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density
than itself. Now, a number of liquids are known which are considerably
heavier than water, but whose specific gravity may be lowered very
gradually by the addition of water or other liquid to them. If a stone
placed in one of these liquids remains floating it is lighter than the
liquid. By reducing the density of the liquid a point may be reached
where the specific gravity of the two is equal, and the stone will
remain suspended somewhere within the liquid. Ascertaining the
specific gravity of the liquid gives, therefore, that of the stone. Or
of two stones supposed to be identical, if one sinks in the liquid as
it is gradually diluted, long before the other, a considerable
difference of specific gravity is indicated, and the stones are
doubtless of different species. Here, as in making determinations of
specific gravity by weight, the relative size of the stones does not
need ordinarily to be taken into consideration.
The
heavy liquids principally employed for the above purposes are the
following: A solution of potassium mercuric iodide, known as Thoulet's
or Sonstadt's solution, having a specific gravity of 3.15; methylen
iodide, whose maximum specific gravity is 3.32; and silver thallium
nitrate, which on fusing yields a liquid with a maximum specific
gravity of 4.5-5. The first, or Thoulet's solution, is prepared by
treating five parts by weight of mercuric iodide and four parts by
weight of potassium iodide in a porcelain dish with a little water and
evaporating until a crust begins to form. The solution can then be
reduced to a desired density by adding distilled water, or can be
brought back to the maximum specific gravity by evaporating the water.
It can be kept indefinitely if placed in closely stopped bottles,
especially if a few drops of mercury be added. It is poisonous. In
using it, steel pincers or glass rods should be employed for immersing
the stones, as the insertion of brass instruments causes a
decomposition of the liquid, and a deposition of mercury upon the metal.
While
the Thoulet solution is the cheapest and easiest of the heavy liquids
to manipulate, its rather low density prevents its use for gems having
a specific gravity much over 3, and hence some of the other liquids are
often preferred. Methylen iodide is recommended by Bauer as best suited
for the general purposes of the student of gems. This has, as stated, a
maximum density of 3.32; but by saturating with iodine and iodoform a
density of 3.6 may be obtained. The usefulness of the latter mixture
is somewhat impaired, however, by its very dark color.
The
dilution of methylen iodide is performed by means of benzol rather than
by water. Bauer recommends having for use four differ-
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