Silver

SILVER, a metal known from very early times and, on ac count of its comparative scarcity, brilliant white colour, and resistance to atmospheric oxidation, has long been used for articles of value—coins, ornaments and jewellery. Silver (symbol Ag, atomic number 47, atomic weight 507.88) was called Luna or Diana by the alchemists, who assigned to it the symbol of the crescent moon; the term lunar caustic is still in general use for silver nitrate.

Silver is very widely diffused throughout nature as is shown by spectrum analysis; it is always present in sea water in minute amounts, an estimate of the total amount present being 2,000,000 tons. Silver is sometimes found native—usually in only small quantities, though occasionally masses of several hundredweights have been discovered.

Gold is almost invariably found associated with silver. The principal ores of silver are the sulphides, and to a lesser extent it is found combined with tellurium, selenium, arsenic and antimony. Galena, PbS, always contains silver and most of the metal pro duced in Europe comes from this source.

Physical Properties.

Silver in the massive state is the whitest of all metals, has a perfect metallic lustre, and is the most malle able and ductile of all metals except gold : one gram of the pure metal can be drawn out into wire considerably more than one mile long, and it can be beaten into leaves of a thickness of 0.00025 mm. Hammering or rolling hardens the metal consider ably, but the malleability is restored by annealing at a dull red heat. Pure silver is somewhat harder than gold but softer than copper; it is too soft in the pure state for use as coinage or jewellery, and for these purposes it is best hardened by alloying it with a small percentage of copper which neither affects its colour appreciably nor renders it brittle. The specific gravity of silver varies considerably according to the previous method of treat ment, the density of different samples varying from 9.87 to 10.55, whereas the density of silver precipitated from solution by fer rous sulphate is 10.62. A mean value of 10.5 may be taken as fairly correct. The specific heat of silver is about 0.56 and its coefficient of linear expansion between o° and ioo° C is about 0.0000194, but this increases rapidly at higher temperatures.

Silver is by far the best conductor of heat and electricity. Ac cording to the law of Wiedemann and Franz, the ratio of heat conductivity to electrical-conductivity of good conducting metals is constant at the same temperature. Taking the thermal conduc tivity of silver as standard (I00), the thermal conductivities of other metals are as follows: The melting point of silver is 960.5° C, and its boiling point under atmospheric pressure is about 2,000° C. It gives rise to a blue vapour which on dilution in the atmosphere appears bright green. Its vapour density has been determined at a temperature above its boiling point, and the molecule thereby found to be mon atomic. In the molten state silver has the property of "occluding" about 20 times its volume of oxygen. This oxygen is not in a state of chemical combination and is violently ejected on cooling to near the solidifying point, producing a phenomenon known as the "spitting" of silver. The absorption of oxygen from the air can be prevented by covering the surface either with powdered charcoal or with some non-oxidising flux.

Chemical Properties.

Silver does not combine directly with oxygen, nor does it decompose water or steam at any temperature; it is, however, oxidized by ozone which becomes converted into ordinary oxygen without alteration in volume. The ordinary blackening or discoloration of silver is invariably caused by sul phur, either in the free state or in compounds other than oxides. This stain is black silver sulphide, Silver dissolves readily in nitric acid, either strong or dilute, producing the nitrate, with evolution of nitric oxide ; it is also readily soluble in hot strong sulphuric acid, silver sulphate, being produced with evolution of sulphur dioxide. Hydro chloric acid acts superficially on silver, but further action is stopped by the coating of silver chloride produced. Hydriodic acid dissolves silver readily, producing the iodide but hydrofluoric acid is without action on it. Silver combines readily with the free halogens.