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Ch. 1: Properties and Characteristics Gemstones

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14 Properties and Characteristics of
maline properly disposed. As alluded to before, double refraction may sometimes be conveniently determined from this property of light; for which purpose the sub­stance to be examined should be interposed between the two reflecting plates, when so arranged that the original ray refuses to be reflected, in which case, should the body be doubly refracting, a position will be found in which a great portion of the previously intercepted light will be more or less transmitted. The refractive index of a body may also be ascertained from the polar­izing angle by the following relation :—
Specific Gravity. By specific gravity is meant the proportion the weight of any body bears to an equal volume of water; that is to say, supposing four substances whose volume was so exactly equal as to fill exactly the same mould, and that each had different weight when weighed in the balance, then these different weights, considered in relation to a common term of comparison, would sig­nify the specific gravity of the various substances. But as the bodies of which the specific gravity is required must have different volumes, the way to ascertain the specific gravity of any body is to weigh first in air and then in water, and to divide the weight in air by the difference between the weight in air and the weight in water. This principle is too well known to require any further explanation.
Ch. 1: Properties and Characteristics Gemstones Page of 295 Ch. 1: Properties and Characteristics Gemstones
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