SILVER, one of the best known metals, has been in use for a long period, probably since before the beginning of history. It is widely distributed and can be easily extracted from some of its ores.
Physical Silver is very malle able and ductile, being surpassed only by gold in these qualities. It is harder than gold, but softer than copper. It can be rolled into sheets 1/1000 inch thick, and silver foil is made thin enough to transmit light. Silver leads all the metals as a conductor of electricity and is second only to gold as a conductor of heat. Its specific gravity is about 10.46; its melting point C., and it can be distilled as a greenish vapor in the electric arc, its boiling point being over C. When molten it can absorb oxygen to 22 times its own volume; this is given off when the metal cools, causing the phe nomenon known as This occurs only when the silver is pure and a very small per .cent of copper, zinc or bismuth prevents it, as does a layer of some substance having no oxy dizing action of the molten metal, like powder charcoal or common salt.
Chemical Silver alloys read ily with mercury, lead, zinc, gold and copper. The alloy with mercury, known as silver amal gam, is formed in various processes for extract ing silver from ores, mercury being able to extract metallic silver from its compounds with chlorine, bromine and iodine. If such an amal gam be heated above the boiling point of mer cury, the mercury volatilizes, leaving the silver. Lead has the power of extracting silver from its compounds with copper, sulphur, arsenic, antimony and silver salts by the formation of lead-silver alloys, a property of great metallur gical importance in the treatment of silver ores. Alloys of silver and copper are formed in smelting argentiferous copper ores, the silver and copper being subsequently separated by refining. As pure silver is soft and abrades easily, all silver used in coinage or for making silverware is alloyed with copper to give hard ness. The United States silver coinage con tains 900 parts silver and 100 parts copper; British coinage contains 925 parts silver and 75 parts copper.
Silver is not affected by either dry or moist air at ordinary temperatures, neither is it af fected by caustic alkalies or vegetable acids. It is dissolved by hot, concentrated sulphuric acid and easily dissolved by nitric acid. Sul phuretted hydrogen forms black silver sulphide and to this is due the familiar tarnishing or blackening of silverware, either by articles of food containing sulphur, as eggs, or by the minute percentage of sulphuretted hydrogen contained in city air or the air of rooms when coal gas is used as an illuminant. So-called oxydized silver is silver with a superficial black coating obtained by immersing in potassium sulphide or platinum chloride. Frosted silver has a rough dead surface produced by heating with nitre.
Silver forms three oxides, but none is of any particular importance. Silver chloride (AgCI), readily formed as a curdy white pre cipitate on adding a soluble chloride to an acid solution of a silver salt, is insoluble in acids but readily soluble in ammonia, in potassium cya nide and in sodium hyposulphite and less soluble in a saturated solution of any one of several chlorides. It is also formed in treating silver
ores with common salt. Silver nitrate (AgNO3), formed by dissolving silver in nitric acid, is a white crystalline salt readily soluble in water. Fused and cast into sticks, it is known as lunar caustic and is used in surgery. In solution it is used as a chemical reagent. Silver bromide (AgBr) and silver iodide (AgI) resemble the chloride and like it are used in photography. Silver sulphate (Ag,So4), a slightly yellow crystalline salt, is readily soluble in hot water but less in cold. Silver sulphide, black, is formed by adding a soluble sulphide to a silver salt.
Silver Ores.— The minerals which, singly or in combination, are mined as silver ores may be divided into three classes: Silver minerals proper, lead silver minerals and lead copper minerals. In the first class come native sil ver; cerargyrite, argentite, pyrargyite, prousite. Native silver occurs in irregular bunches or thread-like or blanching masses, either by itself or associated with other silver ores or with native copper. It adds to the value of an ore body, but rarely occurs in sufficient quantity to be worked for itself. The once famous Silver Islet mine on the north shore of Lake Superior produced splendid specimens of native silver. Cerargyrite, horn silver, is the chloride and contains 75.3 per cent of silver when pure. It occurs in horn-like masses, of a grayish color which turns black on exposure to light, is so soft that it can be easily cut with a knife and is of common occurrence in what is known as the oxydized zone of mines worked for silver. Rich masses have been taken from the mines at Leadville, Colo., and there are many other mines producing this ore in the West. Larger quantities have come from some Mexican mines, particularly those in Guanajuato. Embolite, a chlorobromide, Ag(CIBr), bromyrite, the bro mide, and iodyrite, the iodide, resemble horn silver closely and are found associated with it. Argentite, or silver-glance, silver sulphide (Ag,S) is the commonest of the silver minerals proper. It is black, has a metallic lustre and cuts easily, though harder than horn-silver. When pure it contains 87.1 per cent of silver. Silver sulphide combines readily with the sul phides of antimony or arsenic, forming double salts. Of these pyrargyrite (3Ag2S.Sb2S0 or ruby silver ore contains 60 per cent of silver, proustite (3Ag1S.As2S.), also called ruby sil ver ore, contains 65.5 per cent ; stcphanite (5AgiS,Sb.S.) 68.5 per cent and polybasite (Ag,S.Sb,S,) nominally contains 75.6 per cent silver, but usually part of the silver is replaced by copper and part of the antimony by arsenic. These double sulphides at American silver mines seldom occur in bodies of sufficient size and purity to be worked alone, but are not un common. One or more are found at mines in Montana, Idaho, Utah, Colorado and Arizona. Large masses of pyrargyrite have been taken from mines in the state of Guanajuato, Mex ico. Mention may be made of the mines in the Ruby district of Gunnison County, Colo., and at Silver City, Idaho. The silver ores usually occur in veins or in irregular replace ment deposits formed by hot magmatic waters in the vicinity of igneous intrusions.