he _weights_ would
differ, but like-sized pieces of one and the same material always have
the same weight. It is the custom among scientists to compare the
densities of substances with the density of water. The number which
expresses the relation between the density of any substance and the
density of water is called the _specific gravity_ number of the
substance. For example, if, size for size, a material, say diamond, is
3.51 times as heavy as water, its _specific gravity_ is 3.51. It will be
seen that since each substance always has, when pure, the same _specific
gravity_, we have here a means of distinguishing precious stones. It is
very seldom, if ever, the case that we find any two precious stones of
the same specific gravity. A few stones have nearly the same specific
gravities, and in such cases it is well to apply other tests also. _In
fact one should always make sure of a stone by seeing that two or three
different tests point to the same species._

We must next find out how to determine the specific gravity of a
precious stone. If the shape of a stone were such that the volume could
be readily calculated, then one could easily compare the weight with the
volume or with the weight of the same volume of water, and thus get the
specific gravity (for a specific gravity number really tells how much
heavier a piece of material is than the same volume of water).

Unfortunately the form of most precious stones is such that it would be
very difficult to calculate the volume from the measurements, and the
latter would be hard to make accurately with small stones. To avoid
these difficulties the following ingenious method has been devised:

If a stone is dropped into water it pushes aside, or _displaces_, a body
of water exactly equal in volume to itself. If the water thus displaced
were caught and weighed, and the weight of the stone then divided by the
weight of the water displaced, we would have the specific gravity number
of the stone.

This is precisely what is done in getting the specific gravity of small
stones. To make sure of getting an accurate result for the weight of
water displaced the following apparatus is used.

[Illustration: FIG. 5.

A, Flask-like Bottle; B, Indicates Ground Glass Stopper; C, Shows Hole
Drilled through Stopper.]

THE SPECIFIC GRAVITY BOTTLE. A small flask-like bottle (see Fig. 5) is
obtained. This has a tightly fitting _ground_ glass stopper (B). The
stopper has a small hole (C