Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: and thus forms, by comparison, a graduated table, which is useful afterwards for new trials and experiments. In many crystals, perpendicular planes at right angles exist, and it appears that between these planes the cohesion is so slight that they can be separated with greater facility there than in any other place. This particular property is called " cleavage." In some gems it is very easy to ascertain the point of cleavage, but in others it is found with difficulty. Many gems, as, for example, rock-crystal and emerald, will only break into irregular fragments. SPECIFIC GRAVITY. In order to determine the specific gravity of different bodies, distilled water is chosen as a means of comparison. If it be a gem which weighs 17 carats in the air, and in the water but 12, there will be : Weighed in the air .. .. 17 carats, "Weighed in the water .. 12 ,, Difference .. 5 Kow, dividing the weight of the gem 17 by the difference 5, the quotient will be—17:5 = 3-4, which Mobs has made a graduated table as follows :—1. White talc. 2. Sal gemma. 3. Calcareous spar. 4. Fluoric spar. 5. A]xitite. 6. Adularia. 7. Book crystal. 8. Topaz. 9. Corundum. 10. Diamond. chapter{Section 4represents the specific gravity of the gem which has been put to the proof. It is often very useful to know the specific gravity of gems, as a means of determining their quality, and to prevent the danger of their being changed for others, similar in appearance but very different in value; as, for instance, the diamond and the jargoon. In the case of cut and polished gems it is therefore of great use in ascertaining the class to which they belong, without the possibility of injuring them by the otherwise necessary operation of filing. The comparison of the specific weight of gems...