It is possible to employ a similar method for still denser stones by having recourse to Retgers’s salt, silver-thallium nitrate. This double salt is solid at ordinary room-temperatures, but has the remarkable property of melting at a temperature, 75° C., which is well below the point of fusion of either of its constituents, to a clear, mobile yellow liquid, which is miscible in any proportion with water, and has, when pure, a density of 4·6. The salt may be purchased, or it may be prepared by mixing 100 grams of thallium nitrate and 64 grams of silver nitrate, or similar proportions, in a little water, and heating the whole over a water-bath, keeping it constantly stirred with a glass rod until it is liquefied. The two salts must be mixed in the correct proportions, because otherwise the mixture might form other double salts, which do not melt at so low a temperature. A glance at the table of specific gravities shows that Retgers’s salt may be used for all the gem-stones with the single exception of zircon (b). There are, however, some objections to its use. It is expensive, and, unless kept constantly melted, it is not immediately available. It darkens on exposure to strong sunlight like all silver salts, stains the skin a peculiar shade of purple which is with difficulty removed, and in fact only by abrasion of the skin, and, like all thallium compounds, is highly poisonous.
It is convenient to have three tubes, fitted as before with stoppers or corks, to contain the following liquids, when heated:—
(e) Silver-thallium nitrate, reduced to 3·5; using as indicators, peridot or idocrase 3·40 and topaz 3·53.
(f) Silver-thallium nitrate, reduced to 4·0; indicators, topaz 3·53 and sapphire 4·03.
(g) Silver-thallium nitrate, undiluted, 4·6.
The tubes must be heated in some form of water-bath; an ordinary glass beaker serves the purpose satisfactorily. The pure salt should never be diluted; but the density of the contents of tubes (e) and (f) may be varied at will, water being added in order to lower the density, and concentration by means of evaporation or addition of the nitrate being employed in order to increase it. To avoid the discoloration of the skin, rubber finger-stalls may be used, and the stones should not be handled until after they have been washed in warm water. The staining may be minimized if the hands be well washed in hot water before being exposed to sunlight. It is advisable to warm the stone to be tested in a tube containing water beforehand lest the sudden heating develop cracks. A piece of platinum, or, failing that, copper wire is of service for removing stones from the tubes; a glass rod, spoon-shaped at one end, does equally well. It must be noted that although Retgers’s salt is absolutely harmless to the ordinary gem-stones—with the exception of opal and turquoise, which, as has already been stated, being to some extent porous, should not be immersed in liquids—it attacks certain substances, for instance, sulphides and cannot be applied indiscriminately to minerals.
The procedure described above is intended only as a suggestion; the method may be varied to any extent at will, depending upon the particular requirements. If such tests are made only occasionally, a smaller number of tubes may be used. Thus one tube may be substituted for the two marked a and b, the liquid contained in it being diluted as required, and a series of indicators may be kept apart in small glass tubes. On the other hand, any one having constantly to test stones might increase the number of tubes with advantage, and might find it useful to have at hand fragments of all the principal species in order to make direct comparison.
(2) Direct Weighing
The balance which is necessary in both the methods described under this head should be capable of giving results accurate to milligrams, i.e. the thousandth part of a gram, and consistent with that restriction the beam may be as short as possible so as to give rapid swings and thus shorten the time taken in the observations. A good assay balance answers the purpose admirably. Of course, it is never necessary to wait till the balance has come to rest. The mean of the extreme readings of the pointer attached to the beam will give the position in which it would ultimately come to rest. Thus, if the pointer just touches the eighth division on the right-hand side and the second on the other, the mean position is the third division on the right-hand side (½(8 − 2) = 3). Instead of the ordinary form of chemical balance, Westphal’s form or Joly’s spring-balance may be employed. Weighings are made more quickly, but are not so accurate.
In refined physical work the practice known as double-weighing is employed to obviate any slight error there may be in the suspension of the balance. A counterpoise which is heavier than anything to be weighed is placed in one pan, and weighed. The counterpoise is retained in its pan throughout the whole course of the weighings. Any substance whose weight is to be found is placed in the other pan, and weights added till the balance swings truly again. The difference between the two sets of weights evidently gives the weight of the substance. Balances, however, are so accurately constructed that for testing purposes such refined precautions are not really necessary.