PRECIOUS STONES. 39
liquid
of specific gravity of 4'8 ; a very small quantity of water
considerably lowers the density of the resulting liquid ; in using this
method care must be taken to allow for temperature—in other words, the
density of the solution must be determined at the same temperature as
the solution was at when the stone was being tested in it.
(c) Method with the Hydrostatic Balance.
In
this method any delicate balance may be used, but there are many
special forms made for the purpose which are very convenient in use.
The method is more adapted to the examination of larger specimens. If
the scale pans come close down to the platform of the balance one pan
must be removed and a long fine hair or strand of silk attached in its
place; the remaining pan is now exactly counterpoised by attaching a
suitable weight at the end of the beam where the detached pan was. A
small beaker partly full of water is arranged so that a gem attached to
the end of the hair will be under the surface of the water when the
instrument is in a balanced position ; this beaker can then be removed
from its support and the gem attached to the end of the silk or hair by
slinging it in a small neat loop. The exact weight of the gem in air is
then ascertained ; the beaker is now replaced, allowing the stone to
be completely immersed in water but having as little of the hair as
possible in water. The weight in water is now determined. The
difference between the weight in air and the weight in water is the
weight of a volume of water equal to the volume of the stone, therefore
the weight of the stone in air divided by this difference is the
specific gravity of the stone.
