SPECIFIC GRAVITY 11 



of crystalline individuals is known as a crystalline or massive aggi-egate. The constituent 

 particles of such an aggregate may be of various shapes ; they may be developed fairly 

 equally in all directions, or they may be considerably elongated or shortened in one or more 

 directions. Thus arise granular, columnar, fibrous, shelly, scaly, or other kinds of ap;gre- 

 gates. A granular aggregate is coarsely or finely granular according to the size of the 

 constituent particles. 



Sometimes the particles are so fine that they cannot be distinguished with the naked 

 eye, nor e^en with the help of a simple lens, and the mass then appears to be perfectly homo- 

 geneous. The microscope, however, reveals the fact that it is in reality an aggregate of 

 minute grains, fibres, or scales. A truly homogeneous body appears homogeneous even 

 under the highest powers of the microscope. A mass built up of minute particles, but with 

 an external appearance of apparent homogeneity, is known as a compact aggregate. It 

 often shows the rounded exterior of an amorphous body, and while its constituent particles 

 may show regular crystal-faces the aggregate as a whole never does. 



Specimens of such compact aggregates are frequently opaque, and their microscopic 

 examination necessitates the preparation of a slice sufficiently thin to be transparent. A 

 plate with parallel sides is cut and one side is polished. The plate is then fixed to a slip 

 of glass with Canada-balsam, the unpolished surface being uppermost. The plate is then 

 ground down till it is so thin as to be transparent, when the upper surface is polished. To 

 preserve the section a glass cover-slip is cemented over it with Canada- balsam. Many 

 important and interesting facts respecting the character of minerals and precious stones have 

 been learnt from the microscopic examination of such thiji sections, as they are called. The 

 method has been specially useful in the examination of turquoise, chalcedony, and agate, 

 where special difficulties lie in the way of other methods. 



C. PHYSICAL CHARACTERS. 



A. SPECIFIC GRAVITY. 



One of the most important characters of a precious stone is its density. On this 

 quality depends the weight of a stone of any given size. Thus of two bodies of equal size 

 but of different material, the one having the greater density will exceed the other in weight. 

 To give a concrete example, a cubic inch of iron weighs rather more than a quarter of a 

 pound, while a cubic inch of oak weighs half an ounce. The cube of iron is, therefore, 

 eight times heavier than the cube of wood. 



Instead of measuring the density of a substance it is more convenient to compare the 

 weight of any given volume with the weight of an equal volume of some standard substance. 

 The substance usually selected as the standard is water at a temperature of 4° C. The ratio 

 of the weight of any volume of a substance to the weight of an equal volume of water at the 

 above temperature is known as the specific gravity of that substance. The specific gravity 

 of a body is found, thei-efore, by dividing its weight by the weight of an equal volume of 

 water. To calculate how many times one substance is denser than another the specific 

 gravity of the former must be divided by that of the latter. 



Experience has shown that each chemical substance, each mineral, and also each 

 precious stone, has a definite specific gravity, which in most cases differentiates it from 

 all other substances. This character furnishes an important means of identification, 

 vhich is specially valuable in the case of precious stones, for by this method the 

 most costly cut stone can be identified and suffer no injury in the process. With the 



