46 PRECIOUS STONES
pure
methylene iodide, which has' a specific gravity of 3.2981, and into
this drop a few stones selected indiscriminately, the effect will be
curious : first, some will sink plump to the bottom like lead : second,
some will fall so far quickly, then remain for a considerable time
fairly stationary ; third, some will sink very slowly ; fourth, some
will be partially immersed, that is, a portion of their substance being
above the surface of the liquid and a portion covered by it; fifth,
some will float on the surface without any apparent immersion. In the
first case, the stones will be much heavier than 8.2981 ; in the
second, the stones will be about 3.50; in the third and fourth
instances, the stones will be about the same specific gravity as the
liquid, whilst in the fifth, they will be much lighter, and thus a
rough but tolerably accurate isolation may be made.
On
certain stones being extracted and placed in other liquids of lighter
or denser specific gravity, as the case may be, their proper
classification may easily be arrived at, and if the results are checked
by actual weight, in a specific gravity balance, they will be found to
be fairly accurate. The solution commonly used for the heaviest stones
is a mixture of nitrate of thallium and nitrate of silver. This double
nitrate has a specific gravity of 4-7963, therefore such a stone as
zircon, which is the heaviest known, will float in it. For use, the
mixture should be slightly warmed till it runs thin and clear ; this is
necessary, because at 60° (taking this as ordinary atmospheric
temperature) it is a stiff mass. A lighter liquid is a mixture of
iodide of mercury in iodide of potassium, but this is such an extremely
corrosive and
