dust,
evaporation, etc., in suitable stoppered and capped bottles or short
wide tubes in the case of A and B. A number of weighted glass bulbs or
a series of small mineral fragments of ascertained specific gravity are
very useful as " indicators." It is a good plan to keep one or more of
these indicators in each liquid to be employed. To avoid doubt and
confusion these indicators, whether bulbs or mineral fragments, should
present so characteristic a form or colour or marking that their
identity and value can be recognized at once. It is worth while adding
the remark that liquids A, B, and C, are required much less frequently
than the less dense liquids, and that when the position of a doubtful
stone has been once fixed by the density test so as to prove that it
belongs to a particular group, then it may be necessary to call in the
aid of the dichroiScope and of the scale of hardness in order to learn
to what species in that group the stone really belongs.
2.
By weighing a stone in air and then in some liquid of known density,
the weight of the bulk of the latter displaced by the stone is
ascertained. If, for example, a sapphire weighing 4 grains in air
weighs but 3 grains in water, it has evidently displaced 1 grain of
water, becoming lighter by that amount. So the number 4 represents the
specific gravity of sapphire, showing, as it does, the number of times
that the weight of any bulk of that stone contains the weight of an
equal bulk of water. An example of an actual experiment of this kind
will serve to illustrate this, the ordinary method of taking specific
gravities, better than any further explanation of the principle
involved.
