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Ch. 2: Minerals: Physical Properties

Ch. 2: Minerals: Physical Properties Page of 515 Ch. 2: Minerals: Physical Properties Text size:minus plus Restore normal size   Mail page  Print this page
84                        A POPULAR TREATISE ON GEMS.
difference of weight, within or without the water, of both together, and then of the heavier body alone, the specific gravity of the lighter substance is the result. And for de­termining the specific gravity of liquids, by means of the Hydrostatic balance, a glass ball is applied to one of the arms (its loss of weight in pure water being known), and, dipping the same in the liquid to be examined, any addition and abstraction will result in the specific gravity of the liquid. The hydrometers of Beaume for the different liquids to be examined, are employed with satisfactory results.
That the specific gravity has been known as far back as the thirteenth century, and applied by the Oriental nations for determining the character of precious stones, is suffi­ciently proved by a work written in that century by Mo­hammed Ben Manner. In fact, the specific gravity is often, in connection with the color, quite essential in determining a gem.
Optical Properties of Jfinerals.
There are few more interesting departments of science than the relations of mineral bodies to light, and the modi­fications which it undergoes either when passing through them or when reflected from their surface. In this place, however, we can only notice these phenomena so far as they point out distinctions in the internal constitution of minerals, or furnish characters for distinguishing one species from another.
Minerals, and even different specimens of the same spe­cies, vary much in pellucidity or in the quantity of light which can pass through them. Some transmit so much light, that small objects can be clearly seen, or letters read when placed behind them, and are named transparent. They are semi-transparent when the object is only seen
Ch. 2: Minerals: Physical Properties Page of 515 Ch. 2: Minerals: Physical Properties
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