YEAST

By Thomas H. Huxley

I HAVE selected to-night the particular subject of Yeast for tworeasons--or, rather, I should say for three. In the first place, becauseit is one of the simplest and the most familiar objects with which weare acquainted. In the second place, because the facts and phenomenawhich I have to describe are so simple that it is possible to put thembefore you without the help of any of those pictures or diagrams whichare needed when matters are more complicated, and which, if I had torefer to them here, would involve the necessity of my turning away fromyou now and then, and thereby increasing very largely my difficulty(already sufficiently great) in making myself heard. And thirdly, I havechosen this subject because I know of no familiar substance forming partof our every-day knowledge and experience, the examination of which, with a little care, tends to open up such very considerable issues asdoes this substance--yeast. 

In the first place, I should like to call your attention to a fact withwhich the whole of you are, to begin with, perfectly acquainted, I meanthe fact that any liquid containing sugar, any liquid which is formed bypressing out the succulent parts of the fruits of plants, or a mixtureof honey and water, if left to itself for a short time, begins toundergo a peculiar change. No matter how clear it might be at starting, yet after a few hours, or at most a few days, if the temperature ishigh, this liquid begins to be turbid, and by-and-by bubbles make theirappearance in it, and a sort of dirty-looking yellowish foam or scumcollects at the surface; while at the same time, by degrees, a similarkind of matter, which we call the "lees, " sinks to the bottom. 

The quantity of this dirty-looking stuff, that we call the scum and thelees, goes on increasing until it reaches a certain amount, and thenit stops; and by the time it stops, you find the liquid in which thismatter has been formed has become altered in its quality. To begin withit was a mere sweetish substance, having the flavour of whatever mightbe the plant from which it was expressed, or having merely the taste andthe absence of smell of a solution of sugar; but by the time that thischange that I have been briefly describing to you is accomplished theliquid has become completely altered, it has acquired a peculiar smell, and, what is still more remarkable, it has gained the property ofintoxicating the person who drinks it. Nothing can be more innocent thana solution of sugar; nothing can be less innocent, if taken in excess, as you all know, than those fermented matters which are producedfrom sugar. Well, again, if you notice that bubbling, or, as it were, seething of the liquid, which has accompanied the whole of this process, you will find that it is produced by the evolution of little bubbles ofair-like substance out of the liquid; and I dare say you all know thisair-like substance is not like common air; it is not a substance whicha man can breathe with impunity. You often hear of accidents which takeplace in brewers' vats when men go in carelessly, and get suffocatedthere without knowing that there was anything evil awaiting them. And ifyou tried the experiment with this liquid I am telling of while itwas fermenting, you would find that any small animal let down into thevessel would be similarly stifled; and you would discover that a lightlowered down into it would go out. Well, then, lastly, if after thisliquid has been thus altered you expose it to that process which iscalled distillation; that is to say, if you put it into a still, andcollect the matters which are sent over, you obtain, when you first heatit, a clear transparent liquid, which, however, is something totallydifferent from water; it is much lighter; it has a strong smell, and ithas an acrid taste; and it possesses the same intoxicating power as theoriginal liquid, but in a much more intense degree. If you put a lightto it, it burns with a bright flame, and it is that substance which weknow as spirits of wine. 

Now these facts which I have just put before you--all but the last--havebeen known from extremely remote antiquity. It is, I hope one of thebest evidences of the antiquity of the human race, that among theearliest records of all kinds of men, you find a time recorded when theygot drunk. We may hope that that must have been a very late period intheir history. Not only have we the record of what happened to Noah, butif we turn to the traditions of a different people, those forefathersof ours who lived in the high lands of Northern India, we find that theywere not less addicted to intoxicating liquids; and I have no doubtthat the knowledge of this process extends far beyond the limits ofhistorically recorded time. And it is a very curious thing to observethat all the names we have of this process, and all that belongs toit, are names that have their roots not in our present language, but inthose older languages which go back to the times at which this countrywas peopled. That word "fermentation" for example, which is the titlewe apply to the whole process, is a Latin term; and a term which isevidently based upon the fact of the effervescence of the liquid. Thenthe French, who are very fond of calling themselves a Latin race, have aparticular word for ferment, which is 'levure'. And, in the same way, wehave the word "leaven, " those two words having reference to the heavingup, or to the raising of the substance which is fermented. Now those arewords which we get from what I may call the Latin side of our parentage;but if we turn to the Saxon side, there are a number of names connectedwith this process of fermentation. For example, the Germans callfermentation--and the old Germans did so--"gahren;" and they callanything which is used as a ferment by such names, such as "gheist" and"geest, " and finally in low German, "yest"; and that word you know isthe word our Saxon forefathers used, and is almost the same as the wordwhich is commonly employed in this country to denote the common fermentof which I have been speaking. So they have another name, the word"hefe, " which is derived from their verb "heben, " which signifies toraise up; and they have yet a third name, which is also one common inthis country (I do not know whether it is common in Lancashire, but itis certainly very common in the Midland countries), the word "barm, "which is derived from a root which signifies to raise or to bear up. Barm is a something borne up; and thus there is much more real relationthan is commonly supposed by those who make puns, between the beer whicha man takes down his throat and the bier upon which that process, ifcarried to excess, generally lands him, for they are both derivedfrom the root signifying bearing up; the one thing is borne upon men'sshoulders, and the other is the fermented liquid which was borne up bythe fermentation taking place in itself. 

Again, I spoke of the produce of fermentation as "spirit of wine. " Nowwhat a very curious phrase that is, if you come to think of it. The oldalchemists talked of the finest essence of anything as if it had thesame sort of relation to the thing itself as a man's spirit is supposedto have to his body; and so they spoke of this fine essence of thefermented liquid as being the spirit of the liquid. Thus came aboutthat extraordinary ambiguity of language, in virtue of which you applyprecisely the same substantive name to the soul of man and to a glassof gin! And then there is still yet one other most curious piece ofnomenclature connected with this matter, and that is the word "alcohol"itself, which is now so familiar to everybody. Alcohol originally meanta very fine powder. The women of the Arabs and other Eastern people arein the habit of tinging their eyelashes with a very fine black powderwhich is made of antimony, and they call that "kohol;" and the "al" issimply the article put in front of it, so as to say "the kohol. " Andup to the 17th century in this country the word alcohol was employed tosignify any very fine powder; you find it in Robert Boyle's works thathe uses "alcohol" for a very fine subtle powder. But then this name ofanything very fine and very subtle came to be specially connected withthe fine and subtle spirit obtained from the fermentation of sugar; andI believe that the first person who fairly fixed it as the proper nameof what we now commonly call spirits of wine, was the great Frenchchemist Lavoisier, so comparatively recent is the use of the wordalcohol in this specialised sense. 

So much by way of general introduction to the subject on which I have tospeak to-night. What I have hitherto stated is simply what we may callcommon knowledge, which everybody may acquaint himself with. Andyou know that what we call scientific knowledge is not any kindof conjuration, as people sometimes suppose, but it is simply theapplication of the same principles of common sense that we apply tocommon knowledge, carried out, if I may so speak, to knowledge which isuncommon. And all that we know now of this substance, yeast, and all thevery strange issues to which that knowledge has led us, have simply comeout of the inveterate habit, and a very fortunate habit for the humanrace it is, which scientific men have of not being content until theyhave routed out all the different chains and connections of apparentlysimple phenomena, until they have taken them to pieces and understoodthe conditions upon which they depend. I will try to point out to younow what has happened in consequence of endeavouring to apply thisprocess of "analysis, " as we call it, this teazing out of an apparentlysimple fact into all the little facts of which it is made up, to theascertained facts relating to the barm or the yeast; secondly, what hascome of the attempt to ascertain distinctly what is the nature of theproducts which are produced by fermentation; then what has come of theattempt to understand the relation between the yeast and the products;and lastly, what very curious side issues if I may so call them--havebranched out in the course of this inquiry, which has now occupiedsomewhere about two centuries. 

The first thing was to make out precisely and clearly what was thenature of this substance, this apparently mere scum and mud that wecall yeast. And that was first commenced seriously by a wonderful oldDutchman of the name of Leeuwenhoek, who lived some two hundred yearsago, and who was the first person to invent thoroughly trustworthymicroscopes of high powers. Now, Leeuwenhoek went to work upon thisyeast mud, and by applying to it high powers of the microscope, hediscovered that it was no mere mud such as you might at first suppose, but that it was a substance made up of an enormous multitude of minutegrains, each of which had just as definite a form as if it were a grainof corn, although it was vastly smaller, the largest of these not beingmore than the two-thousandth of an inch in diameter; while, as youknow, a grain of corn is a large thing, and the very smallest ofthese particles were not more than the seven-thousandth of an inch indiameter. Leeuwenhoek saw that this muddy stuff was in reality a liquid, in which there were floating this immense number of definitely shapedparticles, all aggregated in heaps and lumps and some of them separate. That discovery remained, so to speak, dormant for fully a century, andthen the question was taken up by a French discoverer, who, payinggreat attention and having the advantage of better instruments thanLeeuwenhoek had, watched these things and made the astounding discoverythat they were bodies which were constantly being reproduced andgrowing; than when one of these rounded bodies was once formed and hadgrown to its full size, it immediately began to give off a little budfrom one side, and then that bud grew out until it had attained thefull size of the first, and that, in this way, the yeast particle wasundergoing a process of multiplication by budding, just as effectual andjust as complete as the process of multiplication of a plant bybudding; and thus this Frenchman, Cagniard de la Tour, arrived atthe conclusion--very creditable to his sagacity, and which has beenconfirmed by every observation and reasoning since--that this apparentlymuddy refuse was neither more nor less than a mass of plants, of minuteliving plants, growing and multiplying in the sugary fluid in which theyeast is formed. And from that time forth we have known this substancewhich forms the scum and the lees as the yeast plant; and it hasreceived a scientific name--which I may use without thinking of it, and which I will therefore give you--namely, "Torula. " Well, this was acapital discovery. The next thing to do was to make out how this torulawas related to the other plants. I won't weary you with the whole courseof investigation, but I may sum up its results, and they are these--thatthe torula is a particular kind of a fungus, a particular staterather, of a fungus or mould. There are many moulds which under certainconditions give rise to this torula condition, to a substance which isnot distinguishable from yeast, and which has the same properties asyeast--that is to say, which is able to decompose sugar in the curiousway that we shall consider by-and-by. So that the yeast plant is a plantbelonging to a group of the Fungi, multiplying and growing and living inthis very remarkable manner in the sugary fluid which is, so to speak, the nidus or home of the yeast. 

That, in a few words, is, as far as investigation--by the help of one'seye and by the help of the microscope--has taken us. But now there is anobserver whose methods of observation are more refined than those of menwho use their eye, even though it be aided by the microscope; a man whosees indirectly further than we can see directly--that is, the chemist;and the chemist took up this question, and his discovery was not lessremarkable than that of the microscopist. The chemist discovered thatthe yeast plant being composed of a sort of bag, like a bladder, insidewhich is a peculiar soft, semifluid material--the chemist found thatthis outer bladder has the same composition as the substance of wood, that material which is called "cellulose, " and which consists of theelements carbon and hydrogen and oxygen, without any nitrogen. But thenhe also found (the first person to discover it was an Italian chemist, named Fabroni, in the end of the last century) that this inner matterwhich was contained in the bag, which constitutes the yeast plant, was asubstance containing the elements carbon and hydrogen and oxygen andnitrogen; that it was what Fabroni called a vegeto-animal substance, andthat it had the peculiarities of what are commonly called "animalproducts. "

This again was an exceedingly remarkable discovery. It lay neglectedfor a time, until it was subsequently taken up by the great chemists ofmodern times, and they, with their delicate methods of analysis, havefinally decided that, in all essential respects, the substance whichforms the chief part of the contents of the yeast plant is identicalwith the material which forms the chief part of our own muscles, whichforms the chief part of our own blood, which forms the chief part ofthe white of the egg; that, in fact, although this little organism isa plant, and nothing but a plant, yet that its active living contentscontain a substance which is called "protein, " which is of the samenature as the substance which forms the foundation of every animalorganism whatever. 

Now we come next to the question of the analysis of the products, ofthat which is produced during the process of fermentation. So far backas the beginning of the 16th century, in the times of transition betweenthe old alchemy and the modern chemistry, there was a remarkable man, Von Helmont, a Dutchman, who saw the difference between the air whichcomes out of a vat where something is fermenting and common air. He wasthe man who invented the term "gas, " and he called this kind of gas "gassilvestre"--so to speak gas that is wild, and lives in out of the wayplaces--having in his mind the identity of this particular kind of airwith that which is found in some caves and cellars. Then, the gradualprocess of investigation going on, it was discovered that thissubstance, then called "fixed air, " was a poisonous gas, and it wasfinally identified with that kind of gas which is obtained by burningcharcoal in the air, which is called "carbonic acid. " Then thesubstance alcohol was subjected to examination, and it was found to bea combination of carbon, and hydrogen, and oxygen. Then the sugar whichwas contained in the fermenting liquid was examined and that was foundto contain the three elements carbon, hydrogen, and oxygen. So thatit was clear there were in sugar the fundamental elements which arecontained in the carbonic acid, and in the alcohol. And then came thatgreat chemist Lavoisier, and he examined into the subject carefully, and possessed with that brilliant thought of his which happens to bepropounded exactly apropos to this matter of fermentation--that nomatter is ever lost, but that matter only changes its form and changesits combinations--he endeavoured to make out what became of the sugarwhich was subjected to fermentation. He thought he discovered that thewhole weight of the sugar was represented by the carbonic acid produced;that in other words, supposing this tumbler to represent the sugar, thatthe action of fermentation was as it were the splitting of it, the onehalf going away in the shape of carbonic acid, and the other half goingaway in the shape of alcohol. Subsequent inquiry, careful research withthe refinements of modern chemistry, have been applied to this problem, and they have shown that Lavoisier was not quite correct; that what hesays is quite true for about 95 per cent. Of the sugar, but that theother 5 per cent. , or nearly so, is converted into two other things;one of them, matter which is called succinic acid, and the othermatter which is called glycerine, which you all know now as one of thecommonest of household matters. It may be that we have not got to theend of this refined analysis yet, but at any rate, I suppose I maysay--and I speak with some little hesitation for fear my friendProfessor Roscoe here may pick me up for trespassing upon hisprovince--but I believe I may say that now we can account for 99 percent. At least of the sugar, and that 99 per cent. Is split up intothese four things, carbonic acid, alcohol, succinic acid, and glycerine. So that it may be that none of the sugar whatever disappears, andthat only its parts, so to speak, are re-arranged, and if any of itdisappears, certainly it is a very small portion. 

Now these are the facts of the case. There is the fact of the growth ofthe yeast plant; and there is the fact of the splitting up of the sugar. What relation have these two facts to one another?

For a very long time that was a great matter of dispute. The earlyFrench observers, to do them justice, discerned the real state of thecase, namely, that there was a very close connection between the actuallife of the yeast plant and this operation of the splitting up of thesugar; and that one was in some way or other connected with the other. All investigation subsequently has confirmed this original idea. It hasbeen shown that if you take any measures by which other plants of likekind to the torula would be killed, and by which the yeast plant iskilled, then the yeast loses its efficiency. But a capital experimentupon this subject was made by a very distinguished man, Helmholz, whoperformed an experiment of this kind. He had two vessels--one of them wewill suppose full of yeast, but over the bottom of it, as this might be, was tied a thin film of bladder; consequently, through that thin film ofbladder all the liquid parts of the yeast would go, but the solid partswould be stopped behind; the torula would be stopped, the liquid partsof the yeast would go. And then he took another vessel containing afermentable solution of sugar, and he put one inside the other; and inthis way you see the fluid parts of the yeast were able to pass throughwith the utmost ease into the sugar, but the solid parts could not getthrough at all. And he judged thus: if the fluid parts are those whichexcite fermentation, then, inasmuch as these are stopped, the sugar willnot ferment; and the sugar did not ferment, showing quite clearly, that an immediate contact with the solid, living torula was absolutelynecessary to excite this process of splitting up of the sugar. Thisexperiment was quite conclusive as to this particular point, and has hadvery great fruits in other directions. 

Well, then, the yeast plant being essential to the production offermentation, where does the yeast plant come from? Here, again, wasanother great problem opened up, for, as I said at starting, you have, under ordinary circumstances in warm weather, merely to expose somefluid containing a solution of sugar, or any form of syrup or vegetablejuice to the air, in order, after a comparatively short time, to see allthese phenomena of fermentation. Of course the first obvious suggestionis, that the torula has been generated within the fluid. In fact, itseems at first quite absurd to entertain any other conviction; but thatbelief would most assuredly be an erroneous one. 

Towards the beginning of this century, in the vigorous times of the oldFrench wars, there was a Monsieur Appert, who had his attention directedto the preservation of things that ordinarily perish, such as meats andvegetables, and in fact he laid the foundation of our modern method ofpreserving meats; and he found that if he boiled any of these substancesand then tied them so as to exclude the air, that they would bepreserved for any time. He tried these experiments, particularly withthe must of wine and with the wort of beer; and he found that if thewort of beer had been carefully boiled and was stopped in such a waythat the air could not get at it, it would never ferment. What was thereason of this? That, again, became the subject of a long string ofexperiments, with this ultimate result, that if you take precautions toprevent any solid matters from getting into the must of wine or the wortof beer, under these circumstances--that is to say, if the fluid hasbeen boiled and placed in a bottle, and if you stuff the neck of thebottle full of cotton wool, which allows the air to go through and stopsanything of a solid character however fine, then you may let it be forten years and it will not ferment. But if you take that plug out andgive the air free access, then, sooner or later fermentation will setup. And there is no doubt whatever that fermentation is excited only bythe presence of some torula or other, and that that torula proceeds inour present experience, from pre-existing torulae. These little bodiesare excessively light. You can easily imagine what must be the weight oflittle particles, but slightly heavier than water, and not more than thetwo-thousandth or perhaps seven-thousandth of an inch in diameter. Theyare capable of floating about and dancing like motes in the sunbeam;they are carried about by all sorts of currents of air; the greatmajority of them perish; but one or two, which may chance to enter intoa sugary solution, immediately enter into active life, find there theconditions of their nourishment, increase and multiply, and may giverise to any quantity whatever of this substance yeast. And, whatevermay be true or not be true about this "spontaneous generation, " as itis called in regard to all other kinds of living things, it is perfectlycertain, as regards yeast, that it always owes its origin to thisprocess of transportation or inoculation, if you like so to call it, from some other living yeast organism; and so far as yeast is concerned, the doctrine of spontaneous generation is absolutely out of court. And not only so, but the yeast must be alive in order to exert thesepeculiar properties. If it be crushed, if it be heated so far that itslife is destroyed, that peculiar power of fermentation is not excited. Thus we have come to this conclusion, as the result of our inquiry, thatthe fermentation of sugar, the splitting of the sugar into alcohol andcarbonic acid, glycerine, and succinic acid, is the result of nothingbut the vital activity of this little fungus, the torula. 

And now comes the further exceedingly difficult inquiry--how is itthat this plant, the torula, produces this singular operation of thesplitting up of the sugar? Fabroni, to whom I referred some time ago, imagined that the effervescence of fermentation was produced in just thesame way as the effervescence of a sedlitz powder, that the yeast was akind of acid, and that the sugar was a combination of carbonic acid andsome base to form the alcohol, and that the yeast combined withthis substance, and set free the carbonic acid; just as when you addcarbonate of soda to acid you turn out the carbonic acid. But of coursethe discovery of Lavoisier that the carbonic acid and the alcohol takentogether are very nearly equal in weight to the sugar, completely upsetthis hypothesis. Another view was therefore taken by the French chemist, Thenard, and it is still held by a very eminent chemist, M. Pasteur, andtheir view is this, that the yeast, so to speak, eats a little of thesugar, turns a little of it to its own purposes, and by so doing givessuch a shape to the sugar that the rest of it breaks up into carbonicacid and alcohol. 

Well, then, there is a third hypothesis, which is maintained by anothervery distinguished chemist, Liebig, which denies either of the othertwo, and which declares that the particles of the sugar are, as it were, shaken asunder by the forces at work in the yeast plant. Now I am notgoing to take you into these refinements of chemical theory, I cannotfor a moment pretend to do so, but I may put the case before you by ananalogy. Suppose you compare the sugar to a card house, and suppose youcompare the yeast to a child coming near the card house, then Fabroni'shypothesis was that the child took half the cards away; Thenard's andPasteur's hypothesis is that the child pulls out the bottom card andthus makes it tumble to pieces; and Liebig's hypothesis is that thechild comes by and shakes the table and tumbles the house down. Iappeal to my friend here (Professor Roscoe) whether that is not a fairstatement of the case. 

Having thus, as far as I can, discussed the general state of thequestion, it remains only that I should speak of some of thosecollateral results which have come in a very remarkable way out of theinvestigation of yeast. I told you that it was very early observed thatthe yeast plant consisted of a bag made up of the same material as thatwhich composes wood, and of an interior semifluid mass which containsa substance, identical in its composition, in a broad sense, withthat which constitutes the flesh of animals. Subsequently, afterthe structure of the yeast plant had been carefully observed, it wasdiscovered that all plants, high and low, are made up of separatebags or "cells, " as they are called; these bags or cells having thecomposition of the pure matter of wood; having the same composition, broadly speaking, as the sac of the yeast plant, and having in theirinterior a more or less fluid substance containing a matter of the samenature as the protein substance of the yeast plant. And therefore thisremarkable result came out--that however much a plant may differ froman animal, yet that the essential constituent of the contents of thesevarious cells or sacs of which the plant is made up, the nitrogenousprotein matter, is the same in the animal as in the plant. And not onlywas this gradually discovered, but it was found that these semifluidcontents of the plant cell had, in many cases, a remarkable power ofcontractility quite like that of the substance of animals. And about24 or 25 years ago, namely, about the year 1846, to the best of myrecollection, a very eminent German botanist, Hugo Von Mohl, conferredupon this substance which is found in the interior of the plant cell, and which is identical with the matter found in the inside of the yeastcell, and which again contains an animal substance similar to that ofwhich we ourselves are made up--he conferred upon this that title of"protoplasm, " which has brought other people a great deal of troublesince! I beg particularly to say that, because I find many peoplesuppose that I was the inventor of that term, whereas it has been inexistence for at least twenty-five years. And then other observers, taking the question up, came to this astonishing conclusion (workingfrom this basis of the yeast), that the differences between animals andplants are not so much in the fundamental substances which compose them, not in the protoplasm, but in the manner in which the cells of whichtheir bodies are built up have become modified. There is a sense inwhich it is true--and the analogy was pointed out very many years ago bysome French botanists and chemists--there is a sense in which it istrue that every plant is substantially an enormous aggregation ofbodies similar to yeast cells, each having to a certain extent its ownindependent life. And there is a sense in which it is also perfectlytrue--although it would be impossible for me to give the statementto you with proper qualifications and limitations on an occasion likethis--but there is also a sense in which it is true that every animalbody is made up of an aggregation of minute particles of protoplasm, comparable each of them to the individual separate yeast plant. Andthose who are acquainted with the history of the wonderful revolutionwhich has been worked in our whole conception of these matters in thelast thirty years, will bear me out in saying that the first germ ofthem, to a very great extent, was made to grow and fructify by the studyof the yeast plant, which presents us with living matter in almost itssimplest condition. 

Then there is yet one last and most important bearing of this yeastquestion. There is one direction probably in which the effects of thecareful study of the nature of fermentation will yield results morepractically valuable to mankind than any other. Let me recall to yourminds the fact which I stated at the beginning of this lecture. Supposethat I had here a solution of pure sugar with a little mineral matterin it; and suppose it were possible for me to take upon the point of aneedle one single, solitary yeast cell, measuring no more perhaps thanthe three-thousandth of an inch in diameter--not bigger than one ofthose little coloured specks of matter in my own blood at this moment, the weight of which it would be difficult to express in the fractionof a grain--and put it into this solution. From that single one, if thesolution were kept at a fair temperature in a warm summer's day, therewould be generated, in the course of a week, enough torulae to forma scum at the top and to form lees at the bottom, and to change theperfectly tasteless and entirely harmless fluid, syrup, into a solutionimpregnated with the poisonous gas carbonic acid, impregnated with thepoisonous substance alcohol; and that, in virtue of the changes workedupon the sugar by the vital activity of these infinitesimally smallplants. Now you see that this is a case of infection. And from the timethat the phenomenon of fermentation were first carefully studied, ithas constantly been suggested to the minds of thoughtful physicians thatthere was a something astoundingly similar between this phenomena ofthe propagation of fermentation by infection and contagion, and thephenomena of the propagation of diseases by infection and contagion. Out of this suggestion has grown that remarkable theory of many diseaseswhich has been called the "germ theory of disease, " the idea, in fact, that we owe a great many diseases to particles having a certain life oftheir own, and which are capable of being transmitted from one livingbeing to another, exactly as the yeast plant is capable of beingtransmitted from one tumbler of saccharine substance to another. Andthat is a perfectly tenable hypothesis, one which in the present stateof medicine ought to be absolutely exhausted and shown not to be true, until we take to others which have less analogy in their favour. Andthere are some diseases most assuredly in which it turns out to beperfectly correct. There are some forms of what are called malignantcarbuncle which have been shown to be actually effected by a sort offermentation, if I may use the phrase, by a sort of disturbance anddestruction of the fluids of the animal body, set up by minute organismswhich are the cause of this destruction and of this disturbance; andonly recently the study of the phenomena which accompany vaccinationhas thrown an immense light in this direction, tending to show byexperiments of the same general character as that to which I referred asperformed by Helmholz, that there is a most astonishing analogy betweenthe contagion of that healing disease and the contagion of destructivediseases. For it has been made out quite clearly, by investigationscarried on in France and in this country, that the only part of thevaccine matter which is contagious, which is capable of carrying on itsinfluence in the organism of the child who is vaccinated, is the solidparticles and not the fluid. By experiments of the most ingenious kind, the solid parts have been separated from the fluid parts, and it hasthen been discovered that you may vaccinate a child as much as you likewith the fluid parts, but no effect takes place, though an excessivelysmall portion of the solid particles, the most minute that can beseparated, is amply sufficient to give rise to all the phenomena ofthe cow pock, by a process which we can compare to nothing but thetransmission of fermentation from one vessel into another, by thetransport to the one of the torula particles which exist in the other. And it has been shown to be true of some of the most destructivediseases which infect animals, such diseases as the sheep pox, suchdiseases as that most terrible and destructive disorder of horses, glanders, that in these, also, the active power is the living solidparticle, and that the inert part is the fluid. However, do not supposethat I am pushing the analogy too far. I do not mean to say that theactive, solid parts in these diseased matters are of the same nature asliving yeast plants; but, so far as it goes, there is a most surprisinganalogy between the two; and the value of the analogy is this, that byfollowing it out we may some time or other come to understand how thesediseases are propagated, just as we understand, now, about fermentation;and that, in this way, some of the greatest scourges which afflict thehuman race may be, if not prevented, at least largely alleviated. 

This is the conclusion of the statements which I wished to put beforeyou. You see we have not been able to have any accessories. If you willcome in such numbers to hear a lecture of this kind, all I can say is, that diagrams cannot be made big enough for you, and that it is notpossible to show any experiments illustrative of a lecture on such asubject as I have to deal with. Of course my friends the chemists andphysicists are very much better off, because they can not only show youexperiments, but you can smell them and hear them! But in my case suchaids are not attainable, and therefore I have taken a simple subject andhave dealt with it in such a way that I hope you all understand it, at least so far as I have been able to put it before you in words; andhaving once apprehended such of the ideas and simple facts of the caseas it was possible to put before you, you can see for yourselves thegreat and wonderful issues of such an apparently homely subject.