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SPECIFIC GRAVITY
The
specific gravity of mineral species is also one of their fundamental
and constant characters, and furnishes a reliable means of
distinguishing between gems of different kinds, and of separating
false from real stones. To be sure, a variation of composition may
cause a variation of specific gravity in the same species; but this is
usually within comparatively narrow limits. The different kinds of
garnet, or of tourmaline, for example, possess specific gravities
varying within one integer; but the varieties are usually distinguished
by colors by which the appropriate specific gravity can be judged. One
great advantage of using specific gravity as a means of identifying
gems is, that the determination can be made without danger of injuring
the stone, which is more than can be said of tests of hardness,
fusibility, or behavior with acids. While specific gravity can usually
be used for distinguishing between gems, as, for example, between
quartz as compared with diamond, it cannot always be used for
identifying glass, since by the addition of different ingredients it is
possible to make glass of varying specific gravity, and similar to
that of the gem which it is sought to imitate.
The
specific gravity of a substance is its weight as compared with that of
an equal volume of water. When it is stated that the specific gravity
of topaz, for example, is 3.55, the figures simply mean that a given
volume of topaz is 3.55 times heavier than the same volume of water.
Various
means may be taken to determine the specific gravity of a body, the
most obvious and simple depending upon the fact that a body heavier
than water loses, when weighed in that liquid, a weight equal to that
of an equal volume of water. Hence by weighing a body first in air and
then in water, and dividing the weight in air by the difference
between the weight in air and the weight in water, or in other words,
by the loss of weight in water, the quotient will be the specific
gravity.
The following example of a determination of the specific gravity of a sapphire will illustrate this:
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