ent
strengths of the liquid, which may be preserved in four labeled bottles
side by side. By dropping the stone whose specific gravity it is
desired to test, into different bottles successively, its specific
gravity can be learned within narrow limits. It is recommended to place
in the first bottle a solution of methylen iodide saturated with iodine
and iodoform. Its specific gravity would be 3.6. The second bottle
should contain pure methylen iodide, marking a specific gravity of 3.3.
The third bottle may contain the same, diluted until its specific
gravity becomes 3.0, and the fourth the liquid reduced to a specific
gravity of 2.65, which is that of quartz. Topaz and diamond would then
float upon the first liquid, but sink in the second. Hyacinth, ruby,
and sapphire, would sink in all. Beryl, emerald, etc., would float upon
the first three, but sink in the last, and so on.
The
bottles containing the solutions should be kept tightly stoppered, as
any evaporation affects the density of the liquid. Methylen iodide is a
somewhat expensive chemical, costing, as it does, $1.25 an ounce; but
when a supply is once obtained it will last almost indefinitely.
Instead
of having solutions of different densities at hand, some prefer to
have ready for use fragments of minerals of different densities
arranged in series. These are called indicators, and are used by
placing one or more in the liquid with the stone whose specific gravity
is desired, and diluting until the indicator sinks or rises at the same
time with the unknown mineral. The unknown mineral must then be of the
same specific gravity as the indicator.
For
minerals with a specific gravity above 3.6, the only heavy liquid
available is silver thallium nitrate. This, as previously stated, fuses
to a clear liquid having a density of 4.5 to 5. The temperature
required for fusion is about 75° Centigrade (167° F.), and the work can
be conveniently done by heating the salt in a beaker upon a water
bath. The dilution can be performed by adding hot water. The necessity
of working always with hot liquids is of course a drawback to the use
of this substance, and it is also a costly chemical. In other respects
it answers well the purposes of a heavy liquid. It should be noted in
the use of all the heavy liquids that the addition of a very small
amount of water, or other diluting liquid, is sufficient to
considerably reduce the specific gravity. Hence, the addition of the
diluting liquid should be made very slowly and carefully, with frequent
stirring, and a constant watch on the position of the stone that is
being tested. For the purpose of determining exactly the specific
gravity of the liquid at any point, some form of balance is usually
employed, that known as Westphal's giving quick and accurate results.
It is, however,
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