Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/700

628   CADMIUM, a metal closely allied to zinc. It was discovered in 1817 by Stromeyer and Hermann, independ ently, but in a similar manner. The former chemist, in the execution of his duties as inspector of pharmaceutical products in Hanover, found a substance, sold as oxide, to be really carbonate of zinc, and, applying to the manufacturer for explanation of the reason of the substitution of the latter product for the former was informed that, although the best zinc, in which no iron could be detected, was employed, the oxide could not be produced without a slight discoloration from oxide of iron. On investigation by Stromeyev, it was found that the discoloration was due not to iron but to the oxide of a new metal, which he succeeded in isolating, and named cadmium, from the old chemical name for zinc oxide (C admia fossilis). About the same time, the sale of an oxide of zinc supplied by Hermann, a chemical manufacturer, who produced it from the waste of the Silesian zinc furnaces, was stopped in Prussia as being contaminated with arsenic, the reason obviously being that the acid solution of the substance in question gave a bright yellow precipitate when heated with sulphu retted hydrogen. The erroneous character of this inference was, however, soon demonstrated by Hermann, who made a careful investigation e of the subject, and discovered the nature of the new metal, but not before Stromeyer had published the results of his observations. Cadmium does not occur in the metallic state in nature, and there is only one definite mineral known which contains it in quantity, namely, the sulphide, or green- ockite, which occurs at Bishopstown, in Renfrewshire, in small isolated crystals of a bright orange-yellow colour, belonging to the hexagonal system, in a doleritic rock associated with prehnitc. This contains 77 7 per cent, of cadmium and 22 3 per cent, of sulphur, corresponding to the formula CdS, and is isomorphous with voltzite, the rhombohedral form of sulphide of zinc. Although an extremely rare mineral in the pure state, being confined to the single locality mentioned atove, sulphide of cadmium is often present in zinc blende, the richest varieties contain ing -3 per cent, of cadmium. Among these are the yellow radiated blende of Przibram in Bohemia, Eaton in New Hampshire, and Engis and Corfali in Belgium. It is also found in the carbonates and silicates of zinc from most of the localities producing these ores, but in what state of com bination is doubtful, as it is not generally found in quantity a Ancient to be appreciated by the analysis of samples, being only discoverable when the ore is treated for zinc on the large scale, in the first products of the reducing processes. Cadmium is a white metal with a slight bluish tinge by reflected light ; it is whiter than lead or zinc, but less so than silver, has a high lustre when polished, and breaks, under a gradually increasing strain, with the fibrous or scaly fracture characteristic of a soft tough metal. It may be readily crystallized in octohedra, differing in this respect from the allied metal zinc which is rhombohedral. It is somewhat harder than tin, but less so than zinc, and like the former metal it emits a peculiar crackling sound when bent. It is malleable, and may be rolled into thin sheets. The specific gravity after fusion is 8 604, which is increased by hammering to 8 - 694. The specific heat is 0-05669 (Regnault), or 0576 (Dulong and Petit). The electric conductivity is 22 10, or somewhat lower than that of zinc ; the thermal conductivity does not appear to have been determined. It melts at a temperature below redness (315 to 320 C.),and boils at the temperature of 860 C., giving off a vapour of an orange-yellow tint. The principal coloured lines with their relative intensity observed in the spectrum of cadmium vapour are, according to Huggins s notation, 502 4, 639 1, 656 s , 889 5 5 , 918 1 , 953 1 , 986 1 , 1473 10 , 1517 10, 1536 1 , 1747 1 , 1843 10 , 2315 s , 2562 6 , 3239 4. The most brilliant of these are chiefly in the green and blue field. Chemically cadmium belongs to the diatomic group of elements ; its symbol is Cd, and its equivalent 58. It unites readily with most of the heavy metals, forming alloys, which with gold, copper, and platinum, are brittle, while those Avith lead and tin are malleable and ductile. The alloy of silver and i cadmium is very tenacious ; but that, in the reverse proportion, of ^ silver and cadmium is brittle. An alloy of two parts of cadmium, two of lead, and four of tin, known as Wood s fusible metal, melts at a somewhat lower point than the similar alloy where bismuth takes the place of cadmium, or Darcet s fusible metal (see ). It forms several amalgams, among which those containing equal parts of mercury and cadmium and two of mercury to one of cadmium are remarkable for their cohesive power and malleability ; whereas that containing 22 per cent, of cadmium is hard and brittle. The amalgams of the former class have been proposed at different times for use in stop ping teeth, but are not now so employed. When exposed to damp air cadmium becomes rapidly covered with a dull film of suboxide, but as with zinc the oxidation is only super ficial, the crust formed protecting the metal below from further change. When heated to a redness in air it burns, forming a yellowish brown oxide. It also, when in a state of vapour, decomposes water at a red heat, with the forma tion of oxide of [cadmium, hydrogen being evolved. It is soluble with evolution of hydrogen, in sulphuric, hydro chloric, nitric, and even acetic acid, forming colourless salts. When treated with an aqueous solution of sulphurous acid, it dissolves without evolution of hydrogen, sulphite and sulphide of cadmium being found in the liquid. Oxide of cadmium, CdO, is a yellowish brown powder of the specific gravity 6 95, varying in depth of tint accord ing to the temperature at which it is prepared. It may be produced lay burning the metal in air or by calcining the nitrate or carbonate. It is readily reducible by hydrogen or carbon, at a high temperature, but below that necessary for the reduction of zinc oxide. If a mixture of the oxides of the two metals be heated in a current of hydrogen in a glass tube, the oxide of cadmium is reduced, volatilizes, and condenses in the cooler part of the tube, while the oxide of zinc remains unchanged. Oxide of cadmium is a strong base, forming salts similar in constitution to those formed by oxide of zinc, and those of the earthy and alkaline metals. The most important of these is the sulphate, 