38 PRECIOUS STONES.
density,
the numerical equivalent of which is marked on the bead. Thus, when a
mineral is placed in, say, No. 1 bottle, and is found to sink, we know
it is of a density greater than 3.6. A mineral of a sp. gr. of 2'8
would float in solution No. 3, but would sink in solution No. 4. Thus
we may divide all the specimens tested into five groups, and thus
greatly aid their identification. If it is desired to accurately
determine the specific gravity of a specimen we may select a solution
in which it floats, and then dilute with benzine till it neither floats
nor sinks. It is then of the same density as the solution, and this is
determined by the specific gravity bottle described in method (a), then
weighing the same bottle full of water at 4° C, again emptying and
drying the bottle, and finally weighing it full of the solution in
question. Then (weight of bottle with solution less weight of bottle)
divided by (weight of bottle with water less weight of bottle), is
equal to the specific gravity of the solution, and therefore of the gem
to be determined. In removing a gem from one bottle of solution to
another care must be taken to dry it, or else the solutions will become
mixed and altered in density. The bottles containing the four solutions
must be kept in the dark also, to prevent decomposition with separation
of iodine, which would not only cause a dark colour to appear, but
would also alter the density. Such solutions may also be used to
separate particles of various minerals for analysis.
Still more recently thallium-silver nitrate, 
a solid salt, has been used ; with the addition of a little water it
remains fluid at as low a temperature as 50° C. The pure salt melts at
75° C, and forms a transparent
