Page:The American Cyclopædia (1879) Volume XV.djvu/56

light and heat when highly polished; but its radiating capacity in the same condition is very small. By virtue of these properties vessels of silver are best adapted to retain the heat of liquids. It melts at a full red heat, about 1000° C. (1832° F.). It shrinks in cooling, and hence fills but imperfectly the moulds in which it is cast. At a very high temperature it is volatile. Melting silver mechanically absorbs 20 volumes of oxygen, which in solidifying it expels, sometimes with sufficient force to throw off particles of metal. Alloyed with 1 or 2 per cent. of copper or with gold, it apparently loses this property. Silver is oxidized neither by exposure at ordinary temperature to dry or moist air, nor by heating in air; but it burns to an oxide when melted upon charcoal in the oxyhydrogen flame, or when exposed to a galvanic current of great intensity, or to ozone. Chlorine, bromine, and iodine act upon it at ordinary temperatures. It has strong affinity for sulphur (with which it can be easily fused to a sulphide), and is hence readily tarnished by sulphuretted hydrogen, which is present in small quantities in the ordinary air of cities. To protect silver vessels not in use, they may be wrapped in paper saturated with wax, which keeps out the impure air, or in paper painted with white lead, which decomposes sulphuretted hydrogen. Articles of food, with the exception of eggs and salt, scarcely affect silver, and it is therefore a favorite material for table ware. The discoloration from eggs is due to sulphur; that from salt, to chlorine, which forms argentic chloride. This may be removed by rubbing with a linen rag moistened with aqua ammoniæ. The caustic alkalies in solution or fusion do not attack silver as they do platinum, and it is consequently employed for the evaporation of such solutions, and for crucibles in which minerals are fused with potassium or sodium hydrate. Silver foil is sometimes used in blowpipe analyses, for detecting sulphur and the sulphides of the metals. Melted with carbonaceous matter, silver forms a carburet, white like the metal. This is also formed when compounds of silver oxide are decomposed by organic acids.—Silver may be easily alloyed by melting with most metals. The alloys with base metals are in general not useful enough to counterbalance the cost of the silver. The alloy with copper, which in subordinate quantity enhances the valuable qualities of the silver, is an exception. The alloys with lead and zinc, serving an important purpose in metallurgy, will be mentioned further on. An alloy of 100 parts of aluminum with 5 of silver gives a handsome white malleable compound, susceptible of high polish. A small quantity of iron, chromium, cobalt, or nickel imparts great hardness to silver. Steel may be made to retain about 1/500 of its weight of silver, which is said to improve its quality; the alloy is called silver-steel. Combined with mercury, silver forms a most brilliant amalgam for mirrors. An alloy of 20 to 30 parts of silver with 30 of nickel and 50 of copper is said to be equal in all respects to the ordinary standard silver, which is 9 parts of silver with 1 of copper. Small coins have been made in Switzerland of an alloy of silver and copper with 10 per cent. nickel. Two parts zinc and one part silver give a ductile, white, fine-grained alloy. Three parts of silver to one of tin give a hard, and one part of silver to two of tin a soft alloy. Bismuth, antimony, and arsenic yield brittle alloys. The alloys of silver and copper are the most important of all, being used both in coinage and in the arts. The copper alloy is harder than pure silver, takes a finer polish, and wears better; and the white color of silver may be retained if the contents of copper do not exceed a certain proportion, while even those alloys containing a larger proportion of copper may be so treated by “pickling” in acid as to deprive them of copper on the surface, and thus restore their silver-white color. The standard silver for coinage, on the continent of Europe and in the United States, is a compound of 9 parts of silver to 1 of copper; in England, of 37 silver to 3 copper. For plate the legal fineness varies in different countries, or is, as in the United States, left to the choice of the manufacturer. In North Germany the usual fineness is inferior to that of coin.—Silver does not dissolve in any hydrated acids by taking the place of the hydro- gen; on the contrary, hydrogen displaces it from the solutions of its salts and precipitates it in metallic form. Concentrated sulphuric acid oxidizes silver at boiling heat, forming argentic sulphate and sulphurous acid. Nitric acid, even when diluted with an equal bulk of water, acts rapidly upon silver, and at high temperature with great violence, argentic nitrate and nitric oxide being formed. A solution of chromic acid changes silver to a red argentic chromate. Muriatic acid, even at a high temperature, has little effect upon silver. Argentic oxide combines at high temperatures with silicic acid; hence, silver heated or melted with glass or other silicious compounds becomes oxidized and colors the mass yellow. All of the more easily oxidizable metals and many compounds susceptible of higher oxidation (so-called deoxidizing substances), as well as many organic substances, precipitate silver from solution. Silver forms three oxides: a suboxide, Ag₄O; argentic oxide, Ag₂O; and a peroxide (probably Ag₂O₂), which does not combine with acids. The second of these is of special interest as the basis of the salts of the metal. It is separated from the nitrate, or any soluble silver salt, by adding an alkaline solution, as a brown hydrated oxide, which parts with its water at 60° C. (140° F.), and with its oxygen at a red heat. Its solution in ammonia deposits on exposure to the air a black micaceous powder supposed to be a compound of silver oxide and ammonia (Ag₂O, H₃N), or amidide of silver (AgH₂N),