Page:Handbook of Precious Stones.djvu/30

 thus learning the weight of water which the stone displaces—that is, the weight of an equal bulk of water. (3) By measuring or weighing, directly or by difference, the water which the stone diplaces when immersed in water in a small vessel of known capacity. We will now briefly describe these three methods.

1. Several different liquids have been used for the purpose of ascertaining the density of minerals, and even for separating species having different densities from one another. One of these liquids, which has done good service in its day, is a saturated solution of potassio-mercuric iodide. This may be prepared so as to have a density of 3.18 at 15° C. It is a yellow liquid called after its discoverer Sonstadt's Solution. Unfortunately this liquid is very poisonous and rapidly destroys, by amalgamating the metal, any brass apparatus with which it may come into contact. Two substitutes for this liquid are now in use. One is an aqueous solution, which may be diluted at will with water, of the compound known as cadmium boro-tungstate. The crystals of this salt, to which the formula has been assigned, when fused, over a water-bath in their own water of crystallization, yield a liquid which at 75 C. has the specific gravity 3.55. At 22° C. this cadmium tungstoborate in crystals requires but 1-10th of its weight of water for solution: a very small further addition of water enables one to secure a solution which at 15° C. presents the specific gravity of 3.28. The other heavy liquid to which reference has been made, as a second substitute for Sonstadt's Solution, is methylene iodide, the formula of which is. This compound has the density of 3.32 at 15° C. Its density may be lowered by the addition of toluene, which, at the same temperature, has the density 0.869. On the other hand, the density of methylene iodide - may be raised by saturating it with iodoform and iodine. It is well to be content with the addition of iodoform only, for iodine makes the liquid too dark in colour for the movements of a stone put therein to be observed. It will be