theextrication of the two electricities in the form of caloric, which takesplace when bodies unite; and also sometimes in part from a change ofcapacity of the bodies for heat, which always takes place when thecombination is attended with an increase of density, but more especiallywhen the compound passes from the liquid to the solid form. I shall nowshow you a striking instance of a change of temperature from chemicalunion, merely by pouring some nitrous acid on this small quantity of oilof turpentine--the oil will instantly combine with the oxygen of theacid, and produce a considerable change of temperature. 

CAROLINE. 

What a blaze! The temperature of the oil and the acid must be greatlyraised, indeed, to produce such a violent combustion. 

MRS. B. 

There is, however, a peculiarity in this combustion, which is, that theoxygen, instead of being derived from the atmosphere alone, isprincipally supplied by the acid itself. 

EMILY. 

And are not all combustions instances of the change of temperatureproduced by the chemical combination of two bodies?

MRS. B. 

Undoubtedly; when oxygen loses its gaseous form, in order to combinewith a solid body, it becomes condensed, and the caloric evolvedproduces the elevation of temperature. The specific gravity of bodies isat the same time altered by chemical combination; for in consequence ofa change of capacity for heat, a change of density must be produced. 

CAROLINE. 

That was the case with the sulphuric acid and water, which, by beingmixed together, gave out a great deal of heat, and increased in density. 

MRS. B. 

The 5th law of chemical attraction is, that _the properties whichcharacterise bodies, when separate, are altered or destroyed by theircombination_. 

CAROLINE. 

Certainly; what, for instance, can be so different from water as thehydrogen and oxygen gases?

EMILY. 

Or what more unlike sulphat of iron than iron or sulphuric acid?

MRS. B. 

Every chemical combination is an illustration of this rule. But let usproceed--

The 6th law is, that _the force of chemical affinity between theconstituents of a body is estimated by that which is required for theirseparation_. This force is not always proportional to the facility withwhich bodies unite; for manganese, for instance, which, you know, is somuch disposed to unite with oxygen that it is never found in a metallicstate, yields it more easily than any other metal. 

EMILY. 

But, Mrs. B. , you speak of estimating the force of attraction betweenbodies, by the force required to separate them; how can you measurethese forces?

MRS. B. 

They cannot be precisely measured, but they are comparativelyascertained by experiment, and can be represented by numbers whichexpress the relative degrees of attraction. 

The 7th law is, that _bodies have amongst themselves different degreesof attraction_. Upon this law, (which you may have discovered yourselveslong since, ) the whole science of chemistry depends; for it is by meansof the various degrees of affinity which bodies have for each other, that all the chemical compositions and decompositions are effected. Every chemical fact or experiment is an instance of the same kind; andwhenever the decomposition of a body is performed by the addition of anysingle new substance, it is said to be effected by _simple electiveattractions_. But it often happens that no simple substance willdecompose a body, and that, in order to effect this, you must offer tothe compound a body which is itself composed of two, or sometimes threeprinciples, which would not, each separately, perform the decomposition. In this case there are two new compounds formed in consequence of areciprocal decomposition and recomposition. All instances of this kindare called _double elective attractions_. 

CAROLINE. 

I confess I do not understand this clearly. 

MRS. B. 

You will easily comprehend it by the assistance of this diagram, inwhich the reciprocal forces of attraction are represented by numbers:

 _Original Compound_ Sulphat of Soda. 

 Soda 8 Sulphuric Acid

 | | _Quies-_ | | _cent_ | _Result_ _Result_ Nitrat 7 _Divellent Attractions_ 6} 13 Sulphat of Soda of Lime | | | _Attrac-_ | | _tions_ |

 Nitric Acid 4 Lime -- 12

 _Original Compound_ Nitrat of Lime. 

We here suppose that we are to decompose sulphat of soda; that is, toseparate the acid from the alkali; if, for this purpose, we add somelime, in order to make it combine with the acid, we shall fail in ourattempt, because the soda and the sulphuric acid attract each other by aforce which is superior, and (by way of supposition) is represented bythe number 8; while the lime tends to unite with this acid by anaffinity equal only to the number 6. It is plain, therefore, that thesulphat of soda will not be decomposed, since a force equal to 8 cannotbe overcome by a force equal only to 6. 

CAROLINE. 

So far, this appears very clear. 

MRS. B. 

If, on the other hand, we endeavour to decompose this salt by nitricacid, which tends to combine with soda, we shall be equallyunsuccessful, as nitric acid tends to unite with the alkali by a forceequal only to 7. 

In neither of these cases of simple elective attraction, therefore, canwe accomplish our purpose. But let us previously combine together thelime and nitric acid, so as to form a nitrat of lime, a compound salt, the constituents of which are united by a power equal to 4. If then wepresent this compound to the sulphat of soda, a decomposition willensue, because the sum of the forces which tend to preserve the twosalts in their actual state is not equal to that of the forces whichtend to decompose them, and to form new combinations. The nitric acid, therefore, will combine with the soda, and the sulphuric acid with thelime. 

CAROLINE. 

I understand you now very well. This double effect takes place becausethe numbers 8 and 4, which represent the degrees of attraction of theconstituents of the two original salts, make a sum less than the numbers7 and 6, which represent the degrees of attraction of the two newcompounds that will in consequence be formed. 

MRS. B. 

Precisely so. 

CAROLINE. 

But what is the meaning of _quiescent_ and _divellent_ forces, which arewritten in the diagram?

MRS. B. 

Quiescent forces are those which tend to preserve compounds in a stateof rest, or such as they actually are: divellent forces, those whichtend to destroy that state of combination, and to form new compounds. 

These are the principal circumstances relative to the doctrine ofchemical attractions, which have been laid down as rules by modernchemists; a few others might be mentioned respecting the same theory, but of less importance, and such as would take us too far from our plan. I should, however, not omit to mention that Mr. Berthollet, a celebratedFrench chemist, has questioned the uniform operation of electiveattraction, and has advanced the opinion, that, in chemicalcombinations, the changes which take place depend not only upon theaffinities, but also, in some degree, on the respective quantities ofthe substances concerned, on the heat applied during the process, andsome other circumstances. 

CAROLINE. 

In that case, I suppose, there would hardly be two compounds exactlysimilar, though composed of the same materials?

MRS. B. 

On the contrary, it is found that a remarkable uniformity prevails, asto proportions, between the ingredients of bodies of similarcomposition. Thus water, as you may recollect to have seen in a formerconversation, is composed of two volumes of hydrogen gas to one ofoxygen, and this is always found to be precisely the proportion of itsconstituents, from whatever source the water be derived. The sameuniformity prevails with regard to the various salts; the acid andalkali, in each kind of salt, being always found to combine in the sameproportions. Sometimes, it is true, the same acid, and the same alkali, are capable of making two distinct kinds of salts; but in all thesecases it is found that one of the salts contains just twice, or in someinstances, thrice as much acid, or alkali, as the other. 

EMILY. 

If the proportions in which bodies combine are so constant and so welldefined, how can Mr. Berthollet’s remark be reconciled with this uniformsystem of combination?

MRS. B. 

Great as that philosopher’s authority is in chemistry, it is nowgenerally supposed that his doubts on this subject were in a greatdegree groundless, and that the exceptions he has observed in the lawsof definite proportions, have been only apparent, and may be accountedfor consistently with those laws. 

CAROLINE. 

Pray, Mrs. B. , can you decompose a salt by means of electricity, in thesame way as we decompose water?

MRS. B. 

Undoubtedly; and I am glad this question occurred to you, because itgives me an opportunity of showing you some very interesting experimentson the subject. 

If we dissolve a quantity, however small, of any salt in a glass ofwater, and if we plunge into it the extremities of the wires whichproceed from the two ends of the Voltaic battery, the salt will begradually decomposed, the acid being attracted by the positive, and thealkali by the negative wire. 

EMILY. 

But how can you render that decomposition perceptible?

MRS. B. 

By placing in contact with the extremities of each wire, in thesolution, pieces of paper stained with certain vegetable colours, whichare altered by the contact of an acid or an alkali. Thus this bluevegetable preparation called litmus becomes red when touched by an acid;and the juice of violets becomes green by the contact of an alkali. 

But the experiment can be made in a much more distinct manner, byreceiving the extremities of the wires into two different vessels, sothat the alkali shall appear in one vessel and the acid in the other. 

CAROLINE. 

But then the Voltaic circle will not be completed; how can any effect beproduced?

MRS. B. 

You are right; I ought to have added that the two vessels must beconnected together by some interposed substance capable of conductingelectricity. A piece of moistened cotton-wick answers this purpose verywell. You see that the cotton (PLATE XIII. Fig. 2.  c. ) has one endimmersed in one glass and the other end in the other, so as to establisha communication between any fluids contained in them. We shall now putinto each of the glasses a little glauber salt, or sulphat of soda, (which consists of an acid and an alkali, ) and then we shall fill theglasses with water, which will dissolve the salt. Let us now connect theglasses by means of the wires (e,  d, ) with the two ends of the battery, thus .  .  .  . 

 [Illustration: Plate XIII. Vol. II. Page 16. 

 Fig. 1. Voltaic Battery of improved construction with the Plates out of the Cells. 

 Fig. 2. 3 & 4. Instances of Chemical decomposition by the Voltaic Battery. ]

CAROLINE. 

The wires are already giving out small bubbles; is this owing to thedecomposition of the salt?

MRS. B. 

No; these are bubbles produced by the decomposition of the water, as yousaw in a former experiment. In order to render the separation of theacid from the alkali visible, I pour into the glass (a), which isconnected with the positive wire, a few drops of a solution of litmus, which the least quantity of acid turns red; and into the otherglass (b), which is connected with the negative wire, I pour a few dropsof the juice of violets .  .  .  . 

EMILY. 

The blue solution is already turning red all round the wire. 

CAROLINE. 

And the violet solution is beginning to turn green. This is indeed verysingular!

MRS. B. 

You will be still more astonished when we vary the experiment in thismanner:-- These three glasses (fig. 3. F, g,  h, ) are, as in the formerinstance, connected together by wetted cotton, but the middle one alonecontains a saline solution, the two others containing only distilledwater, coloured as before by vegetable infusions. Yet, on making theconnection with the battery, the alkali will appear in the negativeglass (h), and the acid in the positive glass (f), though neither ofthem contained any saline matter. 

EMILY. 

So that the acid and alkali must be conveyed right and left from thecentral glass, into the other glasses, by means of the connectingmoistened cotton?

MRS. B. 

Exactly so; and you may render the experiment still more striking, byputting into the central glass (k,  fig.  3. ) an alkaline solution, theglauber salt being placed into the negative glass (l), and the positiveglass (i) containing only water. The acid will be attracted by thepositive wire (m), and will actually appear in the vessel (i), afterpassing through the alkaline solution (k), without combining with it, although, you know, acids and alkalies are so much disposed to combine. --But this conversation has already much exceeded our usual limits, andwe cannot enlarge more upon this interesting subject at present. 

CONVERSATION XIV. 

ON ALKALIES. 

MRS. B. 

Having now given you some idea of the laws by which chemical attractionsare governed, we may proceed to the examination of bodies which areformed in consequence of these attractions. 

The first class of compounds that present themselves to our notice, inour gradual ascent to the most complicated combinations, are bodiescomposed of only two principles. The sulphurets, phosphurets, carburets, &c. Are of this description; but the most numerous and important ofthese compounds are the combinations of oxygen with the various simplesubstances with which it has a tendency to unite. Of these you havealready acquired some knowledge, but it will be necessary to enter intofurther particulars respecting the nature and properties of those mostdeserving our notice. Of this class are the ALKALIES and the EARTHS, which we shall successively examine. 

We shall first take a view of the alkalies, of which there are three, viz. POTASH, SODA, and AMMONIA. The two first are called _fixedalkalies_, because they exist in a solid form at the temperature of theatmosphere, and require a great heat to be volatilised. They consist, asyou already know, of metallic bases combined with oxygen. In potash, theproportions are about eighty-six parts of potassium to fourteen ofoxygen; and in soda, seventy-seven parts of sodium to twenty-three ofoxygen. The third alkali, ammonia, has been distinguished by the name of_volatile alkali_, because its natural form is that of gas. Itscomposition is of a more complicated nature, of which we shall speakhereafter. 

Some of the earths bear so strong a resemblance in their properties tothe alkalies, that it is difficult to know under which head to placethem. The celebrated French chemist, Fourcroy, has classed two of them(barytes and strontites) with the alkalies; but as lime and magnesiahave almost an equal title to that rank, I think it better not toseparate them, and therefore have adopted the common method of classingthem with the earths, and of distinguishing them by the name of_alkaline earths_. 

The general properties of alkalies are, an acrid burning taste, a pungent smell, and a caustic action on the skin and flesh. 

CAROLINE. 

I wonder they should be caustic, Mrs. B. , since they contain so littleoxygen. 

MRS. B. 

Whatever substance has an affinity for any one of the constituents ofanimal matter, sufficiently powerful to decompose it, is entitled to theappellation of caustic. The alkalies, in their pure state, have a verystrong attraction for water, for hydrogen, and for carbon, which, youknow, are the constituent principles of oil, and it is chiefly byabsorbing these substances from animal matter that they effect itsdecomposition; for, when diluted with a sufficient quantity of water, orcombined with any oily substance, they lose their causticity. 

But, to return to the general properties of alkalies--they change, as wehave already seen, the colour of syrup of violets, and other bluevegetable infusions, to green; and have, in general, a very greattendency to unite with acids, although the respective qualities of thesetwo classes of bodies form a remarkable contrast. 

We shall examine the result of the combination of acids and alkaliesmore particularly hereafter. It will be sufficient at present to informyou, that whenever acids are brought in contact with alkalies, oralkaline earths, they unite with a remarkable eagerness, and formcompounds perfectly different from either of their constituents; thesebodies are called _neutral_ or _compound salts_. 

The dry white powder which you see in this phial is pure caustic POTASH;it is very difficult to preserve it in this state, as it attracts, withextreme avidity, the moisture from the atmosphere, and if the air werenot perfectly excluded, it would, in a very short time, be actuallymelted. 

EMILY. 

It is then, I suppose, always found in a liquid state?

MRS. B. 

No; it exists in nature in a great variety of forms and combinations, but is never found in its pure separate state; it is combined withcarbonic acid, with which it exists in every part of the vegetablekingdom, and is most commonly obtained from the ashes of vegetables, which are the residue that remains after all the other parts have beenvolatilised by combustion. 

CAROLINE. 

But you once said, that after all the volatile parts of a vegetable wereevaporated, the substance that remained was charcoal?

MRS. B. 

I am surprised that you should still confound the processes ofvolatilisation and combustion. In order to procure charcoal, weevaporate such parts as can be reduced to vapour by the operation ofheat alone; but when we _burn_ the vegetable, we burn the carbon also, and convert it into carbonic acid gas. 

CAROLINE. 

That is true; I hope I shall make no more mistakes in my favouritetheory of combustion. 

MRS. B. 

Potash derives its name from the _pots_ in which the vegetables, fromwhich it was obtained, used formerly to be burnt; the alkali remainedmixed with the ashes at the bottom, and was thence called potash. 

EMILY. 

The ashes of a wood-fire, then, are potash, since they are vegetableashes?

MRS. B. 

They always contain more or less potash, but are very far fromconsisting of that substance alone, as they are a mixture of variousearths and salts which remain after the combustion of vegetables, andfrom which it is not easy to separate the alkali in its pure form. Theprocess by which potash is obtained, even in the imperfect state inwhich it is used in the arts, is much more complicated than simplecombustion. It was once deemed impossible to separate it entirely fromall foreign substances, and it is only in chemical laboratories that itis to be met with in the state of purity in which you find it in thisphial. Wood-ashes are, however, valuable for the alkali which theycontain, and are used for some purposes without any further preparation. Purified in a certain degree, they make what is commonly called_pearlash_, which is of great efficacy in taking out grease, in washinglinen, &c. ; for potash combines readily with oil or fat, with which itforms a compound well known to you under the name of _soap_. 

CAROLINE. 

Really! Then I should think it would be better to wash all linen withpearlash than with soap, as, in the latter case, the alkali beingalready combined with oil, must be less efficacious in extractinggrease. 

MRS. B. 

Its effect would be too powerful on fine linen, and would injure itstexture; pearlash is therefore only used for that which is of a strongcoarse kind. For the same reason you cannot wash your hands with plainpotash; but, when mixed with oil in the form of soap, it is soft as wellas cleansing, and is therefore much better adapted to the purpose. 

Caustic potash, as we already observed, acts on the skin, and animalfibre, in virtue of its attraction for water and oil, and converts allanimal matter into a kind of saponaceous jelly. 

EMILY. 

Are vegetables the only source from which potash can be derived?

MRS. B. 

No: for though far most abundant in vegetables, it is by no meansconfined to that class of bodies, being found also on the surface of theearth, mixed with various minerals, especially with earths and stones, whence it is supposed to be conveyed into vegetables by the roots of theplant. It is also met with, though in very small quantities, in someanimal substances. The most common state of potash is that of_carbonat_; I suppose you understand what that is?

EMILY. 

I believe so; though I do not recollect that you ever mentioned the wordbefore. If I am not mistaken, it must be a compound salt, formed by theunion of carbonic acid with potash. 

MRS. B. 

Very true; you see how admirably the nomenclature of modern chemistry isadapted to assist the memory; when you hear the name of a compound, younecessarily learn what are its constituent parts; and when you areacquainted with these constituents, you can immediately name thecompound which they form. 

CAROLINE. 

Pray, how were bodies arranged and distinguished before thisnomenclature was introduced?

MRS. B. 

Chemistry was then a much more difficult study; for every substance hadan arbitrary name, which it derived either from the person whodiscovered it, as _Glauber’s salts_ for instance; or from some othercircumstance relative to it, though quite unconnected with its realnature, as potash. 

These names have been retained for some of the simple bodies; for asthis class is not numerous, and therefore can easily be remembered, ithas not been thought necessary to change them. 

EMILY. 

Yet I think it would have rendered the new nomenclature more complete tohave methodised the names of the elementary, as well as of the compoundbodies, though it could not have been done in the same manner. But thenames of the simple substances might have indicated their nature, or, atleast, some of their principal properties; and if, like the acids andcompound salts, all the simple bodies had a similar termination, theywould have been immediately known as such. So complete and regular anomenclature would, I think, have given a clearer and more comprehensiveview of chemistry than the present, which is a medley of the old and newterms. 

MRS. B. 

But you are not aware of the difficulty of introducing into science anentire set of new terms; it obliges all the teachers and professors togo to school again, and if some of the old names, that are leastexceptionable, were not left as an introduction to the new ones, fewpeople would have had industry and perseverance enough to submit to thestudy of a completely new language; and the inferior classes of artists, who can only act from habit and routine, would, at least for a time, have felt material inconvenience from a total change of their habitualterms. From these considerations, Lavoisier and his colleagues, whoinvented the new nomenclature, thought it most prudent to leave a fewlinks of the old chain, in order to connect it with the new one. Besides, you may easily conceive the inconvenience which might arisefrom giving a regular nomenclature to substances, the simple nature ofwhich is always uncertain; for the new names might, perhaps, have provedto have been founded in error. And, indeed, cautious as the inventors ofthe modern chemical language have been, it has already been foundnecessary to modify it in many respects. In those few cases, however, inwhich new terms have been adopted to designate simple bodies, thesenames have been so contrived as to indicate one of the chief propertiesof the body in question; this is the case with oxygen, which, as Iexplained to you, signifies generator of acids; and hydrogen generatorof water. If all the elementary bodies had a similar termination, as youpropose, it would be necessary to change the name of any that mighthereafter be found of a compound nature, which would be veryinconvenient in this age of discovery. 

But to return to the alkalies. --We shall now try to melt some of thiscaustic potash in a little water, as a circumstance occurs during itssolution very worthy of observation. --Do you feel the heat that isproduced?

CAROLINE. 

Yes, I do; but is not this directly contrary to our theory of latentheat, according to which heat is disengaged when fluids become solid, and cold produced when solids are melted?

MRS. B. 

The latter is really the case in all solutions; and if the solution ofcaustic alkalies seems to make an exception to the rule, it does not, I believe, form any solid objection to the theory. The matter may beexplained thus: When water first comes in contact with the potash, itproduces an effect similar to the slaking of lime, that is, the water issolidified in combining with the potash, and thus loses its latent heat;this is the heat that you now feel, and which is, therefore, producednot by the melting of the solid, but by the solidification of the fluid. But when there is more water than the potash can absorb and solidify, the latter then yields to the solvent power of the water; and if we donot perceive the cold produced by its melting, it is because it iscounterbalanced by the heat previously disengaged. *

A very remarkable property of potash is the formation of glass by itsfusion with siliceous earth. You are not yet acquainted with this lastsubstance, further than its being in the list of simple bodies. It issufficient, for the present, that you should know that sand and flintare chiefly composed of it; alone, it is infusible, but mixed withpotash, it melts when exposed to the heat of a furnace, combines withthe alkali, and runs into glass. 

 [Footnote *: This defence of the general theory, however plausible, is liable to some obvious objections. The phenomenon might perhaps be better accounted for by supposing that a solution of alkali in water has less capacity for heat than either water or alkali in their separate state. ]

CAROLINE. 

Who would ever have supposed that the same substance which convertstransparent oil into such an opake body as soap, should transform thatopake substance, sand, into transparent glass!

MRS. B. 

The transparency, or opacity of bodies, does not, I conceive, depend somuch upon their intimate nature, as upon the arrangement of theirparticles: we cannot have a more striking instance of this, than isafforded by the different states of carbon, which, though it commonlyappears in the form of a black opake body, sometimes assumes the mostdazzling transparent form in nature, that of diamond, which, yourecollect, is carbon, and which, in all probability, derives itsbeautiful transparency from the peculiar arrangement of its particlesduring their crystallisation. 

EMILY. 

I never should have supposed that the formation of glass was so simple aprocess as you describe it. 

MRS. B. 

It is by no means an easy operation to make perfect glass; for if thesand, or flint, from which the siliceous earth is obtained, be mixedwith any metallic particles, or other substance, which cannot bevitrified, the glass will be discoloured, or defaced, by opake specks. 

CAROLINE. 

That, I suppose, is the reason why objects so often appear irregular andshapeless through a common glass-window. 

MRS. B. 

This species of imperfection proceeds, I believe, from another cause. Itis extremely difficult to prevent the lower part of the vessels, inwhich the materials of glass are fused, from containing a more densevitreous matter than the upper, on account of the heavier ingredientsfalling to the bottom. When this happens, it occasions the appearance ofveins or waves in the glass, from the difference of density in itsseveral parts, which produces an irregular refraction of the rays oflight that pass through it. 

Another species of imperfection sometimes arises from the fusion notbeing continued for a length of time sufficient to combine the twoingredients completely, or from the due proportion of potash and silex(which are as two to one) not being carefully observed; the glass, inthose cases, will be liable to alteration from the action of the air, ofsalts, and especially of acids, which will effect its decomposition bycombining with the potash, and forming compound salts. 

EMILY. 

What an extremely useful substance potash is!

MRS. B. 

Besides the great importance of potash in the manufactures of glass andsoap, it is of very considerable utility in many of the other arts, andin its combinations with several acids, particularly the nitric, withwhich it forms saltpetre. 

CAROLINE. 

Then saltpetre must be a _nitrat of potash_? But we are not yetacquainted with the nitric acid?

MRS. B. 

We shall therefore defer entering into the particulars of thesecombinations till we come to a general review of the compound salts. Inorder to avoid confusion, it will be better at present to confineourselves to the alkalies. 

EMILY. 

Cannot you show us the change of colour which you said the alkaliesproduced on blue vegetable infusions?

MRS. B. 

Yes; very easily. I shall dip a piece of white paper into this syrup ofviolets, which, you see, is of a deep blue, and dyes the paper of thesame colour. --As soon as it is dry, we shall dip it into a solution ofpotash, which, though itself colourless, will turn the paper green--

CAROLINE. 

So it has, indeed! And do the other alkalies produce a similar effect?

MRS. B. 

Exactly the same. --We may now proceed to SODA, which, howeverimportant, will detain us but a very short time; as in all its generalproperties it very strongly resembles potash; indeed, so great is theirsimilitude, that they have been long confounded, and they can nowscarcely be distinguished, except by the difference of the salts whichthey form with acids. 

The great source of this alkali is the sea, where, combined with apeculiar acid, it forms the salt with which the waters of the ocean areso strongly impregnated. 

EMILY. 

Is not that the common table salt?

MRS. B. 

The very same; but again we must postpone entering into the particularsof this interesting combination, till we treat of the neutral salts. Soda may be obtained from common salt; but the easiest and most usualmethod of procuring it is by the combustion of marine plants, anoperation perfectly analogous to that by which potash is obtained fromvegetables. 

EMILY. 

From what does soda derive its name?

MRS. B. 

From a plant called by us _soda_, and by the Arabs _kali_, which affordsit in great abundance. Kali has, indeed, given its name to the alkaliesin general. 

CAROLINE. 

Does soda form glass and soap in the same manner as potash?

MRS. B. 

Yes, it does; it is of equal importance in the arts, and is evenpreferred to potash for some purposes; but you will not be able todistinguish their properties till we examine the compound salts whichthey form with acids; we must therefore leave soda for the present, andproceed to AMMONIA, or the VOLATILE ALKALI. 

EMILY. 

I long to hear something of this alkali; is it not of the same nature ashartshorn?

MRS. B. 

Yes, it is, as you will see by-and-bye. This alkali is seldom found innature in its pure state; it is most commonly extracted from a compoundsalt, called _sal ammoniac_, which was formerly imported from _Ammonia_, a region of Libya, from which both these salts and the alkali derivetheir names. The crystals contained in this bottle are specimens of thissalt, which consists of a combination of ammonia and muriatic acid. 

CAROLINE. 

Then it should be called _muriat of ammonia_; for though I am ignorantwhat muriatic acid is, yet I know that its combination with ammoniacannot but be so called; and I am surprised to see sal ammoniacinscribed on the label. 

MRS. B. 

That is the name by which it has been so long known, that the modernchemists have not yet succeeded in banishing it altogether; and it isstill sold under that name by druggists, though by scientific chemistsit is more properly called muriat of ammonia. 

CAROLINE. 

Both the popular and the common name should be inscribed on labels--thiswould soon introduce the new nomenclature. 

EMILY. 

By what means can the ammonia be separated from the muriatic acid?

MRS. B. 

By chemical attractions; but this operation is too complicated for youto understand, till you are better acquainted with the agency ofaffinities. 

EMILY. 

And when extracted from the salt, what kind of substance is ammonia?

MRS. B. 

Its natural form, at the temperature of the atmosphere, when free fromcombination, is that of gas; and in this state it is called _ammoniacalgas_. But it mixes very readily with water, and can be thus obtained ina liquid form. 

CAROLINE. 

You said that ammonia was more complicated in its composition than theother alkalies; pray of what principles does it consist?

MRS. B. 

It was discovered a few years since, by Berthollet, a celebrated Frenchchemist, that it consisted of about one part of hydrogen to four partsof nitrogen. Having heated ammoniacal gas under a receiver, by causingthe electrical spark to pass repeatedly through it, he found that itincreased considerably in bulk, lost all its alkaline properties, andwas actually converted into hydrogen and nitrogen gases; and from thelatest and most accurate experiments, the proportions appear to be, onevolume of nitrogen gas to three of hydrogen gas. 

CAROLINE. 

Ammonia, therefore, has not, like the two other alkalies, a metallicbasis?

MRS. B. 

It is believed it has, though it is extremely difficult to reconcilethat idea with what I have just stated of its chemical nature. But thefact is, that although this supposed metallic basis of ammonia has neverbeen obtained distinct and separate, yet both Professor Berzelius, ofStockholm, and Sir H. Davy, have succeeded in forming a combination ofmercury with the basis of ammonia, which has so much the appearance ofan amalgam, that it strongly corroborates the idea of ammonia having ametallic basis. * But these theoretical points are full of difficultiesand doubts, and it would be useless to dwell any longer upon them. 

Let us therefore return to the properties of volatile alkali. Ammoniacalgas is considerably lighter than oxygen gas, and only about half theweight of atmospherical air. It possesses most of the properties of thefixed alkalies; but cannot be of so much use in the arts on account ofits volatile nature. It is, therefore, never employed in the manufactureof glass, but it forms soap with oils equally as well as potash andsoda; it resembles them likewise in its strong attraction for water; forwhich reason it can be collected in a receiver over mercury only. 

 [Footnote *: This amalgam is easily obtained, by placing a globule of mercury upon a piece of muriat, or carbonat of ammonia, and electrifying this globule by the Voltaic battery. The globule instantly begins to expand to three or four times its former size, and becomes much less fluid, though without losing its metallic lustre, a change which is ascribed to the metallic basis of ammonia uniting with the mercury. This is an extremely curious experiment. ]

CAROLINE. 

I do not understand this?

MRS. B. 

Do you recollect the method which we used to collect gases in aglass-receiver over water?

CAROLINE. 

Perfectly. 

MRS. B. 

Ammoniacal gas has so strong a tendency to unite with water, that, instead of passing through that fluid, it would be instantaneouslyabsorbed by it. We can therefore neither use water for that purpose, norany other liquid of which water is a component part; so that, in orderto collect this gas, we are obliged to have recourse to mercury, (a liquid which has no action upon it, ) and a mercurial bath is usedinstead of a water bath, such as we employed on former occasions. Waterimpregnated with this gas is nothing more than the fluid which youmentioned at the beginning of the conversation--hartshorn; it is theammoniacal gas escaping from the water which gives it so powerful asmell. 

EMILY. 

But there is no appearance of effervescence in hartshorn. 

MRS. B. 

Because the particles of gas that rise from the water are too subtle andminute for their effect to be visible. 

Water diminishes in density, by being impregnated with ammoniacal gas;and this augmentation of bulk increases its capacity for caloric. 

EMILY. 

In making hartshorn, then, or impregnating water with ammonia, heat mustbe absorbed, and cold produced?

MRS. B. 

That effect would take place if it was not counteracted by anothercircumstance; the gas is liquefied by incorporating with the water, andgives out its latent heat. The condensation of the gas more thancounterbalances the expansion of the water; therefore, upon the whole, heat is produced. --But if you dissolve ammoniacal gas with ice or snow, cold is produced. --Can you account for that?

EMILY. 

The gas, in being condensed into a liquid, must give out heat; and, onthe other hand, the snow or ice, in being rarefied into a liquid, mustabsorb heat; so that, between the opposite effects, I should havesupposed the original temperature would have been preserved. 

MRS. B. 

But you have forgotten to take into the account the rarefaction of thewater (or melted ice) by the impregnation of the gas; and this is thecause of the cold which is ultimately produced. 

CAROLINE. 

Is the _sal volatile_ (the smell of which so strongly resembleshartshorn) likewise a preparation of ammonia?

MRS. B. 

It is carbonat of ammonia dissolved in water; and which, in its concretestate, is commonly called salts of hartshorn. Ammonia is caustic, likethe fixed alkalies, as you may judge by the pungent effects ofhartshorn, which cannot be taken internally, nor applied to delicateexternal parts, without being plentifully diluted with water. --Oil andacids are very excellent antidotes for alkaline poisons; can you guesswhy?

CAROLINE. 

Perhaps, because the oil combines with the alkali, and forms soap, andthus destroys its caustic properties; and the acid converts it into acompound salt, which, I suppose, is not so pernicious as caustic alkali. 

MRS. B. 

Precisely so. 

Ammoniacal gas, if it be mixed with atmospherical air, and a burningtaper repeatedly plunged into it, will burn with a large flame of apeculiar yellow colour. 

EMILY. 

But pray tell me, can ammonia be procured from this Lybian salt only?

MRS. B. 

So far from it, that it is contained in, and may be extracted from, allanimal substances whatever. Hydrogen and nitrogen are two of the chiefconstituents of animal matter; it is therefore not surprising that theyshould occasionally meet and combine in those proportions that composeammonia. But this alkali is more frequently generated by the spontaneousdecomposition of animal substances; the hydrogen and nitrogen gases thatarise from putrefied bodies combine, and form the volatile alkali. 

Muriat of ammonia, instead of being exclusively brought from Lybia, asit originally was, is now chiefly prepared in Europe, by chemicalprocesses. Ammonia, although principally extracted from this salt, canalso be produced by a great variety of other substances. The horns ofcattle, especially those of deer, yield it in abundance, and it is fromthis circumstance that a solution of ammonia in water has been calledhartshorn. It may likewise be procured from wool, flesh, and bones; in aword, any animal substance whatever yields it by decomposition. 

We shall now lay aside the alkalies, however important the subject maybe, till we treat of their combination with acids. The next time we meetwe shall examine the earths. 

CONVERSATION XV. 

ON EARTHS. 

MRS. B. 

The EARTHS, which we are to-day to examine, are nine in number:

 SILEX, ALUMINE, BARYTES, LIME, MAGNESIA, STRONTITES, YTTRIA, GLUCINA, ZIRCONIA. 

The last three are of late discovery; their properties are butimperfectly known; and, as they have not yet been applied to use, itwill be unnecessary to enter into any particulars respecting them; weshall confine our remarks, therefore, to the first five. They arecomposed, as you have already learnt, of a metallic basis combined withoxygen; and, from this circumstance, are incombustible. 

CAROLINE. 

Yet I have seen turf burnt in the country, and it makes an excellentfire; the earth becomes red hot, and produces a very great quantity ofheat. 

MRS. B. 

It is not the earth that burns, my dear, but the roots, grass, and otherremnants of vegetables that are intermixed with it. The caloric, whichis produced by the combustion of these substances, makes the earth redhot, and this being a bad conductor of heat, retains its caloric a longtime; but were you to examine it when cooled, you would find that it hadnot absorbed one particle of oxygen, nor suffered any alteration fromthe fire. Earth is, however, from the circumstance just mentioned, anexcellent radiator of heat, and owes its utility, when mixed with fuel, solely to that property. It is in this point of view that Count Rumfordhas recommended balls of incombustible substances to be arranged infire-places, and mixed with the coals, by which means the caloricdisengaged by the combustion of the latter is more perfectly reflectedinto the room, and an expense of fuel is saved. 

EMILY. 

I expected that the list of earths would be much more considerable. WhenI think of the great variety of soils, I am astonished that there is nota greater number of earths to form them. 

MRS. B. 

You might, indeed, almost confine that number to four; for barytes, strontites, and the others of late discovery, act but so small a part inthis great theatre, that they cannot be reckoned as essential to thegeneral formation of the globe. And you must not confine your idea ofearths to the formation of soil; for rock, marble, chalk, slate, sand, flint, and all kinds of stones, from the precious jewels to thecommonest pebbles; in a word, all the immense variety of mineralproducts, may be referred to some of these earths, either in a simplestate, or combined the one with the other, or blended with otheringredients. 

CAROLINE. 

Precious stones composed of earth! That seems very difficult toconceive. 

EMILY. 

Is it more extraordinary than that the most precious of all jewels, diamond, should be composed of carbon? But diamond forms an exception, Mrs.  B. ; for, though a stone, it is not composed of earth. 

MRS. B. 

I did not specify the exception, as I knew you were so well acquaintedwith it. Besides, I would call a diamond a mineral rather than a stone, as the latter term always implies the presence of some earth. 

CAROLINE. 

I cannot conceive how such coarse materials can be converted into suchbeautiful productions. 

MRS. B. 

We are very far from understanding all the secret resources of nature;but I do not think the spontaneous formation of the crystals, which wecall precious stones, one of the most difficult phenomena to comprehend. 

By the slow and regular work of ages, perhaps of hundreds of ages, theseearths may be gradually dissolved by water, and as gradually depositedby their solvent in the undisturbed process of crystallisation. Theregular arrangement of their particles, during their reunion in a solidmass, gives them that brilliancy, transparency, and beauty, for whichthey are so much admired; and renders them in appearance so totallydifferent from their rude and primitive ingredients. 

CAROLINE. 

But how does it happen that they are spontaneously dissolved, andafterwards crystallised?

MRS. B. 

The scarcity of many kinds of crystals, as rubies, emeralds, topazes, &c. Shows that their formation is not an operation very easily carriedon in nature. But cannot you imagine that when water, holding insolution some particles of earth, filters through the crevices of hillsor mountains, and at length dribbles into some cavern, each successivedrop may be slowly evaporated, leaving behind it the particle of earthwhich it held in solution? You know that crystallisation is more regularand perfect, in proportion as the evaporation of the solvent is slow anduniform; nature, therefore, who knows no limit of time, has, in allworks of this kind, an infinite advantage over any artist who attemptsto imitate such productions. 

EMILY. 

I can now conceive that the arrangement of the particles of earth, during crystallisation, may be such as to occasion transparency, byadmitting a free passage to the rays of light; but I cannot understandwhy crystallised earths should assume such beautiful colours as most ofthem do. Sapphire, for instance, is of a celestial blue; ruby, a deepred; topaz, a brilliant yellow?

MRS. B. 

Nothing is more simple than to suppose that the arrangement of theirparticles is such, as to transmit some of the coloured rays of light, and to reflect others, in which case the stone must appear of the colourof the rays which it reflects. But besides, it frequently happens thatthe colour of a stone is owing to a mixture of some metallic matter. 

CAROLINE. 

Pray, are the different kinds of precious stones each composed of oneindividual earth, or are they formed of a combination of several earths?

MRS. B. 

A great variety of materials enters into the composition of most ofthem; not only several earths, but sometimes salts and metals. Theearths, however, in their simple state, frequently form very beautifulcrystals; and, indeed, it is in that state only that they can beobtained perfectly pure. 

EMILY. 

Is not the Derbyshire spar produced by the crystallisation of earths, inthe way you have just explained? I have been in some of thesubterraneous caverns where it is found, which are similar to those youhave described. 

MRS. B. 

Yes; but this spar is a very imperfect specimen of crystallisation; itconsists of a variety of ingredients confusedly blended together, as youmay judge by its opacity, and by the various colours and appearanceswhich it exhibits. 

But, in examining the earths in their most perfect and agreeable form, we must not lose sight of that state in which they are commonly found, and which, if less pleasing to the eye, is far more interesting by itsutility. 

All the earths are more or less endowed with alkaline properties; butthere are four, barytes, magnesia, lime, and strontites, which arecalled _alkaline earths_, because they possess those qualities in sogreat a degree, as to entitle them, in most respects, to the rank ofalkalies. They combine and form compound salts with acids, in the sameway as alkalies; they are, like them, susceptible of a considerabledegree of causticity, and are acted upon in a similar manner by chemicaltests. --The remaining earths, silex and alumine, with one or two othersof late discovery, are in some degree more earthy, that is to say, theypossess more completely the properties common to all the earths, whichare, insipidity, dryness, unalterableness in the fire, infusibility,  &c. 

CAROLINE. 

Yet, did you not tell us that silex, or siliceous earth, when mixed withan alkali, was fusible, and run into glass?

MRS. B. 

Yes, my dear; but the characteristic properties of earths, which I havementioned, are to be considered as belonging to them in a state ofpurity only; a state in which they are very seldom to be met with innature. --Besides these general properties, each earth has its ownspecific characters, by which it is distinguished from any othersubstance. --Let us therefore review them separately. 

SILEX, or SILICA, abounds in flint, sand, sandstone, agate, jasper, &c. ;it forms the basis of many precious stones, and particularly of thosewhich strike fire with steel. It is rough to the touch, scratches andwears away metals; it is acted upon by no acid but the fluoric, and isnot soluble in water by any known process; but nature certainlydissolves it by means with which we are unacquainted, and thus producesa variety of siliceous crystals, and amongst these _rock crystal_, whichis the purest specimen of this earth. Silex appears to have beenintended by Providence to form the solid basis of the globe, to serve asa foundation for the original mountains, and give them that hardness anddurability which has enabled them to resist the various revolutionswhich the surface of the earth has successively undergone. From thesemountains siliceous rocks have, during the course of ages, beengradually detached by torrents of water, and brought down in fragments;these, in the violence and rapidity of their descent, are sometimescrumbled to sand, and in this state form the beds of rivers and of thesea, chiefly composed of siliceous materials. Sometimes the fragmentsare broken without being pulverised by their fall, and assume the formof pebbles, which gradually become rounded and polished. 

EMILY. 

Pray what is the true colour of silex, which forms such a variety ofdifferent coloured substances? Sand is brown, flint is nearly black, andprecious stones are of all colours. 

MRS. B. 

Pure silex, such as is found only in the chemist’s laboratory, isperfectly white, and the various colours which it assumes, in thedifferent substances you have just mentioned, proceed from the differentingredients with which it is mixed in them. 

CAROLINE. 

I wonder that silex is not more valuable, since it forms the basis of somany precious stones. 

MRS. B. 

You must not forget that the value we set upon precious stones dependsin a great measure upon the scarcity with which nature affords them;for, were those productions either common or perfectly imitable by art, they would no longer, notwithstanding their beauty, be so highlyesteemed. But the real value of siliceous earth, in many of the mostuseful arts, is very extensive. Mixed with clay, it forms the basis ofall the various kinds of earthen ware, from the most common utensils tothe most refined ornaments. 

EMILY. 

And we must recollect its importance in the formation of glass withpotash. 

MRS. B. 

Nor should we omit to mention, likewise, many other important uses ofsilex, such as being the chief ingredient of some of the most durablecements, of mortar,  &c. 

I said before, that siliceous earth combined with no acid but thefluoric; it is for this reason that glass is liable to be attacked bythat acid only, which, from its strong affinity for silex, forces thatsubstance from its combination with the potash, and thus destroys theglass. 

We will now hasten to proceed to the other earths, for I am ratherapprehensive of your growing weary of this part of our subject. 

CAROLINE. 

The history of the earths is not quite so entertaining as that of thesimple substances. 

MRS. B. 

Perhaps not; but it is absolutely indispensable that you should knowsomething of them; for they form the basis of so many interesting andimportant compounds, that their total omission would throw greatobscurity on our general outline of chemical science. We shall, however, review them in as cursory a manner as the subject can admit of. 

ALUMINE derives its name from a compound salt called _alum_, of which itforms the basis. 

CAROLINE. 

But it ought to be just the contrary, Mrs.  B. ; the simple body shouldgive, instead of taking, its name from the compound. 

MRS. B. 

That is true; but as the compound salt was known long before its basiswas discovered, it was very natural that when the earth was at lengthseparated from the acid, it should derive its name from the compoundfrom which it was obtained. However, to remove your scruples, we willcall the salt according to the new nomenclature, _sulphat of alumine_. From this combination, alumine may be obtained in its pure state; it isthen soft to the touch, makes a paste with water, and hardens in thefire. In nature, it is found chiefly in clay, which contains aconsiderable proportion of this earth; it is very abundant in fuller’searth, slate, and a variety of other mineral productions. There isindeed scarcely any mineral substance more useful to mankind thanalumine. In the state of clay, it forms large strata of the earth, givesconsistency to the soil of valleys, and of all low and damp spots, suchas swamps and marshes. The beds of lakes, ponds, and springs, are almostentirely of clay; instead of allowing of the filtration of water, assand does, it forms an impenetrable bottom, and by this means water isaccumulated in the caverns of the earth, producing those reservoirswhence springs issue, and spout out at the surface. 

EMILY. 

I always thought that these subterraneous reservoirs of water werebedded by some hard stone, or rock, which the water could not penetrate. 

MRS. B. 

That is not the case; for in the course of time water would penetrate, or wear away silex, or any other kind of stone, while it is effectuallystopped by clay, or alumine. 

The solid compact soils, such as are fit for corn, owe their consistencein a great measure to alumine; this earth is therefore used to improvesandy or chalky soils, which do not retain a sufficient quantity ofwater for the purpose of vegetation. 

Alumine is the most essential ingredient in all potteries. It entersinto the composition of brick, as well as that of the finest porcelain;the addition of silex and water hardens it, renders it susceptible of adegree of vitrification, and makes it perfectly fit for its variouspurposes. 

CAROLINE. 

I can scarcely conceive that brick and china should be made of the samematerials. 

MRS. B. 

Brick consists almost entirely of baked clay; but a certain proportionof silex is essential to the formation of earthen or stone ware. Incommon potteries sand is used for that purpose; a more pure silex is, I believe, necessary for the composition of porcelain, as well as afiner kind of clay; and these materials are, no doubt, more carefullyprepared, and curiously wrought, in the one case than in the other. Porcelain owes its beautiful semitransparency to a commencement ofvitrification. 

EMILY. 

But the commonest earthen-ware, though not transparent, is covered witha kind of glazing. 

MRS. B. 

That precaution is equally necessary for use as for beauty, as the warewould be liable to be spoiled and corroded by a variety of substances, if not covered with a coating of this kind. In porcelain it consists ofenamel, which is a fine white opake glass, formed of metallic oxyds, sand, salts, and such other materials as are susceptible ofvitrification. The glazing of common earthen-ware is made chiefly ofoxyd of lead, or sometimes merely of salt, which, when thinly spreadover earthen vessels, will, at a certain heat, run into opake glass. 

CAROLINE. 

And of what nature are the colours which are used for paintingporcelain?

MRS. B. 

They are all composed of metallic oxyds, so that these colours, insteadof receiving injury from the application of fire, are strengthened anddeveloped by its action, which causes them to undergo different degreesof oxydation. 

Alumine and silex are not only often combined by art, but they have innature a very strong tendency to unite, and are found combined, indifferent proportions, in various gems and other minerals. Indeed, manyof the precious stones, such as ruby, oriental sapphire, amethyst, &c. Consist chiefly of alumine. 

We may now proceed to the alkaline earths, I shall say but a few wordson BARYTES, as it is hardly ever used, except in chemical laboratories. It is remarkable for its great weight, and its strong alkalineproperties, such as destroying animal substances, turning green someblue vegetable colours, and showing a powerful attraction for acids;this last property it possesses to such a degree, particularly withregard to the sulphuric acid, that it will always detect its presence inany substance or combination whatever, by immediately uniting with it, and forming a sulphat of barytes. This renders it a very valuablechemical test. It is found pretty abundantly in nature in the state ofcarbonat, from which the pure earth can be easily separated. 

The next earth we have to consider is LIME. This is a substance of toogreat and general importance to be passed over so slightly as the last. 

Lime is strongly alkaline. In nature it is not met with in its simplestate, as its affinity for water and carbonic acid is so great, that itis always found combined with these substances, with which it forms thecommon lime-stone; but it is separated in the kiln from theseingredients, which are volatilised whenever a sufficient degree of heatis applied. 

EMILY. 

Pure lime, then, is nothing but lime-stone, which has been deprived, inthe kiln, of its water and carbonic acid?

MRS. B. 

Precisely: in this state it is called _quick-lime_, and it is socaustic, that it is capable of decomposing the dead bodies of animalsvery rapidly, without their undergoing the process of putrefaction. --I have here some quick lime, which is kept carefully corked up in abottle to prevent the access of air; for were it at all exposed to theatmosphere, it would absorb both moisture and carbonic acid gas from it, and be soon slaked. Here is also some lime-stone--we shall pour a littlewater on each, and observe the effects that result from it. 

CAROLINE. 

How the quick-lime hisses! It is become excessively hot! --It swells, and now it bursts and crumbles to powder, while the water appears toproduce no kind of alteration on the lime-stone. 

MRS. B. 

Because the lime-stone is already saturated with water, whilst thequick-lime, which has been deprived of it in the kiln, combines with itwith very great avidity, and produces this prodigious disengagement ofheat, the cause of which I formerly explained to you; do yourecollect it?

EMILY. 

Yes; you said that the heat did not proceed from the lime, but from thewater which was _solidified_, and thus parted with its heat ofliquidity. 

MRS. B. 

Very well. If we continue to add successive quantities of water to thelime after being slaked and crumbled as you see, it will then graduallybe diffused in the water, till it will at length be dissolved in it, andentirely disappear; but for this purpose it requires no less than 700times its weight of water. This solution is called _lime-water_. 

CAROLINE. 

How very small, then, is the proportion of lime dissolved!

MRS. B. 

Barytes is still of more difficult solution; it dissolves only in 900times its weight of water: but it is much more soluble in the state ofcrystals. The liquid contained in this bottle is lime-water; it is oftenused as a medicine, chiefly, I believe, for the purpose of combiningwith, and neutralising, the superabundant acid which it meets with inthe stomach. 

EMILY. 

I am surprised that it is so perfectly clear; it does not at all partakeof the whiteness of the lime. 

MRS. B. 

Have you forgotten that, in solutions, the solid body is so minutelysubdivided by the fluid as to become invisible, and therefore will notin the least degree impair the transparency of the solvent?

I said that the attraction of lime for carbonic acid was so strong, thatit would absorb it from the atmosphere. We may see this effect byexposing a glass of lime-water to the air; the lime will then separatefrom the water, combine with the carbonic acid, and re-appear on thesurface in the form of a white film, which is carbonat of lime, commonlycalled _chalk_. 

CAROLINE. 

Chalk is, then, a compound salt! I never should have supposed that thoseimmense beds of chalk, that we see in many parts of the country, were asalt. --Now, the white film begins to appear on the surface of thewater; but it is far from resembling hard solid chalk. 

MRS. B. 

That is owing to its state of extreme division; in a little time it willcollect into a more compact mass, and subside at the bottom of theglass. 

If you breathe into lime-water, the carbonic acid, which is mixed withthe air that you expire, will produce the same effect. It is anexperiment very easily made; --I shall pour some lime-water into thisglass tube, and, by breathing repeatedly into it, you will soon perceivea precipitation of chalk--

EMILY. 

I see already a small white cloud formed. 

MRS. B. 

It is composed of minute particles of chalk; at present it floats in thewater, but it will soon subside. 

Carbonat of lime, or chalk, you see, is insoluble in water, since thelime which was dissolved re-appears when converted into chalk; but youmust take notice of a very singular circumstance, which is, that chalkis soluble in water impregnated with carbonic acid. 

CAROLINE. 

It is very curious, indeed, that carbonic acid gas should render limesoluble in one instance, and insoluble in the other!

MRS. B. 

I have here a bottle of Seltzer water, which, you know, is stronglyimpregnated with carbonic acid:-- let us pour a little of it into aglass of lime-water. You see that it immediately forms a precipitationof carbonat of lime?

EMILY. 

Yes, a white cloud appears. 

MRS. B. 

I shall now pour an additional quantity of the Seltzer water into thelime-water--

EMILY. 

How singular! The cloud is re-dissolved, and the liquid is againtransparent. 

MRS. B. 

All the mystery depends upon this circumstance, that carbonat of lime issoluble in carbonic acid, whilst it is insoluble in water; the firstquantity of carbonic acid, therefore, which I introduce into thelime-water, was employed in forming the carbonat of lime, which remainedvisible, until an additional quantity of carbonic acid dissolved it. Thus, you see, when the lime and carbonic acid are in proper proportionsto form chalk, the white cloud appears, but when the acid predominates, the chalk is no sooner formed than it is dissolved. 

CAROLINE. 

That is now the case; but let us try whether a further addition oflime-water will again precipitate the chalk. 

EMILY. 

It does, indeed! The cloud re-appears, because, I suppose, there is nowno more of the carbonic acid than is necessary to form chalk; and, inorder to dissolve the chalk, a superabundance of acid is required. 

MRS. B. 

We have, I think, carried this experiment far enough; every repetitionwould but exhibit the same appearances. 

Lime combines with most of the acids, to which the carbonic (as beingthe weakest) readily yields it; but these combinations we shall have anopportunity of noticing more particularly hereafter. It unites withphosphorus, and with sulphur, in their simple state; in short, of allthe earths, lime is that which nature employs most frequently, and mostabundantly, in its innumerable combinations. It is the basis of allcalcareous earths and stones; we find it likewise in the animal and thevegetable creations. 

EMILY. 

And in the arts is not lime of very great utility?

MRS. B. 

Scarcely any substance more so; you know that it is a most essentialrequisite in building, as it constitutes the basis of all cements, suchas mortar, stucco, plaister,  &c. 

Lime is also of infinite importance in agriculture; it lightens andwarms soils that are too cold, and compact, in consequence of too greata proportion of clay. --But it would be endless to enumerate the variouspurposes for which it is employed; and you know enough of it to formsome idea of its importance; we shall, therefore, now proceed to thethird alkaline earth, MAGNESIA. 

CAROLINE. 

I am already pretty well acquainted with that earth; it is a medicine. 

MRS. B. 

It is in the state of carbonat that magnesia is usually employedmedicinally; it then differs but little in appearance from its simpleform, which is that of a very fine light white powder. It dissolves in2000 times its weight of water, but forms with acids extremely solublesalts. It has not so great an attraction for acids as lime, andconsequently yields them to the latter. It is found in a great varietyof mineral combinations, such as slate, mica, amianthus, and moreparticularly in a certain lime stone, which has lately been discoveredby Mr. Tennant to contain it in very great quantities. It does notattract and solidify water, like lime: but when mixed with water andexposed to the atmosphere, it slowly absorbs carbonic acid from thelatter, and thus loses its causticity. Its chief use in medicine is, like that of lime, derived from its readiness to combine with, andneutralise, the acid which it meets with in the stomach. 

EMILY. 

Yet, you said that it was taken in the state of carbonat, in which caseit has already combined with an acid?

MRS. B. 

Yes; but the carbonic is the last of all the acids in the order ofaffinities; it will therefore yield the magnesia to any of the others. It is, however, frequently taken in its caustic state as a remedy forflatulence. Combined with sulphuric acid, magnesia forms another andmore powerful medicine, commonly called _Epsom salt_. 

CAROLINE. 

And properly, _sulphat of magnesia_, I suppose? Pray why was it evercalled Epsom salt?

MRS. B. 

Because there is a spring in the neighbourhood of Epsom which containsthis salt in great abundance. 

The last alkaline earth which we have to mention is STRONTIAN, orSTRONTITES, discovered by Dr. Hope a few years ago. It so stronglyresembles barytes in its properties, and is so sparingly found innature, and of so little use in the arts, that it will not be necessaryto enter into any particulars respecting it. One of the remarkablecharacteristic properties of strontites is, that its salts, whendissolved in spirit of wine, tinge the flame of a deep red, or bloodcolour. 

CONVERSATION XVI. 

ON ACIDS. 

MRS. B. 

We may now proceed to the acids. Of the metallic oxyds, you have alreadyacquired some general notions. This subject, though highly interestingin its details, is not of sufficient importance to our concise view ofchemistry, to be particularly treated of; but it is absolutely necessarythat you should be better acquainted with the acids, and likewise withtheir combinations with the alkalies, which form the triple compoundscalled NEUTRAL SALTS. 

The class of acids is characterised by very distinct properties. Theyall change blue vegetable infusions to a red colour: they are all moreor less sour to the taste; and have a general tendency to combine withthe earths, alkalies, and metallic oxyds. 

You have, I believe, a clear idea of the nomenclature by which the base(or radical) of the acid, and the various degrees of acidification, areexpressed?

EMILY. 

Yes, I think so; the acid is distinguished by the name of its base, andits degree of oxydation, that is, the quantity of oxygen it contains, bythe termination of that name in _ous_ or _ic_; thus sulphure_ous_ acidis that formed by the smallest proportion of oxygen combined withsulphur; sulphur_ic_ acid that which results from the combination ofsulphur with the greatest quantity of oxygen. 

MRS. B. 

A still greater latitude may, in many cases, be allowed to theproportions of oxygen than can be combined with acidifiable radicals;for several of these radicals are susceptible of uniting with a quantityof oxygen so small as to be insufficient to give them the properties ofacids; in these cases, therefore, they are converted into oxyds. Such issulphur, which by exposure to the atmosphere with a degree of heatinadequate to produce inflammation, absorbs a small proportion ofoxygen, which colours it red or brown. This, therefore, is the firstdegree of oxygenation of sulphur; the 2d converts it into sulphur_ous_acid; the 3d into the sulphur_ic_ acid; and 4thly, if it was foundcapable of combining with a still larger proportion of oxygen, it wouldthen be termed _super-oxygenated sulphuric acid_. 

EMILY. 

Are these various degrees of oxygenation common to all the acids?

MRS. B. 

No; they vary much in this respect: some are susceptible of only onedegree of oxygenation; others, of two, or three; there are but very fewthat will admit of more. 

CAROLINE. 

The modern nomenclature must be of immense advantage in pointing out soeasily the nature of the acids, and their various degrees ofoxygenation. 

MRS. B. 

Till lately many of the acids had not been decomposed; but analogyafforded so strong a proof of their compound nature, that I never couldreconcile myself to classing them with the simple bodies, though thisdivision has been adopted by several chemical writers. At present thereare only the muriatic and the fluoric acids, which have not had theirbases distinctly separated. 

CAROLINE. 

We have heard of a great variety of acids; pray how many are there inall?

MRS. B. 

I believe there are reckoned at present thirty-four, and their number isconstantly increasing, as the science improves; but the most important, and those to which we shall almost entirely confine our attention, arebut few. I shall, however, give you a general view of the whole; andthen we shall more particularly examine those that are the mostessential. 

This class of bodies was formerly divided into mineral, vegetable, andanimal acids, according to the substances from which they were commonlyobtained. 

CAROLINE. 

That, I should think, must have been an excellent arrangement; why wasit altered?

MRS. B. 

Because in many cases it produced confusion. In which class, forinstance, would you place carbonic acid?

CAROLINE. 

Now I see the difficulty. I should be at a loss where to place it, asyou have told us that it exists in the animal, vegetable, and mineralkingdoms. 

EMILY. 

There would be the same objection with respect to phosphoric acid, which, though obtained chiefly from bones, can also, you said, be foundin small quantities in stones, and likewise in some plants. 

MRS. B. 

You see, therefore, the propriety of changing this mode ofclassification. These objections do not exist in the presentnomenclature; for the composition and nature of each individual acid isin some degree pointed out, instead of the class of bodies from which itis extracted; and, with regard to the more general division of acids, they are classed under these three heads:

First, Acids of known or supposed simple bases, which are formed by theunion of these bases with oxygen. They are the following:

The _Sulphuric_ _Carbonic_ _Nitric_ _Phosphoric_ _Arsenical_ Acids, of known and simple bases. _Tungstenic_ _Molybdenic_ _Boracic_ _Fluoric_ _Muriatic_

This class comprehends the most anciently known and most importantacids. The sulphuric, nitric, and muriatic were formerly, and are stillfrequently, called _mineral acids_. 

2dly, Acids that have double or binary radicals, and which consequentlyconsist of triple combinations. These are the vegetable acids, whosecommon radical is a compound of hydrogen and carbon. 

CAROLINE. 

But if the basis of all the vegetable acids be the same, it should formbut one acid; it may indeed combine with different proportions ofoxygen, but the nature of the acid must be the same. 

MRS. B. 

The only difference that exists in the basis of vegetable acids, is thevarious proportions of hydrogen and carbon from which they are severallycomposed. But this is enough to produce a number of acids apparentlyvery dissimilar. That they do not, however, differ essentially, isproved by their susceptibility of being converted into each other, bythe addition or subtraction of a portion of hydrogen or of carbon. Thenames of these acids are, The _Acetic_ _Oxalic_ _Tartarous_ _Citric_ _Malic_ Acids, of double bases, being of vegetable origin. _Gallic_ _Mucous_ _Benzoic_ _Succinic_ _Camphoric_ _Suberic_

The 3d class of acids consists of those which have triple radicals, andare therefore of a still more compound nature. This class comprehendsthe animal acids, which are, The _Lactic_ _Prussic_ _Formic_ Acids, of triple bases, or animal acids. _Bombic_ _Sebacic_ _Zoonic_ _Lithic_

I have given you this summary account or enumeration of the acids, asyou may find it more satisfactory to have at once an outline or ageneral notion of the extent of the subject; but we shall now confineourselves to the first class, which requires our more immediateattention; and defer the few remarks which we shall have to make on theothers, till we treat of the chemistry of the animal and vegetablekingdoms. 

The acids of simple and known radicals are all capable of beingdecomposed by combustible bodies, to which they yield their oxygen. If, for instance, I pour a drop of sulphuric acid on this piece of iron, itwill produce a spot of rust, you know what that is?

CAROLINE. 

Yes; it is an oxyd, formed by the oxygen of the acid combining with theiron. 

MRS. B. 

In this case you see the sulphur deposits the oxygen by which it wasacidified on the metal. And again, if we pour some acid on a compoundcombustible substance, (we shall try it on this piece of wood, ) it willcombine with one or more of the constituents of that substance, andoccasion a decomposition. 

EMILY. 

It has changed the colour of the wood to black. How is that?

MRS. B. 

The oxygen deposited by the acid has burnt it; you know that wood inburning becomes black before it is reduced to ashes. Whether it derivesthe oxygen which burns it from the atmosphere, or from any other source, the chemical effect on the wood is the same. In the case of realcombustion, wood becomes black, because it is reduced to the state ofcharcoal by the evaporation of its other constituents. But can you tellme the reason why wood turns black when burnt by the application of anacid?

CAROLINE. 

First, tell me what are the ingredients of wood?

MRS. B. 

Hydrogen and carbon are the chief constituents of wood, as of all othervegetable substances. 

CAROLINE. 

Well, then, I suppose that the oxygen of the acid combines with thehydrogen of the wood, to form water; and that the carbon of the wood, remaining alone, appears of its usual black colour. 

MRS. B. 

Very well indeed, my dear; that is certainly the most plausibleexplanation. 

EMILY. 

Would not this be a good method of making charcoal?

MRS. B. 

It would be an extremely expensive, and, I believe, very imperfectmethod; for the action of the acid on the wood, and the heat produced byit, are far from sufficient to deprive the wood of all its evaporableparts. 

CAROLINE. 

What is the reason that vinegar, lemon, and the acid of fruits, do notproduce this effect on wood?

MRS. B. 

They are vegetable acids, whose bases are composed of hydrogen andcarbon; the oxygen, therefore, will not be disposed to quit thisradical, where it is already united with hydrogen. The strongest ofthese may, perhaps, yield a little of their oxygen to the wood, andproduce a stain upon it; but the carbon will not be sufficientlyuncovered to assume its black colour. Indeed, the several mineral acidsthemselves possess this power of charring wood in very differentdegrees. 

EMILY. 

Cannot vegetable acids be decomposed, by any combustibles?

MRS. B. 

No; because their radical is composed of two substances which have agreater attraction for oxygen than any known body. 

CAROLINE. 

And are those strong acids, which burn and decompose wood, capable ofproducing similar effects on the skin and flesh of animals?

MRS. B. 

Yes; all the mineral acids, and one of them more especially, possesspowerful caustic qualities. They actually corrode and destroy the skinand flesh; but they do not produce upon these exactly the samealteration they do on wood, probably because there is a great proportionof nitrogen and other substances in animal matter, which prevents theseparation of carbon from being so conspicuous. 

CONVERSATION XVII. 

OF THE SULPHURIC AND PHOSPHORIC ACIDS; OR THE COMBINATIONS OF OXYGENWITH SULPHUR AND PHOSPHORUS; AND OF THE SULPHATS AND PHOSPHATS. 

MRS. B. 

In addition to the general survey which we have taken of acids, I thinkyou will find it interesting to examine individually a few of the mostimportant of them, and likewise some of their principal combinationswith the alkalies, alkaline earths, and metals. The first of the acids, in point of importance, is the SULPHURIC, formerly called _oil ofvitriol_. 

CAROLINE. 

I have known it a long time by that name, but had no idea that it wasthe same fluid as sulphuric acid. What resemblance or connection canthere be between oil of vitriol and this acid?

MRS. B. 

Vitriol is the common name for sulphat of iron, a salt which is formedby the combination of sulphuric acid and iron; the sulphuric acid wasformerly obtained by distillation from this salt, and it very naturallyreceived its name from the substance which afforded it. 

CAROLINE. 

But it is still usually called oil of vitriol?

MRS. B. 

Yes; a sufficient length of time has not yet elapsed, since theinvention of the new nomenclature, for it to be generally disseminated;but, as it is adopted by all scientific chemists, there is every reasonto suppose that it will gradually become universal. When I received thisbottle from the chemists, _oil of vitriol_ was inscribed on the label;but, as I knew you were very punctilious in regard to the nomenclature, I changed it, and substituted the words _sulphuric acid_. 

EMILY. 

This acid has neither colour nor smell, but it appears much thicker thanwater. 

MRS. B. 

It is nearly twice as heavy as water, and has, you see, an oilyconsistence. 

CAROLINE. 

And it is probably from this circumstance that it has been called anoil, for it can have no real claim to that name, as it does not containeither hydrogen or carbon, which are the essential constituents of oil. 

MRS. B. 

Certainly; and therefore it would be the more absurd to retain a namewhich owed its origin to such a mistaken analogy. 

Sulphuric acid, in its purest state, would probably be a concretesubstance, but its attraction for water is such, that it is impossibleto obtain that acid perfectly free from it; it is, therefore, alwaysseen in a liquid form, such as you here find it. One of the moststriking properties of sulphuric acid is that of evolving a considerablequantity of heat when mixed with water; this I have already shown you. 

EMILY. 

Yes, I recollect it; but what was the degree of heat produced by thatmixture?

MRS. B. 

The thermometer may be raised by it to 300 degrees, which isconsiderably above the temperature of boiling water. 

CAROLINE. 

Then water might be made to boil in that mixture?

MRS. B. 

Nothing more easy, provided that you employ sufficient quantities ofacid and of water, and in the due proportions. The greatest heat isproduced by a mixture of one part of water to four of the acid: we shallmake a mixture of these proportions, and immerse in it this thin glasstube, which is full of water. 

CAROLINE. 

The vessel feels extremely hot, but the water does not boil yet. 

MRS. B. 

You must allow some time for the heat to penetrate the tube, and raisethe temperature of the water to the boiling point--

CAROLINE. 

Now it boils--and with increasing violence. 

MRS. B. 

But it will not continue boiling long; for the mixture gives out heatonly while the particles of the water and the acid are mutuallypenetrating each other: as soon as the new arrangement of thoseparticles is effected, the mixture will gradually cool, and the waterreturn to its former temperature. 

You have seen the manner in which sulphuric acid decomposes allcombustible substances, whether animal, vegetable, or mineral, and burnsthem by means of its oxygen?

CAROLINE. 

I have very unintentionally repeated the experiment on my gown, byletting a drop of the acid fall upon it, and it has made a stain, which, I suppose, will never wash out. 

MRS. B. 

No, certainly; for before you can put it into water, the spot willbecome a hole, as the acid has literally burnt the muslin. 

CAROLINE. 

So it has, indeed! Well, I will fasten the stopper, and put the bottleaway, for it is a dangerous substance. --Oh, now I have done worsestill, for I have spilt some on my hand!

MRS. B. 

It is then burned, as well as your gown, for you know that oxygendestroys animal as well as vegetable matters; and, as far as thedecomposition of the skin of your finger is effected, there is noremedy; but by washing it immediately in water, you will dilute theacid, and prevent any further injury. 

CAROLINE. 

It feels extremely hot, I assure you. 

MRS. B. 

You have now learned, by experience, how cautiously this acid must beused. You will soon become acquainted with another acid, the nitric, which, though it produces less heat on the skin, destroys it stillquicker, and makes upon it an indelible stain. You should never handleany substances of this kind, without previously dipping your fingers inwater, which will weaken their caustic effects. But, since you will notrepeat the experiment, I must put in the stopper, for the acid attractsthe moisture from the atmosphere, which would destroy its strength andpurity. 

EMILY. 

Pray, how can sulphuric acid be extracted from sulphat of iron bydistillation?

MRS. B. 

The process of distillation, you know, consists in separating substancesfrom one another by means of their different degrees of volatility, andby the introduction of a new chemical agent, caloric. Thus, if sulphatof iron be exposed in a retort to a proper degree of heat, it will bedecomposed, and the sulphuric acid will be volatilised. 

EMILY. 

But now that the process of forming acids by the combustion of theirradicals is known, why should not this method be used for makingsulphuric acid?

MRS. B. 

This is actually done in most manufactures; but the usual method ofpreparing sulphuric acid does not consist in burning the sulphur inoxygen gas (as we formerly did by the way of experiment), but in heatingit together with another substance, nitre, which yields oxygen insufficient abundance to render the combustion in common air rapid andcomplete. 

CAROLINE. 

This substance, then, answers the same purpose as oxygen gas?

MRS. B. 

Exactly. In manufactures the combustion is performed in a leadenchamber, with water at the bottom, to receive the vapour and assist itscondensation. The combustion is, however, never so perfect but that aquantity of _sulphureous_ acid is formed at the same time; for yourecollect that the sulphureous acid, according to the chemicalnomenclature, differs from the sulphuric only by containing less oxygen. 

From its own powerful properties, and from the various combinations intowhich it enters, sulphuric acid is of great importance in many of thearts. 

It is used also in medicine in a state of great dilution; for were ittaken internally, in a concentrated state, it would prove a mostdangerous poison. 

CAROLINE. 

I am sure it would burn the throat and stomach. 

MRS. B. 

Can you think of any thing that would prove an antidote to this poison?

CAROLINE. 

A large draught of water to dilute it. 

MRS. B. 

That would certainly weaken the caustic power of the acid, but it wouldincrease the heat to an intolerable degree. Do you recollect nothingthat would destroy its deleterious properties more effectually?

EMILY. 

An alkali might, by combining with it; but, then, a pure alkali isitself a poison, on account of its causticity. 

MRS. B. 

There is no necessity that the alkali should be caustic. Soap, in whichit is combined with oil; or magnesia, either in the state of carbonat, or mixed with water, would prove the best antidotes. 

EMILY. 

In those cases then, I suppose, the potash and the magnesia would quittheir combinations to form salts with the sulphuric acid?

MRS. B. 

Precisely. 

We may now make a few observations on the sulphure_ous_ acid, which wehave found to be the product of sulphur slowly and imperfectly burnt. This acid is distinguished by its pungent smell, and its gaseous form. 

CAROLINE. 

Its aëriform state is, I suppose, owing to the smaller proportion ofoxygen, which renders it lighter than sulphur_ic_ acid?

MRS. B. 

Probably; for by adding oxygen to the weaker acid, it may be convertedinto the stronger kind. But this change of state may also be connectedwith a change of affinity with regard to caloric. 

EMILY. 

And may sulphureous acid be obtained from sulphuric acid by a diminutionof oxygen?

MRS. B. 

Yes; it can be done by bringing any combustible substance in contactwith the acid. This decomposition is most easily performed by some ofthe metals; these absorb a portion of the oxygen from the sulphuricacid, which is thus converted into the sulphureous, and flies off in itsgaseous form. 

CAROLINE. 

And cannot the sulphureous acid itself be decomposed and reduced tosulphur?

MRS. B. 

Yes; if this gas be heated in contact with charcoal, the oxygen of thegas will combine with it, and the pure sulphur is regenerated. 

Sulphureous acid is readily absorbed by water; and in this liquid stateit is found particularly useful in bleaching linen and woollen cloths, and is much used in manufactures for those purposes. I can show you itseffect in destroying colours, by taking out vegetable stains--I think Isee a spot on your gown, Emily, on which we may try the experiment. 

EMILY. 

It is the stain of mulberries; but I shall be almost afraid of exposingmy gown to the experiment, after seeing the effect which the sulphuricacid produced on that of Caroline--

MRS. B. 

There is no such danger from the sulphureous; but the experiment must bemade with great caution, for, during the formation of sulphureous acidby combustion, there is always some sulphuric produced. 

CAROLINE. 

But where is your sulphureous acid?

MRS. B. 

We may easily prepare some ourselves, simply by burning a match; we mustfirst wet the stain with water, and now hold it in this way, at a littledistance, over the lighted match: the vapour that arises from it issulphureous acid, and the stain, you see, gradually disappears. 

EMILY. 

I have frequently taken out stains by this means, without understandingthe nature of the process. But why is it necessary to wet the stainbefore it is exposed to the acid fumes?

MRS. B. 

The moisture attracts and absorbs the sulphureous acid; and it serveslikewise to dilute any particles of sulphuric acid which might injurethe linen. 

Sulphur is susceptible of a third combination with oxygen, in which theproportion of the latter is too small to render the sulphur acid. Itacquires this slight oxygenation by mere exposure to the atmosphere, without any elevation of temperature: in this case, the sulphur does notchange its natural form, but is only discoloured, being changed to redor brown; and in this state it is an oxyd of sulphur. 

Before we take leave of the sulphuric acid, we shall say a few words ofits principal combinations. It unites with all the alkalies, alkalineearths and metals, to form compound salts. 

CAROLINE. 

Pray, give me leave to interrupt you for a moment: you have nevermentioned any other salts than the compound or neutral salts; is thereno other kind?

MRS. B. 

The term _salt_ has been used, from time immemorial, as a kind ofgeneral name for any substance that has savour, odour, is soluble inwater, and crystallisable, whether it be of an acid, an alkaline, orcompound nature; but the compound salts alone retain that appellation inmodern chemistry. 

The most important of the salts, formed by the combinations of thesulphuric acid, are, first, _sulphat of potash_, formerly called _salpolychrest_: this is a very bitter salt, much used in medicine; it isfound in the ashes of most vegetables, but it may be preparedartificially by the immediate combination of sulphuric acid and potash. This salt is easily soluble in boiling water. Solubility is, indeed, a property common to all salts; and they always produce cold in melting. 

EMILY. 

That must be owing to the caloric which they absorb in passing from asolid to a fluid form. 

MRS. B. 

That is, certainly, the most probable explanation. 

_Sulphat of soda_, commonly called Glauber’s salt, is another medicinalsalt, which is still more bitter than the preceding. We must preparesome of these compounds, that you may observe the phenomena which takeplace during their formation. We need only pour some sulphuric acid overthe soda which I have put into this glass. 

CAROLINE. 

What an amazing heat is disengaged! --I thought you said that cold wasproduced by the melting of salts?

MRS. B. 

But you must observe that we are now _making_, not _melting_ a salt. Heat is disengaged during the formation of compound salts, and a faintlight is also emitted, which may sometimes be perceived in the dark. 

EMILY. 

And is this heat and light produced by the union of the oppositeelectricities of the alkali and the acid?

MRS. B. 

No doubt it is, if that theory be true. 

CAROLINE. 

The union of an acid and an alkali is then an actual combustion?

MRS. B. 

Not precisely, though there is certainly much analogy in theseprocesses. 

CAROLINE. 

Will this sulphat of soda become solid?

MRS. B. 

We have not, I suppose, mixed the acid and the alkali in the exactproportions that are required for the formation of the salt, otherwisethe mixture would have been almost immediately changed to a solid mass;but, in order to obtain it in crystals, as you see it in this bottle, itwould be necessary first to dilute it with water, and afterwards toevaporate the water, during which operation the salt would graduallycrystallise. 

CAROLINE. 

But of what use is the addition of water, if it is afterwards to beevaporated?

MRS. B. 

When suspended in water, the acid and the alkali are more at liberty toact on each other, their union is more complete, and the salt assumesthe regular form of crystals during the slow evaporation of its solvent. 

Sulphat of soda liquefies by heat, and effloresces in the air. 

EMILY. 

Pray what is the meaning of the word _effloresces_? I do not recollectyour having mentioned it before. 

MRS. B. 

A salt is said to effloresce when it loses its water of crystallisationon being exposed to the atmosphere, and is thus gradually converted intoa dry powder: you may observe that these crystals of sulphat of soda arefar from possessing the transparency which belongs to their crystallinestate; they are covered with a white powder, occasioned by their havingbeen exposed to the atmosphere, which has deprived their surface of itslustre, by absorbing its water of crystallisation. Salts are, ingeneral, either _efflorescent_ or _deliquescent_: this latter propertyis precisely the reverse of the former; that is to say, deliquescentsalts absorb water from the atmosphere, and are moistened and graduallymelted by it. Muriat of lime is an instance of great deliquescence. 

EMILY. 

But are there no salts that have the same degree of attraction for wateras the atmosphere, and that will consequently not be affected by it?

MRS. B. 

Yes; there are many such salts, as, for instance, common salt, sulphatof magnesia, and a variety of others. 

_Sulphat of lime_ is very frequently met with in nature, and constitutesthe well-known substance called _gypsum_, or _plaster of Paris_. 

_Sulphat of magnesia_, commonly called _Epsom salt_, is another verybitter medicine, which is obtained from sea-water and from severalsprings, or may be prepared by the direct combination of itsingredients. 

We have formerly mentioned _sulphat of alumine_ as constituting thecommon _alum_; it is found in nature chiefly in the neighbourhood ofvolcanos, and is particularly useful in the arts, from its strongastringent qualities. It is chiefly employed by dyers andcalico-printers, to fix colours; and is used also in the manufacture ofsome kinds of leather. 

Sulphuric acid combines also with the metals. 

CAROLINE. 

One of these combinations, _sulphat of iron_, we are already wellacquainted with. 

MRS. B. 

That is the most important metallic salt formed by sulphuric acid, andthe only one that we shall here notice. It is of great use in the arts;and, in medicine, it affords a very valuable tonic: it is of this saltthat most of those preparations called _steel medicines_ are composed. 

CAROLINE. 

But does any carbon enter into these compositions to form steel?

MRS. B. 

Not an atom: they are, therefore, very improperly called steel: but itis the vulgar appellation, and medical men themselves often comply withthe general custom. 

Sulphat of iron may be prepared, as you have seen, by dissolving iron insulphuric acid; but it is generally obtained from the natural productioncalled _Pyrites_, which being a sulphuret of iron, requires onlyexposure to the atmosphere to be oxydated, in order to form the salt;this, therefore, is much the most easy way of procuring it on a largescale. 

EMILY. 

I am surprised to find that both acids and compound salts are generallyobtained from their various combinations, rather than from the immediateunion of their ingredients. 

MRS. B. 

Were the simple bodies always at hand, their combinations wouldnaturally be the most convenient method of forming compounds; but youmust consider that, in most instances, there is great difficulty andexpense in obtaining the simple ingredients from their combinations; itis, therefore, often more expedient to procure compounds from thedecomposition of other compounds. But, to return to the sulphat of iron. --There is a certain vegetable acid called _Gallic acid_, which has theremarkable property of precipitating this salt black--I shall pour a fewdrops of the gallic acid into this solution of sulphat of iron--

CAROLINE. 

It is become as black as ink!

MRS. B. 

And it is ink in reality. Common writing ink is a precipitate of sulphatof iron by gallic acid; the black colour is owing to the formation ofgallat of iron, which being insoluble, remains suspended in the fluid. 

This acid has also the property of altering the colour of iron in itsmetallic state. You may frequently see its effect on the blade of aknife, that has been used to cut certain kinds of fruits. 

CAROLINE. 

True; and that is, perhaps, the reason that a silver knife is preferredto cut fruits; the gallic acid, I suppose, does not act upon silver. --Is this acid found in all fruits?

MRS. B. 

It is contained, more or less, in the rind of most fruits and roots, especially the radish, which, if scraped with a steel or iron knife, hasits bright red colour changed to a deep purple, the knife being at thesame time blackened. But the vegetable substance in which the gallicacid most abounds is _nutgall_, a kind of excrescence that grows onoaks, and from which the acid is commonly obtained for its variouspurposes. 

MRS. B. 

We now come to the PHOSPHORIC and PHOSPHOROUS ACIDS. In treating ofphosphorus, you have seen how these acids may be obtained from it bycombustion?

EMILY. 

Yes; but I should be much surprised if it was the usual method ofobtaining them, since it is so very difficult to procure phosphorus inits pure state. 

MRS. B. 

You are right, my dear; the phosphoric acid, for general purposes, isextracted from bones, in which it is contained in the state of phosphatof lime; from this salt the phosphoric acid is separated by means of thesulphuric, which combines with the lime. In its pure state, phosphoricacid is either liquid or solid, according to its degree ofconcentration. 

Among the salts formed by this acid, _phosphat of lime_ is the only onethat affords much interest; and this, we have already observed, constitutes the basis of all bones. It is also found in very smallquantities in some vegetables. 

CONVERSATION XVIII. 

OF THE NITRIC AND CARBONIC ACIDS: OR THE COMBINATIONS OF OXYGEN WITHNITROGEN AND CARBON; AND OF THE NITRATS AND CARBONATS. 

MRS. B. 

I am almost afraid of introducing the subject of the NITRIC ACID, as Iam sure that I shall be blamed by Caroline for not having made heracquainted with it before. 

CAROLINE. 

Why so, Mrs. B. ?

MRS. B. 

Because you have long known its radical, which is nitrogen or azote; andin treating of that element, I did not even hint that it was the basisof an acid. 

CAROLINE. 

And what could be your reason for not mentioning this acid sooner?

MRS. B. 

I do not know whether you will think the reason sufficiently good toacquit me; but the omission, I assure you, did not proceed fromnegligence. You may recollect that nitrogen was one of the first simplebodies which we examined; you were then ignorant of the theory ofcombustion, which I believe was, for the first time, mentioned in thatlesson; and therefore it would have been in vain, at that time, to haveattempted to explain the nature and formation of acids. 

CAROLINE. 

I wonder, however, that it never occurred to us to enquire whethernitrogen could be acidified; for, as we knew it was classed among thecombustible bodies, it was natural to suppose that it might produce anacid. 

MRS. B. 

That is not a necessary consequence; for it might combine with oxygenonly in the degree requisite to form an oxyd. But you will find thatnitrogen is susceptible of various degrees of oxygenation, some of whichconvert it merely into an oxyd, and others give it all the acidproperties. 

The acids, resulting from the combination of oxygen and nitrogen, arecalled the NITROUS and NITRIC acids. We will begin with the NITRIC, inwhich nitrogen is in the highest state of oxygenation. This acidnaturally exists in the form of gas; but is so very soluble in water, and has so great an affinity for it, that one grain of water will absorband condense ten grains of acid gas, and form the limpid fluid which yousee in this bottle. 

CAROLINE. 

What a strong offensive smell it has!

MRS. B. 

This acid contains a greater abundance of oxygen than any other, but itretains it with very little force. 

EMILY. 

Then it must be a powerful caustic, both from the facility with which itparts with its oxygen, and the quantity which it affords?

MRS. B. 

Very well, Emily; both cause and effect are exactly such as youdescribe: nitric acid burns and destroys all kinds of organised matter. It even sets fire to some of the most combustible substances. --We shallpour a little of it over this piece of dry warm charcoal--you see itinflames it immediately; it would do the same with oil of turpentine, phosphorus, and several other very combustible bodies. This shows youhow easily this acid is decomposed by combustible bodies, since theseeffects must depend upon the absorption of its oxygen. 

Nitric acid has been used in the arts from time immemorial, but it isonly within these twenty-five years that its chemical nature has beenascertained. The celebrated Mr. Cavendish discovered that it consistedof about 10 parts of nitrogen and 25 of oxygen. * These principles, intheir gaseous state, combine at a high temperature; and this may beeffected by repeatedly passing the electrical spark through a mixture ofthe two gases. 

 [Footnote *: The proportion stated by Sir H. Davy, in his Chemical Researches, is as 1 to 2. 389. ]

EMILY. 

The nitrogen and oxygen gases, of which the atmosphere is composed, donot combine, I suppose, because their temperature is not sufficientlyelevated?

CAROLINE. 

But in a thunder-storm, when the lightning repeatedly passes throughthem, may it not produce nitric acid? We should be in a strangesituation, if a violent storm should at once convert the atmosphere intonitric acid. 

MRS. B. 

There is no danger of it, my dear; the lightning can affect but a verysmall portion of the atmosphere, and though it were occasionally toproduce a little nitric acid, yet this never could happen to such anextent as to be perceivable. 

EMILY. 

But how could the nitric acid be known, and used, before the method ofcombining its constituents was discovered?

MRS. B. 

Before that period the nitric acid was obtained, and it is indeed stillextracted, for the common purposes of art, from the compound salt whichit forms with potash, commonly called _nitre_. 

CAROLINE. 

Why is it so called? Pray, Mrs. B. , let these old unmeaning names beentirely given up, by us at least; and let us call this salt _nitrat ofpotash_. 

MRS. B. 

With all my heart; but it is necessary that I should, at least, mentionthe old names, and more especially those which are yet in common use;otherwise, when you meet with them, you would not be able to understandtheir meaning. 

EMILY. 

And how is the acid obtained from this salt?

MRS. B. 

By the intervention of sulphuric acid, which combines with the potash, and sets the nitric acid at liberty. This I can easily show you, bymixing some nitrat of potash and sulphuric acid in this retort, andheating it over a lamp; the nitric acid will come over in the form ofvapour, which we shall collect in a glass bell. This acid, diluted inwater, is commonly called _aqua fortis_, if Caroline will allow me tomention that name. 

CAROLINE. 

I have often heard that aqua fortis will dissolve almost all metals; itis no doubt because it yields its oxygen so easily. 

MRS. B. 

Yes; and from this powerful solvent property, it derived the name ofaqua fortis, or strong water. Do you not recollect that we oxydated, andafterwards dissolved, some copper in this acid?

EMILY. 

If I remember right, the nitrat of copper was the first instance yougave us of a compound salt. 

CAROLINE. 

Can the nitric acid be completely decomposed and converted into nitrogenand oxygen?

EMILY. 

That cannot be the case, Caroline; since the acid can be decomposed onlyby the combination of its constituents with other bodies. 

MRS. B. 

True; but caloric is sufficient for this purpose. By making the acidpass through a red hot porcelain tube, it is decomposed; the nitrogenand oxygen regain the caloric which they had lost in combining, and arethus both restored to their gaseous state. 

The nitric acid may also be partly decomposed, and is by this meansconverted into NITROUS ACID. 

CAROLINE. 

This conversion must be easily effected, as the oxygen is so slightlycombined with the nitrogen. 

MRS. B. 

The partial decomposition of nitric acid is readily effected by mostmetals; but it is sufficient to expose the nitric acid to a very stronglight to make it give out oxygen gas, and thus be converted into nitrousacid. Of this acid there are various degrees, according to theproportions of oxygen which it contains; the strongest, and that intowhich the nitric is first converted, is of a yellow colour, as you seein this bottle. 

CAROLINE. 

How it fumes when the stopper is taken out!

MRS. B. 

The acid exists naturally in a gaseous state, and is here so stronglyconcentrated in water, that it is constantly escaping. 

Here is another bottle of nitrous acid, which, you see, is of an orangered; this acid is weaker, the nitrogen being combined with a smallerquantity of oxygen; and with a still less proportion of oxygen it is anolive-green colour, as it appears in this third bottle. In short, theweaker the acid, the deeper is its colour. 

Nitrous acid acts still more powerfully on some inflammable substancesthan the nitric. 

EMILY. 

I am surprised at that, as it contains less oxygen. 

MRS. B. 

But, on the other hand, it parts with its oxygen much more readily: youmay recollect that we once inflamed oil with this acid. 

The next combinations of nitrogen and oxygen form only oxyds ofnitrogen, the first of which is commonly called _nitrous air_; or moreproperly _nitric oxyd gas_. This may be obtained from nitric acid, byexposing the latter to the action of metals, as in dissolving them itdoes not yield the whole of its oxygen, but retains a portion of thisprinciple sufficient to convert it into this peculiar gas, a specimen ofwhich I have prepared, and preserved within this inverted glass bell. 

EMILY. 

It is a perfectly invisible elastic fluid. 

MRS. B. 

Yes; and it may be kept any length of time in this manner over water, asit is not, like the nitric and nitrous acids, absorbable by it. It israther heavier than atmospherical air, and is incapable of supportingeither combustion or respiration. I am going to incline the glass gentlyon one side, so as to let some of the gas escape--

EMILY. 

How very curious! --It produces orange fumes like the nitrous acid! thatis the more extraordinary, as the gas within the glass is perfectlyinvisible. 

MRS. B. 

It would give me much pleasure if you could make out the reason of thiscurious change without requiring any further explanation. 

CAROLINE. 

It seems, by the colour and smell, as if it were converted into nitrousacid gas: yet that cannot be, unless it combines with more oxygen; andhow can it obtain oxygen the very instant it escapes from the glass?

EMILY. 

From the atmosphere, no doubt. Is it not so, Mrs.  B. ?

MRS. B. 

You have guessed it; as soon as it comes in contact with the atmosphere, it absorbs from it the additional quantity of oxygen necessary toconvert it into nitrous acid gas. And, if I now remove the bottleentirely from the water, so as to bring at once the whole of the gasinto contact with the atmosphere, this conversion will appear still morestriking--

EMILY. 

Look, Caroline, the whole capacity of the bottle is instantly tinged ofan orange colour!

MRS. B. 

Thus, you see, it is the most easy process imaginable to convert_nitrous oxyd gas_ into _nitrous acid gas_. The property of attractingoxygen from the atmosphere, without any elevation of temperature, hasoccasioned this gaseous oxyd being used as a test for ascertaining thedegree of purity of the atmosphere. I am going to show you how it isapplied to this purpose. --You see this graduated glass tube, which isclosed at one end, (PLATE X. Fig.  2. ) --I first fill it with water, andthen introduce a certain measure of nitrous gas, which, not beingabsorbable by water, passes through it, and occupies the upper part ofthe tube. I must now add rather above two-thirds of oxygen gas, whichwill just be sufficient to convert the nitrous oxyd gas into nitrousacid gas. 

CAROLINE. 

So it has! --I saw it turn of an orange colour; but it immediatelyafterwards disappeared entirely, and the water, you see, has risen, andalmost filled the tube. 

MRS. B. 

That is because the acid gas is absorbable by water, and in proportionas the gas impregnates the water, the latter rises in the tube. When theoxygen gas is very pure, and the required proportion of nitrous oxyd gasvery exact, the whole is absorbed by the water; but if any other gas bemixed with the oxygen, instead of combining with the nitrous oxygen, itwill remain and occupy the upper part of the tube; or, if the gases benot in the due proportion, there will be a residue of that whichpredominates. --Before we leave this subject, I must not forget toremark that nitrous acid may be formed by dissolving nitrous oxyd gas innitric acid. This solution may be effected simply by making bubbles ofnitrous oxyd gas pass through nitric acid. 

EMILY. 

That is to say, that nitrogen at its highest degree of oxygenation, being mixed with nitrogen at its lowest degree of oxygenation, willproduce a kind of intermediate substance, which is nitrous acid. 

MRS. B. 

You have stated the fact with great precision. --There are various othermethods of preparing nitrous oxyd, and of obtaining it from compoundbodies; but it is not necessary to enter into these particulars. Itremains for me only to mention another curious modification ofoxygenated nitrogen, which has been distinguished by the name of_gaseous oxyd of nitrogen_. It is but lately that this gas has beenaccurately examined, and its properties have been investigated chieflyby Sir H. Davy. It has obtained also the name of _exhilarating_ gas, from the very singular property which that gentleman has discovered init, of elevating the animal spirits, when inhaled into the lungs, to adegree sometimes resembling delirium or intoxication. 

CAROLINE. 

Is it respirable, then?

MRS. B. 

It can scarcely be called respirable, as it would not support life forany length of time; but it may be breathed for a few moments without anyother effects, than the singular exhilaration of spirits I have justmentioned. It affects different people, however, in a very differentmanner. Some become violent, even outrageous: others experience alanguor, attended with faintness; but most agree in opinion, that thesensations it excites are extremely pleasant. 

CAROLINE. 

I think I should like to try it--how do you breathe it?

MRS. B. 

By collecting the gas in a bladder, to which a short tube with astop-cock is adapted; this is applied to the mouth with one hand, whilstthe nostrils are kept closed with the other, that the common air mayhave no access. You then alternately inspire, and expire the gas, tillyou perceive its effects. But I cannot consent to your making theexperiment; for the nerves are sometimes unpleasantly affected by it, and I would not run any risk of that kind. 

EMILY. 

I should like, at least, to see somebody breathe it; but pray by whatmeans is this curious gas obtained?

MRS. B. 

It is procured from _nitrat of ammonia_, an artificial salt which yieldsthis gas on the application of a gentle heat. I have put some of thesalt into a retort, and by the aid of a lamp the gas will beextricated. --

CAROLINE. 

Bubbles of air begin to escape through the neck of the retort into thewater apparatus; will you not collect them?

MRS. B. 

The gas that first comes over need not be preserved, as it consists oflittle more than the common air that was in the retort; besides, thereis always in this experiment a quantity of watery vapour which must comeaway before the nitrous oxyd appears. 

EMILY. 

Watery vapour! Whence does that proceed? There is no water in nitrat ofammonia?

MRS. B. 

You must recollect that there is in every salt a quantity of water ofcrystallisation, which may be evaporated by heat alone. But, besidesthis, water is actually generated in this experiment, as you will seepresently. First tell me, what are the constituent parts of nitrat ofammonia?

EMILY. 

Ammonia, and nitric acid: this salt, therefore, contains three differentelements, nitrogen and hydrogen, which produce the ammonia; and oxygen, which, with nitrogen, forms the acid. 

MRS. B. 

Well then, in this process the ammonia is decomposed; the hydrogen quitsthe nitrogen to combine with some of the oxygen of the nitric acid, andforms with it the watery vapour which is now coming over. When that iseffected, what will you expect to find?

EMILY. 

Nitrous acid instead of nitric acid, and nitrogen instead of ammonia. 

MRS. B. 

Exactly so; and the nitrous acid and nitrogen combine, and form thegaseous oxyd of nitrogen, in which the proportion of oxygen is 37 partsto 63 of nitrogen. 

You may have observed, that for a little while no bubbles of air havecome over, and we have perceived only a stream of vapour condensing asit issued into the water. --Now bubbles of air again make theirappearance, and I imagine that by this time all the watery vapour iscome away, and that we may begin to collect the gas. We may try whetherit is pure, by filling a phial with it, and plunging a taper intoit--yes, it will do now, for the taper burns brighter than in the commonair, and with a greenish flame. 

CAROLINE. 

But how is that? I thought no gas would support combustion but oxygen orchlorine. 

MRS. B. 

Or any gas that contains oxygen, and is ready to yield it, which is thecase with this in a considerable degree; it is not, therefore, surprising that it should accelerate the combustion of the taper. 

You see that the gas is now produced in great abundance; we shallcollect a large quantity of it, and I dare say that we shall find someof the family who will be curious to make the experiment of respiringit. Whilst this process is going on, we may take a general survey of themost important combinations of the nitric and nitrous acids with thealkalies. 

The first of these is _nitrat of potash_, commonly called _nitre_ or_saltpetre_. 

CAROLINE. 

Is not that the salt with which gunpowder is made?

MRS. B. 

Yes. Gunpowder is a mixture of five parts of nitre to one of sulphur, and one of charcoal. --Nitre from its great proportion of oxygen, andfrom the facility with which it yields it, is the basis of mostdetonating compositions. 

EMILY. 

But what is the cause of the violent detonation of gunpowder when setfire to?

MRS. B. 

Detonation may proceed from two causes; the sudden formation ordestruction of an elastic fluid. In the first case, when either a solidor liquid is instantaneously converted into an elastic fluid, theprodigious and sudden expansion of the body strikes the air with greatviolence, and this concussion produces the sound called detonation. 

CAROLINE. 

That I comprehend very well; but how can a similar effect be produced bythe destruction of a gas?

MRS. B. 

A gas can be destroyed only by condensing it to a liquid or solid state;when this takes place suddenly, the gas, in assuming a new and morecompact form, produces a vacuum, into which the surrounding air rusheswith great impetuosity; and it is by that rapid and violent motion thatthe sound is produced. In all detonations, therefore, gases are eithersuddenly formed, or destroyed. In that of gunpowder, can you tell mewhich of these two circumstances takes place?

EMILY. 

As gunpowder is a solid, it must, of course, produce the gases in itsdetonation; but how, I cannot tell. 

MRS. B. 

The constituents of gunpowder, when heated to a certain degree, enterinto a number of new combinations, and are instantaneously convertedinto a variety of gases, the sudden expansion of which gives rise to thedetonation. 

CAROLINE. 

And in what instance does the destruction or condensation of gasesproduce detonation?

MRS. B. 

I can give you one with which you are well acquainted; the suddencombination of the oxygen and hydrogen gases. 

CAROLINE. 

True; I recollect perfectly that hydrogen detonates with oxygen when thetwo gases are converted into water. 

MRS. B. 

But let us return to the nitrat of potash. --This salt is decomposedwhen exposed to heat, and mixed with any combustible body, such ascarbon, sulphur, or metals, these substances oxydating rapidly at theexpense of the nitrat. I must show you an instance of this. --I exposeto the fire some of the salt in a small iron ladle, and, when it issufficiently heated, add to it some powdered charcoal; this will attractthe oxygen from the salt, and be converted into carbonic acid. --

EMILY. 

But what occasions that crackling noise, and those vivid flashes thataccompany it?

MRS. B. 

The rapidity with which the carbonic acid gas is formed occasions asuccession of small detonations, which, together with the emission offlame, is called _deflagration_. 

_Nitrat of ammonia_ we have already noticed, on account of the gaseousoxyd of nitrogen which is obtained from it. 

_Nitrat of silver_ is the lunar caustic, so remarkable for its propertyof destroying animal fibre, for which purpose it is often used bysurgeons. --We have said so much on a former occasion, on the mode inwhich caustics act on animal matter, that I shall not detain you anylonger on this subject. 

We now come to the CARBONIC ACID, which we have already had manyopportunities of noticing. You recollect that this acid may be formed bythe combustion of carbon, whether in its imperfect state of charcoal, orin its purest form of diamond. And it is not necessary, for thispurpose, to burn the carbon in oxygen gas, as we did in the precedinglecture; for you need only light a piece of charcoal and suspend itunder a receiver on the water bath. The charcoal will soon beextinguished, and the air in the receiver will be found mixed withcarbonic acid. The process, however, is much more expeditious if thecombustion be performed in pure oxygen gas. 

CAROLINE. 

But how can you separate the carbonic acid, obtained in this manner, from the air with which it is mixed?

MRS. B. 

The readiest mode is to introduce under the receiver a quantity ofcaustic lime, or caustic alkali, which soon attracts the whole of thecarbonic acid to form a carbonat. --The alkali is found increased inweight, and the volume of the air is diminished by a quantity equal tothat of the carbonic acid which was mixed with it. 

EMILY. 

Pray is there no method of obtaining pure carbon from carbonic acid?

MRS. B. 

For a long time it was supposed that carbonic acid was notdecompoundable; but Mr. Tennant discovered, a few years ago, that thisacid may be decomposed by burning phosphorus in a closed vessel withcarbonat of soda or carbonat of lime: the phosphorus absorbs the oxygenfrom the carbonat, whilst the carbon is separated in the form of a blackpowder. This decomposition, however, is not effected simply by theattraction of the phosphorus for oxygen, since it is weaker than that ofcharcoal; but the attraction of the alkali of lime for the phosphoricacid, unites its power at the same time. 

CAROLINE. 

Cannot we make that experiment?

MRS. B. 

Not easily; it requires being performed with extreme nicety, in order toobtain any sensible quantity of carbon, and the experiment is much toodelicate for me to attempt it. But there can be no doubt of the accuracyof Mr. Tennant’s results; and all chemists now agree, that one hundredparts of carbonic acid gas consists of about twenty-eight parts ofcarbon to seventy-two of oxygen gas. But if you recollect, we decomposedcarbonic acid gas the other day by burning potassium in it. 

CAROLINE. 

True, so we did; and found the carbon precipitated on the regeneratedpotash. 

MRS. B. 

Carbonic acid gas is found very abundantly in nature; it is supposed toform about one thousandth part of the atmosphere, and is constantlyproduced by the respiration of animals; it exists in a great variety ofcombinations, and is exhaled from many natural decompositions. It iscontained in a state of great purity in certain caves, such as the_Grotto del Cane_, near Naples. 

EMILY. 

I recollect having read an account of that grotto, and of the cruelexperiments made on the poor dogs, to gratify the curiosity ofstrangers. But I understood that the vapour exhaled by this cave wascalled _fixed air_. 

MRS. B. 

That is the name by which carbonic acid was known before its chemicalcomposition was discovered. --This gas is more destructive of life thanany other; and if the poor animals that are submitted to its effects arenot plunged into cold water as soon as they become senseless, they donot recover. It extinguishes flame instantaneously. I have collectedsome in this glass, which I will pour over the candle. 

CAROLINE. 

This is extremely singular--it seems to extinguish it as it were byenchantment, as the gas is invisible. I never should have imagined thatgas could have been poured like a liquid. 

MRS. B. 

It can be done with carbonic acid only, as no other gas is sufficientlyheavy to be susceptible of being poured out in the atmospherical airwithout mixing with it. 

EMILY. 

Pray by what means did you obtain this gas?

MRS. B. 

I procured it from marble. Carbonic acid gas has so strong an attractionfor all the alkalies and alkaline earths, that these are always found innature in the state of carbonats. Combined with lime, this acid formschalk, which may be considered as the basis of all kinds of marbles, andcalcareous stones. From these substances carbonic acid is easilyseparated, as it adheres so slightly to its combinations, that thecarbonats are all decomposable by any of the other acids. I can easilyshow you how I obtained this gas; I poured some diluted sulphuric acidover pulverised marble in this bottle (the same which we used the otherday to prepare hydrogen gas), and the gas escaped through the tubeconnected with it; the operation still continues, as you may easilyperceive--

EMILY. 

Yes, it does; there is a great fermentation in the glass vessel. Whatsingular commotion is excited by the sulphuric acid taking possession ofthe lime, and driving out the carbonic acid!

CAROLINE. 

But did the carbonic acid exist in a gaseous state in the marble?

MRS. B. 

Certainly not; the acid, when in a state of combination, is capable ofexisting in a solid form. 

CAROLINE. 

Whence, then, does it obtain the caloric necessary to convert it intogas?

MRS. B. 

It may be supplied in this case from the mixture of sulphuric acid andwater, which produces an evolution of heat, even greater than isrequired for the purpose; since, as you may perceive by touching theglass vessel, a considerable quantity of the caloric disengaged becomessensible. But a supply of caloric may be obtained also from a diminutionof capacity for heat, occasioned by the new combination which takesplace; and, indeed, this must be the case when other acids are employedfor the disengagement of carbonic acid gas, which do not, like thesulphuric, produce heat on being mixed with water. Carbonic acid maylikewise be disengaged from its combinations by heat alone, whichrestores it to its gaseous state. 

CAROLINE. 

It appears to me very extraordinary that the same gas, which is producedby the burning of wood and coals, should exist also in such bodies asmarble, and chalk, which are incombustible substances. 

MRS. B. 

I will not answer that objection, Caroline, because I think I can putyou in a way of doing it yourself. Is carbonic acid combustible?

CAROLINE. 

Why, no--because it is a body that has been already burnt; it is carbononly, and not the acid, that is combustible. 

MRS. B. 

Well, and what inference do you draw from this?

CAROLINE. 

That carbonic acid cannot render the bodies with which it is unitedcombustible; but that simple carbon does, and that it is in thiselementary state that it exists in wood, coals, and a great variety ofother combustible bodies. --Indeed, Mrs.  B. , you are very ungenerous;you are not satisfied with convincing me that my objections arefrivolous, but you oblige me to prove them so myself. 

MRS. B. 

You must confess, however, that I make ample amends for the detection oferror, when I enable you to discover the truth. You, understand, now, I hope, that carbonic acid is equally produced by the decomposition ofchalk, or by the combustion of charcoal. These processes are certainlyof a very different nature; in the first case the acid is alreadyformed, and requires nothing more than heat to restore it to its gaseousstate; whilst, in the latter, the acid is actually made by the processof combustion. 

CAROLINE. 

I understand it now perfectly. But I have just been thinking of anotherdifficulty, which, I hope, you will excuse my not being able to removemyself. How does the immense quantity of calcareous earth, which isspread all over the globe, obtain the carbonic acid with which it iscombined?

MRS. B. 

The question is, indeed, not very easy to answer; but I conceive thatthe general carbonisation of calcareous matter may have been the effectof a general combustion, occasioned by some revolution of our globe, andproducing an immense supply of carbonic acid, with which the calcareousmatter became impregnated; or that this may have been effected by agradual absorption of carbonic acid from the atmosphere. --But thiswould lead us to discussions which we cannot indulge in, withoutdeviating too much from our subject. 

EMILY. 

How does it happen that we do not perceive the pernicious effects of thecarbonic acid which is floating in the atmosphere?

MRS. B. 

Because of the state of very great dilution in which it exists there. But can you tell me, Emily, what are the sources which keep theatmosphere constantly supplied with this acid?

EMILY. 

I suppose the combustion of wood, coals, and other substances, thatcontain carbon. 

MRS. B. 

And also the breath of animals. 

CAROLINE. 

The breath of animals! I thought you said that this gas was not at allrespirable, but on the contrary, extremely poisonous. 

MRS. B. 

So it is; but although animals cannot breathe in carbonic acid gas, yet, in the process of respiration, they have the power of forming this gasin their lungs; so that the air which we _expire_, or reject from thelungs, always contains a certain proportion of carbonic acid, which ismuch greater than that which is commonly found in the atmosphere. 

CAROLINE. 

But what is it that renders carbonic acid such a deadly poison?

MRS. B. 

The manner in which this gas destroys life, seems to be merely bypreventing the access of respirable air; for carbonic acid gas, unlessvery much diluted with common air, does not penetrate into the lungs, asthe windpipe actually contracts and refuses it admittance. --But we mustdismiss this subject at present, as we shall have an opportunity oftreating of respiration much more fully, when we come to the chemicalfunctions of animals. 

EMILY. 

Is carbonic acid as destructive to the life of vegetables as it is tothat of animals?

MRS. B. 

If a vegetable be completely immersed in it, I believe it generallyproves fatal to it; but mixed in certain proportions with atmosphericalair, it is, on the contrary, very favourable to vegetation. 

You remember, I suppose, our mentioning the mineral waters, both naturaland artificial, which contain carbonic acid gas?

CAROLINE. 

You mean the Seltzer water?

MRS. B. 

That is one of those which are the most used; there are, however, a variety of others into which carbonic acid enters as an ingredient:all these waters are usually distinguished by the name of _acidulous_ or_gaseous mineral waters_. 

The class of salts called _carbonats_ is the most numerous in nature; wemust pass over them in a very cursory manner, as the subject is far tooextensive for us to enter on it in detail. The state of carbonat is thenatural state of a vast number of minerals, and particularly of thealkalies and alkaline earths, as they have so great an attraction forthe carbonic acid, that they are almost always found combined with it;and you may recollect that it is only by separating them from this acid, that they acquire that causticity and those striking qualities which Ihave formerly described. All marbles, chalks, shells, calcareous spars, and lime-stones of every description, are neutral salts, in which_lime_, their common basis, has lost all its characteristic properties. 

EMILY. 

But if all these various substances are formed by the union of lime withcarbonic acid, whence arises their diversity of form and appearance?

MRS. B. 

Both from the different proportions of their component parts, and from avariety of foreign ingredients which may be occasionally blended withthem: the veins and colours of marbles, for instance, proceed from amixture of metallic substances; silex and alumine also frequently enterinto these combinations. The various carbonats, therefore, that I haveenumerated, cannot be considered as pure unadulterated neutral salts, although they certainly belong to that class of bodies. 

CONVERSATION XIX. 

ON THE BORACIC, FLUORIC, MURIATIC, AND OXYGENATED MURIATIC ACIDS; AND ONMURIATS. --ON IODINE AND IODIC ACID. 

MRS. B. 

We now come to the three remaining acids with simple bases, the compoundnature of which, though long suspected, has been but recently proved. The chief of these is the muriatic; but I shall first describe the twoothers, as their bases have been obtained more distinctly than that ofthe muriatic acid. 

You may recollect I mentioned the BORACIC ACID. This is found verysparingly in some parts of Europe, but for the use of manufactures wehave always received it from the remote country of Thibet, where it isfound in some lakes, combined with soda. It is easily separated from thesoda by sulphuric acid, and appears in the form of shining scales, asyou see here. 

CAROLINE. 

I am glad to meet with an acid which we need not be afraid to touch; forI perceive, from your keeping it in a piece of paper, that it is moreinnocent than our late acquaintance, the sulphuric and nitric acids. 

MRS. B. 

Certainly; but being more inert, you will not find its properties sointeresting. However, its decomposition, and the brilliant spectacle itaffords when its basis again unites with oxygen, atones for its want ofother striking qualities. 

Sir H. Davy succeeded in decomposing the boracic acid, (which had tillthen been considered as undecompoundable, ) by various methods. Onexposing this acid to the Voltaic battery, the positive wire gave outoxygen, and on the negative wire was deposited a black substance, inappearance resembling charcoal. This was the basis of the acid, whichSir H. Davy has called _Boracium_, or _Boron_. 

The same substance was obtained in more considerable quantities, byexposing the acid to a great heat in an iron gun-barrel. 

A third method of decomposing the boracic acid consisted in burningpotassium in contact with it in vacuo. The potassium attracts the oxygenfrom the acid, and leaves its basis in a separate state. 

The recomposition of this acid I shall show you, by burning some of itsbasis, which you see here, in a retort full of oxygen gas. The heat of acandle is all that is required for this combustion. --

EMILY. 

The light is astonishingly brilliant, and what beautiful sparks itthrows out!

MRS. B. 

The result of this combustion is the boracic acid, the nature of which, you see, is proved both by analytic and synthetic means. Its basis hasnot, it is true, a metallic appearance; but it makes very hard alloyswith other metals. 

EMILY. 

But pray, Mrs. B. , for what purpose is the boracic acid used inmanufactures?

MRS. B. 

Its principal use is in conjunction with soda, that is, in the state of_borat of soda_, which in the arts is commonly called borax. This salthas a peculiar power of dissolving metallic oxyds, and of promoting thefusion of substances capable of being melted; it is accordingly employedin various metallic arts; it is used, for example, to remove the oxydfrom the surface of metals, and is often employed in the assaying ofmetallic ores. 

Let us now proceed to the FLUORIC ACID. This acid is obtained from asubstance which is found frequently in mines, and particularly in thoseof Derbyshire, called _fluor_, a name which it acquired from thecircumstance of its being used to render the ores of metals more fluidwhen heated. 

CAROLINE. 

Pray is not this the Derbyshire spar, of which so many ornaments aremade?

MRS. B. 

The same; but though it has long been employed for a variety ofpurposes, its nature was unknown until Scheele, the great Swedishchemist, discovered that it consisted of lime united with a peculiaracid, which obtained the name of _fluoric acid_. It is easily separatedfrom the lime by the sulphuric acid, and unless condensed in water, ascends in the form of gas. A very peculiar property of this acid is itsunion with siliceous earths, which I have already mentioned. If thedistillation of this acid is performed in glass vessels, they arecorroded, and the siliceous part of the glass comes over, united withthe gas; if water is then admitted, part of the silex is deposited, asyou may observe in this jar. 

CAROLINE. 

I see white flakes forming on the surface of the water; is that silex?

MRS. B. 

Yes it is. This power of corroding glass has been used for engraving, orrather etching, upon it. The glass is first covered with a coat of wax, through which the figures to be engraved are to be scratched with a pin;then pouring the fluoric acid over the wax, it corrodes the glass wherethe scratches have been made. 

CAROLINE. 

I should like to have a bottle of this acid, to make engravings. 

MRS. B. 

But you could not have it in a _glass_ bottle, for in that case the acidwould be saturated with silex, and incapable of executing an engraving;the same thing would happen were the acid kept in vessels of porcelainor earthen-ware; this acid must therefore be both prepared and preservedin vessels of silver. 

If it be distilled from fluor spar and vitriolic acid, in silver orleaden vessels, the receiver being kept very cold during thedistillation, it assumes the form of a dense fluid, and in that state isthe most intensely corrosive substance known. This seems to be the acidcombined with a little water. It may be called _hydro-fluoric acid_; andSir H. Davy has been led, from some late experiments on the subject, toconsider _pure_ fluoric acid as a compound of a certain unknownprinciple, which he calls _fluorine_, with hydrogen. 

Sir H. Davy has also attempted to decompose the fluoric acid by burningpotassium in contact with it; but he has not yet been able by this orany other method, to obtain its basis in a distinct separate state. 

We shall conclude our account of the acids with that of the MURIATICACID, which is perhaps the most curious and interesting of all of them. It is found in nature combined with soda, lime, and magnesia. _Muriat ofsoda_ is the common sea-salt, and from this substance the acid isusually disengaged by means of the sulphuric acid. The natural state ofthe muriatic acid is that of an invisible permanent gas, at the commontemperature of the atmosphere; but it has a remarkably strong attractionfor water, and assumes the form of a whitish cloud whenever it meets anymoisture to combine with. This acid is remarkable for its peculiar andvery pungent smell, and possesses, in a powerful degree, most of theacid properties. Here is a bottle containing muriatic acid in a liquidstate. 

CAROLINE. 

And how is it liquefied?

MRS. B. 

By impregnating water with it; its strong attraction for water makes itvery easy to obtain it in a liquid form. Now, if I open the phial, youmay observe a kind of vapour rising from it, which is muriatic acid gas, of itself invisible, but made apparent by combining with the moisture ofthe atmosphere. 

EMILY. 

Have you not any of the pure muriatic acid gas?

MRS. B. 

This jar is full of that acid in its gaseous state--it is inverted overmercury instead of water, because, being absorbable by water, this gascannot be confined by it. --I shall now raise the jar a little on oneside, and suffer some of the gas to escape. --You see that itimmediately becomes visible in the form of a cloud. 

EMILY. 

It must be, no doubt, from its uniting with the moisture of theatmosphere, that it is converted into this dewy vapour. 

MRS. B. 

Certainly; and for the same reason, that is to say, its extremeeagerness to unite with water, this gas will cause snow to melt asrapidly as an intense fire. 

This acid proved much more refractory when Sir H. Davy attempted todecompose it than the other two undecompounded acids. It is singularthat potassium will burn in muriatic acid, and be converted into potash, without decomposing the acid, and the result of this combustion is a_muriat of potash_; for the potash, as soon as it is regenerated, combines with the muriatic acid. 

CAROLINE. 

But how can the potash be regenerated if the muriatic acid does notoxydate the potassium?

MRS. B. 

The potassium, in this process, obtains oxygen from the moisture withwhich the muriatic acid is always combined, and accordingly hydrogen, resulting from the decomposition of the moisture, is invariably evolved. 

EMILY. 

But why not make these experiments with dry muriatic acid?

MRS. B. 

Dry acids cannot be acted on by the Voltaic battery, because acids arenon-conductors of electricity, unless moistened. In the course of anumber of experiments which Sir H. Davy made upon acids in a state ofdryness, he observed that the presence of water appeared alwaysnecessary to develop the acid properties, so that acids are not evencapable of reddening vegetable blues if they have been carefullydeprived of moisture. This remarkable circumstance led him to suspect, that water, instead of oxygen, may be the acidifying principle; but thishe threw out rather as a conjecture than as an established point. 

Sir H. Davy obtained very curious results from burning potassium in amixture of phosphorus and muriatic acid, and also of sulphur andmuriatic acid; the latter detonates with great violence. All hisexperiments, however, failed in presenting to his view the basis of themuriatic acid, of which he was in search; and he was at last induced toform an opinion respecting the nature of this acid, which I shallpresently explain. 

EMILY. 

Is this acid susceptible of different degrees of oxygenation?

MRS. B. 

Yes, for though we cannot deoxygenate this acid, yet we may add oxygento it. 

CAROLINE. 

Why, then, is not the least degree of oxygenation of the acid called the_muriatous_, and the higher degree the _muriatic_ acid?

MRS. B. 

Because, instead of becoming, like other acids, more dense, and moreacid by an addition of oxygen, it is rendered on the contrary morevolatile, more pungent, but less acid, and less absorbable by water. These circumstances, therefore, seem to indicate the propriety of makingan exception to the nomenclature. The highest degree of oxygenation ofthis acid has been distinguished by the additional epithet of_oxygenated_, or, for the sake of brevity, _oxy_, so that it is calledthe _oxygenated_, or _oxy-muriatic acid_. This likewise exists in agaseous form, at the temperature of the atmosphere; it is alsosusceptible of being absorbed by water, and can be congealed, orsolidified, by a certain degree of cold. 

EMILY. 

And how do you obtain the oxy-muriatic acid?

MRS. B. 

In various ways; but it may be most conveniently obtained by distillingliquid muriatic acid over oxyd of manganese, which supplies the acidwith the additional oxygen. One part of the acid being put into aretort, with two parts of the oxyd of manganese, and the heat of a lampapplied, the gas is soon disengaged, and may be received over water, asit is but sparingly absorbed by it. --I have collected some in thisjar--

CAROLINE. 

It is not invisible, like the generality of gases; for it is of ayellowish colour. 

MRS. B. 

The muriatic acid extinguishes flame, whilst, on the contrary, theoxy-muriatic makes the flame larger, and gives it a dark red colour. Canyou account for this difference in the two acids?

EMILY. 

Yes, I think so; the muriatic acid will not supply the flame with theoxygen necessary for its support; but when this acid is furtheroxygenated, it will part with its additional quantity of oxygen, and inthis way support combustion. 

MRS. B. 

That is exactly the case; indeed the oxygen added to the muriatic acid, adheres so slightly to it, that it is separated by mere exposure to thesun’s rays. This acid is decomposed also by combustible bodies, many ofwhich it burns, and actually inflames, without any previous increase oftemperature. 

CAROLINE. 

That is extraordinary, indeed! I hope you mean to indulge us with someof these experiments?

MRS. B. 

I have prepared several glass jars of oxy-muriatic acid gas for thatpurpose. In the first we shall introduce some Dutch gold leaf. --Do youobserve that it takes fire?

EMILY. 

Yes, indeed it does--how wonderful it is! It became immediately red hot, but was soon smothered in a thick vapour. 

CAROLINE. 

What a disagreeable smell!

MRS. B. 

We shall try the same experiment with phosphorus in another jar of thisacid. --You had better keep your handkerchief to your nose when I openit--now let us drop into it this little piece of phosphorus--

CAROLINE. 

It burns really; and almost as brilliantly as in oxygen gas! But, whatis most extraordinary, these combustions take place without the metal orphosphorus being previously lighted, or even in the least heated. 

MRS. B. 

All these curious effects are owing to the very great facility withwhich this acid yields oxygen to such bodies as are strongly disposed tocombine with it. It appears extraordinary indeed to see bodies, andmetals in particular, melted down and inflamed, by a gas without anyincrease of temperature, either of the gas, or of the combustible. Thephenomenon, however, is, you see, well accounted for. 

EMILY. 

Why did you burn a piece of Dutch gold leaf rather than a piece of anyother metal?

MRS. B. 

Because, in the first place, it is a composition of metals (consistingchiefly of copper) which burns readily; and I use a thin metallic leafin preference to a lump of metal, because it offers to the action of thegas but a small quantity of matter under a large surface. Filings, orshavings, would answer the purpose nearly as well; but a lump of metal, though the surface would oxydate with great rapidity, would not takefire. Pure gold is not inflamed by oxy-muriatic acid gas, but it israpidly oxydated, and dissolved by it; indeed, this acid is the only onethat will dissolve gold. 

EMILY. 

This, I suppose, is what is commonly called _aqua regia_, which you knowis the only thing that will act upon gold. 

MRS. B. 

That is not exactly the case either; for aqua regia is composed of amixture of muriatic acid and nitric acid. --But, in fact, the result ofthis mixture is the formation of oxy-muriatic acid, as the muriatic acidoxygenates itself at the expence of the nitric; this mixture, therefore, though it bears the name of _nitro-muriatic acid_, acts on gold merelyin virtue of the oxy-muriatic acid which it contains. 

Sulphur, volatile oils, and many other substances, will burn in the samemanner in oxy-muriatic acid gas; but I have not prepared a sufficientquantity of it, to show you the combustion of all these bodies. 

CAROLINE. 

There are several jars of the gas yet remaining. 

MRS. B. 

We must reserve these for future experiments. The oxy-muriatic acid doesnot, like other acids, redden the blue vegetable colours; but it totallydestroys any colour, and turns all vegetables perfectly white. Let uscollect some vegetable substances to put into this glass, which is fullof gas. 

EMILY. 

Here is a sprig of myrtle--

CAROLINE. 

And here some coloured paper--

MRS. B. 

We shall also put in this piece of scarlet riband, and a rose--

EMILY. 

Their colours begin to fade immediately! But how does the gas producethis effect?

MRS. B. 

The oxygen combines with the colouring matter of these substances, anddestroys it; that is to say, destroys the property which these colourshad of reflecting only one kind of rays, and renders them capable ofreflecting them all, which, you know, will make them appear white. Oldprints may be cleaned by this acid, for the paper will be whitenedwithout injury to the impression, as printer’s ink is made of materials(oil and lamp black) which are not acted upon by acids. 

This property of the oxy-muriatic acid has lately been employed inmanufactures in a variety of bleaching processes; but for these purposesthe gas must be dissolved in water, as the acid is thus rendered muchmilder and less powerful in its effects; for, in a gaseous state, itwould destroy the texture, as well as the colour of the substancesubmitted to its action. 

CAROLINE. 

Look at the things which we put into the gas; they have now entirelylost their colour!

MRS. B. 

The effect of the acid is almost completed; and, if we were to examinethe quantity that remains, we should find it to consist chiefly ofmuriatic acid. 

The oxy-muriatic acid has been used to purify the air in fever hospitalsand prisons, as it burns and destroys putrid effluvia of every kind. Theinfection of the small-pox is likewise destroyed by this gas, and matterthat has been submitted to its influence will no longer generate thatdisorder. 

CAROLINE. 

Indeed, I think the remedy must be nearly as bad as the disease; theoxy-muriatic acid has such a dreadfully suffocating smell. 

MRS. B. 

It is certainly extremely offensive; but by keeping the mouth shut, andwetting the nostrils with liquid ammonia, in order to neutralize thevapour as it reaches the nose, its prejudicial effects may be in somedegree prevented. At any rate, however, this mode of disinfection canhardly be used in places that are inhabited. And as the vapour of nitricacid, which is scarcely less efficacious for this purpose, is not at allprejudicial, it is usually preferred on such occasions. 

CAROLINE. 

You have not told us yet what is Sir H. Davy’s new opinion respectingthe nature of muriatic acid, to which you alluded a few minutes ago?

MRS. B. 

True; I avoided noticing it then, because you could not have understoodit without some previous knowledge of the oxy-muriatic acid, which Ihave but just introduced to your acquaintance. 

Sir H. Davy’s idea is that muriatic acid, instead of being a compound, consisting of an unknown basis and oxygen, is formed by the union ofoxy-muriatic gas with hydrogen. 

EMILY. 

Have you not told us just now that oxy-muriatic gas was itself acompound of muriatic acid and oxygen?

MRS. B. 

Yes; but according to Sir H. Davy’s hypothesis, oxy-muriatic gas isconsidered as a simple body, which contains no oxygen--as a substance ofits own kind, which has a great analogy to oxygen in most of itsproperties, though in others it differs entirely from it. --According tothis view of the subject, the name of _oxy-muriatic acid_ can no longerbe proper, and therefore Sir H. Davy has adopted that of _chlorine_, or_chlorine gas_, a name which is simply expressive of its greenishcolour; and in compliance with that philosopher’s theory, we have placedchlorine in our table among the simple bodies. 

CAROLINE. 

But what was Sir H. Davy’s reason for adopting an opinion so contrary tothat which had hitherto prevailed?

MRS. B. 

There are many circumstances which are favourable to the new doctrine;but the clearest and simplest fact in its support is, that if hydrogengas and oxy-muriatic gas be mixed together, both these gases disappear, and muriatic acid gas is formed. 

EMILY. 

That seems to be a complete proof; is it not considered as perfectlyconclusive?

MRS. B. 

Not so decisive as it appears at first sight; because it is argued bythose who still incline to the old doctrine, that muriatic acid gas, however dry it may be, always contains a certain quantity of water, which is supposed essential to its formation. So that, in the experimentjust mentioned, this water is supplied by the union of the hydrogen gaswith the oxygen of the oxy-muriatic acid; and therefore the mixtureresolves itself into the base of muriatic acid and water, that is, muriatic acid gas. 

CAROLINE. 

I think the old theory must be the true one; for otherwise how could youexplain the formation of oxy-muriatic gas, from a mixture of muriaticacid and oxyd of manganese?

MRS. B. 

Very easily; you need only suppose that in this process the muriaticacid is decomposed; its hydrogen unites with the oxygen of the manganeseto form water, and the chlorine appears in its separate state. 

EMILY. 

But how can you explain the various combustions which take place inoxy-muriatic gas, if you consider it as containing no oxygen?

MRS. B. 

We need only suppose that combustion is the result of intense chemicalaction; so that chlorine, like oxygen, in combining with bodies, formscompounds which have less capacity for caloric than their constituentprinciples, and, therefore, caloric is evolved at the moment of theircombination. 

EMILY. 

If, then, we may explain every thing by either theory, to which of thetwo shall we give the preference?

MRS. B. 

It will, perhaps, be better to wait for more positive proofs, if suchcan be obtained, before we decide positively upon the subject. The newdoctrine has certainly gained ground very rapidly, and may be consideredas nearly established; but several competent judges still refuse theirassent to it, and until that theory is very generally adopted, it may beas well for us still occasionally to use the language to which chemistshave long been accustomed. --But let us proceed to the examination ofsalts formed by muriatic acid. 

Among the compound salts formed by muriatic acid, the _muriat of soda_, or common salt, is the most interesting. * The uses and properties ofthis salt are too well known to require much comment. Besides thepleasant flavour it imparts to the food, it is very wholesome, when notused to excess, as it assists the process of digestion. 

Sea-water is the great source from which muriat of soda is extracted byevaporation. But it is also found in large solid masses in the bowels ofthe earth, in England, and in many other parts of the world. 

 [Footnote *: According to Sir H. Davy’s views of the nature of the muriatic and oxy-muriatic acids, dry muriat of soda is a compound of sodium and chlorine, for it may be formed by the direct combination of oxy-muriatic gas and sodium. In his opinion, therefore, what we commonly call muriat of soda contains neither soda nor muriatic acid. ]

EMILY. 

I thought that salts, when solid, were always in the state of crystals;but the common table-salt is in the form of a coarse white powder. 

MRS. B. 

Crystallisation depends, as you may recollect, on the slow and regularreunion of particles dissolved in a fluid; common sea-salt is only in astate of imperfect crystallisation, because the process by which it isprepared is not favourable to the formation of regular crystals. But ifyou dissolve it, and afterwards evaporate the water slowly, you willobtain a regular crystallisation. 

_Muriat of ammonia_ is another combination of this acid, which we havealready mentioned as the principal source from which ammonia is derived. 

I can at once show you the formation of this salt by the immediatecombination of muriatic acid with ammonia. --These two glass jarscontain, the one muriatic acid gas, the other ammoniacal gas, both ofwhich are perfectly invisible--now, if I mix them together, you see theyimmediately form an opake white cloud, like smoke. --If a thermometerwas placed in the jar in which these gases are mixed, you would perceivethat some heat is at the same time produced. 

EMILY. 

The effects of chemical combinations are, indeed, wonderful! --Howextraordinary it is that two invisible bodies should become visible bytheir union!

MRS. B. 

This strikes you with astonishment, because it is a phenomenon whichnature seldom exhibits to our view; but the most common of heroperations are as wonderful, and it is their frequency only thatprevents our regarding them with equal admiration. What would be moresurprising, for instance, than combustion, were it not rendered sofamiliar by custom?

EMILY. 

That is true. --But pray, Mrs. B. , is this white cloud the salt thatproduces ammonia? How different it is from the solid muriat of ammoniawhich you once showed us!

MRS. B. 

It is the same substance which first appears in the state of vapour, butwill soon be condensed by cooling against the sides of the jar, in theform of very minute crystals. 

We may now proceed to the _oxy-muriats_. In this class of salts the_oxy-muriat of potash_ is the most worthy of our attention, for itsstriking properties. The acid, in this state of combination, contains astill greater proportion of oxygen than when alone. 

CAROLINE. 

But how can the oxy-muriatic acid acquire an increase of oxygen bycombining with potash?

MRS. B. 

It does not really acquire an additional quantity of oxygen, but itloses some of the muriatic acid, which produces the same effect, as theacid which remains is proportionably super-oxygenated. *

If this salt be mixed, and merely rubbed together with sulphur, phosphorus, charcoal, or indeed any other combustible, it explodesstrongly. 

 [Footnote *: According to Sir H. Davy’s new views, just explained, oxy-muriat of potash is a compound of chlorine with oxyd of potassium. ]

CAROLINE. 

Like gun-powder, I suppose, it is suddenly converted into elasticfluids?

MRS. B. 

Yes; but with this remarkable difference, that no increase oftemperature, any further than is produced by gentle friction, isrequired in this instance. Can you tell me what gases are generated bythe detonation of this salt with charcoal?

EMILY. 

Let me consider . . . . . The oxy-muriatic acid parts with its excess ofoxygen to the charcoal, by which means it is converted into muriaticacid gas; whilst the charcoal, being burnt by the oxygen, is changed tocarbonic acid gas. --What becomes of the potash I cannot tell. 

MRS. B. 

That is a fixed product which remains in the vessel. 

CAROLINE. 

But since the potash does not enter into the new combinations, I do notunderstand of what use it is in this operation. Would not theoxy-muriatic acid and the charcoal produce the same effect without it?

MRS. B. 

No; because there would not be that very great concentration of oxygenwhich the combination with the potash produces, as I have justexplained. 

I mean to show you this experiment, but I would advise you not to repeatit alone; for if care be not taken to mix only very small quantities ata time, the detonation will be extremely violent, and may be attendedwith dangerous effects. You see I mix an exceedingly small quantity ofthe salt with a little powdered charcoal, in this Wedgwood mortar, andrub them together with the pestle--

CAROLINE. 

Heavens! How can such a loud explosion be produced by so small aquantity of matter?

MRS. B. 

You must consider that an extremely small quantity of solid substancemay produce a very great volume of gases; and it is the sudden evolutionof these which occasions the sound. 

EMILY. 

Would not oxy-muriat of potash make stronger gunpowder than nitrat ofpotash?

MRS. B. 

Yes; but the preparation, as well as the use of this salt, is attendedwith so much danger, that it is never employed for that purpose. 

CAROLINE. 

There is no cause to regret it, I think; for the common gunpowder isquite sufficiently destructive. 

MRS. B. 

I can show you a very curious experiment with this salt; but it mustagain be on condition that you will never attempt to repeat it byyourselves. I throw a small piece of phosphorus into this glass ofwater; then a little oxy-muriat of potash; and, lastly, I pour in (bymeans of this funnel, so as to bring it in contact with the two otheringredients at the bottom of the glass) a small quantity of sulphuricacid--

CAROLINE. 

This is, indeed, a beautiful experiment! The phosphorus takes fire andburns from the bottom of the water. 

EMILY. 

How wonderful it is to see flame bursting out under water, and risingthrough it! Pray, how is this accounted for?

MRS. B. 

Cannot you find it out, Caroline?

EMILY. 

Stop--I think I can explain it. Is it not because the sulphuric aciddecomposes the salt by combining with the potash, so as to liberate theoxy-muriatic acid gas by which the phosphoric is set on fire?

MRS. B. 

Very well, Emily; and with a little more reflection you would havediscovered another concurring circumstance, which is, that an increaseof temperature is produced by the mixture of the sulphuric acid andwater, which assists in promoting the combustion of the phosphorus. 

I must, before we part, introduce to your acquaintance thenewly-discovered substance IODINE, which you may recollect we placednext to oxygen and chlorine in our table of simple bodies. 

CAROLINE. 

Is this also a body capable of maintaining combustion like oxygen andchlorine?

MRS. B. 

It is; and although it does not so generally disengage light and heatfrom inflammable bodies, as oxygen and chlorine do, yet it is capable ofcombining with most of them; and sometimes, as in the instance ofpotassium and phosphorus, the combination is attended with an actualappearance of light and heat. 

CAROLINE. 

But what sort of a substance is iodine: what is its form, and colour?

MRS. B. 

It is a very singular body, in many respects. At the ordinarytemperature of the atmosphere, it commonly appears in the form ofblueish black crystalline scales, such as you see in this tube. 

CAROLINE. 

They shine like black lead, and some of the scales have the shape oflozenges. 

MRS. B. 

That is actually the form which the crystals of iodine often assume. Butif we heat them gently, by holding the tube over the flame of a candle, see what a change takes place in them. 

CAROLINE. 

How curious! They seem to melt, and the tube immediately fills with abeautiful violet vapour. But look, Mrs.  B. , the same scales are nowappearing at the other end of the tube. 

MRS. B. 

This is in fact a sublimation of iodine, from one part of the tube toanother; but with this remarkable peculiarity, that, while in thegaseous state, iodine assumes that bright violet colour, which, as youmay already perceive, it loses as the tube cools, and the substanceresumes its usual solid form. --It is from the violet colour of the gasthat iodine has obtained its name. 

CAROLINE. 

But how is this curious substance obtained?

MRS. B. 

It is found in the ley of ashes of sea-weeds, after the soda has beenseparated by crystallisation; and it is disengaged by means of sulphuricacid, which expels it from the alkaline ley in the form of a violet gas, which may be collected and condensed in the way you have just seen. --This interesting discovery was made in the year 1812, by M. Courtois, a manufacturer of saltpetre at Paris. 

CAROLINE. 

And pray, Mrs. B. , what is the proof of iodine being a simple body?

MRS. B. 

It is considered as a simple body, both because it is not capable ofbeing resolved into other ingredients; and because it is itself capableof combining with other bodies, in a manner analogous to oxygen andchlorine. The most curious of these combinations is that which it formswith hydrogen gas, the result of which is a peculiar gaseous acid. 

CAROLINE. 

Just as chlorine and hydrogen gas form muriatic acid? In this respectchlorine and iodine seem to bear a strong analogy to each other. 

MRS. B. 

That is indeed the case; so that if the theory of the constitution ofeither of these two bodies be true, it must be true also in regard tothe other; if erroneous in the one, the theory must fall in both. 

But it is now time to conclude; we have examined such of the acids andsalts as I conceived would appear to you most interesting. --I shall notenter into any particulars respecting the metallic acids, as they offernothing sufficiently striking for our present purpose. 

CONVERSATION XX. 

ON THE NATURE AND COMPOSITION OF VEGETABLES. 

MRS. B. 

We have hitherto treated only of the simplest combinations of elements, such as alkalies, earths, acids, compound salts, stones, &c. ; all ofwhich belong to the mineral kingdom. It is time now to turn ourattention to a more complicated class of compounds, that of ORGANISEDBODIES, which will furnish us with a new source of instruction andamusement. 

EMILY. 

By organised bodies, I suppose, you mean the vegetable and animalcreation? I have, however, but a very vague idea of the word_organisation_, and I have often wished to know more precisely what itmeans. 

MRS. B. 

Organised bodies are such as are endowed by nature with various parts, peculiarly constructed and adapted to perform certain functionsconnected with life. Thus you may observe, that mineral compounds areformed by the simple effect of mechanical or chemical attraction, andmay appear to some to be in a great measure the productions of chance;whilst organised bodies bear the most striking and impressive marks ofdesign, and are eminently distinguished by that unknown principle, called _life_, from which the various organs derive the power ofexercising their respective functions. 

CAROLINE. 

But in what manner does life enable these organs to perform theirseveral functions?

MRS. B. 

That is a mystery which, I fear, is enveloped in such profound darknessthat there is very little hope of our ever being able to unfold it. Wemust content ourselves with examining the effects of this principle; asfor the cause, we have been able only to give it a name, withoutattaching any other meaning to it than the vague and unsatisfactory ideaof au unknown agent. 

CAROLINE. 

And yet I think I can form a very clear idea of life. 

MRS. B. 

Pray let me hear how you would define it?

CAROLINE. 

It is perhaps more easy to conceive than to express--let me consider--Is not life the power which enables both the animal and the vegetablecreation to perform the various functions which nature has assigned tothem?

MRS. B. 

I have nothing to object to your definition; but you will allow me toobserve, that you have only mentioned the effects which the unknowncause produces, without giving us any notion of the cause itself. 

EMILY. 

Yes, Caroline, you have told us what life _does_, but you have not toldus what it _is_. 

MRS. B. 

We may study its operations, but we should puzzle ourselves to nopurpose by attempting to form an idea of its real nature. 

We shall begin with examining its effects in the vegetable world, whichconstitutes the simplest class of organised bodies; these we shall finddistinguished from the mineral creation, not only by their morecomplicated nature, but by the power which they possess withinthemselves, of forming new chemical arrangements of their constituentparts, by means of appropriate organs. Thus, though all vegetables areultimately composed of hydrogen, carbon, and oxygen, (with a few otheroccasional ingredients, ) they separate and combine these principles bytheir various organs, in a thousand ways, and form, with them, differentkinds of juices and solid parts, which exist ready made in vegetables, and may, therefore, be considered as their immediate materials. 

These are:

 _Sap_, _Mucilage_, _Sugar_, _Fecula_, _Gluten_, _Fixed Oil_, _Volatile Oil_, _Camphor_, _Resins_, _Gum Resins_, _Balsams_, _Caoutchouc_, _Extractive colouring Matter_, _Tannin_, _Woody Fibre_, _Vegetable Acids_, _&c. _

CAROLINE. 

What a long list of names! I did not suppose that a vegetable wascomposed of half so many ingredients. 

MRS. B. 

You must not imagine that every one of these materials is formed in eachindividual plant. I only mean to say, that they are all derivedexclusively from the vegetable kingdom. 

EMILY. 

But does each particular part of the plant, such as the root, the bark, the stem, the seeds, the leaves, consist of one of these ingredientsonly, or of several of them combined together?

MRS. B. 

I believe there is no part of a plant which can be said to consistsolely of any one particular ingredient; a certain number of vegetablematerials must always be combined for the formation of any particularpart, (of a seed for instance, ) and these combinations are carried on bysets of vessels, or minute organs, which select from other parts, andbring together, the several principles required for the development andgrowth of those particular parts which they are intended to form and tomaintain. 

EMILY. 

And are not these combinations always regulated by the laws of chemicalattraction?

MRS. B. 

No doubt; the organs of plants cannot force principles to combine thathave no attraction for each other; nor can they compel superiorattractions to yield to those of inferior power; they probably actrather mechanically, by bringing into contact such principles, and insuch proportions, as will, by their chemical combination, form thevarious vegetable products. 

CAROLINE. 

We may then consider each of these organs as a curiously constructedapparatus, adapted for the performance of a variety of chemicalprocesses. 

MRS. B. 

Exactly so. As long as the plant lives and thrives, the carbon, hydrogen, and oxygen, (the chief constituents of its immediatematerials, ) are so balanced and connected together, that they are notsusceptible of entering into other combinations; but no sooner doesdeath take place, than this state of equilibrium is destroyed, and newcombinations produced. 

EMILY. 

But why should death destroy it; for these principles must remain in thesame proportions, and consequently, I should suppose, in the same orderof attractions?

MRS. B. 

You must remember, that in the vegetable, as well as in the animalkingdom, it is by the principle of _life_ that the organs are enabled toact; when deprived of that agent or stimulus, their power ceases, and anorder of attractions succeeds similar to that which would take place inmineral or unorganised matter. 

EMILY. 

It is this new order of attractions, I suppose, that destroys theorganisation of the plant after death; for if the same combinationsstill continued to prevail, the plant would always remain in the statein which it died?

MRS. B. 

And that, you know, is never the case; plants may be partially preservedfor some time after death, by drying; but in the natural course ofevents they all return to the state of simple elements; a wise andadmirable dispensation of Providence, by which dead plants are renderedfit to enrich the soil, and become subservient to the nourishment ofliving vegetables. 

CAROLINE. 

But we are talking of the dissolution of plants, before we have examinedthem in their living state. 

MRS. B. 

That is true, my dear. But I wished to give you a general idea of thenature of vegetation, before we entered into particulars. Besides, it isnot so irrelevant as you suppose to talk of vegetables in their deadstate, since we cannot analyse them without destroying life; and it isonly by hastening to submit them to examination, immediately after theyhave ceased to live, that we can anticipate their natural decomposition. There are two kinds of analysis of which vegetables are susceptible;first, that which separates them into their immediate materials, such assap, resin, mucilage, &c. ; secondly, that which decomposes them intotheir primitive elements, as carbon, hydrogen, and oxygen. 

EMILY. 

Is there not a third kind of analysis of plants, which consists inseparating their various parts, as the stem, the leaves, and the severalorgans of the flower?

MRS. B. 

That, my dear, is rather the department of the botanist; we shallconsider these different parts of plants only, as the organs by whichthe various secretions or separations are performed; but we must firstexamine the nature of these secretions. 

The _sap_ is the principal material of vegetables, since it contains theingredients that nourish every part of the plant. The basis of thisjuice, which the roots suck up from the soil, is water; this holds insolution the various other ingredients required by the several parts ofthe plant, which are gradually secreted from the sap by the differentorgans appropriated to that purpose, as it passes them in circulatingthrough the plant. 

_Mucus_, or _mucilage_, is a vegetable substance, which, like all theothers, is secreted from the sap; when in excess, it exudes from treesin the form of gum. 

CAROLINE. 

Is that the gum so frequently used instead of paste or glue?

MRS. B. 

It is; almost all fruit-trees yield some sort of gum, but that mostcommonly used in the arts is obtained from a species of acacia-tree inArabia, and is called _gum arabic_; it forms the chief nourishment ofthe natives of those parts, who obtain it in great quantities fromincisions which they make in the trees. 

CAROLINE. 

I did not know that gum was eatable. 

MRS. B. 

There is an account of a whole ship’s company being saved from starvingby feeding on the cargo, which was gum senegal. I should not, however, imagine, that it would be either a pleasant or a particularly eligiblediet to those who have not, from their birth, been accustomed to it. Itis, however, frequently taken medicinally, and considered as verynourishing. Several kinds of vegetable acids may be obtained, byparticular processes, from gum or mucilage, the principal of which iscalled the _mucous acid_. 

_Sugar_ is not found in its simple state in plants, but is always mixedwith gum, sap, or other ingredients; this saccharine matter is to be metwith in every vegetable, but abounds most in roots, fruits, andparticularly in the sugar-cane. 

EMILY. 

If all vegetables contain sugar, why is it extracted exclusively fromthe sugar-cane?

MRS. B. 

Because it is both most abundant in that plant, and most easily obtainedfrom it. Besides, the sugars produced by other vegetables differ alittle in their nature. 

During the late troubles in the West-Indies, when Europe was butimperfectly supplied with sugar, several attempts were made to extractit from other vegetables, and very good sugar was obtained from parsnipsand from carrots; but the process was too expensive to carry thisenterprize to any extent. 

CAROLINE. 

I should think that sugar might be more easily obtained from sweetfruits, such as figs, dates,  &c. 

MRS. B. 

Probably; but it would be still more expensive, from the high price ofthose fruits. 

EMILY. 

Pray, in what manner is sugar obtained from the sugar-cane?

MRS. B. 

The juice of this plant is first expressed by passing it between twocylinders of iron. It is then boiled with lime-water, which makes athick scum rise to the surface. The clarified liquor is let off belowand evaporated to a very small quantity, after which it is suffered tocrystallise by standing in a vessel, the bottom of which is perforatedwith holes, that are imperfectly stopped, in order that the syrup maydrain off. The sugar obtained by this process is a coarse brown powder, commonly called raw or moist sugar; it undergoes another operation to berefined and converted into loaf sugar. For this purpose it is dissolvedin water, and afterwards purified by an animal fluid called albumen. White of eggs chiefly consist of this fluid, which is also one of theconstituent parts of blood; and consequently eggs, or bullocks’ blood, are commonly used for this purpose. 

The albuminous fluid being diffused through the syrup, combines with allthe solid impurities contained in it, and rises with them to thesurface, where it forms a thick scum; the clear liquor is then againevaporated to a proper consistence, and poured into moulds, in which, bya confused crystallisation, it forms loaf-sugar. But an additionalprocess is required to whiten it; to this effect the mould is inverted, and its open base is covered with clay, through which water is made topass; the water slowly trickling through the sugar, combines with andcarries off the colouring matter. 

CAROLINE. 

I am very glad to hear that the blood that is used to purify sugar doesnot remain in it; it would be a disgusting idea. I have heard of someimprovements by the late Mr. Howard, in the process of refining sugar. Pray what are they?

MRS. B. 

It would be much too long to give you an account of the process indetail. But the principal improvement relates to the mode of evaporatingthe syrup, in order to bring it to the consistency of sugar. Instead ofboiling the syrup in a large copper, over a strong fire, Mr. Howardcarries off the water by means of a large air-pump, in a way similar tothat used in Mr. Leslie’s experiment for freezing water by evaporation;that is, the syrup being exposed to a vacuum, the water evaporatesquickly, with no greater heat than that of a little steam, which isintroduced round the boiler. The air-pump is of course of largedimensions, and is worked by a steam engine. A great saving is thusobtained, and a striking instance afforded of the power of science insuggesting useful economical improvements. 

EMILY. 

And pray how is sugar-candy and barley-sugar prepared?

MRS. B. 

Candied sugar is nothing more than the regular crystals, obtained byslow evaporation from a solution of sugar. Barley-sugar is sugar meltedby heat, and afterwards cooled in moulds of a spiral form. 

Sugar may be decomposed by a red heat, and, like all other vegetablesubstances, resolved into carbonic acid and hydrogen. The formation andthe decomposition of sugar afford many very interesting particulars, which we shall fully examine, after having gone through the othermaterials of vegetables. We shall find that there is reason to supposethat sugar is not, like the other materials, secreted from the sap byappropriate organs; but that it is formed by a peculiar process withwhich you are not yet acquainted. 

CAROLINE. 

Pray, is not honey of the same nature as sugar?

MRS. B. 

Honey is a mixture of saccharine matter and gum. 

EMILY. 

I thought that honey was in some measure an animal substance, as it isprepared by the bees. 

MRS. B. 

It is rather collected by them from flowers, and conveyed to theirstore-houses, the hives. It is the wax only that undergoes a realalteration in the body of the bee, and is thence converted into ananimal substance. 

Manna is another kind of sugar, which is united with a nauseousextractive matter, to which it owes its peculiar taste and colour. Itexudes like gum from various trees in hot climates, some of which havetheir leaves glazed by it. 

The next of the vegetable materials is _fecula_; this is the generalname given to the farinaceous substance contained in all seeds, and insome roots, as the potatoe, parsnip, &c. It is intended by nature forthe first aliment of the young vegetable; but that of one particulargrain is become a favourite and most common food of a large part ofmankind. 

EMILY. 

You allude, I suppose, to bread, which is made of wheat-flower?

MRS. B. 

Yes. The fecula of wheat contains also another vegetable substance whichseems peculiar to that seed, or at least has not as yet been obtainedfrom any other. This is _gluten_, which is of a sticky, ropy, elasticnature; and it is supposed to be owing to the viscous qualities of thissubstance, that wheat-flour forms a much better paste than any other. 

EMILY. 

Gluten, by your description, must be very like gum?

MRS. B. 

In their sticky nature they certainly have some resemblance; but glutenis essentially different from gum in other points, and especially in itsbeing insoluble in water, whilst gum, you know, is extremely soluble. 

The _oils_ contained in vegetables all consist of hydrogen and carbon invarious proportions. They are of two kinds, _fixed_ and _volatile_, bothof which we formerly mentioned. Do you remember in what the differencebetween fixed and volatile oil consists?

EMILY. 

If I recollect rightly, the former are decomposed by heat, whilst thelatter are merely volatilised by it. 

MRS. B. 

Very well. Fixed oil is contained only in the seeds of plants, exceptingin the olive, in which it is produced in, and expressed from, the fruit. We have already observed that seeds contain also fecula; these twosubstances, united with a little mucilage, form the white substancecontained in the seeds or kernels of plants, and is destined for thenourishment of the young plant, to which the seed gives birth. The milkof almonds, which is expressed from the seed of that name, is composedof these three substances. 

EMILY. 

Pray, of what nature is the linseed oil which is used in painting?

MRS. B. 

It is a fixed oil, obtained from the seed of flax. Nut oil, which isfrequently used for the same purpose, is expressed from walnuts. 

Olive oil is that which is best adapted to culinary purposes. 

CAROLINE. 

And what are the oils used for burning?

MRS. B. 

Animal oils most commonly; but the preference given to them is owing totheir being less expensive; for vegetable oils burn equally well, andare more pleasant, as their smell is not offensive. 

EMILY. 

Since oil is so good a combustible, what is the reason that lamps sofrequently require trimming?

MRS. B. 

This sometimes proceeds from the construction of the lamp, which may notbe sufficiently favourable to a perfect combustion; but there iscertainly a defect in the nature of oil itself, which renders itnecessary for the best-constructed lamps to be occasionally trimmed. This defect arises from a portion of mucilage which it is extremelydifficult to separate from the oil, and which being a bad combustible, gathers round the wick, and thus impedes its combustion, andconsequently dims the light. 

CAROLINE. 

But will not oils burn without a wick?

MRS. B. 

Not unless their temperature be elevated to five or six hundred degrees;the wick answers this purpose, as I think I once before explained toyou. The oil rises between the fibres of the cotton by capillaryattraction, and the heat of the burning wick volatilises it, and bringsit successively to the temperature at which it is combustible. 

EMILY. 

I suppose the explanation which you have given with regard to thenecessity of trimming lamps, applies also to candles, which so oftenrequire snuffing?

MRS. B. 

I believe it does; at least, in some degree. But besides thecircumstance just explained, the common sorts of oils are not veryhighly combustible, so that the heat produced by a candle, which is acoarse kind of animal oil, being insufficient to volatilise themcompletely, a quantity of soot is gradually deposited on the wick, whichdims the light, and retards the combustion. 

CAROLINE. 

Wax candles then contain no incombustible matter, since they do notrequire snuffing?

MRS. B. 

Wax is a much better combustible than tallow, but still not perfectlyso, since it likewise contains some particles that are unfit forburning; but when these gather round the wick, (which in a wax light iscomparatively small, ) they weigh it down on one side, and fall offtogether with the burnt part of the wick. 

CAROLINE. 

As oils are such good combustibles, I wonder that they should require sogreat an elevation of temperature before they begin to burn?

MRS. B. 

Though fixed oils will not enter into actual combustion below thetemperature of about four hundred degrees, yet they will slowly absorboxygen at the common temperature of the atmosphere. Hence arises avariety of changes in oils which modify their properties and uses in thearts. 

If oil simply absorbs, and combines with oxygen, it thickens and changesto a kind of wax. This change is observed to take place on the externalparts of certain vegetables, even during their life. But it happens inmany instances that the oil does not retain all the oxygen which itattracts, but that part of it combines with, or burns, the hydrogen ofthe oil, thus forming a quantity of water, which gradually goes off byevaporation. In this case the alteration of the oil consists not only inthe addition of a certain quantity of oxygen, but in the diminution ofthe hydrogen. These oils are distinguished by the name of _drying oils_. Linseed, poppy, and nut-oils, are of this description. 

EMILY. 

I am well acquainted with drying oils, as I continually use them inpainting. But I do not understand why the acquisition of oxygen on onehand, and a loss of hydrogen on the other, should render them drying?

MRS. B. 

This, I conceive, may arise from two reasons; either from the oxygenwhich is added being less favourable to the state of fluidity than thehydrogen, which is subtracted; or from this additional quantity ofoxygen giving rise to new combinations, in consequence of which the mostfluid parts of the oil are liberated and volatilised. 

For the purpose of painting, the drying quality of oil is furtherincreased by adding a quantity of oxyd of lead to it, by which means itis more rapidly oxygenated. 

The rancidity of oil is likewise owing to their oxygenation. In thiscase a new order of attraction takes place, from which a peculiar acidis formed, called the _sebacic acid_. 

CAROLINE. 

Since the nature and composition of oil is so well known, pray could notoil be actually _made_, by combining its principles?

MRS. B. 

That is by no means a necessary consequence; for there are innumerablevarieties of compound bodies which we can decompose, although we areunable to reunite their ingredients. This, however, is not the case withoil, as it has very lately been discovered, that it is possible to formoil, by a peculiar process, from the action of oxygenated muriatic acidgas on hydro-carbonate. 

We now pass to the _volatile_ or _essential oils_. These form the basisof all the vegetable perfumes, and are contained, more or less, in everypart of the plant excepting the seed; they are, at least, never found inthat part of the seed which contains the embrio plant. 

EMILY. 

The smell of flowers, then, proceeds from volatile oil?

MRS. B. 

Certainly; but this oil is often most abundant in the rind of fruits, asin oranges, lemons,  &c. From which it may be extracted by the slightestpressure; it is found also in the leaves of plants, and even in thewood. 

CAROLINE. 

Is it not very plentiful in the leaves of mint, and of thyme, and allthe sweet-smelling herbs?

MRS. B. 

Yes, remarkably so; and in geranium leaves also, which have a much morepowerful odour than the flowers. 

The perfume of sandal fans is an instance of its existence in wood. Inshort, all vegetable odours or perfumes are produced by the evaporationof particles of these volatile oils. 

EMILY. 

They are, I suppose, very light, and of very thin consistence, sincethey are so volatile?

MRS. B. 

They vary very much in this respect, some of them being as thick asbutter, whilst others are as fluid as water. In order to be prepared forperfumes, or essences, these oils are first properly purified, and theneither distilled with spirit of wine, as in the case with lavenderwater, or simply mixed with a large proportion of water, as is oftendone with regard to peppermint. Frequently, also, these odoriferouswaters are prepared merely by soaking the plants in water, anddistilling. The water then comes over impregnated with the volatile oil. 

CAROLINE. 

Such waters are frequently used to take spots of grease out of cloth, orsilk; how do they produce that effect?

MRS. B. 

By combining with the substance that forms these stains; for volatileoils, and likewise the spirit in which they are distilled, will dissolvewax, tallow, spermaceti, and resins; if, therefore, the spot proceedsfrom any of these substances, it will remove it. Insects of every kindhave a great aversion to perfumes, so that volatile oils are employedwith success in museums for the preservation of stuffed birds and otherspecies of animals. 

CAROLINE. 

Pray does not the powerful smell of camphor proceed from a volatile oil?

MRS. B. 

_Camphor_ seems to be a substance of its own kind, remarkable by manypeculiarities. But if not exactly of the same nature as volatile oil, itis at least very analogous to it. It is obtained chiefly from thecamphor-tree, a species of laurel which grows in China, and in theIndian isles, from the stem and roots of which it is extracted. Smallquantities have also been distilled from thyme, sage, and other aromaticplants; and it is deposited in pretty large quantities by some volatileoils after long standing. It is extremely volatile and inflammable. Itis insoluble in water, but is soluble in oils, in which state, as wellas in its solid form, it is frequently applied to medicinal purposes. Amongst the particular properties of camphor, there is one too singularto be passed over in silence. If you take a small piece of camphor, andplace it on the surface of a bason of pure water, it will immediatelybegin to move round and round with great rapidity; but if you pour intothe bason a single drop of any odoriferous fluid, it will instantly puta stop to this motion. You can at any time try this very simpleexperiment; but you must not expect that I shall be able to account forthis phenomenon, as nothing satisfactory has yet been advanced for itsexplanation. 

CAROLINE. 

It is very singular indeed; and I will certainly try the experiment. Pray what are _resins_, which you just now mentioned?

MRS. B. 

They are volatile oils, that have been acted on, and peculiarlymodified, by oxygen. 

CAROLINE. 

They are, therefore, oxygenated volatile oils?

MRS. B. 

Not exactly; for the process does not appear to consist so much in theoxygenation of the oil, as in the combustion of a portion of itshydrogen, and a small portion of its carbon. For when resins areartificially made by the combination of volatile oils with oxygen, thevessel in which the process is performed is bedewed with water, and theair included within is loaded with carbonic acid. 

EMILY. 

This process must be, in some respects, similar to that for preparingdrying oils?

MRS. B. 

Yes; and it is by this operation that both of them acquire a greaterdegree of consistence. Pitch, tar, and turpentine, are the most commonresins; they exude from the pine and fir trees. Copal, mastic, andfrankincense, are also of this class of vegetable substances. 

EMILY. 

Is it of these resins that the mastic and copal varnishes, so much usedin painting, are made?

MRS. B. 

Yes. Dissolved either in oil, or in alcohol, resins form varnishes. Fromthese solutions they may be precipitated by water, in which they areinsoluble. This I can easily show you. --If you will pour some waterinto this glass of mastic varnish, it will combine with the alcohol inwhich the resin is dissolved, and the latter will be precipitated in theform of a white cloud--

EMILY. 

It is so. And yet how is it that pictures or drawings, varnished withthis solution, may safely be washed with water?

MRS. B. 

As the varnish dries, the alcohol evaporates, and the dry varnish orresin which remains, not being soluble in water, will not be acted onby it. 

There is a class of compound resins called _gum-resins_, which areprecisely what their name denotes, that is to say, resins combined withmucilage. Myrrh and assafœtida are of this description. 

CAROLINE. 

Is it possible that a substance of so disagreeable a smell as assafœtidacan be formed from a volatile oil?

MRS. B. 

The odour of volatile oils is by no means always grateful. Onions andgarlic derive their smell from volatile oils, as well as roses andlavender. 

There is still another form under which volatile oils presentthemselves, which is that of _balsams_. These consist of resinous juicescombined with a peculiar acid, called the benzoic acid. Balsams appearto have been originally volatile oils, the oxygenation of which hasconverted one part into a resin, and the other part into an acid, which, combined together, form a balsam; such are the balsams of Peru, Tolu, &c. 

We shall now take leave of the oils and their various modifications, andproceed to the next vegetable substance, which is _caoutchouc_. This isa white milky glutinous fluid, which acquires consistence, and blackensin drying, in which state it forms the substance with which you are sowell acquainted, under the name of gum-elastic. 

CAROLINE. 

I am surprised to hear that gum-elastic was ever white, or ever fluid!And from what vegetable is it procured?

MRS. B. 

It is obtained from two or three different species of trees, in theEast-Indies, and South-America, by making incisions in the stem. Thejuice is collected as it trickles from these incisions, and moulds ofclay, in the form of little bottles of gum-elastic, are dipped into it. A layer of this juice adheres to the clay and dries on it; and severallayers are successively added by repeating this till the bottle is ofsufficient thickness. It is then beaten to break down the clay, which iseasily shaken out. The natives of the countries where this substance isproduced sometimes make shoes and boots of it by a similar process, andthey are said to be extremely pleasant and serviceable, both from theirelasticity, and their being water-proof. 

The substance which comes next in our enumeration of the immediateingredients of vegetables, is _extractive matter_. This is a term, which, in a general sense, may be applied to any substance extractedfrom vegetables; but it is more particularly understood to relate to theextractive _colouring matter_ of plants. A great variety of colours areprepared from the vegetable kingdom, both for the purposes of paintingand of dying; all the colours called _lakes_ are of this description;but they are less durable than mineral colours, for, by long exposure tothe atmosphere, they either darken or turn yellow. 

EMILY. 

I know that in painting, the lakes are reckoned far less durable coloursthan the ochres; but what is the reason of it?

MRS. B. 

The change which takes place in vegetable colours is owing chiefly tothe oxygen of the atmosphere slowly burning their hydrogen, and leaving, in some measure, the blackness of the carbon exposed. Such change cannottake place in ochre, which is altogether a mineral substance. 

Vegetable colours have a stronger affinity for animal than for vegetablesubstances, and this is supposed to be owing to a small quantity ofnitrogen which they contain. Thus, silk and worsted will take a muchfiner vegetable dye than linen and cotton. 

CAROLINE. 

Dying, then, is quite a chemical process?

MRS. B. 

Undoubtedly. The condition required to form a good dye is, that thecolouring matter should be precipitated, or fixed, on the substance tobe dyed, and should form a compound not soluble in the liquids to whichit will probably be exposed. Thus, for instance, printed or dyed linensor cottons must be able to resist the action of soap and water, to whichthey must necessarily be subject in washing; and woollens and silksshould withstand the action of grease and acids, to which they mayaccidentally be exposed. 

CAROLINE. 

But if linen and cotton have not a sufficient affinity for colouringmatter, how are they made to resist the action of washing, which theyalways do when they are well printed?

MRS. B. 

When the substance to be dyed has either no affinity for the colouringmatter, or not sufficient power to retain it, the combination iseffected, or strengthened, by the intervention of a third substance, called a _mordant_, or basis. The mordant must have a strong affinityboth for the colouring matter and the substance to be dyed, by whichmeans it causes them to combine and adhere together. 

CAROLINE. 

And what are the substances that perform the office of thus reconcilingthe two adverse parties?

MRS. B. 

The most common mordant is sulphat of alumine, or alum. Oxyds of tin andiron, in the state of compound salts, are likewise used for thatpurpose. 

_Tannin_ is another vegetable ingredient of great importance in thearts. It is obtained chiefly from the bark of trees; but it is foundalso in nut-galls, and in some other vegetables. 

EMILY. 

Is that the substance commonly called _tan_, which is used inhot-houses?

MRS. B. 

Tan is the prepared bark in which the peculiar substance, tannin, iscontained. But the use of tan in hot-houses is of much less importancethan in the operation of _tanning_, by which skin is converted intoleather. 

EMILY. 

Pray, how is this operation performed?

MRS. B. 

Various methods are employed for this purpose, which all consist inexposing skin to the action of tannin, or of substances containing thisprinciple, in sufficient quantities, and disposed to yield it to theskin. The most usual way is to infuse coarsely powdered oak bark inwater, and to keep the skin immersed in this infusion for a certainlength of time. During this process, which is slow and gradual, the skinis found to have increased in weight, and to have acquired aconsiderable tenacity and impermeability to water. This effect may bemuch accelerated by using strong saturations of the tanning principle(which can be extracted from bark), instead of employing the barkitself. But this quick mode of preparation does not appear to makeequally good leather. 

Tannin is contained in a great variety of astringent vegetablesubstances, as galls, the rose-tree, and wine; but it is nowhere soplentiful as in bark. All these substances yield it to water, from whichit may be precipitated by a solution of isinglass, or glue, with whichit strongly unites and forms an insoluble compound. Hence its valuableproperty of combining with skin (which consists chiefly of glue), and ofenabling it to resist the action of water. 

EMILY. 

Might we not see that effect by pouring a little melted isinglass into aglass of wine, which you say contains tannin?

MRS. B. 

Yes. I have prepared a solution of isinglass for that very purpose. --Doyou observe the thick muddy precipitate? --That is the tannin combinedwith the isinglass. 

CAROLINE. 

This precipitate must then be of the same nature as leather?

MRS. B. 

It is composed of the same ingredients; but the organisation and textureof the skin being wanting, it has neither the consistence nor thetenacity of leather. 

CAROLINE. 

One might suppose that men who drink large quantities of red wine standa chance of having the coats of their stomachs converted into leather, since tannin has so strong an affinity for skin. 

MRS. B. 

It is not impossible but that the coats of their stomachs may be, insome measure, tanned, or hardened by the constant use of this liquor;but you must remember that where a number of other chemical agents areconcerned, and, above all, where life exists, no certain chemicalinference can be drawn. 

I must not dismiss this subject, without mentioning a recent discoveryof Mr. Hatchett, which relates to it. This gentleman found that asubstance very similar to tannin, possessing all its leading properties, and actually capable of tanning leather, may be produced by exposingcarbon, or any substance containing carbonaceous matter, whethervegetable, animal, or mineral, to the action of nitric acid. 

CAROLINE. 

And is not this discovery very likely to be of use to manufactures?

MRS. B. 

That is very doubtful, because tannin, thus artificially prepared, mustprobably always be more expensive than that which is obtained from bark. But the fact is extremely curious, as it affords one of those very rareinstances of chemistry being able to imitate the proximate principles oforganised bodies. 

The last of the vegetable materials is _woody fibre_; it is the hardestpart of plants. The chief source from which this substance is derived iswood, but it is also contained, more or less, in every solid part ofthat plant. It forms a kind of skeleton of the part to which it belongs, and retains its shape after all the other materials have disappeared. Itconsists chiefly of carbon, united with a small proportion of salts, andthe other constituents common to all vegetables. 

EMILY. 

It is of woody fibre, then, that the common charcoal is made?

MRS. B. 

Yes. Charcoal, as you may recollect, is obtained from wood, by theseparation of all its evaporable parts. 

Before we take leave of the vegetable materials, it will be proper, atleast, to enumerate the several vegetable acids which we either havehad, or may have occasion to mention. I believe I formerly told you thattheir basis, or radical, was uniformly composed of hydrogen and carbon, and that their difference consisted only in the various proportions ofoxygen which they contained. 

The following are the names of the vegetable acids:

The _Mucous Acid_, obtained from gum or mucilage; _Suberic_ - - - from cork; _Camphoric_ - - - from camphor; _Benzoic_ - - - from balsams; _Gallic_ - - - from galls, bark,  &c. _Malic_ - - - from ripe fruits; _Citric_ - - - from lemon juice; _Oxalic_ - - - from sorrel; _Succinic_ - - - from amber; _Tartarous_ - - - from tartrit of potash: _Acetic_ - - - from vinegar. 

They are all decomposable by heat, soluble in water, and turn vegetableblue colours red. The _succinic_, the _tartarous_, and the _acetousacids_, are the products of the decomposition of vegetables; we shall, therefore, reserve their examination for a future period. 

The _oxalic acid_, distilled from sorrel, is the highest term ofvegetable acidification; for, if more oxygen be added to it, it losesits vegetable nature, and is resolved into carbonic acid and water;therefore, though all the other acids may be converted into the oxalicby an addition of oxygen, the oxalic itself is not susceptible of afurther degree of oxygenation; nor can it be made, by any chemicalprocesses, to return to a state of lower acidification. 

To conclude this subject, I have only to add a few words on the _gallicacid_.  .  .  .  . 

CAROLINE. 

Is not this the same acid before mentioned, which forms ink, byprecipitating sulphat of iron from its solution?

MRS. B. 

Yes. Though it is usually extracted from galls, on account of its beingmost abundant in that vegetable substance, it may also be obtained froma great variety of plants. It constitutes what is called the _astringentprinciple_ of vegetables; it is generally combined with tannin, and youwill find that an infusion of tea, coffee, bark, red-wine, or anyvegetable substance that contains the astringent principle, will make ablack precipitate with a solution of sulphat of iron. 

CAROLINE. 

But pray what are galls?

MRS. B. 

They are excrescences which grow on the bark of young oaks, and areoccasioned by an insect which wounds the bark of trees, and lays itseggs in the aperture. The lacerated vessels of the tree then dischargetheir contents, and form an excrescence, which affords a defensivecovering for these eggs. The insect, when come to life, first feeds onthis excrescence, and some time afterward eats its way out, as itappears from a hole which is formed in all gall-nuts that no longercontain an insect. It is in hot climates only that strongly astringentgall-nuts are found; those which are used for the purpose of making inkare brought from Aleppo. 

EMILY. 

But are not the oak-apples, which grow on the leaves of the oak in thiscountry, of a similar nature?

MRS. B. 

Yes; only they are an inferior species of galls, containing less of theastringent principle, and therefore less applicable to useful purposes. 

CAROLINE. 

Are the vegetable acids never found but in their pure uncombined state?

MRS. B. 

By no means; on the contrary, they are frequently met with in the stateof compound salts; these, however, are in general not fully saturatedwith the salifiable bases, so that the acid predominates; and, in thisstate, they are called _acidulous_ salts. Of this kind is the saltcalled cream of tartar. 

CAROLINE. 

Is not the salt of lemon, commonly used to take out ink-spots andstains, of this nature?

MRS. B. 

No; that salt consists of the oxalic acid, combined with a littlepotash. It is found in that state in sorrel. 

CAROLINE. 

And pray how does it take out ink-spots?

MRS. B. 

By uniting with the iron, and rendering it soluble in water. 

Besides the vegetable materials which we have enumerated, a variety ofother substances, common to the three kingdoms, are found in vegetables, such as potash, which was formerly supposed to belong exclusively toplants, and was, in consequence, called the vegetable alkali. 

Sulphur, phosphorus, earths, and a variety of metallic oxyds, are alsofound in vegetables, but only in small quantities. And we meet sometimeswith neutral salts, formed by the combination of these ingredients. 

CONVERSATION XXI. 

ON THE DECOMPOSITION OF VEGETABLES. 

CAROLINE. 

The account which you have given us, Mrs.  B. , of the materials ofvegetables, is, doubtless, very instructive; but it does not completelysatisfy my curiosity. I wish to know how plants obtain the principlesfrom which their various materials are formed; by what means these areconverted into vegetable matter, and how they are connected with thelife of the plant?

MRS. B. 

This implies nothing less than a complete history of the chemistry andphysiology of vegetation, subjects on which we have yet but veryimperfect notions. Still I hope that I shall be able, in some measure, to satisfy your curiosity. But, in order to render the subject moreintelligible, I must first make you acquainted with the various changeswhich vegetables undergo, when the vital power no longer enables them toresist the common laws of chemical attraction. 

The composition of vegetables being more complicated than that ofminerals, the former more readily undergo chemical changes than thelatter: for the greater the variety of attractions, the more easily isthe equilibrium destroyed, and a new order of combinations introduced. 

EMILY. 

I am surprised that vegetables should be so easily susceptible ofdecomposition; for the preservation of the vegetable kingdom iscertainly far more important than that of minerals. 

MRS. B. 

You must consider, on the other hand, how much more easily the former isrenewed than the latter. The decomposition of the vegetable takes placeonly after the death of the plant, which, in the common course ofnature, happens when it has yielded fruit and seeds to propagate itsspecies. If, instead of thus finishing its career, each plant was toretain its form and vegetable state, it would become an useless burdento the earth and its inhabitants. When vegetables, therefore, cease tobe productive, they cease to live, and nature then begins her process ofdecomposition, in order to resolve them into their chemicalconstituents, hydrogen, carbon, and oxygen; those simple and primitiveingredients, which she keeps in store for all her combinations. 

EMILY. 

But since no system of combination can be destroyed, except by theestablishment of another order of attractions, how can the decompositionof vegetables reduce them to their simple elements?

MRS. B. 

It is a very long process, during which a variety of new combinationsare successively established and successively destroyed: but, in each ofthese changes, the ingredients of vegetable matter tend to unite in amore simple order of compounds, till they are at length brought to theirelementary state, or, at least, to their most simple order ofcombinations. Thus you will find that vegetables are in the end almostentirely reduced to water and carbonic acid; the hydrogen and carbondividing the oxygen between them, so as to form with it these twosubstances. But the variety of intermediate combinations that take placeduring the several stages of the decomposition of vegetables, present uswith a new set of compounds, well worthy of our examination. 

CAROLINE. 

How is it possible that vegetables, while putrefying, should produce anything worthy of observation?

MRS. B. 

They are susceptible of undergoing certain changes before they arrive atthe state of putrefaction, which is the final term of decomposition; andof these changes we avail ourselves for particular and importantpurposes. But, in order to make you understand this subject, which is ofconsiderable importance, I must explain it more in detail. 

The decomposition of vegetables is always attended by a violent internalmotion, produced by the disunion of one order of particles, and thecombination of another. This is called FERMENTATION. There are severalperiods at which this process stops, so that a state of rest appears tobe restored, and the new order of compounds fairly established. But, unless means be used to secure these new combinations in their actualstate, their duration will be but transient, and a new fermentation willtake place, by which the compound last formed will be destroyed; andanother, and less complex order, will succeed. 

EMILY. 

The fermentations, then, appear to be only the successive steps by whicha vegetable descends to its final dissolution. 

MRS. B. 

Precisely so. Your definition is perfectly correct. 

CAROLINE. 

And how many fermentations, or new arrangements, does a vegetableundergo before it is reduced to its simple ingredients?

MRS. B. 

Chemists do not exactly agree in this point; but there are, I think, four distinct fermentations, or periods, at which the decomposition ofvegetable matter stops and changes its course. But every kind ofvegetable matter is not equally susceptible of undergoing all thesefermentations. 

There are likewise several circumstances required to producefermentation. Water and a certain degree of heat are both essential tothis process, in order to separate the particles, and thus weaken theirforce of cohesion, that the new chemical affinities may be brought intoaction. 

CAROLINE. 

In frozen climates, then, how can the spontaneous decomposition ofvegetables take place?

MRS. B. 

It certainly cannot; and, accordingly, we find scarcely any vestiges ofvegetation where a constant frost prevails. 

CAROLINE. 

One would imagine that, on the contrary, such spots would be coveredwith vegetables; for, since they cannot be decomposed, their number mustalways increase. 

MRS. B. 

But, my dear, heat and water are quite as essential to the formation ofvegetables, as they are to their decomposition. Besides, it is from thedead vegetables, reduced to their elementary principles, that the risinggeneration is supplied with sustenance. No young plant, therefore, cangrow unless its predecessors contribute both to its formation andsupport; and these not only furnish the seed from which the new plantsprings, but likewise the food by which it is nourished. 

CAROLINE. 

Under the torrid zone, therefore, where water is never frozen, and theheat is very great, both the processes of vegetation and of fermentationmust, I suppose, be extremely rapid?

MRS. B. 

Not so much as you imagine: for in such climates great part of the waterwhich it requires for these processes is in an aëriform state, which isscarcely more conducive either to the growth or formation of vegetablesthan that of ice. In those latitudes, therefore, it is only in low dampsituations, sheltered by woods from the sun’s rays, that the smallertribes of vegetables can grow and thrive during the dry season, as deadvegetables seldom retain water enough to produce fermentation, but are, on the contrary, soon dried up by the heat of the sun, which enablesthem to resist that process; so that it is not till the fall of theautumnal rains (which are very violent in such climates), thatspontaneous fermentation can take place. 

The several fermentations derive their names from their principalproducts. The first is called the _saccharine fermentation_, because itsproduct is _sugar_. 

CAROLINE. 

But sugar, you have told us, is found in all vegetables; it cannot, therefore, be the product of their decomposition. 

MRS. B. 

It is true that this fermentation is not confined to the decompositionof vegetables, as it continually takes place during their life; and, indeed, this circumstance has, till lately, prevented it from beingconsidered as one of the fermentations. But the process appears soanalogous to the other fermentations, and the formation of sugar, whether in living or dead vegetable matter is so evidently a newcompound, proceeding from the destruction of the previous order ofcombinations, and essential to the subsequent fermentations, that it isnow, I believe, generally esteemed the first step, or necessarypreliminary, to decomposition, if not an actual commencement of thatprocess. 

CAROLINE. 

I recollect your hinting to us that sugar was supposed not to besecreted from the sap, in the same manner as mucilage, fecula, oil, andthe other ingredients of vegetables. 

MRS. B. 

It is rather from these materials, than from the sap itself, that sugaris formed; and it is developed at particular periods, as you may observein fruits, which become sweet in ripening, sometimes even after theyhave been gathered. Life, therefore, is not essential to the formationof sugar, whilst on the contrary, mucilage, fecula, and the othervegetable materials that are secreted from the sap by appropriateorgans, whose powers immediately depend on the vital principle, cannotbe produced but during the existence of that principle. 

EMILY. 

The ripening of fruits is, then, their first step to destruction, aswell as their last towards perfection?

MRS. B. 

Exactly. --A process analogous to the saccharine fermentation takesplace also during the cooking of certain vegetables. This is the casewith parsnips, carrots, potatoes, &c. In which sweetness is developed byheat and moisture; and we know that if we carried the process a littlefarther, a more complete decomposition would ensue. The same processtakes place also in seeds previous to their sprouting. 

CAROLINE. 

How do you reconcile this to your theory, Mrs.  B. ? Can you suppose thata decomposition is the necessary precursor of life?

MRS. B. 

That is indeed the case. The materials of the seed must be decomposed, and the seed disorganized, before a plant can sprout from it. Seeds, besides the embrio plant, contain (as we have already observed) fecula, oil, and a little mucilage. These substances are destined for thenourishment of the future plant; but they undergo some change beforethey can be fit for this function. The seeds, when buried in the earth, with a certain degree of moisture and of temperature, absorb water, which dilates them, separates their particles, and introduces a neworder of attractions, of which sugar is the product. The substance ofthe seed is thus softened, sweetened, and converted into a sort of whitemilky pulp, fit for the nourishment of the embrio plant. 

The saccharine fermentation of seeds is artificially produced, for thepurpose of making _malt_, by the following process:-- A quantity ofbarley is first soaked in water for two or three days: the water beingafterwards drained off, the grain heats spontaneously, swells, bursts, sweetens, shows a disposition to germinate, and actually sprouts to thelength of an inch, when the process is stopped by putting it into akiln, where it is well dried at a gentle heat. In this state it is crispand friable, and constitutes the substance called _malt_, which is theprincipal ingredient of beer. 

EMILY. 

But I hope you will tell us how malt is made into beer?

MRS. B. 

Certainly; but I must first explain to you the nature of the secondfermentation, which is essential to that operation. This is called the_vinous fermentation_, because its product is _wine_. 

EMILY. 

How very different the decomposition of vegetables is from what I hadimagined! The products of their disorganisation appear almost superiorto those which they yield during their state of life and perfection. 

MRS. B. 

And do you not, at the same time, admire the beautiful economy ofNature, which, whether she creates, or whether she destroys, directs allher operations to some useful and benevolent purpose? --It appears thatthe saccharine fermentation is extremely favourable, if not absolutelyessential, as a previous step, to the vinous fermentation; so that ifsugar be not developed during the life of the plant, the saccharinefermentation must be artificially produced before the vinousfermentation can take place. This is the case with barley, which doesnot yield any sugar until it is made into malt; and it is in that stateonly that it is susceptible of undergoing the vinous fermentation bywhich it is converted into beer. 

CAROLINE. 

But if the product of the vinous fermentation is always wine, beercannot have undergone that process, for beer is certainly not wine. 

MRS. B. 

Chemically speaking, beer may be considered as the wine of grain. For itis the product of the fermentation of malt, just as wine is that of thefermentation of grapes, or other fruits. 

The consequence of the vinous fermentation is the decomposition of thesaccharine matter, and the formation of a spirituous liquor from theconstituents of the sugar. But, in order to promote this fermentation, not only water and a certain degree of heat are necessary, but also someother vegetable ingredients, besides the sugar, as fecula, mucilage, acids, salts, extractive matter, &c. All of which seem to contribute tothis process; and give to the liquor its peculiar taste. 

EMILY. 

It is, perhaps, for this reason that wine is not obtained from thefermentation of pure sugar; but that fruits are chosen for that purpose, as they contain not only sugar, but likewise the other vegetableingredients which promote the vinous fermentation, and give the peculiarflavour. 

MRS. B. 

Certainly. And you must observe also, that the relative quantity ofsugar is not the only circumstance to be considered in the choice ofvegetable juices for the formation of wine; otherwise the sugar-canewould be best adapted for that purpose. It is rather the manner andproportion in which the sugar is mixed with other vegetable ingredientsthat influences the production and qualities of wine. And it is foundthat the juice of the grape not only yields the most considerableproportion of wine, but that it likewise affords it of the most gratefulflavour. 

EMILY. 

I have seen a vintage in Switzerland, and I do not recollect that heatwas applied, or water added, to produce the fermentation of the grapes. 

MRS. B. 

The common temperature of the atmosphere in the cellars in which thejuice of the grape is fermented is sufficiently warm for this purpose;and as the juice contains an ample supply of water, there is no occasionfor any addition of it. But when fermentation is produced in dry malt, a quantity of water must necessarily be added. 

EMILY. 

But what are precisely the changes that happen during the vinousfermentation?

MRS. B. 

The sugar is decomposed, and its constituents are recombined into twonew substances; the one a peculiar liquid substance, called _alcohol_ or_spirit of wine_, which remains in the fluid; the other, carbonic acidgas, which escapes during the fermentation. Wine, therefore, as I beforeobserved, in a general point of view, may be considered as a liquid ofwhich alcohol constitutes the essential part. And the varieties ofstrength and flavour of the different kinds of wine are to be attributedto the different qualities of the fruits from which they are obtained, independently of the sugar. 

CAROLINE. 

I am astonished to hear that so powerful a liquid as spirit of wineshould be obtained from so mild a substance as sugar. 

MRS. B. 

Can you tell me in what the principal difference consists betweenalcohol and sugar?

CAROLINE. 

Let me reflect . . . . . Sugar consists of carbon, hydrogen, and oxygen. If carbonic acid be subtracted from it, during the formation of alcohol, the latter will contain less carbon and oxygen than sugar does;therefore hydrogen must be the prevailing principle of alcohol. 

MRS. B. 

It is exactly so. And this very large proportion of hydrogen accountsfor the lightness and combustible property of alcohol, and of spirits ingeneral, all of which consist of alcohol variously modified. 

EMILY. 

And can sugar be recomposed from the combination of alcohol and carbonicacid?

MRS. B. 

Chemists have never been able to succeed in effecting this; but fromanalogy, I should suppose such a recomposition possible. Let us nowobserve more particularly the phenomena that take place during thevinous fermentation. At the commencement of this process, heat isevolved, and the liquor swells considerably from the formation of thecarbonic acid, which is disengaged in such prodigious quantities aswould be fatal to any person who should unawares inspire it; an accidentwhich has sometimes happened. If the fermentation be stopped by puttingthe liquor into barrels, before the whole of the carbonic acid isevolved, the wine is brisk, like Champagne, from the carbonic acidimprisoned in it, and it tastes sweet, like cyder, from the sugar notbeing completely decomposed. 

EMILY. 

But I do not understand why heat should be evolved during thisoperation. For, as there is a considerable formation of gas, in which aproportionable quantity of heat must become insensible, I should haveimagined that cold, rather than heat, would have been produced. 

MRS. B. 

It appears so on first consideration; but you must recollect thatfermentation is a complicated chemical process; and that, during thedecompositions and recompositions attending it, a quantity of chemicalheat may be disengaged, sufficient both to develope the gas, and toeffect an increase of temperature. When the fermentation is completed, the liquid cools and subsides, the effervescence ceases, and the thick, sweet, sticky juice of the fruit is converted into a clear, transparent, spirituous liquor, called wine. 

EMILY. 

How much I regret not having been acquainted with the nature of thevinous fermentation, when I had an opportunity of seeing the process!

MRS. B. 

You have an easy method of satisfying yourself in that respect byobserving the process of brewing, which, in every essentialcircumstance, is similar to that of making wine, and is really a verycurious chemical operation. 

Although we cannot actually make wine at this moment, it will be easy toshow you the mode of analyzing it. This is done by distillation. Whenwine of any kind is submitted to this operation, it is found to containbrandy, water, tartar, extractive colouring matter, and some vegetableacids. I have put a little port wine into this alembic of glass (PLATEXIV. Fig.  1. ), and on placing the lamp under it, you will soon see thespirit and water successively come over--

 [Illustration: Plate XIV. Vol. II. P. 213. 

 Fig. 1. A Alembic. B Lamp. C Wine glass. 

 Fig. 2. Alcohol blowpipe. D the Lamp. E the vessel in which the Alcohol is boiling. F a safety valve. G the inflamed jet or steam of alcohol directed towards a glass tube H. ]

EMILY. 

But you do not mention alcohol amongst the _products_ of thedistillation of wine; and yet that is its most essential ingredient?

MRS. B. 

The alcohol is contained in the brandy which is now coming over, anddropping from the still. Brandy is nothing more than a mixture ofalcohol and water; and in order to obtain the alcohol pure, we mustagain distil it from brandy. 

CAROLINE. 

I have just taken a drop on my finger; it tastes like strong brandy, butit is without colour, whilst brandy is of a deep yellow. 

MRS. B. 

It is not so naturally; in its pure state brandy is colourless, and itobtains the yellow tint you observe, by extracting the colouring matterfrom the new oaken casks in which it is kept. But if it does not acquirethe usual tinge in this way, it is the custom to colour the brandy usedin this country artificially, with a little burnt sugar, in order togive it the appearance of having been long kept. 

CAROLINE. 

And is rum also distilled from wine?

MRS. B. 

By no means; it is distilled from the sugar-cane, a plant which containsso great a quantity of sugar, that it yields more alcohol than almostany other vegetable. After the juice of the cane has been pressed outfor making sugar, what still remains in the bruised cane is extracted bywater, and this watery solution of sugar is fermented, and produces rum. 

The spirituous liquor called _arack_ is in a similar manner distilledfrom the product of the vinous fermentation of rice. 

EMILY. 

But rice has no sweetness; does it contain any sugar?

MRS. B. 

Like barley and most other seeds, it is insipid until it has undergonethe saccharine fermentation; and this, you must recollect, is always aprevious step to the vinous fermentation in those vegetables in whichsugar is not already formed. Brandy may in the same manner be obtainedfrom malt. 

CAROLINE. 

You mean from beer, I suppose; for the malt must have previouslyundergone the vinous fermentation. 

MRS. B. 

Beer is not precisely the product of the vinous fermentation of malt. For hops are a necessary ingredient for the formation of that liquor;whilst brandy is distilled from pure fermented malt. But brandy might, no doubt, be distilled from beer as well as from any other liquor thathas undergone the vinous fermentation; for since the basis of brandy isalcohol, it may be obtained from any liquid that contains thatspirituous substance. 

EMILY. 

And pray, from what vegetable is the favourite spirit of the lowerorders of people, gin, extracted?

MRS. B. 

The spirit (which is the same in all fermented liquors) may be obtainedfrom any kind of grain; but the peculiar flavour which distinguishes ginis that of juniper berries, which are distilled together with thegrain--

I think the brandy contained in the wine which we are distilling must, by this time, be all come over. Yes--taste the liquid that is nowdropping from the alembic--

CAROLINE. 

It is perfectly insipid, like water. 

MRS. B. 

It is water, which, as I was telling you, is the second product of wine, and comes over after all the spirit, which is the lightest part, isdistilled. --The tartar and extractive colouring matter we shall find ina solid form at the bottom of the alembic. 

EMILY. 

They look very like the lees of wine. 

MRS. B. 

And in many respects they are of a similar nature; for lees of wineconsist chiefly of tartrit of potash; a salt which exists in the juiceof the grape, and in many other vegetables, and is developed only by thevinous fermentation. During this operation it is precipitated, anddeposits itself on the internal surface of the cask in which the wine iscontained. It is much used in medicine, and in various arts, particularly dying, under the name of _cream of tartar_, and it is fromthis salt that the tartarous acid is obtained. 

CAROLINE. 

But the medicinal cream of tartar is in appearance quite different fromthese dark-coloured dregs; it is perfectly colourless. 

MRS. B. 

Because it consists of the pure salts only, in its crystallised form;whilst in the instance before us it is mixed with the deep-colouredextractive matter, and other foreign ingredients. 

EMILY. 

Pray cannot we now obtain pure alcohol from the brandy which we havedistilled?

MRS. B. 

We might; but the process would be tedious: for in order to obtainalcohol perfectly free from water, it is necessary to distil, or, as thedistillers call it, _rectify_ it several times. You must therefore allowme to produce a bottle of alcohol that has been thus purified. This is avery important ingredient, which has many striking properties, besidesits forming the basis of all spirituous liquors. 

EMILY. 

It is alcohol, I suppose, that produces intoxication?

MRS. B. 

Certainly; but the stimulus and momentary energy it gives to the system, and the intoxication it occasions when taken in excess, arecircumstances not yet accounted for. 

CAROLINE. 

I thought that it produced these effects by increasing the rapidity ofthe circulation of the blood; for drinking wine or spirits, I haveheard, always quickens the pulse. 

MRS. B. 

No doubt; the spirit, by stimulating the nerves, increases the action ofthe muscles; and the heart, which is one of the strongest muscularorgans, beats with augmented vigour, and propels the blood withaccelerated quickness. After such a strong excitation the framenaturally suffers a proportional degree of depression, so that a stateof debility and languor is the invariable consequence of intoxication. But though these circumstances are well ascertained, they are far fromexplaining why alcohol should produce such effects. 

EMILY. 

Liqueurs are the only kind of spirits which I think pleasant. Pray ofwhat do they consist?

MRS. B. 

They are composed of alcohol, sweetened with syrup, and flavoured withvolatile oil. 

The different kinds of odoriferous spirituous waters are likewisesolutions of volatile oil in alcohol, as lavender water, eau deCologne,  &c. 

The chemical properties of alcohol are important and numerous. It is oneof the most powerful chemical agents, and is particularly useful indissolving a variety of substances, which are soluble neither by waternor heat. 

EMILY. 

We have seen it dissolve copal and mastic to form varnishes; and theseresins are certainly not soluble in water, since water precipitates themfrom their solution in alcohol. 

MRS. B. 

I am happy to find that you recollect these circumstances so well. Thesame experiment affords also an instance of another property ofalcohol, --its tendency to unite with water; for the resin isprecipitated in consequence of losing the alcohol, which abandons itfrom its preference for water. It is attended also, as you mayrecollect, with the same peculiar circumstance of a disengagement ofheat and consequent diminution of bulk, which we have supposed to beproduced by a mechanical penetration of particles by which latent heatis forced out. 

Alcohol unites thus readily not only with resins and with water, butwith oils and balsams; these compounds form the extensive class ofelixirs, tinctures, quintessences,  &c. 

EMILY. 

I suppose that alcohol must be highly combustible, since it contains solarge a proportion of hydrogen?

MRS. B. 

Extremely so; and it will burn at a very moderate temperature. 

CAROLINE. 

I have often seen both brandy and spirit of wine burnt; they produce agreat deal of flame, but not a proportional quantity of heat, and nosmoke whatever. 

MRS. B. 

The last circumstance arises from their combustion being complete; andthe disproportion between the flame and heat shows you that these are byno means synonymous. 

The great quantity of flame proceeds from the combustion of the hydrogento which, you know, that manner of burning is peculiar. --Have you notremarked also that brandy and alcohol will burn without a wick? --Theytake fire at so low a temperature, that this assistance is not requiredto concentrate the heat and volatilise the fluid. 

CAROLINE. 

I have sometimes seen brandy burnt by merely heating it in a spoon. 

MRS. B. 

The rapidity of the combustion of alcohol may, however, be prodigiouslyincreased by first volatilising it. An ingenious instrument has beenconstructed on this principle to answer the purpose of a blow-pipe, which may be used for melting glass, or other chemical purposes. Itconsists of a small metallic vessel (PLATE XIV. Fig.  2. ), of a sphericalshape, which contains the alcohol, and is heated by the lamp beneath it;as soon as the alcohol is volatilised, it passes through the spout ofthe vessel, and issues just above the wick of the lamp, whichimmediately sets fire to the stream of vapour, as I shall show you--

EMILY. 

With what amazing violence it burns! The flame of alcohol, in the stateof vapour, is, I fancy, much hotter than when the spirit is merely burntin a spoon?

MRS. B. 

Yes; because in this way the combustion goes on much quicker, and, ofcourse, the heat is proportionally increased. --Observe its effect onthis small glass tube, the middle of which I present to the extremity ofthe flame, where the heat is greatest. 

CAROLINE. 

The glass, in that spot, is become red hot, and bends from its ownweight. 

MRS. B. 

I have now drawn it asunder, and am going to blow a ball at one of theheated ends; but I must previously close it up, and flatten it with thislittle metallic instrument, otherwise the breath would pass through thetube without dilating any part of it. --Now, Caroline, will you blowstrongly into the tube whilst the closed end is red hot. 

EMILY. 

You blowed too hard; for the ball suddenly dilated to a great size, andthen burst in pieces. 

MRS. B. 

You will be more expert another time; but I must caution you, should youever use this blow-pipe, to be very careful that the combustion of thealcohol does not go on with too great violence, for I have seen theflame sometimes dart out with such force as to reach the opposite wallof the room, and set the paint on fire. There is, however, no danger ofthe vessel bursting, as it is provided with a safety tube, which affordsan additional vent for the vapour of alcohol when required. 

The products of the combustion of alcohol consist in a great proportionof water, and a small quantity of carbonic acid. There is no smoke orfixed remains whatever. --How do you account for that, Emily?

EMILY. 

I suppose that the oxygen which the alcohol absorbs in burning, convertsits hydrogen into water and its carbon into carbonic acid gas, and thusit is completely consumed. 

MRS. B. 

Very well. --_Ether_, the lightest of all fluids, and with which you arewell acquainted, is obtained from alcohol, of which it forms thelightest and most volatile part. 

EMILY. 

Ether, then, is to alcohol, what alcohol is to brandy?

MRS. B. 

No: there is an essential difference. In order to obtain alcohol frombrandy, you need only deprive the latter of its water; but for theformation of ether, the alcohol must be decomposed, and one of itsconstituents partly subtracted. I leave you to guess which of themit is--

EMILY. 

It cannot be hydrogen, as ether is more volatile than alcohol, andhydrogen is the lightest of all its ingredients: nor do I suppose thatit can be oxygen, as alcohol contains so small a proportion of thatprinciple; it is, therefore, most probably, carbon, a diminution ofwhich would not fail to render the new compound more volatile. 

MRS. B. 

You are perfectly right. The formation of ether consists simply insubtracting from the alcohol a certain proportion of carbon; this iseffected by the action of the sulphuric, nitric, or muriatic acids, onalcohol. The acid and carbon remain at the bottom of the vessel, whilstthe decarbonised alcohol flies off in the form of a condensable vapour, which is ether. 

Ether is the most inflammable of all fluids, and burns at so slow atemperature that the heat evolved during its combustion is more than isrequired for its support, so that a quantity of ether is volatilised, which takes fire, and gradually increases the violence of thecombustion. 

Sir Humphry Davy has lately discovered a very singular fact respectingthe vapour of ether. If a few drops of ether be poured into awine-glass, and a fine platina wire, heated almost to redness, be heldsuspended in the glass, close to the surface of the ether, the wire soonbecomes intensely red-hot, and remains so for any length of time. We mayeasily try the experiment.  .  .  .  . 

CAROLINE. 

How very curious! The wire is almost white hot, and a pungent smellrises from the glass. Pray how is this accounted for?

MRS. B. 

This is owing to a very peculiar property of the vapour of ether, andindeed of many other combustible gaseous bodies. At a certaintemperature lower than that of ignition, these vapours undergo a slowand imperfect combustion, which does not give rise, in any sensibledegree, to the phenomena of light and flame, and yet extricates aquantity of caloric sufficient to react upon the wire and make itred-hot, and the wire in its turn keeps up the effect as long as theemission of vapour continues. 

CAROLINE. 

But why should not an iron or silver wire produce the same effect?

MRS. B. 

Because either iron or silver, being much better conductors of heat thanplatina, the heat is carried off too fast by those metals to allow theaccumulation of caloric necessary to produce the effect in question. 

Ether is so light that it evaporates at the common temperature of theatmosphere; it is therefore necessary to keep it confined by a wellground glass stopper. No degree of cold known has ever frozen it. 

CAROLINE. 

Is it not often taken medicinally?

MRS. B. 

Yes; it is one of the most effectual antispasmodic medicines, and thequickness of its effects, as such, probably depends on its beinginstantly converted into vapour by the heat of the stomach, through theintervention of which it acts on the nervous system. But the frequentuse of ether, like that of spirituous liquors, becomes prejudicial, and, if taken to excess, it produces effects similar to those ofintoxication. 

We may now take our leave of the vinous fermentation, of which, I hope, you have acquired a clear idea; as well as of the several products thatare derived from it. 

CAROLINE. 

Though this process appears, at first sight, so much complicated, itmay, I think, be summed up in a few words, as it consists in theconversion of sugar and fermentable bodies into alcohol and carbonicacid, which give rise both to the formation of wine, and of all kinds ofspirituous liquors. 

MRS. B. 

We shall now proceed to the _acetous fermentation_, which is thuscalled, because it converts wine into vinegar, by the formation of theacetous acid, which is the basis or radical of vinegar. 

CAROLINE. 

But is not the acidifying principle of the acetous acid the same as thatof all other acids, oxygen?

MRS. B. 

Certainly; and on that account the contact of air is essential to thisfermentation, as it affords the necessary supply of oxygen. Vinegar, inorder to obtain pure acetous acid from it, must be distilled andrectified by certain processes. 

EMILY. 

But pray, Mrs. B. , is not the acetous acid frequently formed withoutthis fermentation taking place? Is it not, for instance, contained inacid fruits, and in every substance that becomes sour?

MRS. B. 

No, not in fruits; you confound it with the citric, the malic, theoxalic, and other vegetable acids, to which living vegetables owe theiracidity. But whenever a vegetable substance turns sour, after it hasceased to live, the acetous acid is developed by means of the acetousfermentation, in which the substance advances a step towards its finaldecomposition. 

Amongst the various instances of acetous fermentation, that of bread isusually classed. 

CAROLINE. 

But the fermentation of bread is produced by yeast; how does thateffect it?

MRS. B. 

It is found by experience that any substance that has already undergonea fermentation, will readily excite it in one that is susceptible ofthat process. If, for instance, you mix a little vinegar with wine, thatis intended to be acidified, it will absorb oxygen more rapidly, and theprocess be completed much sooner, than if left to ferment spontaneously. Thus yeast, which is a product of the fermentation of beer, is used toexcite and accelerate the fermentation of malt, which is to be convertedinto beer, as well as that of paste which is to be made into bread. 

CAROLINE. 

But if bread undergoes the acetous fermentation, why is it not sour?

MRS. B. 

It acquires a certain savour which corrects the heavy insipidity offlour, and may be reckoned a first degree of acidification; or if theprocess were carried further, the bread would become decidedly acid. 

There are, however, some chemists who do not consider the fermentationof bread as being of the acetous kind, but suppose that it is a processof fermentation peculiar to that substance. 

The _putrid fermentation_ is the final operation of Nature, and her laststep towards reducing organised bodies to their simplest combinations. All vegetables spontaneously undergo this fermentation after death, provided there be a sufficient degree of heat and moisture, togetherwith access of air; for it is well known that dead plants may bepreserved by drying, or by the total exclusion of air. 

CAROLINE. 

But do dead plants undergo the other fermentation previous to this last;or do they immediately suffer the putrid fermentation?

MRS. B. 

That depends on a variety of circumstances, such as the degrees oftemperature and of moisture, the nature of the plant itself, &c. But ifyou were carefully to follow and examine the decomposition of plantsfrom their death to their final dissolution, you would generally find asweetness developed in the seeds, and a spirituous flavour in the fruits(which have undergone the saccharine fermentation), previous to thetotal disorganisation and separation of the parts. 

EMILY. 

I have sometimes remarked a kind of spirituous taste in fruits that wereover ripe, especially oranges; and this was just before they becamerotten. 

MRS. B. 

It was then the vinous fermentation which had succeeded the saccharine, and had you followed up these changes attentively, you would probablyhave found the spirituous taste followed by acidity, previous to thefruit passing to the state of putrefaction. 

When the leaves fall from the trees in autumn, they do not (if there isno great moisture in the atmosphere) immediately undergo adecomposition, but are first dried and withered; as soon, however, asthe rain sets in, fermentation commences, their gaseous products areimperceptibly evolved into the atmosphere, and their fixed remains mixedwith their kindred earth. 

Wood, when exposed to moisture, also undergoes the putrid fermentationand becomes rotten. 

EMILY. 

But I have heard that the _dry rot_, which is so liable to destroy thebeams of houses, is prevented by a current of air; and yet you said thatair was essential to the putrid fermentation?

MRS. B. 

True; but it must not be in such a proportion to the moisture as todissolve the latter, and this is generally the case when the rotting ofwood is prevented or stopped by the free access of air. What is commonlycalled dry rot, however, is not I believe a true process ofputrefaction. It is supposed to depend on a peculiar kind of vegetation, which, by feeding on the wood, gradually destroys it. 

Straw and all other kinds of vegetable matter undergo the putridfermentation more rapidly when mixed with animal matter. Much heat isevolved during this process, and a variety of volatile products aredisengaged, as carbonic acid and hydrogen gas, the latter of which isfrequently either sulphurated or phosphorated. --When all these gaseshave been evolved, the fixed products, consisting of carbon, salts, potash, &c. Form a kind of vegetable earth, which makes very finemanure, as it is composed of those elements which form the immediatematerials of plants. 

CAROLINE. 

Pray are not vegetables sometimes preserved from decomposition bypetrification? I have seen very curious specimens of petrifiedvegetables, in which state they perfectly preserve their form andorganisation, though in appearance they are changed to stone. 

MRS. B. 

That is a kind of metamorphosis, which, now that you are tolerably wellversed in the history of mineral and vegetable substances, I leave toyour judgment to explain. Do you imagine that vegetables can beconverted into stone?

EMILY. 

No, certainly; but they might perhaps be changed to a substance inappearance resembling stone. 

MRS. B. 

It is not so, however, with the substances that are called petrifiedvegetables; for these are really stone, and generally of the hardestkind, consisting chiefly of silex. The case is this: when a vegetable isburied under water, or in wet earth, it is slowly and graduallydecomposed. As each successive particle of the vegetable is destroyed, its place is supplied by a particle of siliceous earth, conveyed thitherby the water. In the course of time the vegetable is entirely destroyed, but the silex has completely replaced it, having assumed its form andapparent texture, as if the vegetable itself were changed to stone. 

CAROLINE. 

That is very curious! and I suppose that petrified animal substances areof the same nature?

MRS. B. 

Precisely. It is equally impossible for either animal or vegetablesubstances to be converted into stone. They may be reduced, as we findthey are, by decomposition, to their constituent elements, but cannot bechanged to elements, which do not enter into their composition. 

There are, however, circumstances which frequently prevent the regularand final decomposition of vegetables; as, for instance, when they areburied either in the sea, or in the earth, where they cannot undergo theputrid fermentation for want of air. In these cases they are subject toa peculiar change, by which they are converted into a new class ofcompounds, called _bitumens_. 

CAROLINE. 

These are substances I never heard of before. 

MRS. B. 

You will find, however, that some of them are very familiar to you. Bitumens are vegetables so far decomposed as to retain no organicappearance; but their origin is easily detected by their oily nature, their combustibility, the products of their analysis, and theimpressions of the forms of leaves, grains, fibres of wood, and even ofanimals, which they frequently bear. 

They are sometimes of an oily liquid consistence, as the substancecalled _naptha_, in which we preserved potassium; it is a finetransparent colourless fluid, that issues out of clays in some parts ofPersia. But more frequently bitumens are solid, as _asphaltum_, a smooth, hard, brittle substance, which easily melts, and forms, in itsliquid state, a beautiful dark brown colour for oil painting. _Jet_, which is of a still harder texture, is a peculiar bitumen, susceptibleof so fine a polish, that it is used for many ornamental purposes. 

_Coal_ is also a bituminous substance, to the composition of which boththe mineral and animal kingdoms seem to concur. This most useful mineralappears to consist chiefly of vegetable matter, mixed with the remainsof marine animals and marine salts, and occasionally containing aquantity of sulphuret of iron, commonly called pyrites. 

EMILY. 

It is, I suppose, the earthly, the metallic, and the saline parts ofcoals, that compose the cinders or fixed products of their combustion;whilst the hydrogen and carbon, which they derive from vegetables, constitute their volatile products. 

CAROLINE. 

Pray is not _coke_, (which I have heard is much used in somemanufactures, ) also a bituminous substance?

MRS. B. 

No; it is a kind of fuel artificially prepared from coals. It consistsof coals reduced to a substance analogous to charcoal, by theevaporation of their bituminous parts. Coke, therefore, is composed ofcarbon, with some earthy and saline ingredients. 

_Succin_, or _yellow amber_, is a bitumen which the ancients called_electrum_, from whence the word electricity is derived, as thatsubstance is peculiarly, and was once supposed to be exclusively, electric. It is found either deeply buried in the bowels of the earth, or floating on the sea, and is supposed to be a resinous body which hasbeen acted on by sulphuric acid, as its analysis shows it to consist ofah oil and an acid. The oil is called _oil of amber_, the acid the_succinic_. 

EMILY. 

That oil I have sometimes used in painting, as it is reckoned to changeless than the other kinds of oils. 

MRS. B. 

The last class of vegetable substances that have changed their natureare _fossil-wood_, _peat_, and _turf_. These are composed of wood androots of shrubs, that are partly decomposed by being exposed to moistureunder ground, and yet, in some measure, preserve their form and organicappearance. The peat, or black earth of the moors, retains but fewvestiges of the roots to which it owes its richness and combustibility, these substances being in the course of time reduced to the state ofvegetable earth. But in turf the roots of plants are still discernible, and it equally answers the purpose of fuel. It is the combustible usedby the poor in heathy countries, which supply it abundantly. 

It is too late this morning to enter upon the history of vegetation. Weshall reserve this subject, therefore, for our next interview, when Iexpect that it will furnish us with ample matter for anotherconversation. 

CONVERSATION XXII. 

HISTORY OF VEGETATION. 

MRS. B. 

The VEGETABLE KINGDOM may be considered as the link which unites themineral and animal creation into one common chain of beings; for it isthrough the means of vegetation alone that mineral substances areintroduced into the animal system, since, generally speaking, it is fromvegetables that all animals ultimately derive their sustenance. 

CAROLINE. 

I do not understand that; the human species subsists as much on animalas on vegetable food, and there are some carnivorous animals that willeat only animal food. 

MRS. B. 

That is true; but you do not consider that those that live on animalfood, derive their sustenance equally, though not so immediately, fromvegetables. The meat that we eat is formed from the herbs of the field, and the prey of carnivorous animals proceeds, either directly orindirectly, from the same source. It is, therefore, through this channelthat the simple elements become a part of the animal frame. We should invain attempt to derive nourishment from carbon, hydrogen, and oxygen, either in their separate state, or combined in the mineral kingdom; forit is only by being united in the form of vegetable combination, thatthey become capable of conveying nourishment. 

EMILY. 

Vegetation, then, seems to be the method which Nature employs to preparethe food of animals?

MRS. B. 

That is certainly its principal object. The vegetable creation does notexhibit more wisdom in that admirable system of organisation, by whichit is enabled to answer its own immediate ends of preservation, nutrition, and propagation, than in its grand and ultimate object offorming those arrangements and combinations of principles, which are sowell adapted for the nourishment of animals. 

EMILY. 

But I am very curious to know whence vegetables obtain those principleswhich form their immediate materials?

MRS. B. 

This is a point on which we are yet so much in the dark, that I cannothope fully to satisfy your curiosity; but what little I know on thissubject, I will endeavour to explain to you. 

The soil, which, at first view, appears to be the aliment of vegetables, is found, on a closer investigation, to be little more than the channelthrough which they receive their nourishment; so that it is verypossible to rear plants without any earth or soil. 

CAROLINE. 

Of that we have an instance in the hyacinth and other bulbous roots, which will grow and blossom beautifully in glasses of water. But Iconfess I should think it would be difficult to rear trees in a similarmanner. 

MRS. B. 

No doubt it would, as it is the burying of the roots in the earth thatsupports the stem of the tree. But this office, besides that ofaffording a vehicle for food, is far the most important part which theearthy portion of the soil performs in the process of vegetation; for wecan discover, by analysis, but an extremely small proportion of earth invegetable compounds. 

CAROLINE. 

But if earths do not afford nourishment, why is it necessary to be soattentive to the preparation of the soil?

MRS. B. 

In order to impart to it those qualities which render it a propervehicle for the food of the plant. Water is the chief nourishment ofvegetables; if, therefore, the soil be too sandy, it will not retain aquantity of water sufficient to supply the roots of the plants. If, onthe contrary, it abound too much with clay, the water will lodge in suchquantities as to threaten a decomposition of the roots. Calcareous soilsare, upon the whole, the most favourable to the growth of plants: soilsare, therefore, usually improved by chalk, which, you may recollect, isa carbonat of lime. Different vegetables, however, require differentkinds of soils. Thus rice demands a moist retentive soil; potatoes asoft sandy soil; wheat a firm and rich soil. Forest trees grow better infine sand than in a stiff clay; and a light ferruginous soil is bestsuited to fruit-trees. 

CAROLINE. 

But pray what is the use of manuring the soil?

MRS. B. 

Manure consists of all kinds of substances, whether of vegetable oranimal origin, which have undergone the putrid fermentation, and areconsequently decomposed, or nearly so, into their elementary principles. And it is requisite that these vegetable matters should be in a state ofdecay, or approaching decomposition. The addition of calcareous earth, in the state of chalk or lime, is beneficial to such soils, as itaccelerates the dissolution of vegetable bodies. Now, I ask you, what isthe utility of supplying the soil with these decomposed substances?

CAROLINE. 

It is, I suppose, in order to furnish vegetables with the principleswhich enter into their composition. For manures not only contain carbon, hydrogen, and oxygen, but by their decomposition supply the soil withthese principles in their elementary form. 

MRS. B. 

Undoubtedly; and it is for this reason that the finest crops areproduced in fields that were formerly covered with woods, because theirsoil is composed of a rich mould, a kind of vegetable earth, whichabounds in those principles. 

EMILY. 

This accounts for the plentifulness of the crops produced in America, where the country was but a few years since covered with wood. 

CAROLINE. 

But how is it that animal substances are reckoned to produce the bestmanure? Does it not appear much more natural that the decomposedelements of vegetables should be the most appropriate to the formationof new vegetables?

MRS. B. 

The addition of a much greater proportion of nitrogen, which constitutesthe chief difference between animal and vegetable matter, renders thecomposition of the former more complicated, and consequently morefavourable to decomposition. The use of animal substances is chiefly togive the first impulse to the fermentation of the vegetable ingredientsthat enter into the composition of manures. The manure of a farm-yard isof that description; but there is scarcely any substance susceptible ofundergoing the putrid fermentation that will not make good manure. Theheat produced by the fermentation of manure is another circumstancewhich is extremely favourable to vegetation; yet this heat would be toogreat if the manure was laid on the ground during the height offermentation; it is used in this state only for hot-beds, to producemelons, cucumbers, and such vegetables as require a very hightemperature. 

CAROLINE. 

A difficulty has just occurred to me which I do not know how to remove. Since all organised bodies are, in the common course of nature, ultimately reduced to their elementary state, they must necessarily inthat state enrich the soil, and afford food for vegetation. How is it, then, that agriculture, which cannot increase the quantity of thoseelements that are required to manure the earth, can increase its produceso wonderfully as is found to be the case in all cultivated countries?

MRS. B. 

It is by suffering none of these decaying bodies to be dissipated, butin applying them duly to the soil. It is by a judicious preparation ofthe soil, which consists in fitting it either for the general purposesof vegetation, or for that of the particular seed which is to be sown. Thus, if the soil be too wet, it may be drained; if too loose and sandy, it may be rendered more consistent and retentive of water by theaddition of clay or loam; it may be enriched by chalk, or any kind ofcalcareous earth. On soils thus improved, manures will act with doubleefficacy, and if attention be paid to spread them on the ground at aproper season of the year, to mix them with the soil so that they may begenerally diffused through it, to destroy the weeds which mightappropriate these nutritive principles to their own use, to remove thestones which would impede the growth of the plant, &c. We may obtain aproduce an hundred fold more abundant than the earth would spontaneouslysupply. 

EMILY. 

We have a very striking instance of this in the scanty produce ofuncultivated commons, compared to the rich crops of meadows which areoccasionally manured. 

CAROLINE. 

But, Mrs. B. , though experience daily proves the advantage ofcultivation, there is still a difficulty which I cannot get over. A certain quantity of elementary principles exist in nature, which it isnot in the power of man either to augment or diminish. Of theseprinciples you have taught us that both the animal and vegetablecreation are composed. Now the more of them is taken up by the vegetablekingdom, the less, it would seem, will remain for animals; and, therefore, the more populous the earth becomes, the less it willproduce. 

MRS. B. 

Your reasoning is very plausible; but experience every where contradictsthe inference you would draw from it; for we find that the animal andvegetable kingdoms, instead of thriving, as you would suppose, at eachother’s expense, always increase and multiply together. For you shouldrecollect that animals can derive the elements of which they are formedonly through the medium of vegetables. And you must allow that yourconclusion would be valid only if every particle of the severalprinciples that could possibly be spared from other purposes wereemployed in the animal and vegetable creations. Now we have reason tobelieve that a much greater proportion of these principles than isrequired for such purposes remains either in an elementary state, orengaged in a less useful mode of combination in the mineral kingdom. Possessed of such immense resources as the atmosphere and the watersafford us, for oxygen, hydrogen, and carbon, so far from being in dangerof working up all our simple materials, we cannot suppose that we shallever bring agriculture to such a degree of perfection as to require thewhole of what these resources could supply. 

Nature, however, in thus furnishing us with an inexhaustible stock ofraw materials, leaves it in some measure to the ingenuity of man toappropriate them to its own purposes. But, like a kind parent, shestimulates him to exertion, by setting the example and pointing out theway. For it is on the operations of nature that all the improvements ofart are founded. The art of agriculture consists, therefore, indiscovering the readiest method of obtaining the several principles, either from their grand sources, air and water, or from thedecomposition of organised bodies; and in appropriating them in the bestmanner to the purposes of vegetation. 

EMILY. 

But, among the sources of nutritive principles, I am surprised that youdo not mention the earth itself, as it contains abundance of coals, which are chiefly composed of carbon. 

MRS. B. 

Though coals abound in carbon, they cannot, on account of their hardnessand impermeable texture, be immediately subservient to the purposes ofvegetation. 

EMILY. 

No; but by their combustion carbonic acid is produced; and this enteringinto various combinations on the surface of the earth, may, perhaps, assist in promoting vegetation. 

MRS. B. 

Probably it may in some degree; but at any rate the quantity ofnourishment which vegetables may derive from that source can be but verytrifling, and must entirely depend on local circumstances. 

CAROLINE. 

Perhaps the smoky atmosphere of London is the cause of vegetation beingso forward and so rich in its vicinity?

MRS. B. 

I rather believe that this circumstance proceeds from the very amplesupply of manure, assisted, perhaps, by the warmth and shelter which thetown affords. Far from attributing any good to the smoky atmosphere ofLondon, I confess I like to anticipate the time when we shall have madesuch progress in the art of managing combustion, that every particle ofcarbon will be consumed, and the smoke destroyed at the moment of itsproduction. We may then expect to have the satisfaction of seeing theatmosphere of London as clear as that of the country. --But to return toour subject: I hope that you are now convinced that we shall not easilyexperience a deficiency of nutritive elements to fertilise the earth, and that, provided we are but industrious in applying them to the bestadvantage by improving the art of agriculture, no limits can be assignedto the fruits that we may expect to reap from our labours. 

CAROLINE. 

Yes; I am perfectly satisfied in that respect, and I can assure you thatI feel already much more interested in the progress and improvement ofagriculture. 

EMILY. 

I have frequently thought that the culture of the land was notconsidered as a concern of sufficient importance. Manufactures alwaystake the lead; and health and innocence are frequently sacrificed to theprospect of a more profitable employment. It has often grieved me to seethe poor manufacturers crowded together in close rooms, and confined forthe whole day to the most uniform and sedentary employment, instead ofbeing engaged in that innocent and salutary kind of labour, which Natureseems to have assigned to man for the immediate acquirement of comfort, and for the preservation of his existence. I am sure that you agree withme in thinking so, Mrs.  B. ?

MRS. B. 

I am entirely of your opinion, my dear, in regard to the importance ofagriculture; but as the conveniences of life, which we are all enjoying, are not derived merely from the soil, I am far from wishing todepreciate manufactures. Besides, as the labour of one man is sufficientto produce food for several, those whose industry is not required intillage must do something in return for the food that is provided forthem. They exchange, consequently, the accommodations for thenecessaries of life. Thus the carpenter and the weaver lodge and clothethe peasant, who supplies them with their daily bread. The greater stockof provisions, therefore, which the husbandman produces, the greater isthe quantity of accommodation which the artificer prepares. Such are thehappy effects which naturally result from civilised society. It would bewiser, therefore, to endeavour to improve the situation of those who areengaged in manufactures, than to indulge in vain declamations on thehardships to which they are too frequently exposed. 

But we must not yet take our leave of the subject of agriculture; wehave prepared the soil, it remains for us now to sow the seed. In thisoperation we must be careful not to bury it too deep in the ground, asthe access of air is absolutely necessary to its germination; the earthmust, therefore, lie loose and light over it, in order that the air maypenetrate. Hence the use of ploughing and digging, harrowing and raking, &c. A certain degree of heat and moisture, such as usually takes placein the spring, is likewise necessary. 

CAROLINE. 

One would imagine you were going to describe the decomposition of an oldplant, rather than the formation of a new one; for you have enumeratedall the requisites of fermentation. 

MRS. B. 

Do you forget, my dear, that the young plant derives its existence fromthe destruction of the seed, and that it is actually by the saccharinefermentation that the latter is decomposed?

CAROLINE. 

True; I wonder that I did not recollect that. The temperature andmoisture required for the germination of the seed is then employed inproducing the saccharine fermentation within it?

MRS. B. 

Certainly. But, in order to understand the nature of germination, youshould be acquainted with the different parts of which the seed iscomposed. The external covering or envelope contains, besides the germof the future plant, the substance which is to constitute its firstnourishment; this substance, which is called the _parenchyma_, consistsof fecula, mucilage, and oil, as we formerly observed. 

The seed is generally divided into two compartments, called _lobes_, or_cotyledons_, as is exemplified by this bean (PLATE XV. Fig.  1. )--thedark-coloured kind of string which divides the lobes is called the_radicle_, as it forms the root of the plant, and it is from acontiguous substance, called _plumula_, which is enclosed within thelobes, that the stem arises. The figure and size of the seed depend verymuch upon the cotyledons; these vary in number in different seeds; somehave only one, as wheat, oats, barley, and all the grasses; some havethree, others six. But most seeds, as, for instance, all the varietiesof beans, have two cotyledons. When the seed is buried in the earth, atany temperature above 40 degrees, it imbibes water, which softens andswells the lobes; it then absorbs oxygen, which combines with some ofits carbon, and is returned in the form of carbonic acid. This loss ofcarbon increases the comparative proportion of hydrogen and oxygen inthe seed, and excites the saccharine fermentation, by which theparenchymatous matter is converted into a kind of sweet emulsion. Inthis form it is carried into the radicle by vessels appropriated to thatpurpose; and in the mean time, the fermentation having caused the seedto burst, the cotyledons are rent asunder, the radicle strikes into theground and becomes the root of the plant, and hence the fermented liquidis conveyed to the plumula, whose vessels have been previously distendedby the heat of the fermentation. The plumula being thus swelled, as itwere, by the emulsive fluid, raises itself and springs up to the surfaceof the earth, bearing with it the cotyledons, which, as soon as theycome in contact with the air, spread themselves, and are transformedinto leaves. --If we go into the garden, we shall probably find someseeds in the state which I have described--

 [Illustration: Plate XV. Vol. II. P. 250

 Germination. Fig. 1 & 2. A. B Cotyledons. C Envelope. D Radicle. 

 Fig. 3. A. B Cotyledons. C Plumula. D Radicle. 

 Fig. 4. A. B. Cotyledons. C Plumula. D Radicle. 

 Fig. 5. Apparatus to illustrate the mechanism of breathing. A. A Glass Bell. B Bladder representing the lungs. C Bladder representing the Diaphragm. ]

EMILY. 

Here are some lupines that are just making their appearance aboveground. 

MRS. B. 

We shall take up several of them to observe their different degrees ofprogress in vegetation. Here is one that has but recently burst itsenvelope--do you see the little radicle striking downwards? (PLATE XV. Fig.  2. ) In this the plumula is not yet visible. But here is another ina greater state of forwardness--the plumula, or stem, has risen out ofthe ground, and the cotyledons are converted into seed leaves. (PLATEXV. Fig.  3. )

CAROLINE. 

These leaves are very thick and clumsy, and unlike the other leaves, which I perceive are just beginning to appear. 

MRS. B. 

It is because they retain the remains of the parenchyma, with which theystill continue to nourish the young plant, as it has not yet sufficientroots and strength to provide for its sustenance from the soil. --But, in this third lupine (PLATE XV. Fig.  4. ), the radicle had sunk deepinto the earth, and sent out several shoots, each of which is furnishedwith a mouth to suck up nourishment from the soil; the function of theoriginal leaves, therefore, being no longer required, they are graduallydecaying, and the plumula is become a regular stem, shooting out smallbranches, and spreading its foliage. 

EMILY. 

There seems to be a very striking analogy between a seed and an egg;both require an elevation of temperature to be brought to life; both atfirst supply with aliment the organised being which they produce; and assoon as this has attained sufficient strength to procure its ownnourishment, the egg-shell breaks, whilst in the plant the seed-leavesfall off. 

MRS. B. 

There is certainly some resemblance between these processes; and whenyou become acquainted with animal chemistry, you will frequently bestruck with its analogy to that of the vegetable kingdom. 

As soon as the young plant feeds from the soil, it requires theassistance of leaves, which are the organs by which it throws off itssuper-abundant fluid; this secretion is much more plentiful in thevegetable than in the animal creation, and the great extent of surfaceof the foliage of plants is admirably calculated for carrying it on insufficient quantities. This transpired fluid consists of little morethan water. The sap, by this process, is converted into a liquid ofgreater consistence, which is fit to be assimilated to its severalparts. 

EMILY. 

Vegetation, then, must be essentially injured by destroying the leavesof the plant?

MRS. B. 

Undoubtedly; it not only diminishes the transpiration, but also theabsorption by the roots; for the quantity of sap absorbed is always inproportion to the quantity of fluid thrown off by transpiration. Yousee, therefore, the necessity that a young plant should unfold itsleaves as soon as it begins to derive its nourishment from the soil;and, accordingly, you will find that those lupines which have droppedtheir seed-leaves, and are no longer fed by the parenchyma, have spreadtheir foliage, in order to perform the office just described. 

But I should inform you that this function of transpiration seems to beconfined to the upper surface of the leaves, whilst, on the contrary, the lower surface, which is more rough and uneven, and furnished with akind of hair or down, is destined to absorb moisture, or such otheringredients as the plant derives from the atmosphere. 

As soon as a young plant makes its appearance above ground, light, aswell as air, becomes necessary to its preservation. Light is essentialto the development of the colours, and to the thriving of the plant. Youmay have often observed what a predilection vegetables have for thelight. If you make any plants grow in a room, they all spread theirleaves, and extend their branches towards the windows. 

CAROLINE. 

And many plants close up their flowers as soon as it is dark. 

EMILY. 

But may not this be owing to the cold and dampness of the evening air?

MRS. B. 

That does not appear to be the case; for in a course of curiousexperiments, made by Mr. Senebier, of Geneva, on plants which he rearedby lamp-light, he found that the flowers closed their petals wheneverthe lamps were extinguished. 

EMILY. 

But pray, why is air essential to vegetation, plants do not breathe itlike animals?

MRS. B. 

At least not in the same manner; but they certainly derive someprinciples from the atmosphere, and yield others to it. Indeed, it ischiefly owing to the action of the atmosphere and the vegetable kingdomon each other, that the air continues always fit for respiration. Butyou will understand this better when I have explained the effect ofwater on plants. 

I have said that water forms the chief nourishment of plants; it is thebasis not only of the sap, but of all the vegetable juices. Water is thevehicle which carries into the plant the various salts and otheringredients required for the formation and support of the vegetablesystem. Nor is this all; part of the water itself is decomposed by theorgans of the plant; the hydrogen becomes a constituent part of oil, ofextract, of colouring matter, &c. Whilst a portion of the oxygen entersinto the formation of mucilage, of fecula, of sugar, and of vegetableacids. But the greater part of the oxygen, proceeding from thedecomposition of the water, is converted into a gaseous state by thecaloric disengaged from the hydrogen during its condensation in theformation of the vegetable materials. In this state the oxygen istranspired by the leaves of plants when exposed to the sun’s rays. Thusyou find that the decomposition of water, by the organs of the plant, isnot only a means of supplying it with its chief ingredient, hydrogen, but at the same time of replenishing the atmosphere with oxygen, a principle which requires continual renovation, to make up for thegreat consumption of it occasioned by the numerous oxygenations, combustions, and respirations, that are constantly taking place on thesurface of the globe. 

EMILY. 

What a striking instance of the harmony of nature. 

MRS. B. 

And how admirable the design of Providence, who makes every differentpart of the creation thus contribute to the support and renovation ofeach other!

But the intercourse of the vegetable and animal kingdoms through themedium of the atmosphere extends still further. Animals, in breathing, not only consume the oxygen of the air, but load it with carbonic acid, which, if accumulated in the atmosphere, would, in a short time, renderit totally unfit for respiration. Here the vegetable kingdom againinterferes; it attracts and decomposes the carbonic acid, retains thecarbon for its own purposes, and returns the oxygen for ours. 

CAROLINE. 

How interesting this is! I do not know a more beautiful illustration ofthe wisdom which is displayed in the laws of nature. 

MRS. B. 

Faint and imperfect as are the ideas which our limited perceptionsenable us to form of divine wisdom, still they cannot fail to inspire uswith awe and admiration. What, then, would be our feelings, were thecomplete system of nature at once displayed before us! So magnificent ascene would probably be too great for our limited and imperfectcomprehension, and it is no doubt among the wise dispensations ofProvidence, to veil the splendour of a glory with which we should beoverpowered. But it is well suited to the nature of a rational being toexplore, step by step, the works of the creation, to endeavour toconnect them into harmonious systems; and, in a word, to trace in thechain of beings, the kindred ties and benevolent design which unites itsvarious links, and secure its preservation. 

CAROLINE. 

But of what nature are the organs of plants which are endued with suchwonderful powers?

MRS. B. 

They are so minute that their structure, as well as the mode in whichthey perform their functions, generally elude our examination; but wemay consider them as so many vessels or apparatus appropriated toperform, with the assistance of the principle of life, certain chemicalprocesses, by means of which these vegetable compounds are generated. Wemay, however, trace the tannin, resins, gum, mucilage, and some othervegetable materials, in the organised arrangement of plants, in whichthey form the bark, the wood, the leaves, flowers, and seeds. 

The _bark_ is composed of the _epidermis_, the _parenchyma_, and the_cortical layers_. 

The epidermis is the external covering of the plant. It is a thintransparent membrane, consisting of a number of slender fibres, crossingeach other, and forming a kind of net-work. When of a white glossynature, as in several species of trees, in the stems of corn and ofseeds, it is composed of a thin coating of siliceous earth, whichaccounts for the strength and hardness of those long and slender stems. Sir H. Davy was led to the discovery of the siliceous nature of theepidermis of such plants, by observing the singular phenomenon of sparksof fire emitted by the collision of ratan canes with which two boys werefighting in a dark room. On analysing the epidermis of the cane, hefound it to be almost entirely siliceous. 

CAROLINE. 

With iron then, a cane, I suppose, will strike fire very easily?

MRS. B. 

I understand that it will. --In ever-greens the epidermis is mostlyresinous, and in some few plants is formed of wax. The resin, from itswant of affinity for water, tends to preserve the plant from thedestructive effects of violent rains, severe climates, or inclementseasons, to which this species of vegetables is peculiarly exposed. 

EMILY. 

Resin must preserve wood just like a varnish, as it is the essentialingredient of varnishes?

MRS. B. 

Yes; and by this means it prevents likewise all unnecessary expenditureof moisture. 

The parenchyma is immediately beneath the epidermis; it is that greenrind which appears when you strip a branch of any tree or shrub of itsexternal coat of bark. The parenchyma is not confined to the stem orbranches, but extends over every part of the plant. It forms the greenmatter of the leaves, and is composed of tubes filled with a peculiarjuice. 

The cortical layers are immediately in contact with the wood; theyabound with tannin and gallic acid, and consist of small vessels throughwhich the sap descends after being elaborated in the leaves. Thecortical layers are annually renewed, the old bark being converted intowood. 

EMILY. 

But through what vessels does the sap ascend?

MRS. B. 

That function is performed by the tubes of the alburnum, or wood, whichis immediately beneath the cortical layers. The wood is composed ofwoody fibre, mucilage, and resin. The fibres are disposed in two ways;some of them longitudinally, and these form what is called the silvergrain of the wood. The others, which are concentric, are called thespurious grain. These last are disposed in layers, from the number ofwhich the age of the tree may be computed, a new one being producedannually by the conversion of the bark into wood. The oldest, andconsequently most internal part of the alburnum, is called heart-wood;it appears to be dead, at least no vital functions are discernible init. It is through the tubes of the living alburnum that the sap rises. These, therefore, spread into the leaves, and there communicate with theextremities of the vessels of the cortical layers, into which they pourtheir contents. 

CAROLINE. 

Of what use, then, are the tubes of the parenchyma, since neither theascending nor descending sap passes through them?

MRS. B. 

They are supposed to perform the important function of secreting fromthe sap the peculiar juices from which the plant more immediatelyderives its nourishment. These juices are very conspicuous, as thevessels which contain them are much larger than those through which thesap circulates. The peculiar juices of plants differ much in theirnature, not only in different species of vegetables, but frequently indifferent parts of the same individual plant: they are sometimessaccharine, as in the sugar-cane, sometimes resinous, as in firs andevergreens, sometimes of a milky appearance, as in the laurel. 

EMILY. 

I have often observed, that in breaking a young shoot, or in bruising aleaf of laurel, a milky juice will ooze out in great abundance. 

MRS. B. 

And it is by making incisions in the bark that pitch, tar, andturpentine are obtained from fir-trees. The durability of this speciesof wood is chiefly owing to the resinous nature of its peculiar juices. The volatile oils have, in a great measure, the same preservativeeffects, as they defend the parts, with which they are connected, fromthe attack of insects. This tribe seems to have as great an aversion toperfumes, as the human species have delight in them. They scarcely everattack any odoriferous parts of plants, and it is not uncommon to seeevery leaf of a tree destroyed by a blight, whilst the blossoms remainuntouched. Cedar, sandal, and all aromatic woods, are on this account ofgreat durability. 

EMILY. 

But the wood of the oak, which is so much esteemed for its durability, has, I believe, no smell. Does it derive this quality from its hardnessalone?

MRS. B. 

Not entirely; for the chesnut, though considerably harder and firmerthan the oak, is not so lasting. The durability of the oak is, I believe, in a great measure owing to its having very littleheart-wood, the alburnum preserving its vital functions longer than inother trees. 

CAROLINE. 

If incisions are made into the alburnum and cortical layers, may not theascending and descending sap be procured in the same manner as thepeculiar juice is from the vessels of the parenchyma?

MRS. B. 

Yes; but in order to obtain specimens of these fluids, in any quantity, the experiment must be made in the spring, when the sap circulates withthe greatest energy. For this purpose a small bent glass tube should beintroduced into the incision, through which the sap may flow withoutmixing with any of the other juices of the tree. From the bark the sapwill flow much more plentifully than from the wood, as the ascending sapis much more liquid, more abundant, and more rapid in its motion thanthat which descends; for the latter having been deprived by theoperation of the leaves of a considerable part of its moisture, containsa much greater proportion of solid matter, which retards its motion. Itdoes not appear that there is any excess of descending sap, as none everexudes from the roots of plants; this process, therefore, seems to becarried on only in proportion to the wants of the plant, and the sapdescends no further, and in no greater quantity, than is required tonourish the several organs. Therefore, though the sap rises and descendsin the plant, it does not appear to undergo a real circulation. 

The last of the organs of plants is the _flower_, or _blossom_, whichproduces the _fruits_ and _seed_. These may be considered as theultimate purpose of nature in the vegetable creation. From fruits andseeds animals derive both a plentiful source of immediate nourishment, and an ample provision for the reproduction of the same means ofsubsistence. 

The seed which forms the final product of mature plants, we have alreadyexamined as constituting the first rudiments of future vegetation. 

These are the principal organs of vegetation, by means of which theseveral chemical processes which are carried on during the life of theplant are performed. 

EMILY. 

But how are the several principles which enter into the composition ofvegetables so combined by the organs of the plant as to be convertedinto vegetable matter?

MRS. B. 

By chemical processes, no doubt; but the apparatus in which they areperformed is so extremely minute as completely to elude our examination. We can form an opinion, therefore, only by the result of theseoperations. The sap is evidently composed of water, absorbed by theroots, and holding in solution the various principles which it derivesfrom the soil. From the roots the sap ascends through the tubes of thealburnum into the stem, and thence branches out to every extremity ofthe plant. Together with the sap circulates a certain quantity ofcarbonic acid, which is gradually disengaged from the former by theinternal heat of the plant. 

CAROLINE. 

What! have vegetables a peculiar heat, analogous to animal heat?

MRS. B. 

It is a circumstance that has long been suspected; but late experimentshave decided beyond a doubt that vegetable heat is considerably abovethat of unorganised matter in winter, and below it in summer. The woodof a tree is about sixty degrees, when the thermometer is seventy oreighty degrees. And the bark, though so much exposed, is seldom belowforty in winter. 

It is from the sap, after it has been elaborated by the leaves, thatvegetables derive their nourishment; in its progress through the plantfrom the leaves to the roots, it deposits in the several sets of vesselswith which it communicates, the materials on which the growth andnourishment of each plant depends. It is thus that the various peculiarjuices, saccharine, oily, mucous, acid, and colouring, are formed; asalso the more solid parts, fecula, woody fibre, tannin, resins, concretesalts; in a word, all the immediate materials of vegetables, as well asthe organised parts of plants, which latter, besides the power ofsecreting these from the sap for the general purpose of the plant, havealso that of applying them to their own particular nourishment. 

EMILY. 

But why should the process of vegetation take place only at one seasonof the year, whilst a total inaction prevails during the other?

MRS. B. 

Heat is such an important chemical agent, that its effect, as such, might perhaps alone account for the impulse which the spring gives tovegetation. But, in order to explain the mechanism of that operation, ithas been supposed that the warmth of the spring dilates the vessels ofplants, and produces a kind of vacuum, into which the sap (which hadremained in a state of inaction in the trunk during the winter) rises:this is followed by the ascent of the sap contained in the roots, androom is thus made for fresh sap, which the roots, in their turn, pump upfrom the soil. This process goes on till the plant blossoms and bearsfruit, which terminates its summer career: but when the cold weathersets in, the fibres and vessels contract, the leaves wither, and are nolonger able to perform their office of transpiration; and, as thissecretion stops, the roots cease to absorb sap from the soil. If theplant be an annual, its life then terminates; if not, it remains in astate of torpid inaction during the winter; or the only internal motionthat takes place is that of a small quantity of resinous juice, whichslowly rises from the stem into the branches, and enlarges their budsduring the winter. 

CAROLINE. 

Yet, in evergreens, vegetation must continue throughout the year. 

MRS. B. 

Yes; but in winter it goes on in a very imperfect manner, compared tothe vegetation of spring and summer. 

We have dwelt much longer on the history of vegetable chemistry than Ihad intended; but we have at length, I think, brought the subject to aconclusion. 

CAROLINE. 

I rather wonder that you did not reserve the account of thefermentations for the conclusion; for the decomposition of vegetablesnaturally follows their death, and can hardly, it seems, be introducedwith so much propriety at any other period. 

MRS. B. 

It is difficult to determine at what point precisely it may be mosteligible to enter on the history of vegetation; every part of thesubject is so closely connected, and forms such an uninterrupted chain, that it is by no means easy to divide it. Had I begun with thegermination of the seed, which, at first view, seems to be the mostproper arrangement, I could not have explained the nature andfermentation of the seed, or have described the changes which manuremust undergo, in order to yield the vegetable elements. To understandthe nature of germination, it is necessary, I think, previously todecompose the parent plant, in order to become acquainted with thematerials required for that purpose. I hope, therefore, that, uponsecond consideration, you will find that the order which I have adopted, though apparently less correct, is in fact the best calculated for theelucidation of the subject. 

CONVERSATION XXIII. 

ON THE COMPOSITION OF ANIMALS. 

MRS. B. 

We are now come to the last branch of chemistry, which comprehends themost complicated order of compound beings. This is the animal creation, the history of which cannot but excite the highest degree of curiosityand interest, though we often fail in attempting to explain the laws bywhich it is governed. 

EMILY. 

But since all animals ultimately derive their nourishment fromvegetables, the chemistry of this order of beings must consist merely inthe conversion of vegetable into animal matter. 

MRS. B. 

Very true; but the manner in which this is effected is, in a greatmeasure, concealed from our observation. This process is called_animalisation_, and is performed by peculiar organs. The difference ofthe animal and vegetable kingdoms does not however depend merely on adifferent arrangement of combinations. A new principle abounds in theanimal kingdom, which is but rarely and in very small quantities foundin vegetables; this is nitrogen. There is likewise in animal substancesa greater and more constant proportion of phosphoric acid, and othersaline matters. But these are not essential to the formation of animalmatter. 

CAROLINE. 

Animal compounds contain, then, four fundamental principles; oxygen, hydrogen, carbon, and nitrogen?

MRS. B. 

Yes; and these form the immediate materials of animals, which are_gelatine_, _albumen_, and _fibrine_. 

EMILY. 

Are those all? I am surprised that animals should be composed of fewerkinds of materials than vegetables; for they appear much morecomplicated in their organisation. 

MRS. B. 

Their organisation is certainly more perfect and intricate, and theingredients that occasionally enter into their composition are morenumerous. But notwithstanding the wonderful variety observable in thetexture of the animal organs, we find that the original compounds, fromwhich all the varieties of animal matter are derived, may be reduced tothe three heads just mentioned. Animal substances being the mostcomplicated of all natural compounds, are most easily susceptible ofdecomposition, as the scale of attractions increases in proportion tothe number of constituent principles. Their analysis is, however, bothdifficult and imperfect; for as they cannot be examined in their livingstate, and are liable to alteration immediately after death, it isprobable that, when submitted to the investigation of a chemist, theyare always more or less altered in their combinations and properties, from what they were, whilst they made part of the living animal. 

EMILY. 

The mere diminution of temperature, which they experience by theprivation of animal heat, must, I should suppose, be sufficient toderange the order of attractions that existed during life. 

MRS. B. 

That is one of the causes, no doubt: but there are many othercircumstances which prevent us from studying the nature of living animalsubstances. We must therefore, in a considerable degree, confine ourresearches to the phenomena of these compounds in their inanimate state. 

These three kinds of animal matter, gelatine, albumen, and fibrine, formthe basis of all the various parts of the animal system; either solid, as the _skin_, _flesh_, _nerves_, _membranes_, _cartilages_, and_bones_; or fluid, as _blood_, _chyle_, _milk_, _mucus_, the _gastric_and _pancreatic juices_, _bile_, _perspiration_, _saliva_, _tears_,  &c. 

CAROLINE. 

Is it not surprising that so great a variety of substances, and sodifferent in their nature, should yet all arise from so few materials, and from the same original elements?

MRS. B. 

The difference in the nature of various bodies depends, as I have oftenobserved to you, rather on their state of combination, than on thematerials of which they are composed. Thus, in considering the chemicalnature of the creation in a general point of view, we observe that it isthroughout composed of a very small number of elements. But when wedivide it into the three kingdoms, we find that, in the mineral, thecombinations seem to result from the union of elements casually broughttogether; whilst in the vegetable and animal kingdoms, the attractionsare peculiarly and regularly produced by appropriate organs, whoseaction depends on the vital principle. And we may further observe, thatby means of certain spontaneous changes and decompositions, the elementsof one kind of matter become subservient to the reproduction of another;so that the three kingdoms are intimately connected, and constantlycontributing to the preservation of each other. 

EMILY. 

There is, however, one very considerable class of elements, which seemsto be confined to the mineral kingdom: I mean metals. 

MRS. B. 

Not entirely; they are found, though in very minute quantities, both inthe vegetable and animal kingdoms. A small portion of earths and sulphurenters also into the composition of organised bodies. Phosphorus, however, is almost entirely confined to the animal kingdom; andnitrogen, but with few exceptions, is extremely scarce in vegetables. 

Let us now proceed to examine the nature of the three principalmaterials of the animal system. 

_Gelatine_, or _jelly_, is the chief ingredient of skin, and of all themembranous parts of animals. It may be obtained from these substances, by means of boiling water, under the forms of glue, size, isinglass, andtransparent jelly. 

CAROLINE. 

But these are of a very different nature; they cannot therefore be allpure gelatine. 

MRS. B. 

Not entirely, but very nearly so. Glue is extracted from the skin ofanimals. Size is obtained either from skin in its natural state, or fromleather. Isinglass is gelatine procured from a particular species offish; it is, you know, of this substance that the finest jelly is made, and this is done by merely dissolving the isinglass in boiling water, and allowing the solution to congeal. 

EMILY. 

The wine, lemon, and spices, are, I suppose, added only to flavour thejelly?

MRS. B. 

Exactly so. 

CAROLINE. 

But jelly is often made of hartshorn shavings, and of calves’ feet; dothese substances contain gelatine?

MRS. B. 

Yes. Gelatine may be obtained from almost any animal substance, as itenters more or less into the composition of all of them. The process forobtaining it is extremely simple, as it consists merely in boiling thesubstance that contains it with water. The gelatine dissolves in water, and may be attained of any degree of consistence or strength, byevaporating this solution. Bones in particular produce it veryplentifully, as they consist of phosphat of lime combined or cemented bygelatine. Horns, which are a species of bone, will yield abundance ofgelatine. The horns of the hart are reckoned to produce gelatine of thefinest quality; they are reduced to the state of shavings in order thatthe jelly may be more easily extracted by the water. It is of hartshornshavings that the jellies for invalids are usually made, as they are ofvery easy digestion. 

CAROLINE. 

It appears singular that hartshorn, which yields such a powerfulingredient as ammonia, should at the same time produce so mild andinsipid a substance as jelly?

MRS. B. 

And (what is more surprising) it is from the gelatine of bones thatammonia is produced. You must observe, however, that the processes bywhich these two substances are obtained from bones are very different. By the simple action of water and heat, the gelatine is separated; butin order to procure the ammonia, or what is commonly called hartshorn, the bones must be distilled, by which means the gelatine is decomposed, and hydrogen and nitrogen combined in the form of ammonia. So that thefirst operation is a mere separation of ingredients, whilst the secondrequires a chemical decomposition. 

CAROLINE. 

But when jelly is made from hartshorn shavings, what becomes of thephosphat of lime which constitutes the other part of bones?

MRS. B. 

It is easily separated by straining. But the jelly is afterwards moreperfectly purified, and rendered transparent, by adding white of egg, which being coagulated by heat, rises to the surface along with anyimpurities. 

EMILY. 

I wonder that bones are not used by the common people to make jelly;a great deal of wholesome nourishment, might, I should suppose, beprocured from them, though the jelly would perhaps not be quite so goodas if made from hartshorn shavings?

MRS. B. 

There is a prejudice among the poor against a species of food that isusually thrown to the dogs; and as we cannot expect them to enter intochemical considerations, it is in some degree excusable. Besides, itrequires a prodigious quantity of fuel to dissolve bones and obtain thegelatine from them. 

The solution of bones in water is greatly promoted by an accumulation ofheat. This may be effected by means of an extremely strong metallicvessel, called _Papin’s digester_, in which the bones and water areenclosed, without any possibility of the steam making its escape. A heatcan thus be applied much superior to that of boiling water; and bones, by this means, are completely reduced to a pulp. But the process stillconsumes too much fuel to be generally adopted among the lower classes. 

CAROLINE. 

And why should not a manufacture be established for grinding ormacerating bones, or at least for reducing them to the state ofshavings, when I suppose they would dissolve as readily as hartshornshavings?

MRS. B. 

They could not be collected clean for such a purpose, but they are notlost, as they are used for making hartshorn and sal ammoniac; and suchis the superior science and industry of this country, that we now sendsal ammoniac to the Levant, though it originally came to us from Egypt. 

EMILY. 

When jelly is made of isinglass, does it leave no sediment?

MRS. B. 

No; nor does it so much require clarifying, as it consists almostentirely of pure gelantine, and any foreign matter that is mixed withit, is thrown off during the boiling in the form of scum. --These areprocesses which you may see performed in great perfection in theculinary laboratory, by that very able and most useful chemist the cook. 

CAROLINE. 

To what an immense variety of purposes chemistry is subservient!

EMILY. 

It appears, in that respect, to have an advantage over most other artsand sciences; for these, very often, have a tendency to confine theimagination to their own particular object, whilst the pursuit ofchemistry is so extensive and diversified, that it inspires a generalcuriosity, and a desire of enquiring into the nature of every object. 

CAROLINE. 

I suppose that soup is likewise composed of gelatine; for, when cold, itoften assumes the consistence of jelly?

MRS. B. 

Not entirely; for though soups generally contain a quantity of gelatine, the most essential ingredient is a mucous or extractive matter, a peculiar animal substance, very soluble in water, which has a strongtaste, and is more nourishing than gelatine. The various kinds ofportable soup consist of this extractive matter in a dry state, which, in order to be made into soup, requires only to be dissolved in water. 

Gelatine, in its solid state, is a semiductile transparent substance, without either taste or smell. --When exposed to heat, in contact withair and water, it first swells, then fuses, and finally burns. You mayhave seen the first part of this operation performed in the carpenter’sglue-pot. 

CAROLINE. 

But you said that gelatine had no smell, and glue has a verydisagreeable one. 

MRS. B. 

Glue is not pure gelatine; as it is not designed for eating, it isprepared without attending to the state of the ingredients, which aremore or less contaminated by particles that have become putrid. 

Gelatine may be precipitated from its solution in water by alcohol. --Weshall try this experiment with a glass of warm jelly. --You see that thegelatine subsides by the union of the alcohol and the water. 

EMILY. 

How is it, then, that jelly is flavoured with wine, without producingany precipitation?

MRS. B. 

Because the alcohol contained in wine is already combined with water, and other ingredients, and is therefore not at liberty to act upon thejelly as when in its separate state. Gelatine is soluble both in acidsand in alkalies; the former, you know, are frequently used to seasonjellies. 

CAROLINE. 

Among the combinations of gelatine we must not forget one which youformerly mentioned; that with tannin, to form leather. 

MRS. B. 

True; but you must observe that leather can be produced only by gelatinein a membranous state; for though pure gelatine and tannin will producea substance chemically similar to leather, yet the texture of the skinis requisite to make it answer the useful purposes of that substance. 

The next animal substance we are to examine is _albumen_; this, althoughconstituting a part of most of the animal compounds, is frequently foundinsulated in the animal system; the white of egg, for instance, consistsalmost entirely of albumen; the substance that composes the nerves, theserum, or white part of the blood, and the curds of milk, are littleelse than albumen variously modified. 

In its most simple state, albumen appears in the form of a transparentviscous fluid, possessed of no distinct taste or smell; it coagulates atthe low temperature of 165 degrees, and, when once solidified, it willnever return to its fluid state. 

Sulphuric acid and alcohol are each of them capable of coagulatingalbumen in the same manner as heat, as I am going to show you. 

EMILY. 

Exactly so. --Pray, Mrs. B. , what kind of action is there betweenalbumen and silver? I have sometimes observed, that if the spoon withwhich I eat an egg happens to be wetted, it becomes tarnished. 

MRS. B. 

It is because the white of egg (and, indeed, albumen in general)contains a little sulphur, which, at the temperature of an egg justboiled, will decompose the drop of water that wets the spoon, andproduce sulphurated hydrogen gas, which has the property of tarnishingsilver. 

We may now proceed to _fibrine_. This is an insipid and inodoroussubstance, having somewhat the appearance of fine white threads adheringtogether; it is the essential constituent of muscles or flesh, in whichit is mixed with and softened by gelatine. It is insoluble both in waterand alcohol, but sulphuric acid converts it into a substance veryanalogous to gelatine. 

These are the essential and general ingredients of animal matter; butthere are other substances, which, though not peculiar to the animalsystem, usually enter into its composition, such as oils, acids, salts,  &c. 

_Animal oil_ is the chief constituent of fat; it is contained inabundance in the cream of milk, whence it is obtained in the form ofbutter. 

EMILY. 

Is animal oil the same in its composition as vegetable oils?

MRS. B. 

Not the same, but very analogous. The chief difference is that animaloil contains nitrogen, a principle which seldom enters into thecomposition of vegetable oils, and never in so large a proportion. 

There are a few animal acids, that is to say, acids peculiar to animalmatter, from which they are almost exclusively obtained. 

The animal acids have triple bases of hydrogen, carbon, and nitrogen. Some of them are found native in animal matter; others are producedduring its decomposition. 

Those that we find ready formed are:

The _bombic acid_, which is obtained from silk-worms. 

The _formic acid_, from ants. 

The _lactic acid_, from the whey of milk. 

The _sebacic_, from oil or fat. 

Those produced during the decomposition of animal substances by heat, are the _prussic_ and _zoonic_ acids. This last is produced by theroasting of meat, and gives it a brisk flavour. 

CAROLINE. 

The class of animal acids is not very extensive?

MRS. B. 

No; nor are they, generally speaking, of great importance. The _prussicacid_ is, I think, the only one sufficiently interesting to require anyfurther comment. It can be formed by any artificial process, without thepresence of any animal matter; and it may likewise be obtained from avariety of vegetables, particularly those of the narcotic kind, such aspoppies, laurel, &c. But it is commonly obtained from blood, by stronglyheating that substance with caustic potash; the alkali attracts the acidfrom the blood, and forms with it a _prussiat of potash_. From thisstate of combination the prussic acid can be obtained pure by means ofother substances which have the power of separating it from the alkali. 

EMILY. 

But if this acid does not exist ready formed in blood, how can thealkali attract it from it?

MRS. B. 

It is the triple basis only of this acid that exists in the blood; andthis is developed and brought to the state of acid, during thecombustion. The acid therefore is first formed, and it afterwardscombines with the potash. 

EMILY. 

Now I comprehend it. But how can the prussic acid be artificially made?

MRS. B. 

By passing ammoniacal gas over red-hot charcoal; and hence we learn thatthe constituents of this acid are hydrogen, nitrogen, and carbon. Thetwo first are derived from the volatile alkali, the last from thecombustion of the charcoal. 

CAROLINE. 

But this does not accord with the system of oxygen being the principleof acidity. 

MRS. B. 

The colouring matter of prussian blue is called an acid, because itunites with alkalies and metals, and not from any other characteristicproperties of acids; perhaps the name is not strictly appropriate. Butthis circumstance, together with some others of the same kind, hasinduced several chemists to think that oxygen may not be the exclusivegenerator of acids. Sir H. Davy, I have already informed you, was led byhis experiments on dry acids to suspect that water might be essential toacidity. And it is the opinion of some chemists that acidity maypossibly depend rather on the arrangement than on the presence of anyparticular principles. But we have not yet done with the prussic acid. It has a strong affinity for metallic oxyds, and precipitates thesolutions of iron in acids of a blue colour. This is the prussian blue, or prussiat of iron, so much used in the arts, and with which I thinkyou must be acquainted. 

EMILY. 

Yes, I am; it is much used in painting, both in oil and in watercolours; but it is not reckoned a permanent oil-colour. 

MRS. B. 

That defect arises, I believe, in general, from its being badlyprepared, which is the case when the iron is not so fully oxydated as toform a red oxyd. For a solution of green oxyd of iron (in which themetal is more slightly oxydated), makes only a pale green, or even awhite precipitate, with prussiat of potash; and this gradually changesto blue by being exposed to the air, as I can immediately show you. 

CAROLINE. 

It already begins to assume a pale blue colour. But how does the airproduce this change?

MRS. B. 

By oxydating the iron more perfectly. If we pour some nitrous acid onit, the prussian blue colour will be immediately produced, as the acidwill yield its oxygen to the precipitate, and fully saturate it withthis principle, as you shall see. 

CAROLINE. 

It is very curious to see a colour change so instantaneously. 

MRS. B. 

Hence you perceive that prussian blue cannot be a permanent colour, unless prepared with red oxyd of iron, since by exposure to theatmosphere it gradually darkens, and in a short time is no longer inharmony with the other colours of the painting. 

CAROLINE. 

But it can never become darker, by exposure to the atmosphere, than thetrue prussian blue, in which the oxyd is perfectly saturated?

MRS. B. 

Certainly not. But in painting, the artist not reckoning upon partialalterations in his colours, gives his blue tints that particular shadewhich harmonises with the rest of the picture. If, afterwards, thosetints become darker, the harmony of the colouring must necessarily bedestroyed. 

CAROLINE. 

Pray, of what nature is the paint called _carmine_?

MRS. B. 

It is an animal colour prepared from _cochineal_, an insect, theinfusion of which produces a very beautiful red. 

CAROLINE. 

Whilst we are on the subject of colours, I should like to learn what_ivory black_ is?

MRS. B. 

It is a carbonaceous substance obtained by the combustion of ivory. A more common species of black is obtained from the burning of bone. 

CAROLINE. 

But during the combustion of ivory or bone, the carbon, I should haveimagined, must be converted into carbonic acid gas, instead of thisblack substance?

MRS. B. 

In this, as in most combustions, a considerable part of the carbon issimply volatilised by the heat, and again obtained concrete on cooling. This colour, therefore, may be called the soot produced by the burningof ivory or bone. 

CONVERSATION XXIV. 

ON THE ANIMAL ECONOMY. 

MRS. B. 

We have now acquired some idea of the various materials that compose theanimal system; but if you are curious to know in what manner thesesubstances are formed by the animal organs, from vegetable, as well asfrom animal substances, it will be necessary to have some previousknowledge of the nature and functions of these organs, without which itis impossible to form any distinct idea of the process of_animalisation_ and _nutrition_. 

CAROLINE. 

I do not exactly understand the meaning of the word animalisation?

MRS. B. 

Animalisation is the process by which the food is _assimilated_, that isto say, converted into animal matter; and nutrition is that by which thefood thus assimilated is rendered subservient to the purposes ofnourishing and maintaining the animal system. 

EMILY. 

This, I am sure, must be the most interesting of all the branches ofchemistry!

CAROLINE. 

So I think; particularly as I expect that we shall hear something of thenature of respiration, and of the circulation of the blood?

MRS. B. 

These functions undoubtedly occupy a most important place in the historyof the animal economy. --But I must previously give you a very shortaccount of the principal organs by which the various operations of theanimal system are performed. These are:

The _Bones_; _Muscles_, _Blood vessels_, _Lymphatic vessels_, _Glands_, and _Nerves_. 

The _bones_ are the most solid part of the animal frame, and in a greatmeasure determine its form and dimensions. You recollect, I suppose, what are the ingredients which enter into their composition?

CAROLINE. 

Yes; phosphat of lime, cemented by gelatine. 

MRS. B. 

During the earliest period of animal life, they consist almost entirelyof gelatinous membrane having the form of the bones, but of a loosespongy texture, the cells or cavities of which are destined to be filledwith phosphat of lime; it is the gradual acquisition of this salt whichgives to the bones their subsequent hardness and durability. Infantsfirst receive it from their mother’s milk, and afterwards derive it fromall animal and from most vegetable food, especially farinaceoussubstances, such as wheat-flour, which contain it in sensiblequantities. A portion of the phosphat, after the bones of the infanthave been sufficiently expanded and solidified, is deposited in theteeth, which consist at first only of a gelatinous membrane or case, fitted for the reception of this salt; and which, after acquiringhardness within the gum, gradually protrude from it. 

CAROLINE. 

How very curious this is; and how ingeniously nature has first providedfor the solidification of such bones as are immediately wanted, andafterwards for the formation of the teeth, which would not only beuseless, but detrimental in infancy!

MRS. B. 

In quadrupeds the phosphat of lime is deposited likewise in their horns, and in the hair or wool with which they are generally clothed. 

In birds it serves also to harden the beaks and the quills of theirfeathers. 

When animals are arrived at a state of maturity, and their bones haveacquired a sufficient degree of solidity, the phosphat of lime which istaken with the food is seldom assimilated, excepting when the femalenourishes her young; it is then all secreted into the milk, as aprovision for the tender bones of the nursling. 

EMILY. 

So that whatever becomes superfluous to one being, is immediately wantedby another; and the child acquires strength precisely by the species ofnourishment which is no longer necessary to the mother. Nature is, indeed, an admirable economist!

CAROLINE. 

Pray, Mrs. B. , does not the disease in the bones of children, called therickets, proceed from a deficiency of phosphat of lime?

MRS. B. 

I have heard that this disease may arise from two causes; it issometimes occasioned by the growth of the muscles being too rapid inproportion to that of the bones. In this case the weight of the flesh isgreater than the bones can support, and presses upon them so as toproduce a swelling of the joints, which is the great indication of therickets. The other cause of this disorder is supposed to be an imperfectdigestion and assimilation of the food, attended with an excess of acid, which counteracts the formation of phosphat of lime. In both instances, therefore, care should be taken to alter the child’s diet, not merely byincreasing the quantity of aliment containing phosphat of lime, but alsoby avoiding all food that is apt to turn acid on the stomach, and toproduce indigestion. But the best preservative against complaints ofthis kind is, no doubt, good nursing: when a child has plenty of air andexercise, the digestion and assimilation will be properly performed, noacid will be produced to interrupt these functions, and the muscles andbones will grow together in just proportions. 

CAROLINE. 

I have often heard the rickets attributed to bad nursing, but I nevercould have guessed what connection there was between exercise and theformation of the bones. 

MRS. B. 

Exercise is generally beneficial to all the animal functions. If man isdestined to labour for his subsistence, the bread which he earns isscarcely more essential to his health and preservation than theexertions by which he obtains it. Those whom the gifts of fortune haveplaced above the necessity of bodily labour are compelled to takeexercise in some mode or other, and when they cannot convert it into anamusement, they must submit to it as a task, or their health will soonexperience the effects of their indolence. 

EMILY. 

That will never be my case: for exercise, unless it becomes fatigue, always gives me pleasure; and, so far from being a task, is to me asource of daily enjoyment. I often think what a blessing it is, thatexercise, which is so conducive to health, should be so delightful;whilst fatigue, which is rather hurtful, instead of pleasure, occasionspainful sensations. So that fatigue, no doubt, was intended to moderateour bodily exertions, as satiety puts a limit to our appetites. 

MRS. B. 

Certainly. --But let us not deviate too far from our subject. --Thebones are connected together by ligaments, which consist of a whitethick flexible substance, adhering to their extremities, so far as tosecure the joints firmly, though without impeding their motion. And thejoints are moreover covered by a solid, smooth, elastic, whitesubstance, called _cartilage_, the use of which is to allow, by itssmoothness and elasticity, the bones to slide easily over one another, so that the joints may perform their office without difficulty ordetriment. 

Over the bones the _muscles_ are placed; they consist of bundles offibres which terminate in a kind of string, or ligament, by which theyare fastened to the bones. The muscles are the organs of motion; bytheir power of dilatation and contraction they put into action thebones, which act as levers, in all the motions of the body, and form thesolid support of its various parts. The muscles are of various degreesof strength or consistence in different species of animals. Themammiferous tribe, or those that suckle their young, seem in thisrespect to occupy an intermediate place between birds and cold-bloodedanimals, such as reptiles and fishes. 

EMILY. 

The different degrees of firmness and solidity in the muscles of theseseveral species of animals proceed, I imagine, from the different natureof the food on which they subsist?

MRS. B. 

No; that is not supposed to be the case: for the human species, who areof the mammiferous tribe, live on more substantial food than birds, andyet the latter exceed them in muscular strength. We shall hereafterattempt to account for this difference; but let us now proceed in theexamination of the animal functions. 

The next class of organs is that of the _vessels_ of the body, theoffice of which is to convey the various fluids throughout the frame. These vessels are innumerable. The most considerable of them are thosethrough which the blood circulates, which are of two kinds: the_arteries_, which convey it from the heart to the extremities of thebody, and the _veins_, which bring it back into the heart. 

Besides these, there are a numerous set of small transparent vessels, destined to absorb and convey different fluids into the blood; they aregenerally called the _absorbent_ or _lymphatic_ vessels: but it is to aportion of them only that the function of conveying into the blood thefluid called _lymph_ is assigned. 

EMILY. 

Pray what is the nature of that fluid?

MRS. B. 

The nature and use of the lymph have, I believe, never been perfectlyascertained; but it is supposed to consist of matter that has beenpreviously animalised, and which, after answering the purpose for whichit was intended, must, in regular rotation, make way for the freshsupplies produced by nourishment. The lymphatic vessels pump up thisfluid from every part of the system, and convey it into the veins to bemixed with the blood which runs through them, and which is commonlycalled venous blood. 

CAROLINE. 

But does it not again enter into the animal system through that channel?

MRS. B. 

Not entirely; for the venous blood does not return into the circulationuntil it has undergone a peculiar change, in which it throws offwhatever is become useless. 

Another set of absorbent vessels pump up the _chyle_ from the stomachand intestines, and convey it, after many circumvolutions, into thegreat vein near the heart. 

EMILY. 

Pray what is chyle?

MRS. B. 

It is the substance into which food is converted by digestion. 

CAROLINE. 

One set of the absorbent vessels, then, is employed in bringing away theold materials that are no longer fit for use; whilst the other set isbusy in conveying into the blood the new materials that are to replacethem. 

EMILY. 

What a great variety of ingredients must enter into the composition ofthe blood?

MRS. B. 

You must observe that there is also a great variety of substances to besecreted from it. We may compare the blood to a general receptacle orstorehouse for all kinds of commodities, which are afterwards fashioned, arranged, and disposed of as circumstances require. 

There is another set of absorbent vessels in females which is destinedto secrete milk for the nourishment of the young. 

EMILY. 

Pray is not milk very analogous in its composition to blood; for, sincethe nursling derives its nourishment from that source only, it mustcontain every principle which the animal system requires?

MRS. B. 

Very true. Milk is found, by its analysis, to contain the principalmaterials of animal matter, albumen, oil, and phosphat of lime; so thatthe suckling has but little trouble to digest and assimilate thisnourishment. But we shall examine the composition of milk more fullyafterwards. 

In many parts of the body numbers of small vessels are collectedtogether in little bundles called _glands_, from a Latin word meaningacorn, on account of the resemblance which some of them bear in shape tothat fruit. The function of the glands is to _secrete_, or separatecertain matters from the blood. 

The secretions are not only mechanical, but chemical separations fromthe blood; for the substances thus formed, though contained in theblood, are not ready combined in that fluid. The secretions are of twokinds, those which form peculiar animal fluids, as bile, tears, saliva, &c. ; and those which produce the general materials of the animal system, for the purpose of recruiting and nourishing the several organs of thebody; such as albumen, gelatine, and fibrine; the latter may bedistinguished by the name of _nutritive secretions_. 

CAROLINE. 

I am quite astonished to hear that all the secretions should be derivedfrom the blood. 

EMILY. 

I thought that the bile was produced by the liver?

MRS. B. 

So it is; but the liver is nothing more than a very large gland, whichsecretes the bile from the blood. 

The last of the animal organs which we have mentioned are the _nerves_;these are the vehicles of sensation, every other part of the body being, of itself, totally insensible. 

CAROLINE. 

They must then be spread through every part of the frame, for we areevery where susceptible of feeling. 

EMILY. 

Excepting the nails and the hair. 

MRS. B. 

And those are almost the only parts in which nerves cannot bediscovered. The common source of all the nerves is the brain; thencethey descend, some of them through different holes of the skull, but thegreatest part through the back bone, and extend themselves byinnumerable ramifications throughout the whole body. They spreadthemselves over the muscles, penetrate the glands, wind round thevascular system, and even pierce into the interior of the bones. It ismost probably through them that the communication is carried on betweenthe mind and the other parts of the body; but in what manner they areacted on by the mind, and made to re-act on the body, is still aprofound secret. Many hypotheses have been formed on this very obscuresubject, but they are all equally improbable, and it would be uselessfor us to waste our time in conjectures on an enquiry, which, in allprobability, is beyond the reach of human capacity. 

CAROLINE. 

But you have not mentioned those particular nerves that form the sensesof hearing, seeing, smelling, and tasting?

MRS. B. 

They are considered as being of the same nature as those which aredispersed over every part of the body, and constitute the general senseof feeling. The different sensations which they produce arise from theirpeculiar situation and connection with the several organs of taste, smell, and hearing. 

EMILY. 

But these senses appear totally different from that of feeling?

MRS. B. 

They are all of them sensations, but variously modified according to thenature of the different organs in which the nerves are situated. For, aswe have formerly observed, it is by contact only that the nerves areaffected. Thus odoriferous particles must strike upon the nerves of thenose, in order to excite the sense of smelling; in the same manner thattaste is produced by the particular substance coming in contact with thenerves of the palate. It is thus also that the sensation of sound isproduced by the concussion of the air striking against the auditorynerve; and sight is the effect of the light falling upon the opticnerve. These various senses, therefore, are affected only by the actualcontact of particles of matter, in the same manner as that of feeling. 

The different organs of the animal body, though easily separated andperfectly distinct, are loosely connected together by a kind of spongysubstance, in texture somewhat resembling net-work, called the cellularmembrane; and the whole is covered by the skin. 

The _skin_, as well as the bark of vegetables, is formed of three coats. The external one is called the _cuticle_ or _epidermis_; the second, which is called the _mucous membrane_, is of a thin soft texture, andconsists of a mucous substance, which in negroes is black, and is thecause of their skin appearing of that colour. 

CAROLINE. 

Is then the external skin of negroes white like ours?

MRS. B. 

Yes; but as the cuticle is transparent, as well as porous, the blacknessof the mucous membrane is visible through it. The extremities of thenerves are spread over this skin, so that the sensation of feeling istransmitted through the cuticle. The internal covering of the muscles, which is properly the skin, is the thickest, the toughest, and mostresisting of the whole; it is this membrane which is so essential in thearts, by forming leather when combined with tannin. 

The skin which covers the animal body, as well as those membranes thatform the coats of the vessels, consists almost exclusively of gelatine;and is capable of being converted into glue, size, or jelly. 

The cavities between the muscles and the skin are usually filled withfat, which lodges in the cells of the membranous net before mentioned, and gives to the external form (especially in the human figure) thatroundness, smoothness, and softness, so essential to beauty. 

EMILY. 

And the skin itself is, I think, a very ornamental part of the humanframe, both from the fineness of its texture, and the variety anddelicacy of its tints. 

MRS. B. 

This variety and harmonious graduation of colours, proceed, not so muchfrom the skin itself, as from the internal organs which transmit theirseveral colours through it, these being only softened and blended by thecolour of the skin, which is uniformly of a yellowish white. 

Thus modified, the darkness of the veins appears of a pale blue colour, and the floridness of the arteries is changed to a delicate pink. In themost transparent parts, the skin exhibits the bloom of the rose, whilstwhere it is more opake its own colour predominates; and at the joints, where the bones are most prominent, their whiteness is oftendiscernible. In a word, every part of the human frame seems tocontribute to its external grace; and this not merely by producing apleasing variety of tints, but by a peculiar kind of beauty whichbelongs to each individual part. Thus it is to the solidity andarrangement of the bones that the human figure owes the grandeur of itsstature, and its firm and dignified deportment. The muscles delineatethe form, and stamp it with energy and grace; and the soft substancewhich is spread over them smooths their ruggedness, and gives to thecontours the gentle undulations of the line of beauty. Every organ ofsense is a peculiar and separate ornament; and the skin, which polishesthe surface, and gives it that charm of colouring so inimitable by art, finally conspires to render the whole the fairest work of the creation. 

But now that we have seen in what manner the animal frame is formed, letus observe how it provides for its support, and how the several organs, which form so complete a whole, are nourished and maintained. 

This will lead us to a more particular explanation of the internalorgans: here we shall not meet with so much apparent beauty, becausethese parts were not intended by nature to be exhibited to view; but thebeauty of design, in the internal organisation of the animal frame, is, if possible, still more remarkable than that of the external parts. 

We shall defer this subject till our next interview. 

CONVERSATION XXV. 

ON ANIMALISATION, NUTRITION, AND RESPIRATION. 

MRS. B. 

We have now learnt of what materials the animal system is composed, andhave formed some idea of the nature of its organisation. In order tocomplete the subject, it remains for us to examine in what manner it isnourished and supported. 

Vegetables, we have observed, obtain their nourishment from varioussubstances, either in their elementary state, or in a very simple stateof combination; as carbon, water, and salts, which they pump up from thesoil; and carbonic acid and oxygen, which they absorb from theatmosphere. 

Animals, on the contrary, feed on substances of the most complicatedkind; for they derive their sustenance, some from the animal creation, others from the vegetable kingdom, and some from both. 

CAROLINE. 

And there is one species of animals, which, not satisfied with enjoyingeither kind of food in its simple state, has invented the art ofcombining them together in a thousand ways, and of rendering even themineral kingdom subservient to its refinements. 

EMILY. 

Nor is this all; for our delicacies are collected from the variousclimates of the earth, so that the four quarters of the globe are oftenobliged to contribute to the preparation of our simplest dishes. 

CAROLINE. 

But the very complicated substances which constitute the nourishment ofanimals, do not, I suppose, enter into their system in their actualstate of combination?

MRS. B. 

So far from it, that they not only undergo a new arrangement of theirparts, but a selection is made of such as are most proper for thenourishment of the body, and those only enter into the system, and areanimalised. 

EMILY. 

And by what organs is this process performed?

MRS. B. 

Chiefly by the stomach, which is the organ of digestion, and the primeregulator of the animal frame. 

_Digestion_ is the first step towards nutrition. It consists in reducinginto one homogeneous mass the various substances that are taken asnourishment; it is performed by first chewing and mixing the solidaliment with the saliva, which reduces it to a soft mass, in which stateit is conveyed into the stomach, where it is more completely dissolvedby the _gastric juice_. 

This fluid (which is secreted into the stomach by appropriate glands) isso powerful a solvent that scarcely any substances will resist itsaction. 

EMILY. 

The coats of the stomach, however, cannot be attacked by it, otherwisewe should be in danger of having them destroyed when the stomach wasempty. 

MRS. B. 

They are probably not subject to its action; as long, at least, as lifecontinues. But it appears, that when the gastric juice has no foreignsubstance to act upon, it is capable of occasioning a degree ofirritation in the coats of the stomach, which produces the sensation ofhunger. The gastric juice, together with the heat and muscular action ofthe stomach, converts the aliment into an uniform pulpy mass calledchyme. This passes into the intestines, where it meets with the bile andsome other fluids, by the agency of which, and by the operation of othercauses hitherto unknown, the chyme is changed into chyle, a much thinnersubstance, somewhat resembling milk, which is pumped by immense numbersof small absorbent vessels spread over the internal surface of theintestines. These, after many circumvolutions, gradually meet and uniteinto large branches, till they at length collect the chyle into onevessel, which pours its contents into the great vein near the heart, bywhich means the food, thus prepared, enters into the circulation. 

CAROLINE. 

But I do not yet clearly understand how the blood, thus formed, nourishes the body and supplies all the secretions?

MRS. B. 

Before this can be explained to you, you must first allow me to completethe formation of the blood. The chyle may, indeed, be considered asforming the chief ingredient of blood; but this fluid is not perfectuntil it has passed through the lungs, and undergone (together with theblood that has already circulated) certain necessary changes that areeffected by RESPIRATION. 

CAROLINE. 

I am very glad that you are going to explain the nature of respiration:I have often longed to understand it, for though we talk incessantly of_breathing_, I never knew precisely what purpose it answered. 

MRS. B. 

It is indeed one of the most interesting processes imaginable; but, inorder to understand this function well, it will be necessary to enterinto some previous explanations. Tell me, Emily, --what do youunderstand by respiration?

EMILY. 

Respiration, I conceive, consists simply in alternately _inspiring_ airinto the lungs, and _expiring_ it from them. 

MRS. B. 

Your answer will do very well as a general definition. But, in order toform a tolerably clear notion of the various phenomena of respiration, there are many circumstances to be taken into consideration. 

In the first place, there are two things to be distinguished inrespiration, the _mechanical_ and the _chemical_ part of the process. 

The mechanism of breathing depends on the alternate expansions andcontractions of the chest, in which the lungs are contained. When thechest dilates, the cavity is enlarged, and the air rushes in at themouth, to fill up the vacuum formed by this dilatation; when itcontracts, the cavity is diminished, and the air forced out again. 

CAROLINE. 

I thought that it was the lungs that contracted and expanded inbreathing?

MRS. B. 

They do likewise; but their action is only the consequence of that ofthe chest. The lungs, together with the heart and largest blood vessels, in a manner fill up the cavity of the chest; they could not, therefore, dilate if the chest did not previously expand; and, on the other hand, when the chest contracts, it compresses the lungs and forces the air outof them. 

CAROLINE. 

The lungs, then, are like bellows, and the chest is the power that worksthem. 

MRS. B. 

Precisely so. Here is a curious little figure (PLATE XV. Fig.  5. ), thatwill assist me in explaining the mechanism of breathing. 

CAROLINE. 

What a droll figure! a little head fixed upon a glass bell, with abladder tied over the bottom of it!

MRS. B. 

You must observe that there is another bladder within the glass, theneck of which communicates with the mouth of the figure--this representsthe lungs contained within the chest; the other bladder, which you seeis tied loose, represents a muscular membrane, called the _diaphragm_, which separates the chest from the lower part of the body. By the chest, therefore, I mean that large cavity in the upper part of the bodycontained within the ribs, the neck, and the diaphragm; this membrane ismuscular, and capable of contraction and dilatation. The contraction maybe imitated by drawing the bladder tight over the bottom of thereceiver, when the air in the bladder, which represents the lungs, willbe forced out through the mouth of the figure--

EMILY. 

See, Caroline, how it blows the flame of the candle in breathing!

MRS. B. 

By letting the bladder loose again, we imitate the dilatation of thediaphragm, and the cavity of the chest being enlarged, the lungs expand, and the air rushes in to fill them. 

EMILY. 

This figure, I think, gives a very clear idea of the process ofbreathing. 

MRS. B. 

It illustrates tolerably well the action of the lungs and diaphragm; butthose are not the only powers that are concerned in enlarging ordiminishing the cavity of the chest; the ribs are also possessed of amuscular motion for the same purpose; they are alternately drawn in, edgeways, to assist the contraction, and stretched out, like the hoopsof a barrel, to contribute to the dilatation of the chest. 

EMILY. 

I always supposed that the elevation and depression of the ribs were theconsequence, not the cause of breathing. 

MRS. B. 

It is exactly the reverse. The muscular action of the diaphragm, together with that of the ribs, are the _causes_ of the contraction andexpansion of the chest; and the air rushing into, and being expelledfrom the lungs, are only _consequences_ of those actions. 

CAROLINE. 

I confess that I thought the act of breathing began by opening the mouthfor the air to rush in, and that it was the air alone, which, byalternately rushing in and out, occasioned the dilatations andcontractions of the lungs and chest. 

MRS. B. 

Try the experiment of merely opening your mouth; the air will not rushin, till by an interior muscular action you produce a vacuum--yes, justso, your diaphragm is now dilated, and the ribs expanded. But you willnot be able to keep them long in that state. Your lungs and chest arealready resuming their former state, and expelling the air with whichthey had just been filled. This mechanism goes on more or less rapidly, but, in general, a person at rest and in health will breathe betweenfifteen and twenty-five times in a minute. 

We may now proceed to the chemical effects of respiration; but, for thispurpose, it is necessary that you should previously have some notion ofthe _circulation_ of the blood. Tell me, Caroline, what do youunderstand by the circulation of the blood?

CAROLINE. 

I am delighted that you come to that subject, for it is one that haslong excited my curiosity. But I cannot conceive how it is connectedwith respiration. The idea I have of the circulation is, that the bloodruns from the heart through the veins all over the body, and back againto the heart. 

MRS. B. 

I could hardly have expected a better definition from you; it is, however, not quite correct, for you do not distinguish the _arteries_from the _veins_, which, as we have already observed, are two distinctsets of vessels, each having its own peculiar functions. The arteriesconvey the blood from the heart to the extremities of the body; and theveins bring it back into the heart. 

This sketch will give you an idea of the manner in which some of theprincipal veins and arteries of the human body branch out of the heart, which may be considered as a common centre to both sets of vessels. Theheart is a kind of strong elastic bag, or muscular cavity, whichpossesses a power of dilating and contracting itself, for the purposesof alternately receiving and expelling the blood, in order to carry onthe process of circulation. 

EMILY. 

Why are the arteries in this drawing painted red, and the veins purple?

MRS. B. 

It is to point out the difference of the colour of the blood in thesetwo sets of vessels. 

CAROLINE. 

But if it is the same blood that flows from the arteries into the veins, how can its colour be changed?

MRS. B. 

This change arises from various circumstances. In the first place, during its passage through the arteries, the blood undergoes aconsiderable alteration, some of its constituent parts being graduallyseparated from it for the purpose of nourishing the body, and ofsupplying the various secretions. The consequence of this is, that theflorid arterial colour of the blood changes by degrees to a deep purple, which is its constant colour in the veins. On the other hand, the bloodis recruited during its return through the veins by the fresh chyle, orimperfect blood, which has been produced by food; and it receives alsolymph from the absorbent vessels, as we have before mentioned. Inconsequence of these several changes, the blood returns to the heart ina state very different from that in which it left it. It is loaded witha greater proportion of hydrogen and carbon, and is no longer fit forthe nourishment of the body, or other purposes of circulation. 

EMILY. 

And in this state does it mix in the heart with the pure florid bloodthat runs into the arteries?

MRS. B. 

No. The heart is divided into two cavities or compartitions, called the_right_ and _left ventricles_. The left ventricle is the receptacle forthe pure arterial blood previous to its circulation; whilst the venous, or impure blood, which returns to the heart after having circulated, isreceived into the right ventricle, previous to its purification, which Ishall presently explain. 

CAROLINE. 

For my part, I always thought that the same blood circulated again andagain through the body, without undergoing any change. 

MRS. B. 

Yet you must have supposed that the blood circulated for some purpose?

CAROLINE. 

I knew that it was indispensable to life; but had no idea of its realfunctions. 

MRS. B. 

But now that you understand that the blood conveys nourishment to everypart of the body, and supplies the various secretions, you must besensible that it cannot constantly answer these objects without beingproportionally renovated and purified. 

CAROLINE. 

But does not the chyle answer this purpose?

MRS. B. 

Only in part. It renovates the nutritive principles of the blood, butdoes not relieve it from the superabundance of water and carbon withwhich it is encumbered. 

EMILY. 

How, then, is this effected?

MRS. B. 

By RESPIRATION. This is one of the grand mysteries which modernchemistry has disclosed. When the venous blood enters the rightventricle of the heart, it contracts by its muscular power, and throwsthe blood through a large vessel into the lungs, which are contiguous, and through which it circulates by millions of small ramifications. Hereit comes in contact with the air which we breathe. The action of the airon the blood in the lungs is, indeed, concealed, from our immediateobservation; but we are able to form a tolerably accurate judgment of itfrom the changes which it effects not only in the blood, but also on theair expired. 

The air, after passing through the lungs, is found to contain all thenitrogen inspired, but to have lost part of its oxygen, and to haveacquired a portion of watery vapour and of carbonic acid gas. Hence itis inferred, that when the air comes in contact with the venous blood inthe lungs, the oxygen attracts from it the superabundant quantity ofcarbon with which it has impregnated itself during the circulation, andconverts it into carbonic acid. This gaseous acid, together with theredundant moisture from the lungs*, being then expired, the blood isrestored to its former purity, that is, to the state of arterial blood, and is thus again enabled to perform its various functions. 

 [Footnote *: The quantity of moisture discharged by the lungs in 24 hours, may be computed at eight or nine ounces. ]

CAROLINE. 

This is truly wonderful! Of all that we have yet learned, I do notrecollect any thing that has appeared to me so curious and interesting. I almost believe that I should like to study anatomy now, though I havehitherto had so disgusting an idea of it. Pray, to whom are we indebtedfor these beautiful discoveries?

MRS. B. 

Priestley and Crawford, in this country, and Lavoisier, in France, arethe principal inventors of the theory of respiration. Of late years thesubject has been farther illustrated and simplified by the accurateexperiments of Messrs. Allen and Pepys. But the still more important andmore admirable discovery of the circulation of the blood was made longbefore by our immortal countryman Harvey. 

EMILY. 

Indeed I never heard any thing that delighted me so much as this theoryof respiration. But I hope, Mrs.  B. , that you will enter a little moreinto particulars before you dismiss so interesting a subject. We leftthe blood in the lungs to undergo the salutary change: but how does itthence spread to all the parts of the body?

MRS. B. 

After circulating through the lungs, the blood is collected into fourlarge vessels, by which it is conveyed into the left ventricle of theheart, whence it is propelled to all the different parts of the body bya large artery, which gradually ramifies into millions of small arteriesthrough the whole frame. From the extremities of these littleramifications the blood is transmitted to the veins, which bring it backto the heart and lungs, to go round again and again in the manner wehave just described. You see, therefore, that the blood actuallyundergoes two circulations; the one, through the lungs, by which it isconverted into pure arterial blood; the other, or general circulation, by which nourishment is conveyed to every part of the body; and theseare both equally indispensable to the support of animal life. 

EMILY. 

But whence proceeds the carbon with which the blood is impregnated whenit comes into the lungs?

MRS. B. 

Carbon exists in a greater proportion in blood than in organised animalmatter. The blood, therefore, after supplying its various secretions, becomes loaded with an excess of carbon, which is carried off byrespiration; and the formation of new chyle from the food affords aconstant supply of carbonaceous matter. 

CAROLINE. 

I wonder what quantity of carbon may be expelled from the blood byrespiration in the course of 24 hours?

MRS. B. 

It appears by the experiments of Messrs. Allen and Pepys that about40, 000 cubic inches of carbonic acid gas are emitted from the lungs of ahealthy person, daily; which is equivalent to _eleven ounces_ of solidcarbon every 24 hours. 

EMILY. 

What an immense quantity! And pray how much of carbonic acid gas do weexpel from our lungs at each expiration?

MRS. B. 

The quantity of air which we take into our lungs at each inspiration, isabout 40 cubic inches, which contain a little less than 10 cubic inchesof oxygen; and of those 10 inches, one-eighth is converted into carbonicacid gas on passing once through the lungs*, a change which issufficient to prevent air which has only been breathed once fromsuffering a taper to burn in it. 

 [Footnote *: The bulk of carbonic acid gas formed by respiration, is exactly the same as that of the oxygen gas which disappears. ]

CAROLINE. 

Pray, how does the air come in contact with the blood in the lungs?

MRS. B. 

I cannot answer this question without entering into an explanation ofthe nature and structure of the lungs. You recollect that the venousblood, on being expelled from the right ventricle, enters the lungs togo through what we may call the lesser circulation; the large trunk orvessel that conveys it branches out, at its entrance into the lungs, into an infinite number of very fine ramifications. The windpipe, whichconveys the air from the mouth into the lungs, likewise spreads out intoa corresponding number of air vessels, which follow the same course asthe blood vessels, forming millions of very minute air-cells. These twosets of vessels are so interwoven as to form a sort of net-work, connected into a kind of spongy mass, in which every particle of bloodmust necessarily come in contact with a particle of air. 

CAROLINE. 

But since the blood and the air are contained in different vessels, howcan they come into contact?

MRS. B. 

They act on each other through the membrane which forms the coats ofthese vessels; for although this membrane prevents the blood and the airfrom mixing together in the lungs, yet it is no impediment to theirchemical action on each other. 

EMILY. 

Are the lungs composed entirely of blood vessels and air vessels?

MRS. B. 

I believe they are, with the addition only of nerves and of a smallquantity of the cellular substance before mentioned, which connects thewhole into an uniform mass. 

EMILY. 

Pray, why are the lungs always spoken of in the plural number? Are theremore than one?

MRS. B. 

Yes; for though they form but one organ, they really consist of twocompartments called lobes, which are enclosed in separate membranes orbags, each occupying one side of the chest, and being in close contactwith each other, but without communicating together. This is a beautifulprovision of nature, in consequence of which, if one of the lobes bewounded, the other performs the whole process of respiration till thefirst is healed. 

The blood, thus completed, by the process of respiration, forms the mostcomplex of all animal compounds, since it contains not only the numerousmaterials necessary to form the various secretions, as saliva, tears, &c. But likewise all those that are required to nourish the severalparts of the body, as the muscles, bones, nerves, glands,  &c. 

EMILY. 

There seems to be a singular analogy between the blood of animals andthe sap of vegetables; for each of these fluids contains the severalmaterials destined for the nutrition of the numerous class of bodies towhich they respectively belong. 

MRS. B. 

Nor is the production of these fluids in the animal and vegetablesystems entirely different; for the absorbent vessels, which pump up thechyle from the stomach and intestines, may be compared to the absorbentsof the roots of plants, which suck up the nourishment from the soil. Andthe analogy between the sap and the blood may be still further traced, if we follow the latter in the course of its circulation; for, in theliving animal, we find every where organs which are possessed of a powerto secrete from the blood and appropriate to themselves the ingredientsrequisite for their support. 

CAROLINE. 

But whence do these organs derive their respective powers?

MRS. B. 

From a peculiar organisation, the secret of which no one has yet beenable to unfold. But it must be ultimately by means of the vitalprinciple that both their mechanical and chemical powers are broughtinto action. 

I cannot dismiss the subject of circulation without mentioning_perspiration_, a secretion which is immediately connected with it, andacts a most important part in the animal economy. 

CAROLINE. 

Is not this secretion likewise made by appropriate glands?

MRS. B. 

No; it is performed by the extremities of the arteries, which penetratethrough the skin and terminate under the cuticle, through the pores ofwhich the perspiration issues. When this fluid is not secreted inexcess, it is _insensible_, because it is dissolved by the air as itexudes from the pores; but when it is secreted faster than it can bedissolved, it becomes _sensible_, as it assumes its liquid state. 

EMILY. 

This secretion bears a striking resemblance to the transpiration of thesap of plants. They both consist of the most fluid part, and both exudefrom the surface by the extremities of the vessels through which theycirculate. 

MRS. B. 

And the analogy does not stop there; for, since it has been ascertainedthat the sap returns into the roots of the plants, the resemblancebetween the animal and vegetable circulation is become still moreobvious. The latter, however, is far from being complete, since, as weobserved before, it consists only in a rising and descending of the sap, whilst in animals the blood actually _circulates_ through every part ofthe system. 

We have now, I think, traced the process of nutrition, from theintroduction of the food into the stomach to its finally becoming aconstituent part of the animal frame. This will, therefore, be a fitperiod to conclude our present conversation. What further remarks wehave to make on the animal economy shall be reserved for our nextinterview. 

CONVERSATION XXVI. 

ON ANIMAL HEAT; AND ON VARIOUS ANIMAL PRODUCTS. 

EMILY. 

Since our last interview, I have been thinking much of the theory ofrespiration; and I cannot help being struck with the resemblance whichit appears to bear to the process of combustion. For in respiration, asin most cases of combustion, the air suffers a change, and a portion ofits oxygen combines with carbon, producing carbonic acid gas. 

MRS. B. 

I am much pleased that this idea has occurred to you: these twoprocesses appear so very analogous, that it has been supposed that akind of combustion actually takes place in the lungs; not of the blood, but of the superfluous carbon which the oxygen attracts from it. 

CAROLINE. 

A combustion in our lungs! that is a curious idea indeed! But, Mrs.  B. , how can you call the action of the air on the blood in the lungscombustion, when neither light nor heat are produced by it?

EMILY. 

I was going to make the same objection. --Yet I do not conceive how theoxygen can combine with the carbon, and produce carbonic acid, withoutdisengaging heat?

MRS. B. 

The fact is, that heat is disengaged. * Whether any light be evolved, I cannot pretend to determine; but that heat is produced in considerableand very sensible quantities is certain, and this is the principal, ifnot the only source of ANIMAL HEAT. 

 [Footnote *: It has been calculated that the heat produced by respiration in 12 hours, in the lungs of a healthy person, is such as would melt about 100 pounds of ice. ]

EMILY. 

How wonderful! that the very process which purifies and elaborates theblood, should afford an inexhaustible supply of internal heat?

MRS. B. 

This is the theory of animal heat in its original simplicity, suchnearly as it was first proposed by Black and Lavoisier. It was equallyclear and ingenious; and was at first generally adopted. But it wasobjected, on second consideration, that if the whole of the animal heatwas evolved in the lungs, it would necessarily be much less in theextremities of the body than immediately at its source; which is notfound to be the case. This objection, however, which was by no meansfrivolous, is now satisfactorily removed by the followingconsideration:-- Venous blood has been found by experiment to have _lesscapacity for heat_ than arterial blood; whence it follows that theblood, in gradually passing from the arterial to the venous state, during the circulation, parts with a portion of caloric, by means ofwhich heat is diffused through every part of the body. 

EMILY. 

More and more admirable!

CAROLINE. 

The cause of animal heat was always a perfect mystery to me, and I amdelighted with its explanation. --But pray, Mrs.  B. , can you tell mewhat is the reason of the increase of heat that takes place in a fever?

EMILY. 

Is it not because we then breathe quicker, and therefore more heat isdisengaged in the system?

MRS. B. 

That may be one reason: but I should think that the principal cause ofthe heat experienced in fevers, is, that there is no vent for thecaloric which is generated in the body. One of the most considerablesecretions is the insensible perspiration; this is constantly carryingoff caloric in a latent state; but during the hot stage of a fever, thepores are so contracted, that all perspiration ceases, and theaccumulation of caloric in the body occasions those burning sensationswhich are so painful. 

EMILY. 

This is, no doubt, the reason why the perspiration that often succeedsthe hot stage of a fever affords so much relief. If I had known thistheory of animal heat when I had a fever last summer, I think I shouldhave found some amusement in watching the chemical processes that weregoing on within me. 

CAROLINE. 

But exercise likewise produces animal heat, and that must be quite in adifferent manner. 

MRS. B. 

Not so much so as you think; for the more exercise you take, the morethe body is stimulated, and requires recruiting. For this purpose thecirculation of the blood is quickened, the breath proportionablyaccelerated, and consequently a greater quantity of caloric evolved. 

CAROLINE. 

True; after running very fast, I gasp for breath, my respiration isquick and hard, and it is just then that I begin to feel hot. 

EMILY. 

It would seem, then, that violent exercise should produce fever. 

MRS. B. 

Not if the person is in a good state of health; for the additionalcaloric is then carried off by the perspiration which succeeds. 

EMILY. 

What admirable resources nature has provided for us! By the productionof animal heat she has enabled us to keep up the temperature of ourbodies above that of inanimate objects; and whenever this source becomestoo abundant, the excess is carried off by perspiration. 

MRS. B. 

It is by the same law of nature that we are enabled, in all climates, and in all seasons, to preserve our bodies of an equal temperature, orat least very nearly so. 

CAROLINE. 

You cannot mean to say that our bodies are of the same temperature insummer, and in winter, in England, and in the West-Indies. 

MRS. B. 

Yes, I do; at least if you speak of the temperature of the blood, andthe internal parts of the body; for those parts that are immediately incontact with the atmosphere, such as the hands and face, willoccasionally get warmer, or colder, than the internal or more shelteredparts. But if you put the bulb of a thermometer in your mouth, which isthe best way of ascertaining the real temperature of your body, you willscarcely perceive any difference in its indication, whatever may be thedifference of temperature of the atmosphere. 

CAROLINE. 

And when I feel overcome by heat, I am really not hotter than when I amshivering with cold?

MRS. B. 

When a person in health feels very hot, whether from internal heat, fromviolent exercise, or from the temperature of the atmosphere, his body iscertainly a little warmer than when he feels very cold; but thisdifference is much smaller than our sensations would make us believe;and the natural standard is soon restored by rest and by perspiration. It is chiefly the external parts that are warmer, and I am sure that youwill be surprised to hear that the internal temperature of the bodyscarcely ever descends below ninety-five or ninety-six degrees, andseldom attains one hundred and four or one hundred and five degrees, even in the most violent fevers. 

EMILY. 

The greater quantity of caloric, therefore, that we receive from theatmosphere in summer, cannot raise the temperature of our bodies beyondcertain limits, as it does that of inanimate bodies, because an excessof caloric is carried off by perspiration. 

CAROLINE. 

But the temperature of the atmosphere, and consequently that ofinanimate bodies, is surely never so high as that of animal heat?

MRS. B. 

I beg your pardon. Frequently in the East and West Indies, and sometimesin the southern parts of Europe, the atmosphere is above ninety-eightdegrees, which is the common temperature of animal heat. Indeed, even inthis country, it occasionally happens that the sun’s rays, setting fullon an object, elevate its temperature above that point. 

In illustration of the power which our bodies have to resist the effectsof external heat, Sir Charles Blagden, with some other gentlemen, madeseveral very curious experiments. He remained for some time in an ovenheated to a temperature not much inferior to that of boiling water, without suffering any other inconvenience than a profuse perspiration, which he supported by drinking plentifully. 

EMILY. 

He could scarcely consider the perspiration as an inconvenience, sinceit saved him from being baked by giving vent to the excess of caloric. 

CAROLINE. 

I always thought, I confess, that it was from the heat of theperspiration that we suffered in summer. 

MRS. B. 

You now find that you are quite mistaken. Whenever evaporation takesplace, cold, you know, is produced in consequence of a quantity ofcaloric being carried off in a latent state; this is the case withperspiration, and it is in this way that it affords relief. It is onthat account also that we are so apt to _catch cold_, when in a state ofprofuse perspiration. It is for the same reason that tea is oftenrefreshing in summer, though it appears to heat you at the moment youdrink it. 

EMILY. 

And in winter, on the contrary, tea is pleasant on account of its heat. 

MRS. B. 

Yes; for we have then rather to guard against a deficiency than anexcess of caloric, and you do not find that tea will excite perspirationin winter, unless after dancing, or any other violent exercise. 

CAROLINE. 

What is the reason that it is dangerous to eat ice after dancing, or todrink any thing cold when one is very hot?

MRS. B. 

Because the loss of heat arising from the perspiration, conjointly withthe chill occasioned by the cold draught, produce more cold than can beborne with safety, unless you continue to use the same exercise afterdrinking that you did before; for the heat occasioned by the exercisewill counteract the effects of the cold drink, and the danger will beremoved. You may, however, contrary to the common notion, consider it asa rule, that cold liquids may, at all times, be drunk with perfectsafety, however hot you may feel, provided you are not at the moment ina state of great perspiration, and on condition that you keep yourselfin gentle exercise afterwards. 

EMILY. 

But since we are furnished with such resources against the extremes ofheat or cold, I should have thought that all climates would have beenequally wholesome. 

MRS. B. 

That is true, in a certain degree, with regard to those who have beenaccustomed to them from birth; for we find that the natives of thoseclimates, which we consider as most deleterious, are as healthy asourselves; and if such climates are unwholesome to those who arehabituated to a more moderate temperature, it is because the animaleconomy does not easily accustom itself to considerable changes. 

CAROLINE. 

But pray, Mrs. B. , if the circulation preserves the body of an uniformtemperature, how does it happen that animals are sometimes frozen?

MRS. B. 

Because, if more heat be carried off by the atmosphere than thecirculation can supply, the cold will finally prevail, the heart willcease to beat, and the animal will be frozen. And, likewise, if the bodyremained long exposed to a degree of heat, greater than the perspirationcould carry off, it would at last lose the power of resisting itsdestructive influence. 

CAROLINE. 

Fish, I suppose, have no animal heat, but only partake of thetemperature of the water in which they live?

EMILY. 

And their coldness, no doubt, proceeds from their not breathing?

MRS. B. 

All kinds of fish breathe more or less, though in a much smaller degreethan land animals. Nor are they entirely destitute of animal heat, though, for the same reason, they are much colder than other creatures. They have comparatively but a very small quantity of blood, thereforebut very little oxygen is required, and a proportionally small quantityof animal heat is generated. 

CAROLINE. 

But how can fish breathe under water?

MRS. B. 

They breathe by means of the air which is dissolved in the water, and ifyou put them into water deprived of air by boiling, they are soonsuffocated. 

If a fish is confined in a vessel of water closed from the air, it soondies; and any fish put in afterwards would be killed immediately, as allthe air had been previously consumed. 

CAROLINE. 

Are there any species of animals that breathe more than we do?

MRS. B. 

Yes; birds, of all animals, breathe the greatest quantity of air inproportion to their size; and it is to this that they are supposed toowe the peculiar firmness and strength of their muscles, by which theyare enabled to support the violent exertion of flying. 

This difference between birds and fish, which may be considered as thetwo extremes of the scale of muscular strength, is well worth observing. Birds residing constantly in the atmosphere, surrounded by oxygen, andrespiring it in greater proportions than any other species of animals, are endowed with a superior degree of muscular strength, whilst themuscles of fish, on the contrary, are flaccid and oily; these animalsare comparatively feeble in their motions, and their temperature isscarcely above that of the water in which they live. This is, in allprobability, owing to their imperfect respiration; the quantity ofhydrogen and carbon, that is in consequence accumulated in their bodies, forms the oil which is so strongly characteristic of that species ofanimals, and which relaxes and softens the small quantity of fibrinewhich their muscles contain. 

CAROLINE. 

But, Mrs. B. , there are some species of birds that frequent bothelements, as, for instance, ducks and other water fowl. Of what natureis the flesh of these?

MRS. B. 

Such birds, in general, make but little use of their wings; if they fly, it is but feebly, and only to a short distance. Their flesh, too, partakes of the oily nature, and even in taste sometimes resembles thatof fish. This is the case not only with the various kinds of waterfowls, but with all other amphibious animals, as the otter, thecrocodile, the lizard,  &c. 

CAROLINE. 

And what is the reason that reptiles are so deficient in muscularstrength?

MRS. B. 

It is because they usually live under ground, and seldom come into theatmosphere. They have imperfect, and sometimes no discernible organs ofrespiration; they partake therefore of the soft oily nature of fish;indeed, many of them are amphibious, as frogs, toads, and snakes, andvery few of them find any difficulty in remaining a length of time underwater. Whilst, on the contrary, the insect tribe, that are so strong inproportion to their size, and alert in their motions, partake of thenature of birds, air being their peculiar element, and their organs ofrespiration being comparatively larger than in other classes of animals. 

I have now given you a short account of the principal animal functions. However interesting the subject may appear to you, a fullerinvestigation of it would, I fear, lead us too far from our object. 

EMILY. 

Yet I shall not quit it without much regret; for of all the branches ofchemistry, it is certainly the most curious and most interesting. 

CAROLINE. 

But, Mrs.  B. , I must remind you that you promised to give us someaccount of the nature of _milk_. 

MRS. B. 

True. There are several other animal productions that deserve likewiseto be mentioned. We shall begin with milk, which is certainly the mostimportant and the most interesting of all the animal secretions. 

Milk, like all other animal substances, ultimately yields by analysisoxygen, hydrogen, carbon, and nitrogen. These are combined in it underthe forms of albumen, gelatine, oil, and water. But milk contains, besides, a considerable portion of phosphat of lime, the purposes ofwhich I have already pointed out. 

CAROLINE. 

Yes; it is this salt which serves to nourish the tender bones of thesuckling. 

MRS. B. 

To reduce milk to its elements, would be a very complicated, as well asuseless operation; but this fluid, without any chemical assistance, maybe decomposed into three parts, _cream_, _curds_, and _whey_. Theseconstituents of milk have but a very slight affinity for each other, andyou find accordingly that cream separates from milk by mere standing. Itconsists chiefly of oil, which being lighter than the other parts of themilk, gradually rises to the surface. It is of this, you know, thatbutter is made, which is nothing more than oxygenated cream. 

CAROLINE. 

Butter, then, is somewhat analogous to the waxy substance formed by theoxygenation of vegetable oils. 

MRS. B. 

Very much so. 

EMILY. 

But is the cream oxygenated by churning?

MRS. B. 

Its oxygenation commences previous to churning, merely by standingexposed to the atmosphere, from which it absorbs oxygen. The process isafterwards completed by churning; the violent motion which thisoperation occasions brings every particle of cream in contact with theatmosphere, and thus facilitates its oxygenation. 

CAROLINE. 

But the effect of churning, I have often observed in the dairy, is toseparate the cream into two substances, butter and butter-milk. 

MRS. B. 

That is to say, in proportion as the oily particles of the cream becomeoxygenated, they separate from the other constituent parts of the creamin the form of butter. So by churning you produce, on the one hand, butter, or oxygenated oil; and, on the other, butter-milk, or creamdeprived of oil. But if you make butter by churning new milk instead ofcream, the butter-milk will then be exactly similar in its properties tocreamed or skimmed milk. 

CAROLINE. 

Yet butter-milk is very different from common skimmed milk. 

MRS. B. 

Because you know it is customary, in order to save time and labour, tomake butter from cream alone. In this case, therefore, the butter-milkis deprived of the creamed milk, which contains both the curd and whey. Besides, in consequence of the milk remaining exposed to the atmosphereduring the separation of the cream, the latter becomes more or lessacid, as well as the butter-milk which it yields in churning. 

EMILY. 

Why should not the butter be equally acidified by oxygenation?

MRS. B. 

Animal oil is not so easily acidified as the other ingredients of milk. Butter, therefore, though usually made of sour cream, is not souritself, because the oily part of the cream had not been acidified. Butter, however, is susceptible of becoming acid by an excess of oxygen;it is then said to be rancid, and produces the sebacic acid, the same asthat which is obtained from fat. 

EMILY. 

If that be the case, might not rancid butter be sweetened by mixing withit some substance that would take the acid from it?

MRS. B. 

This idea has been suggested by Sir H. Davy, who supposes, that ifrancid butter were well washed in an alkaline solution, the alkali wouldseparate the acid from the butter. 

CAROLINE. 

You said just now that creamed milk consisted of curd and whey. Pray howare these separated?

MRS. B. 

They may be separated by standing for a certain length of time exposedto the atmosphere; but this decomposition may be almost instantaneouslyeffected by the chemical agency of a variety of substances. Alkalies, rennet*, and indeed almost all animal substances, decompose milk bycombining with the curds. 

Acids and spirituous liquors, on the other hand, produce a decompositionby combining with the whey. In order, therefore, to obtain the wheypure, rennet, or alkaline substances, must be used to attract the curdsfrom it. 

But if it be wished to obtain the curds pure, the whey must be separatedby acids, wine, or other spirituous liquors. 

 [Footnote *: Rennet is the name given to a watery infusion of the coats of the stomach of a sucking calf. Its remarkable efficacy in promoting coagulation is supposed to depend on the gastric juice with which it is impregnated. ]

EMILY. 

This is a very useful piece of information; for I find white-wine whey, which I sometimes take when I have a cold, extremely heating; now, ifthe whey were separated by means of an alkali instead of wine, it wouldnot produce that effect. 

MRS. B. 

Perhaps not. But I would strenuously advise you not to place too muchreliance on your slight chemical knowledge in medical matters. I do notknow why whey is not separated from curd by rennet, or by an alkali, forthe purpose which you mention; but I strongly suspect that there must besome good reason why the preparation by means of wine is generallypreferred. I can, however, safely point out to you a method of obtainingwhey without either alkali, rennet, or wine; it is by substituting lemonjuice, a very small quantity of which will separate it from the curds. 

Whey, as an article of diet, is very wholesome, being remarkable lightof digestion. But its effect, taken medicinally, is chiefly, I believe, to excite perspiration, by being drunk warm on going to bed. 

From whey a substance may be obtained in crystals by evaporation, called_sugar of milk_. This substance is sweet to the taste, and in itscomposition is so analogous to common sugar, that it is susceptible ofundergoing the vinous fermentation. 

CAROLINE. 

Why then is not wine, or alcohol, made from whey?

MRS. B. 

The quantity of sugar contained in milk is so trifling, that it canhardly answer that purpose. I have heard of only one instance of itsbeing used for the production of a spirituous liquor, and this is by theTartan Arabs; their abundance of horses, as well as their scarcity offruits, has introduced the fermentation of mares’ milk, by which theyproduce a liquor called _koumiss_. Whey is likewise susceptible of beingacidified by combining with oxygen from the atmosphere. It then producesthe _lactic acid_, which you may recollect is mentioned amongst theanimal acids, as the acid of milk. 

Let us now see what are the properties of curds. 

EMILY. 

I know that they are made into cheese; but I have heard that for thatpurpose they are separated from the whey by rennet, and yet this youhave just told us is not the method of obtaining pure curds?

MRS. B. 

Nor are pure curds so well adapted for the formation of cheese. For thenature and flavour of the cheese depend, in a great measure, upon thecream or oily matter which is left in the curds; so that if everyparticle of cream be removed from the curds, the cheese is scarcelyeatable. Rich cheeses, such as cream and Stilton cheeses, derive theirexcellence from the quantity, as well as the quality, of the cream thatenters into their composition. 

CAROLINE. 

I had no idea that milk was such an interesting compound. In manyrespects there appears to me to be a very striking analogy between milkand the contents of an egg, both in respect to their nature and theiruse. They are, each of them, composed of the various substancesnecessary for the nourishment of the young animal, and equally destinedfor that purpose. 

MRS. B. 

There is, however, a very essential difference. The young animal isformed, as well as nourished, by the contents of the egg-shell; whilstmilk serves as nutriment to the suckling, only after it is born. 

There are several peculiar animal substances which do not enter into thegeneral enumeration of animal compounds, and which, however, deserve tobe mentioned. 

_Spermaceti_ is of this class; it is a kind of oily substance obtainedfrom the head of the whale, which, however, must undergo a certainpreparation before it is in a fit state to be made into candles. It isnot much more combustible than tallow, but it is pleasanter to burn, asit is less fusible and less greasy. 

_Ambergris_ is another peculiar substance derived from a species ofwhale. It is, however, seldom obtained from the animal itself, but isgenerally found floating on the surface of the sea. 

_Wax_, you know, is a concrete oil, the peculiar product of the bee, part of the constituents of which may probably be derived from flowers, but so prepared by the organs of the bee, and so mixed with its ownsubstance, as to be decidedly an animal product. Bees’ wax is naturallyof a yellow colour, but it is bleached by long exposure to theatmosphere, or may be instantaneously whitened by the oxy-muriatic acid. The combustion of wax is far more perfect than that of tallow, andconsequently produces a greater quantity of light and heat. 

_Lac_ is a substance very similar to wax in the manner of its formation;it is the product of an insect, which collects its ingredients fromflowers, apparently for the purpose of protecting its eggs from injury. It is formed into cells, fabricated with as much skill as those of thehoney-comb, but differently arranged. The principal use of lac is in themanufacture of sealing-wax, and in making varnishes and lacquers. 

_Musk_, _civet_, and _castor_, are other particular productions, fromdifferent species of quadrupeds. The two first are very powerfulperfumes; the latter has a nauseous smell and taste, and is only usedmedicinally. 

CAROLINE. 

Is it from this substance that castor oil is obtained?

MRS. B. 

No. Far from it, for castor oil is a vegetable oil, expressed from theseeds of a particular plant; and has not the least resemblance to themedicinal substance obtained from the castor. 

_Silk_ is a peculiar secretion of the silk-worm, with which it buildsits nest or cocoon. This insect was originally brought to Europe fromChina. Silk, in its chemical nature, is very similar to the hair andwool of animals; whilst in the insect it is a fluid, which iscoagulated, apparently by uniting with oxygen, as soon as it comes incontact with the air. The moth of the silk-worm ejects a liquor whichappears to contain a particular acid, called _bombic_, the properties ofwhich are but very little known. 

EMILY. 

Before we conclude the subject of the animal economy, shall we not learnby what steps dead animals return to their elementary state?

MRS. B. 

Animal matter, although the most complicated of all natural substances, returns to its elementary state by one single spontaneous process, the_putrid fermentation_. By this, the albumen, fibrine, &c. Are slowlyreduced to the state of oxygen, hydrogen, nitrogen, and carbon; and thusthe circle of changes through which these principles have passed isfinally completed. They first quitted their elementary form, or theircombination with unorganised matter, to enter into the vegetable system. Hence they were transmitted to the animal kingdom; and from this theyreturn, again to their primitive simplicity, soon to re-enter the sphereof organised existence. 

When all the circumstances necessary to produce fermentation do not takeplace, animal, like vegetable matter, is liable to a partial orimperfect decomposition, which converts it into a combustible substancevery like spermaceti. I dare say that Caroline, who is so fond ofanalogies, will consider this as a kind of animal bitumen. 

CAROLINE. 

And why should I not, since the processes which produce these substancesare so similar?

MRS. B. 

There is, however, one considerable difference; the state of bitumenseems permanent, whilst that of animal substances, thus imperfectlydecomposed, is only transient; and unless precautions be taken topreserve them in that state, a total dissolution infallibly ensues. Thiscircumstance, of the occasional conversion of animal matter into a kindof spermaceti, is of late discovery. A manufacture has in consequencebeen established near Bristol, in which, by exposing the carcases ofhorses and other animals for a length of time under water, the muscularparts are converted into this spermaceti-like substance. The bonesafterwards undergo a different process to produce hartshorn, or, moreproperly, ammonia, and phosphorus; and the skin is prepared for leather. 

Thus art contrives to enlarge the sphere of useful purposes, for whichthe elements were intended by nature; and the productions of the severalkingdoms are frequently arrested in their course, and variouslymodified, by human skill, which compels them to contribute, under newforms, to the necessities or luxuries of man. 

But all that we enjoy, whether produced by the spontaneous operations ofnature, or the ingenious efforts of art, proceed alike from the goodnessof Providence. --To GOD alone man owes the admirable faculties whichenable him to improve and modify the productions of nature, no less thanthose productions themselves. In contemplating the works of thecreation, or studying the inventions of art, let us, therefore, neverforget the Divine Source from which they proceed; and thus everyacquisition of knowledge will prove a lesson of piety and virtue. 

INDEX. 

A

 Absorbent vessels, ii. 304 Absorption of caloric, i. 59. 66 Acetic acid, ii. 75. 197 Acetous fermentation, ii. 232 ---- acid, ii. 193. 232 Acidulous gaseous mineral waters, ii. 129 ---- salts, ii. 200 Acids, i. 262. Ii. 69 Aeriform, i. 36 Affinity, i. 19. Ii. 1 Agate, ii. 51 Agriculture, ii. 252 Air, i. 182. Ii. 262 Albumen, ii. 277. 288 Alburnum, ii. 267 Alchemists, i. 4 Alcohol, or spirit of wine, ii. 215. 222 Alembic, i. 258 Alkalies, ii. 19 Alkaline earths, ii. 50. 58 Alloys, i. 344 Alum, or sulphat of alumine, ii. 55. 95 Alumine, ii. 54 Alumium, i. 13 Amalgam, i. 347 Ambergris, ii. 358 Amethyst, ii. 58 Amianthus, ii. 66 Ammonia, or volatile alkali, i. 363. Ii. 20. 35 Ammoniacal gas, ii. 36 Ammonium, i. 13 Analysis, i. 287 ---- of vegetables, ii. 165 Animals, ii. 276 Animal acids, ii. 75. 290 ---- colours, ii. 292 ---- heat, ii. 337 ---- oil, ii. 178. 283 Animalization, ii. 276. 297. 315 Antidotes, ii. 41. 87 Antimony, i. 14 Aqua fortis, ii. 105 ---- regia, i. 340. Ii. 144 Arrack, ii. 220 Argand’s Lamp, i. 208 Arsenic, i. 14. 340. 348 Arteries, ii. 304. 323 Arterial blood, ii. 305. 326. 338 Asphaltum, ii. 240 Assafœtida, ii. 188 Assimilation, ii. 298 Astringent principle, ii. 198 Atmosphere, i. 90. 181. Ii. 262 Atmospherical air, i. 182 Attraction of aggregation, or cohesion, i. 16. Ii. 2 ---- of composition, i. 16. Ii. 1 Azot, or nitrogen, i. 182, ii. 100 Azotic gas, i. 182

B

 Balsams, ii. 165. 188 Balloons, i. 245 Bark, ii. 193. 265 Barytes, ii. 44. 58. 61 Bases of acids, i. 263. Ii. 69 ---- gases, i. 183 ---- salts, ii. 5 Beer, ii. 212. 220 Benzoic acid, ii. 74. 197 Bile, ii. 308 Birds, ii. 347 Bismuth, i. 14 Bitumens, ii. 239 Black lead, or plumbago, i. 304 Bleaching, i. 32. Ii. 89. 140. Blow-pipe, i. 324. Ii. 226 Blood, ii. 306. 317 Blood-vessels, ii. 298 Boiling water, i. 93 Bombic acid, ii. 75. 290 Bones, ii. 298, 299 Boracic acid, i. 365. Ii. 131 Boracium, i. 13. Ii. 132 Borat of soda, ii. 133 Brandy, ii. 218 Brass, i. 344 Bread, ii. 233 Bricks, ii. 56 Brittle-metals, i. 14 Bronze, i. 341 Butter, ii. 351 Butter-milk, ii. 352

C

 Calcareous earths, ii. 65 ---- stones, ii. 123 Calcium, i. 13 Caloric, i. 12. 33 ----, absorption of, i. 66 ----, conductors of, i. 70 ----, combined, i. 122 ----, expansive power of i. 35 ----, equilibrium of, i. 50 ----, reflexion of, i. 54. 67 ----, radiation of, i. 52. 61 ----, solvent power of, i. 96. 102 ----, capacity for, i. 124 Calorimeter, i. 156 Calx, i. 183 Camphor, ii. 165. 185 Camphoric acid, ii. 74. 197 Caoutchouc, ii. 165. 189 Carbonats, ii. 25. 129 Carbonat of ammonia, ii. 41 ---- lead, i. 320 ---- lime, ii. 59. 130 ---- magnesia, ii. 67 ---- potash, ii. 25 Carbonated hydrogen gas, i. 302 Carbon, i. 282. Ii. 329 Carbonic acid, i. 290. 359. Ii. 327 Carburet of iron, i. 304. 342 Carmine, ii. 295 Cartilage, ii. 303 Castor, ii. 359 Cellular membrane, ii. 311 Caustics, i. 349 Chalk, ii. 62. 123 Charcoal, i. 282 Cheese, ii. 356 Chemical attraction, i. 15. Ii. 9 Chemistry, i. 3 Chest, ii. 318 China, ii. 54 Chlorine, i. 214 Chrome, i. 14. 340 Chyle, ii. 305. 317 Chyme, ii. 316 Citric acid, ii. 74. 197 Circulation of the blood, ii. 322 Civet, ii. 359 Clay, i. 48. Ii. 55 Coke, ii. 241 Coal, ii. 240. 252 Cobalt, i. 14 Cochineal, ii. 295 Cold, i. 50. 58 ---- from evaporation, i. 102. 113. 150 Colours of metallic oxyds, i. 319 Columbium, i. 14. 340. 348 Combined caloric, i. 122 Combustion, i. 190 ----, volatile products of, i. 207 ----, fixed products of, i. 207 ----, of alcohol, ii. 225 ----, of ammoniacal gas, ii. 42 ----, of boracium, ii. 133 ----, by oxymuriatic acid or chlorine, ii. 142 ----, of carbon, i. 289 ----, of coals, i. 207. 297 ----, of charcoal by nitric acid, ii. 102 ----, of candles, i. 236. 309. Ii. 179 ----, of diamonds, i. 292 ----, of ether, ii. 230 ----, of hydrogen, i. 229. ----, of iron, i. 200. 322 ----, of metals, i. 321 ----, of oils, i. 208. Ii. 178. 309 ----, of oil of turpentine by nitrous acid, ii. 6 ----, of phosphorus, i. 272 ----, of sulphur, i. 261 ---- of potassium, i. 358. Ii. 132. 138, 139 Compound bodies, i. 9. Ii. 14 ---- or neutral salts i. 333. Ii. 4 Conductors of heat, i. 71 ----, solids, i. 73 ----, fluids, i. 78 ----, Count Rumford’s theory, i. 79 Constituent parts, i. 9 Copper, i. 14. 331 Copal, ii. 187. 224 Cortical layers, ii. 265. 267 Cotyledons, or lobes, ii. 256 Cream, ii. 351 Cream of tartar, or tartrit of potash, ii. 200. 222 Cryophorus, i. 154 Crystallisation, i. 338. Ii. 47 Cucurbit, i. 258 Culinary heat, i. 88 Curd, ii. 351. 354 Cuticle, or epidermis, ii. 310

D

 Decomposition, i. 8. 20 ---- of atmospherical air, i. 181. 209 ---- of water by the Voltaic battery, i. 220 ---- of salts by the Voltaic battery, ii. 14 ---- of water by metals, i. 225. 334 ---- ---- by carbon, i. 301 ---- of vegetables, ii. 202 ---- of potash, i. 356 ---- of soda, i. 56 ---- of ammonia, i. 363. Ii. 37 ---- of the boracic acid, ii. 132 ---- of the fluoric acid, ii. 136 ---- of the muriatic acid, ii. 139 Deflagration, ii. 118 Definite proportions, ii. 13 Deliquescence, ii. 95 Detonation, i. 219. Ii. 116 Dew, i. 105 Diamond, i. 285 Diaphragm, ii. 320 Digestion, ii. 316 Dissolution of metals, i. 165. 316. 333 Distillation, i. 259. Ii. 218 ---- of red wine, ii. 218 Divellent forces, ii. 12 Division, i. 7 Drying oils, ii. 181 Dying, ii. 191

E

 Earths, ii. 44 Earthen-ware, ii. 53. 57 Effervescence, i. 298 Efflorescence, ii. 94 Elastic fluids, i. 37 Electricity, i. 12. 25. 160. 220. Ii. 139 Electric machine, i. 169 Elective attractions, ii. 9 Elementary bodies, i. 8. 12 Elixirs, tinctures, or quintessences, ii. 225 Enamel, ii. 57 Epidermis of vegetables, ii. 269 ---- of animals, ii. 310 Epsom salts, ii. 63. 95 Equilibrium of caloric, i. 50 Essences, i. 307. Ii. 183. 224 Essential, or volatile oils, i. 307. Ii. 183 Ether, i. 111. Ii. 229 Evaporation, i. 103 Evergreens, ii. 274 Eudiometer, i. 276 Expansion of caloric, i. 36 Extractive colouring matter, ii. 165. 190

F

 Falling stones, i. 319 Fat, i. 306. Ii. 311 Feathers, ii. 300 Fecula, ii. 176 Fermentation, ii. 205 Fibrine, ii. 277. 289 Fire, i. 7. 27 Fish, ii. 346 Fixed air, or carbonic acid, i. 290. Ii. 125 ---- alkalies, ii. 20 ---- oils, i. 307. Ii. 165. 177 ---- products of combustion, i. 207 Flame, i. 237 Flint, ii. 30. 51 Flower or blossom, ii. 271 Fluoric acid, ii. 54. 134 Fluorium, or Fluorine, i. 12. Ii. 136 Formic acid, ii. 290 Fossil wood, ii. 242 Frankincense, ii. 187 Free or radiant caloric, or heat of temperature, i. 33 Freezing mixtures, i. 142 ---- by evaporation, i. 104. 150,  &c. Frost, i. 94 Fruit, ii. 271 Fuller’s earth, ii. 55 Furnace, i. 304

G

 Galls, ii. 199 Gallat of iron, ii. 98 Gallic acid, ii. 74. 197, 198 Galvanism, i. 163 Gas, i. 182 Gas-lights, i. 240 Gaseous oxyd of carbon, i. 296 ---- nitrogen, ii. 111 Gastric juice, ii. 316 Gelatine, or jelly, ii. 277. 280 Germination, ii. 256 Gin, ii. 221 Glands, ii. 298. 307 Glass, ii. 30 Glauber’s salts, or sulphat of soda, ii. 92 Glazing, ii. 57 Glucium, i. 13 Glue, ii. 281. 287 Gluten, ii. 165. 177 Gold, i. 14. 323 Gum, ii. 170 ---- arabic, ii. 170 ---- elastic, or caoutchouc, ii. 189 ---- resins, ii. 165. 188 Gunpowder, ii. 116 Gypsum, or plaister of Paris, or sulphat of lime, ii. 95

H

 Hair, ii. 300 Harrogate water, i. 268. 341 Hartshorn, ii. 35. 39. 281. 285 Heart, ii. 323 ---- wood, ii. 268 Heat, i. 26. 33 ---- of capacity, i. 127. 135 ---- of temperature, i. 33 Honey, ii. 175 Horns, ii. 282. 300 Hydro-carbonat, i. 241. 303 Hydrogen, i. 214 ---- gas, i. 215

I

 Jasper, ii. 51 Ice, i. 138 Jelly, ii. 281 Jet, ii. 240 Ignes fatui, i. 277 Ignition, i. 119 Imponderable agents, i. 12 Inflammable air, i. 215 Ink, ii. 98. 199 Insects, ii. 349 Integrant pans, i. 9 Iridium, i. 14 Iron, i. 14. 319. 328 Isinglass, ii. 194. 285 Ivory black, ii. 295 Iodine, i. 214. Ii. 157

K

 Kali, ii. 34 Koumiss, ii. 356

L

 Lac, ii. 358 Lactic acid, ii. 75. 290. 356 Lakes, colours, ii. 190 Latent heat, i. 133 Lavender water, ii. 184. 224 Lead, i. 14. 318. 330 Leather, ii. 193. 287 Leaves, ii. 260 Life, ii. 159. 168 Ligaments, ii. 303 Light, i. 12. 26. Ii. 261 Lightning, i. 248 Lime, ii. 59 ---- water, ii. 61 Limestone, ii. 60 Linseed oil, ii. 178 Liqueurs, ii. 224 Liver, ii. 308 Lobes, ii. 256. 332 Lunar caustic, or nitrat of silver, i. 350. Ii. 119 Lungs, ii. 319. 330 Lymph, ii. 304 Lymphatic vessels, ii. 304

M

 Magnesia, ii. 44. 66 Magnium, i. 13 Malic acid, ii. 74. 197 Malt, ii. 211 Malleable metals, i. 14 Manganese, i. 14. 317 Manna, ii. 176 Manure, ii. 247 Marble, ii. 123 Marine acid, or muriatic acid, ii. 136 Mastic, ii. 187. 224 Materials of animals, ii. 277 ---- of vegetables, ii. 165 Mercury, i. 14. 346 ----, new mode of freezing, i. 155. 347 Metallic acids, i. 340 ---- oxyds, i. 316 Metals, i. 12. 314 Meteoric stones, i. 342 Mica, ii. 66 Milk, ii. 299. 306. 350 Minerals, i. 315. Ii. 44. 158 Mineral waters, i. 296. Ii. 129 ---- acids, ii. 73 Miner’s lamp, i. 249 Mixture, i. 99 Molybdena, i. 14. 340 Mordant, ii. 165. 192 Mortar, ii. 53. 65 Mucilage, ii. 170 Mucous acid, ii. 74. 171. 197 ---- membrane, ii. 311 Muriatic acid, or marine acid, ii. 136 Muriats, ii. 151 Muriat of ammonia, ii. 35. 152 ---- lime, i. 100 ---- soda, or common salt, ii. 136. 151 ---- potash, ii. 138 Muriatium, i. 13 Muscles of animals, ii. 298. 303 Musk, ii. 359 Myrrh, ii. 188

N. 

 Naphtha, i. 357. Ii. 240 Negative electricity, i. 25. 161. 185 Nerves, ii. 279. 298. 308 Neutral, or compound salts, i. 333. Ii. 4. 22. 69 Nickel, i. 13. 343 Nitre, or nitrat of potash, or saltpetre, ii. 32. 104. 116 Nitric acid, ii. 100 Nitrogen, or azot, i. 181. Ii. 100 ---- gas, i. 182. 211 Nitro-muriatic acid, or aqua regia, ii. 144 Nitrous acid gas, ii. 101. 106 ---- air, or nitrit oxyd gas, ii. 107 Nitrats, ii. 116 Nitrat of copper, ii. 5 ---- ammonia, ii. 113. 118 ---- potash, or nitre, or saltpetre, ii. 32. 104. 116 ---- silver, or lunar caustic, ii. 19 Nomenclature of acids, i. 264. Ii. 69 ---- compound salts, ii. 4. 22 ---- other binary compounds, i. 278 Nut-galls, ii. 98. 199 Nut-oil, ii. 178 Nutrition, ii. 297

O

 Ochres, i. 320 Oils, i. 285. Ii. 306 Oil of amber, ii. 241 ---- vitriol, or sulphuric acid, ii. 80 Olive oil, ii. 178 Ores, i. 315 Organized bodies, ii. 159 Organs of animals, ii. 290. 310 ---- vegetables, ii. 159. 265. 271 Osmium, i. 14. 348 Oxalic acid, ii. 74. 197 Oxyds, i. 198 Oxyd of manganese, i. 117. 317 ---- iron, i. 204. 319 ---- lead, i. 319 ---- sulphur, ii. 91 Oxydation, or oxygenation, i. 196 Oxygen, i. 11. 181. 201. 211 ---- gas, or vital air, i. 182. 201 Oxy-muriatic acid, ii. 140 Oxy-muriats, ii. 153 Oxy-muriat of potash, ii. 155

P

 Palladium, i. 13. 348 Papin’s digester, i. 120. Ii. 284 Parenchyma, ii. 256. 266 Particles, i. 16 Pearlash, ii. 24 Peat, ii. 242 Peculiar juice of plants, ii. 268 Perfect metals, i. 14. 324 Perfumes, i. 308. Ii. 183 Perspiration, ii. 333. 329 Petrification, ii. 237 Pewter, i. 344 Pharmacy, i. 14 Phosphat of lime, ii. 99. 299 Phosphorated hydrogen gas, i. 277 Phosphorescence, i. 29 Phosphoric acid, i. 273. Ii. 99 Phosphorous acid, i. 274. Ii. 99 Phosphorus, i. 270 Phosphoret of lime, i. 278. 341 ---- sulphur, i. 279. 341 Pitch, ii. 187 Plaster, ii. 65 Platina, i. 14. 323 Plating, i. 345 Plumbago, or black lead, i. 304 Plumula, ii. 257 Porcelain, ii. 56 Positive electricity, i. 25. 161. 185 Potassium, i. 13. 357. Ii. 15 Pottery, ii. 56 Potash, i. 356. Ii. 22 Precipitate, i. 22 Pressure of the atmosphere, i. 112. 116 Printer’s ink, ii. 144 Prussiat of iron, or prussian blue, ii. 291 ---- potash, ii. 291 Prussic acid, ii. 75. 290 Putrid fermentation, ii. 235. 360 Pyrites, i. 341. Ii. 97 Pyrometer, i. 38. 42

Q

 Quick lime, ii. 59 Quiescent forces, ii. 12

R

 Radiation of caloric, i. 52 ----, Prevost’s theory, i. 52 ----, Pictet’s explanations, i. 54 ----, Leslie’s illustrations, i. 61 Radicals, ii. 5. 69 Radicle; or root, ii. 257 Rain, i. 104 Rancidity, ii. 182 Rectification, ii. 223 Reflexion of caloric, i. 54. 64 Reptiles, ii. 349 Resins, ii. 165, 186. 266 Respiration, ii. 317. 326 Reviving of metals, i. 327 Rhodium, i. 14. 348 Roasting metals, i. 316 Rock crystal, ii. 61 Ruby, ii. 53 Rum, ii. 219 Rust, i. 318. 328

S

 Saccharine fermentation, ii. 208 Sal ammoniac, or muriat of ammonia, ii. 35 ---- polychrest, or sulphat of potash, ii. 91 ---- volatile, or carbonat of ammonia, ii. 41 Salifiable bases, ii. 5 Salifying principles, ii. 5 Saltpetre, or nitre, or nitrat of potash, ii. 32. 104. 116 Salt, ii. 91 Sand, ii. 30. 51 Sandstone, ii. 51 Sap of plants, ii. 165. 260. 262. 270. 272 Sapphire, ii. 58 Saturation, i. 101. Sapphire, ii. 58 Saturation, i. 101 Seas, temperature of, i. 33. Sebacic acid, ii. 75. 182. 290. 353 Secretions, ii. 307 Seeds of plants, ii. 210. 271 Seltzer water, i. 289. Ii. 63. 129 Senses, ii. 310 Silex, or silica, ii. 30. 51 Silicium, i. 13. Silk, ii. 359 Silver, i. 321 Simple bodies, i. 10. 12 Size, ii. 281 Skin, ii. 279. 310. 193 Slakeing of lime, i. 147. Ii. 56 Slate, ii. 51. 66 Smelting metals, i. 316 Smoke i. 208 Soap, ii. 24 Soda, i. 363. Ii. 33 ---- water, i. 299 Sodium, i. 13. 363 Soils, i. 42. Ii. 245 Soldering, i. 345 Solubility, ii. 92 Solution, i. 96 ---- by the air, i. 102 ---- of potash, ii. 28 Specific heat, i. 126 Spermaceti, ii. 358 Spirits, ii. 313 Steam, i. 140. 182 Steel, i. 305 Stomach, ii. 315 Stones, ii. 46 Stucco, ii. 65 Strontites, ii. 44. 68 Strontium, i. 13 Suberic acid, ii. 74. 197 Sublimation, i. 257 Succin, or yellow amber, ii. 241 Succinic acid, ii. 74. 197. 241 Sugar, ii. 165. 174. 208 ---- of milk, ii. 355 Sulphats, ii. 5. 91 Super oxygenated sulphuric acid, ii. 70. Sulphat of alumine, or alum, ii. 54. 95 ---- barytes, ii. 58 ---- iron, ii. 96 ---- lime, or gypsum, or plaster of Paris, ii. 95 ---- magnesia, or Epsom salt, ii. 67. 95 ---- potash, or sal polychrest, ii. 91 ---- soda, or Glauber’s salts, ii. 92 Sulphur, i. 256 ---- flowers of, i. 257 Sulphurated hydrogen gas, i. 165. 268 Sulphurets, i. 341 Sulphurous acid, i. 254. Ii. 88 Sulphuric acid, i. 74. Ii. 265 Sympathetic ink, i. 354 Synthesis, i. 287

T

 Tan, ii. 192 Tannin, ii. 165. 192 Tar, ii. 187 Tartarous acid, ii. 74. 197 Tartrit of potash, ii. 222 Teeth, ii. 300 Tellurium, i. 14 Temperature, i. 33 Thaw, i. 158 Thermometers, i. 40 ----, Fahrenheit’s, i. 42 ----, Reaumur’s, i. 42 ----, Centigrade, i. 43 ----, air, i. 44 ----, differential, i. 46 Thunder, i. 248 Tin, i. 14. 344 Titanium, i. 14. 348 Turf, ii. 242 Turpentine, ii. 187 Transpiration of plants, ii. 260 Tungsten, i. 14. 340

V

 Vapour, i. 36. 49. 93. 182 Vaporisation, i. 103 Varnishes, ii. 187 Vegetables, ii. 158 Vegetable acid, i. 310. Ii. 74. 197 ---- colours, ii. 190 ---- heat, ii. 272 ---- oils, ii. 177 Veins, ii. 304. 323. Venous blood, ii. 305. 326. 338 Ventricles, ii. 324 Verdigris, i. 352 Vessels, ii. 304 Vinegar, ii. 232 Vinous fermentation, ii. 212 Vital air, or oxygen gas, i. 182 Vitriol, or sulphat of iron, ii. 81 Volatile oils, i. 307. Ii. 165. 183. 224. 269 ---- products of combustion, i. 207 ---- alkali, i. 363. Ii. 20. 35 Voltaic battery, i. 164. 220. 356. Ii. 15

U

 Uranium, i. 14

W

 Water, i. 215. Ii. 262 ----, decomposition of, by electricity, i. 200. 225 ----, condensation of, i. 32 ---- of the sea, i. 86 ----, boiling, i. 93 ----, solution by, i. 96 ---- of crystallisation, i. 339 Wax, i. 309. Ii. 180. 358 Whey, ii. 351 Wine, ii. 212 Wood, ii. 267 Woody fibre, ii. 156. 196. 267 Wool, ii. 300

Y

 Yeast, ii. 234. Yttria, ii. 44. Yttrium, i. 13. 

Z

 Zinc, ii. 14. 344 Zirconia, ii. 44 Zirconium, i. 14 Zoonic acid, ii. 75. 220

END. 

 Printed by A. Strahan, Printers-Street, London

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Terminology

 oxy-muriatic acid = chlorine (proposed as an element in 1815: see Conversation XIX) “columbium or tantalium” = niobium and tantalum (the two elements always occur together, and were not recognized as separate until much later in the 19th century) phosphat of lime = calcium diphosphate _or_ calcium (the element calcium was isolated in 1808, but is named only once in this 1817 edition) glucium = beryllium (Humphry Davy’s name for the element)

 muriatic acid = hydrochloric acid (still called “muriatic acid” for some commercial uses) muriat of lime = calcium chloride oxymuriate of potash = potassium chlorate muriat of soda = sodium chloride (table salt) carbonic acid = carbon dioxide

Note also:

 simple body, fundamental principle = element fecula = starch (usually spelled “fæcula”) spirit of wine = alcohol philosopher = scientist arts = industry, manufacture, crafts etc. (seldom “fine arts”)

Some essential concepts relating to living things--photosynthesis, microorganisms, the cell, proteins--are either unknown or not mentioned. The atom theory had been proposed, but not by Humphry Davy; it is notmentioned in this book. 

The word “explode” is used at least once in its orginal, figurativesense (“a word that should be exploded in chemistry”) but far moreoften in its later, concrete one. The word “explosion” is always usedconcretely (“an explosion, or a _detonation_ as chemists commonly callit”). 

Calculated Values:

“the point of zero, or the absolute privation of heat, must consequentlybe 1260 degrees below 32 degrees”

 [-1228° F. The calculation is based on wrong premises; the correct figure is about -460° F or -273° C. ]

“Mercury congeals only at seventy-two degrees below the freezing point. ”

 [-40° F, which is also -40° C. This figure is correct, though approximate. ]

“The proportion stated by Sir H. Davy, in his Chemical Researches, is as1 to 2. 389. ”

“[ammonia] consisted of about one part of hydrogen to four parts ofnitrogen. .. . And from the latest and most accurate experiments, theproportions appear to be, one volume of nitrogen gas to three ofhydrogen gas”

 [These and similar calculations involving weight and volume make more sense when one knows the elements’ atomic weights. For nitric acid, HNO_3, the figures are 1:14:48, giving a proportion closer to 1:3. 5. For ammonia, NH_3 (not 4), the figures are 14:3. ]

“The _oxalic acid_, distilled from sorrel, is the highest term ofvegetable acidification; for, if more oxygen be added to it, it losesits vegetable nature, and is resolved into carbonic acid and water;”

 [Oxalic acid = H_2C_2O_4; carbonic acid (carbon dioxide) = CO_2. H_2C_2O_4 + O becomes H_2O + CO_2 + CO_2. ]

 * * * * * * * * *

Contents: Numbering and Changes

The 3rd and 4th editions used the same Conversation numbering. Changes between the 4th and 5th (present text) edition are shown. Some illustrations were also changed. 

_Volume I: On Simple Bodies_

(I, II, III no change)

 IV. On Specific Heat, Latent Heat, and Chemical Heat. IV. On Combined Caloric, Comprehending Specific Heat and Latent Heat. 

 -- V. [New Chapter] On The Chemical Agencies Of Electricity. 

 V. VI. On Oxygen And Nitrogen. 

 VI. VII. On Hydrogen. [5th edn: adds sections on Gas lights and Miner’s Lamp]

 VII. VIII. On Sulphur And Phosphorus. [5th edn: adds section on Decomposition of Sulphur]

 VIII. On Carbone. IX. On Carbon. [4th edn: Section “Diamond is Carbon in a State of perfect purity”; later edn: “Diamond” alone]

 IX. X. On Metals. 

 X. XIV. On Alkalies. XI. XV. On Earths. [5th edn: both moved to Vol. II. ]

_Volume II. On Compound Bodies_

 XII. XIII. On The Attraction Of Composition. 

 [5th edn. XIV, XV = 4th edn. X, XI]

 XIII. On Compound Bodies. XVI. On Acids. [Most of XIII, On Compound Bodies, became XVI, On Acids. Some introductory material was moved to XIV, On Alkalies. ]

 XIV. On The Combinations of Oxygen with Sulphur and with Phosphorus; and of the Sulphats And Phosphats. XVII. Of the Sulphuric and Phosphoric Acids: or, The Combinations of . .. . 

 XV. On The Combination of Oxygen With Nitrogen and with Carbone; and of The Nitrats And Carbonats. XVIII. Of The Nitric And Carbonic Acids: Or The Combination . .. 

 XVI. On Muriatic And Oxygenated Muriatic Acids; and on Muriats. XIX. On The Boracic, Fluoric, Muriatic, and Oxygenated Muriatic Acids; and on Muriats. 

 XVII. XX. On The Nature And Composition Of Vegetables. 

(remaining Conversations: 4th edn. + 3 = 5th edn. )

 * * * * * * * * *

ERRATA

Inconsistencies are generally unchanged; see end of Errata list. 

Two items are noted in the printed Errata, immediately after theContents for Vol. I:

 Vol. I. Page 56. Last line but one, for “caloric, ” read “calorific. ” 179. Note, for “Plate XII. ” r. “Plate XIII. ”

 I. 56 The principal use of the mirrors in this experiment is, to prove that the calorific emanation . .. I. 179 fn. A model of this mode of construction is exhibited in PLATE XIII. Fig.  1. 

Errata Noted by Transcriber:

Contents II . .. Dr.  Herschel’s Experiments [_body text has “Herschell”_] XIV . .. Hartshorn and Sal Volatile [ad]

Conversation I [Emily] the electric spark which is visible, and [aud]

Conversation II [Mrs. B. , parenthesis] (PLATE I. Fig.  1. ) [Fig. I. ] [Mrs. B. ] it has been called _differential_ thermometer [_missing “the”?_] [Mrs. B. , parenthesis] (PLATE III. Fig.  1. ) [Fig. I. ] [Emily] the tin surface should radiate the least caloric [carolic]

Conversation III [Emily] the glass skreen [_spelling unchanged_] [Mrs. B. , parenthesis] (PLATE IV. Fig.  1. ) [_error for Fig. 2. _] [Plate IV caption] Thermometers one in the Ether, the other [_invisible comma after “Thermometers”?_] [Mrs. B. ] he found that it was considerably colder [is was]

Conversation IV [Caroline] But how can you reverse this experiment? [_printed “expe-/periment” at line break_] [Mrs. B. ] instead of being 75 degrees, will be 80 degrees [_error for 88?_] [Emily, footnote] See page 102. [_in Conversation III_] [Mrs. B. ] then soke it in ether [_spelling unchanged_]

Conversation V [Mrs. B. ] at regular distances in wooden troughs [throughs] [Caroline] the nature of the action of the Voltaic battery [Votaic]

Conversation VI [Caroline] the nature of OXYGEN, which come next in our table [_error for “comes”?_]

Conversation VII [Mrs. B. , parenthesis] (c,  d, PLATE VIII. Fig. 2. ) [fig. 2, ] [Caroline] be soon adopted every where, [every where. ] [Plate X] C. Apperture for supplying Oil. [_spelling unchanged_]

Conversation VIII [Mrs. B. ] sulphur is a very combustible substance [sulpur] [Mrs. B. ] I now put into the receiver [_missing “it”?_] [Emily] What is . .. After its detonation? [. For ?]

Conversation IX [Mrs. B. ] we are to burn the carbon [bnrn] [Mrs. B. ] since they may be prepared [thay] [Mrs. B. ] artificial Seltzer water [artifical]

Conversation X [Mrs. B. ] increase the rapidity of its combustion [of of] [Caroline] a pair of scissars [_spelling unchanged_] [Mrs. B. ] as well as by evaporating the liquid. [? for . ] [Mrs. B. , footnote] page 155. Of this volume [_near the end of Conversation IV_] [Caroline] amongst the metals. I had no notion [, for . ] [Emily] But is it not very singular [singulr] [Mrs. B. ] Thenard and Gay Lussac [_usually hyphenated: “Gay-Lussac”_]

Conversation XIII [Emily] . .. Render that decomposition perceptible? [. For ?]

Conversation XIV [Mrs. B. ] an acrid burning taste [on acrid] [Caroline] according to which heat is disengaged [_t in “heat” invisible_] [Mrs. B. ] one volume of nitrogen gas to three of hydrogen gas [_text has “oxygen” for “hydrogen”_]

Conversation XV [Mrs. B. ] so many interesting and important compounds [interesing] [Caroline] And of what nature . .. Painting porcelain? [. For ?] [_speaker’s name missing; supplied from other editions_]

Conversation XVI [Mrs. B. ] combined with acidifiable radicals [acidificiable] [Mrs. B. ] this power of charring wood [charing]

Conversation XVIII [Caroline] not mentioning this acid [_printed “mention-/this” at line break_] [Mrs. B. (footnote)] 1 to 2. 389. ] [_printed “2, 389”: no other decimal numbers occur in the text, but a comma appears once as a thousands separator_] [Mrs. B. ] You, understand, now, I hope, [_all commas in original_]

Conversation XX [Mrs. B. ] of vegetables; we shall, therefore [, for ;]

Conversation XXI [Mrs. B. ] the compound last formed will be destroyed; [detroyed] [Mrs. B. ] in such climates great part of the water [_missing “a” before “great”?_] [Mrs. B. ] a state of debility and languor [langour] [Mrs. B. , parenthesis] (PLATE XIV. Fig.  2. ) [PLATE XIII. ] [Mrs. B. ] burns at so slow a temperature [_text unchanged: error for “low” or correct as printed?_] [Mrs. B. ] the poor in heathy countries [_not an error: “heath-y”, not “healthy”_]

Conversation XXII [Mrs. B. , parenthesis] (PLATE XV. Fig.  4. ) [PLATE XIV. ] [Mrs. B. ] so many vessels or apparatus [_not an error: “apparatus” is the Latin plural form_] [Mrs. B. ] chesnut [_common variant spelling_]

Conversation XXIII [Mrs. B. ] pure gelatine [gelantine]

Conversation XXV [Mrs. B. ] The muscular action of the diaphragm [diaphram]

Conversation XXVI [Mrs. B. ] the air had been previously consumed [previouly] [Mrs. B. ] this is by the Tartan Arabs [_text unchanged: Tartar?_] [Mrs. B. ] or studying the inventions of art, let us, therefore [; for, ]

Index Arrack [_body text has “arack”_] [Cold] ---- from evaporation, i. 102. 113. 150 [_volume number missing_] Culinary heat, i. 88 [_volume number missing_] [Decomposition] ---- of ammonia, i. 363. Ii. 37 [ammonnia; _“i” invisible_] Frankincense [Francincense] [Freezing mixtures] ---- by evaporation, i. 104. 150,  &c. [_volume number invisible_] Glue, ii. 281. 287 [_volume number missing_] N. [_anomalous . Unchanged_] Phosphorous acid, i. 274. Ii. 99 [_volume I body text always has “Phosphorus acid”_] [Sulphat] ---- lime, or gypsum, or [gypsum of] [Sulphur] ---- flowers of, i. 257 [_volume number missing_] [Thermometers] ----, Centigrade, i. 43 [Centrigade] [Thermometers] ----, differential, i. 46 [differentiial] V, U [_alphabetized as shown_] Zirconia, ii. 44 [Zicornia] Zirconium, i. 14 [Zicornium . .. 13]

Inconsistencies and variant spellings:

Standard spellings in this book include: bason, judgment, embrio, volcanos (plural), potatoe (singular)

Inconsistencies include: capitalization of “Fig. ” or “fig. ” hyphenization of words such as “oxy-muriatic” “glauber salt” and “Glauber’s salt” both occur

Variant forms include: opake, opaque aëriform, aeriform (with and without dieresis) gasses, gases phosphoret, phosphuret (but always carburet) Libya, Lybia dy(e)ing [from “dye”] nap(h)tha pla(i)ster slak(e)ing earthen-ware, earthen ware “sulphurous”, “naphtha” are used in the Contents and the Index; “sulphureous”, “naptha” in the body text forms in “-xion” (such as “connexion”) appear only in the Contents and the Index

Volume I has more archaic forms than Volume II: “shew”, “inclose” are sometimes used instead of “show”, “enclose” “carbone” with final “e” appears in one Plate caption. (In the same plate’s header, the “e” appears to have been removed by the engraver. ) “develope(ment)” is more common in Volume I, “develop(ment)” in Volume II “-ize” and “-yze” forms (for later “-ise” and “-yse”) are common in Volume I, rare in Volume II except in the Index

The “Dr. Marcet” mentioned in a few footnotes and figure captions is theauthor’s husband. Humphry Davy (“Sir H. Davy”) was knighted in 1812, between the 3rd and 4th editions of the book. 

Reminder:

DO NOT TRY THIS AT HOME. 

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