Page:The American Cyclopædia (1879) Volume XV.djvu/61

Rh into chloride or sulphate, the former by a chloridizing, the latter by an oxidizing roasting. The chloridizing roasting is essentially that of the Freiberg amalgamation process, and is effected by mixing salt with the charge. The silver chloride is extracted from the mass by lixiviation with hot brine (old Augustin process), cold brine (Hungarian improvement), sodium hyposulphite (Patera process), or calcium hyposulphite (Kiss process in Hungary and Russia, Hofmann in Mexico). The latter extracts also gold chloride if it is present, which brine will not do, unless it has been, as Patera recommends, impregnated with free chlorine gas. Experiments conducted at Wydotte, Mich., by Messrs. Courtis and Hahn, indicate the availability of other chlorides than common salt (particularly calcium chloride, or a solution obtained by treating common limestone with muriatic acid) as a solvent for the silver chloride. The novel and important results of these investigations are given in the “Transactions of the American Institute of Mining Engineers.” From its hyposulphite or chloride solution the silver is precipitated with metallic copper, as cement silver, which is washed, pressed, melted, and cast into bars. Ziervogel's method of extracting silver by roasting the sulphuretted ore to produce silver sulphate, leaching this with hot acidulated water, and precipitating with copper, is the simplest and cheapest of all; but it requires very skilful and delicate roasting, and ores comparatively free from lead, antimony, arsenic, and zinc. The three latter tend to cause volatilization of silver; the sulphide of antimony and lead cause a sintering of the roasting charge; copper dioxide, or too high a temperature in the furnace, leads to the formation of metallic silver, instead of the desired sulphate. Hence the application of this process is limited. Its best field is the treatment of the copper mattes of Mansfeld, containing 70 to 72 per cent. of copper, and 0.33 per cent. of silver. The so-called acid extraction is principally used upon cupriferous furnace products, which contain too much lead, antimony, arsenic, &c., to permit treatment by the Augustin or the Ziervogel method. In this process, the base metals are dissolved out by treatment with sulphuric or muriatic acid, and the residuum, containing gold and silver, is further reduced by smelting, or in rare instances by humid methods. For full discussions of all the foregoing processes, see Percy's “Metallurgy,” and Bruno Kerl's Metallhüttenkunde. The details of American practice, and critical comparisons of different American and foreign methods, are given in the reports of R. W. Raymond, United States commissioner of mining statistics, and in the “Transactions of the American Institute of Mining Engineers.”—The principal uses of silver have been mentioned already in this article; see also , (section on electrotyping), , and. The real value of silver as compared to gold has varied in different ages from one eighth to less than one sixteenth; but the mint rates have often been arbitrarily established by government for the profit of the treasury, in spite of the market price of the metals. At present it is lower than at any previous period. The average ratio of value of silver to gold in the London market for the year ending Dec. 31, 1874, was 1 to 16.27. The following table shows the estimated product of silver at various periods in the present century: