seen
that we have now at our disposal liquids which present a range of
density sufficiently wide to permit of the identification of minerals
having densities up to about 3.4 : and this result can be achieved
without having recourse to those liquids which need to be warmed above
150 C. in order to maintain them in a liquid condition. It
ought to be mentioned that the methylene iodide preparations, owing to
their volatility and to their high coefficient of expansion when
heated, yield results, which, in the absence of the necessary
precautions, may easily be somewhat inexact.
In order to furnish a liquid which will enable one to deal with stones having a density above 3.4,
the double nitrate of thallium and silver may be taken. It is better to
purchase this salt ready prepared, but it may be made by melting
together 150 grams of crystals of commercial thallium nitrate and 64
grams of silver nitrate along with a little water and heating the
mixture with constant stirring until the temperature of 700 C. has been reached. It is possible thus to obtain a liquid which at 750
C has a density of 4.8, but in practice this figure need not be
reached. It must be remembered that all the dilutions of this liquid
have one common property—they are poisonous ; moreover they stain the
skin a dark slaty purple not easy to remove.
For
the purpose of the collector and connoisseur in precious stones it will
suffice to have at hand the follows six heavy liquids :
