CADMIUM, a metallic element resembling zinc in its chemical properties, and discovered by Stromeycr in 1817, in a specimen of zinc carbonate. Cadmium often occurs in ores of zinc to a small extent, blende sometimes con taining as much as 3 per cent of cadmium sul phide. The commercial supply of the element is obtained as a by-product in the smelting of zinc, chiefly in Belgium and Silesia. Cadmium sulphide also occurs native as the mineral greenockite (q.v.), otherwise known as "cad mium blende.x. In the distillation of zinc ores the cadmium, being more volatile, passes over first; and advantage is taken of this fact for the isolation of the metal in the arts. In Silesia, where the zinc ores often contain considerable quantities of cadmium, the first portion of the distillate is likely to contain as much as from 3 to 10 per cent of cadmium. This is mixed with coal or charcoal and redistilled at a low, red heat. Cadmium, mixed with a little zinc, passes over; and by one more distillation the metal is obtained in a fairly pure form. To eliminate the last traces of zinc, the crude metal is dissolved in hydrochloric acid, then diluted and precipitated as a sulphide by a current of sulphuretted hydrogen. The sulphide is then dissolved in concentrated hydrochloric acid, and the subsequent addition of carbonate of soda precipitates the carbonate of cadmium, which is reduced to the oxide upon ignition. The pure oxide thus obtained may then be reduced to the metallic form by distillation with charcoal.
Metallic cadmium is lustrous and resembles tin in appearance, though it has a bluish tinge. It is stronger than tin, but, like that metal, it emits a peculiar crackling sound, or "cry," when bent. At ordinary temperatures it is quite ductile and malleable, and may be drawn into thin wire, rolled into thin sheets, or hammered into foih At about 175° F. it becomes brittle,
however, so that it can be pulverized in a mortar. Cadmium has the chemical symbol Cd. Its specific gravity is about 8.65. It melts at 600° F., and boils at about 1,500° F., yielding a yellow vapor. Its atomic weight is 112.4 if 0 = 16, or 111.6 if H = 1. Its specific heat is about 0.055, and its linear coefficient of ex pansion is about 0.0000185 per Fahrenheit de gree. Metallic cadmium is used to a limited extent in the preparation of alloys, its general effect being to reduce the melting-point of the alloy to which it is added. The total produc tion of the metal per annum is probably about to tons.
In its chemical relations, cadmium, like zinc, is a dyad. Metallic cadmium undergoes a slow, superficial oxidation upon exposure to the air and when sufficiently heated in the presence of air it oxidizes rapidly and may even take fire. The resulting oxide, CdO, is brown in color and readily dissolves in acids, with the produc tion of the corresponding cadmium salts. One of the best known of these salts is the iodide, CdIs which is used in photography and in medicine, and may be obtained by the action of hydriodic acid, HI, upon cadmium carbonate, or metallic cadmium. The bright yellow sul phide, CdS, is formed when the stream of sul phuretted hydrogen gas is passed through a slightly acid solution of a cadmium salt; and this fact is used in the detection and isolation of cadmium in qualitative analysis. The sul phide is used as a pigment, under the name of °cadmitun yellow"; it is brilliant in color, and does not change upon exposure to air or light.