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 15° 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 70° C. has been reached. It is possible thus to obtain a liquid which at 75° 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:
A. Thallium and silver nitrate solution of specific gravity 4·5 maintained at a temperature well above its fusing point.
B. Thallium and silver nitrate solution of specific gravity 4·1 maintained at a temperature well above its fusing point.
C. Thallium and silver nitrate solution of specific gravity 3·9 at 15° C.
D. Thallium and silver nitrate solution of specific gravity 3·5 at 15° C.
E. Cadmium boro-tungstate solution of specific gravity 3·28 at 15° C., or methylene iodide slightly diluted with toluene.
F. Cadmium boro-tungstate solution of specific gravity 2·67: this is prepared by diluting E. with water until a fragment of beryl sinks and a fragment of amethyst floats therein. Or methylene iodide, diluted with toluol to the same density and with the same indicators, may be substituted.
