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.