Page:The American Cyclopædia (1879) Volume XV.djvu/57

or nitride of silver (Ag₃N). It is terribly explosive, and is hence called fulminating silver (Berthollet's). This most dangerous compound may also be unintentionally produced by precipitating an ammoniacal solution of argentic nitrate by the addition of caustic potash. The chlorate of this oxide is likewise very explosive, as is also the fulminate proper (Brugnatelii's). (See Explosives.) The sulphate is formed by treatment of the metal at a high temperature with concentrated sulphuric acid. Upon this reaction is based one method of separating silver and gold. (See Gold.) The nitrate (AgNO₃) is the most important salt of silver. (See Nitrates, vol. xii., p. 463.) It is employed in the preparation of other compounds of silver, the most important of which is the chloride, produced by adding to the nitrate solution chlorine or a soluble chloride, such as common salt. It is a dense white flocculent precipitate, which under exposure to light turns first violet, then black, probably by partial reduction to subchloride. Chlorine restores the white color. The chloride is slightly soluble in boiling concentrated muriatic acid, more readily in strong solutions of chlorides, ammonia, alkaline cyanides, and hyposulphites; insoluble in water and dilute acids; scarcely affected by any oxygen acid, even concentrated sulphuric; reduced to metal by zinc, iron, copper, or any metal more oxidizable than silver, heated hydrogen, organic compounds containing hydrogen, alkalies and alkaline earths, and by heating upon charcoal before the blowpipe. The insolubility of the chloride in oxygen acids permits the precipitation of silver from solutions of almost all its salts by the addition of hydrochloric acid or of other chlorides, thus giving a convenient means of determining its presence or separating it from other metals. On the other hand, the solubility of the chloride in brine or sodium hyposulphite constitutes an important means of silver extraction by the humid method of metallurgy described below. This salt occurs in nature as an ore. It is used in photography, and its ammoniacal solution is employed to color mother-of-pearl. The bromide (AgBr) and the iodide (AgI) also occur in nature, the latter rarely. Their chemical relations are similar to those of the chloride, but the bromide is but slightly dissolved in dilute aqua ammoniæ, and the iodide scarcely at all. They likewise have the property of darkening by exposure to light. (See Photography.)—The Metallurgy of Silver. Silver is obtained partly from true silver ores, partly from other ores containing silver as an accidental or variable constituent. To the former class belongs the native metal, which is usually more or less alloyed with gold, and sometimes with other metals, as above remarked. The occurrence of gold and silver in variable natural alloy is so general that they may almost be said to constitute but one mineral species, ranging from silver with a slight trace of gold to gold with a slight trace of silver. Native silver is found in masses and in arborescent and filiform shapes in veins of quartz, calcite, &c., or as segregations accompanying other silver ores. The masses are sometimes crystalline, showing cubical and octahedral forms. Very pure silver occurs with the native copper at Lake Superior. The most famous masses of native silver, several of which exceeded 500 lbs., have been found at the mines of Kongsberg in Norway, of Freiberg, Schneeberg, and Johann-Georgenstadt in Saxony, and in the Bohemian, Hungarian, Peruvian, and Mexican mines. In the silver mines of Nevada, Idaho, and Utah it is not uncommon, though it has not been found in large masses. Silver amalgam occurs in small quantities in some European mines, and contains 26 to 35 per cent. of silver, the remainder being mercury. The variety known as arguerite, from Coquimbo in Chili, is an important ore in that region, and contains 43 to 63 per cent. of silver. The antimoniuret and the telluret of silver are comparatively rare. The most important silver ores are the chloride, the sulphide, and the combinations of sulphide of silver with other sulphides. The chloride of silver, or horn silver (AgCl), is a common ore in Chili, Peru, Mexico, and the western regions of the United States, particularly in certain districts of Nevada, and in the Owyhee district of Idaho. It has been met with in small quantities in many of the European mines. When pure, its composition is silver 75.2, chlorine 24.8. It has a waxy appearance, resinous lustre, and pearl-gray, greenish, whitish, or bluish color, turning brown in the air; hardness 1 to 1.5; sp. gr. 5.3 to 5.5. It occurs chiefly near the outcrops of argentiferous deposits as a product of the decomposition of other ores. In Chili and Peru, for instance, it is found in cubical crystals in the ferruginous gossan known as pecos and colorados. The bromide and iodide, which also occur in nature, closely resemble it, but are far more rare. The sulphide of silver (Ag₂S, silver glance, vitreous silver, or argentite), containing 87.1 silver and 12.9 sulphur, is, next to the native metal, the richest ore. It has a blackish lead-gray color, metallic lustre, and shining streak; H. 2 to 2.5 ; sp. gr. 7.196 to 7.365; is easily cut with a knife, and readily melts on charcoal before the blowpipe. It forms a considerable portion of the ores of the silver mines of Saxony, Bohemia, Hungary, Mexico, Peru, and the United States. It is commonly associated with other argentiferous minerals, and sometimes is finely disseminated through the gangue or the accompanying ores. The double sulphides of silver and antimony constitute a very valuable class of ores, of which the chief are: stephanite (Ag₅SbS₄), with 68.5 per cent. of silver and sometimes small quantities of iron, copper, and arsenic, having metallic lustre, iron-gray color, black powder, H. 2 to 2.5, sp. gr. 6 to 6.27, occurring in Saxony, Bohemia, Hungary, Mexico, and