xviii INTRODUCTIOK 



7. Quartz : a transparent variety. 



8. Topaz : a transparent variety. 



9. Sapphire : cleavable variety, 

 10. Diamond. 



Toughness. 



This quality (which expresses the resistance which a body offers to being 

 broken or torn) must not be considered identical with Hardness. Some soft 

 minerals may be tough, such as sulphate of lime ; others, as Flint, though 

 hard, may be easily broken ; while others, of which Jade is an instance, are 

 at the same time both hard and tough. 



Specific Gravity. 



The specific gravity of a mineral is a test of very great importance in the 

 identification of minerals, and in some cases (as in those of polished gems 

 for instance) it is almost the only one which can be had recourse to without 

 occasioning injury to the specimen. In such cases the test of hardness does 

 not admit of being applied, and, for the same reason, chemical analysis is out 

 of the question. When, therefore, the test of colour cannot be relied upon, 

 the determination of the specific gravity will almost always solve the diffi- 

 culty. The mistakes that have been, and are constantly being, made by 

 not determining the specific gravity of polished stones (even by those whose 

 business it is to buy and sell such articles) are remarkable. It will be seen 

 in the body of the work that colourless Jargoons are often sold in the 

 East, and even in Europe, for inferior Diamonds, and similar substitu- 

 tions are frequently made by dealers and jewellers in this country, not from 

 any wilful intention to deceive, but in consequence of their relying solely on 

 colour, lustre, and general appearance in the identification of gems. 



These mistakes might generally be avoided by ascertaining the specific 

 gravity. The process is very simple, all that is required being an accurately 

 adjusted balance, and care in the use of it. 



The determination of the specific gravity is effected by first weighing the 

 mineral in the usual manner, and then, in water, suspended by a fine thread 

 or horsehair. As the mineral will be buoyed up by the water in a degree 

 proportionate to the surface it presents, its weight in water will be less than 

 in air, and the difference between the weight in water and the weight in air, 

 or the loss in weight it has sustained by immersion, will represent the weight 

 of a quantity of water equal in bulk to the substance operated on. Now, 

 as the specific gravity of a body is the proportion which its weight bears to 

 that of an equal bulk of water, the weight in air divided by the loss of 

 weight (or the difference of the weight obtained in and out of water) which 

 it has sustained in water, will give the desired relation and be the required 

 specific gravity. 



Taste. 



This test is, of course, only applicable in the case of minerals which are 

 soluble in water. It is of seven kinds, viz. : — 



1. Astringent: as in Sulphate of Iron. 



2. Sweetish astringent: as in Alum. 



