Gem-Stones and Their Distinctive Characters

by George Frederick Herbert Smith

CHAPTER VIII

SPECIFIC GRAVITY

IT is one of our earliest experiences that different substances of the same size have often markedly different weights; thus, there is a great difference between wood and iron, and still greater between wood and lead. It is usual to say that iron is heavier than wood, but the statement is misleading, because it would be possible by selecting a large enough piece of wood to find one at least as heavy as a particular piece of iron. We have, in fact, to compare equal volumes of the two substances, and all ambiguity is removed if we speak of relative density or specific gravity—the former term being usually applied to liquids and the latter to solids—instead of weight or heaviness. The density of water at 4° C. is taken as unity, that being the temperature at which it is highest; at other temperatures it is somewhat lower, as will be seen from [Table IX] given at the end of the book. The direct determination of the volume of an irregular solid presents almost insuperable difficulty; but, fortunately, for finding the specific gravity it is quite unnecessary to know the volume, as will be shown when we proceed to consider the methods in use.

The specific gravity of a stone is a character which is within narrow limits constant for each species, and is therefore very useful for discriminative purposes. It can be determined whatever be the shape of the stone, and it is immaterial whether it be transparent or not; but, on the other hand, the stone must be unmounted and free from the setting.

The methods for the determination of the specific gravity are of two kinds: in the first a liquid is found of the same, or nearly the same, density as the stone, and in the second weighings are made and the use of an accurate balance is required.

(1) Heavy Liquids

Experiment tells us that a solid substance floats in a liquid denser than itself, sinks in one less dense, and remains suspended at any level in one of precisely the same density. If the stone be only slightly less dense than the liquid, it will rise to the surface; if it be just as slightly denser, it will as surely sink to the bottom, a physical fact which has added so much to the difficulty and danger of submarine manœuvring. If then we can find a liquid denser than the stone to be tested, and place the latter in it, the stone will float on the surface. If we take a liquid which is less dense than the stone and capable of mixing with the heavier liquid, and add it to the latter, drop by drop, gently stirring so as to assure that the density of the combination is uniformly the same throughout, a stage is finally reached when the stone begins to move downwards. It has now very nearly the density of the liquid, and, if we find by some means this density, we know simultaneously the specific gravity of the stone.

Various devices and methods are available for ascertaining the density of liquids—for instance, Westphal’s balance; but, apart from the inconvenience attending such a determination, the density of all liquids is somewhat seriously affected by changes in the temperature, and it is therefore better to make direct comparison with fragments of substances of known specific gravity, which are termed indicators. If of two fragments differing slightly in specific gravity one floats on the surface of a uniform column of liquid and the other lies at the bottom of the tube containing the liquid, we may be certain that the density of the liquid is intermediate between the two specific gravities. Such a precaution is necessary because, if the liquid be a mixture of two distinct liquids, the density would tend to increase owing to the greater volatility of the lighter of them, and in any case the density is affected by change of temperature. The specific gravity of stones is not much altered by variation in the temperature.

A more convenient variation of this method is to form a diffusion column, so that the density increases progressively with the depth. If the stone under test floats at a certain level in such a column intermediate between two fragments of known specific gravity, its specific gravity may be found by elementary interpolation. To form a column of this kind the lighter liquid should be poured on to the top of the heavier. Natural diffusion gives the most perfect column, but, being a lengthy process, it may conveniently be quickened by gently shaking the tube, and the column thus formed gives results sufficiently accurate for discriminative purposes.

By far the most convenient liquid for ordinary use is methylene iodide, which has already been recommended for its high refraction. It has, when pure, a density at ordinary room-temperatures of 3·324, and it is miscible in all proportions with benzol, whose density is O·88, or toluol, another hydrocarbon which is somewhat less volatile than benzol, and whose density is about the same, namely, 0·86. When fresh, methylene iodide has only a slight tinge of yellow, but it rapidly darkens on exposure to light owing to the liberation of iodine which is in a colloidal form and cannot be removed by filtration. The liquid may, however, be easily cleared by shaking it up with any substance with which the iodine combines to form an iodide removable by filtration. Copper filings answer the purpose well, though rather slow in action; mercury may also be used, but is not very satisfactory, because a small amount may be dissolved and afterwards be precipitated on to the stone under test, carrying it down to the bottom of the tube. Caustic potash (potassium hydroxide) is also recommended; in this case the operation should preferably be carried out in a special apparatus which permits the clear liquid to be drawn off underneath, because water separates out and floats on the surface. In Fig. 32 three cut stones, a quartz (a), a beryl (b), and a tourmaline (c) are shown floating in a diffusion column of methylene iodide and benzol. Although the beryl is only slightly denser than the quartz, it floats at a perceptibly lower level. These three species are occasionally found as yellow stones of very similar tint.