tone dropped into them, by the position they assume.
Thus, if we take a solution of 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 3.2981; 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 deg. (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 dangerous mixture, that the more common solution is one
in which methylene iodide is saturated with a mixture of iodoform until
it shows a specific gravity of 3.601; and by using the methylene iodide
alone, in its pure state, it having a specific gravity of 3.2981, the
stones to that weight can be isolated, and by diluting this with
benzole, its weight can be brought down to that of the benzole itself,
as in the case of Sonstadt's solution. This solution, in full standard
strength, has a specific gravity of 3.1789, but may be weakened by the
addition of distilled water in varying proportions till the weight
becomes almost that of water.

Knowing the spec