IRIDIUM, a metallic element resembling platinum in its general properties, and occur ring in nature in the metallic form, alloyed, usually, with platinum and osmium. The com mercial supply comes chiefly from the Ural district, from Brazil, and from Borneo. Small amounts have also been obtained from Cali fornia. The existence of iridium was first recognized, in 1802, by Tennant, who observed that an insoluble residue remained behind after dissolving platinum ore in aqua regia. In 1804 he showed that this residue contains two new metals, to which he gave the respective names °iridium° and °osmium.° The separation of iridium from the other metals of the platinum group is a matter of some difficulty; and for details of the processes employed the more ex tended treaties on metallurgy and the chemis try of the metals must be consulted. Iridium is frequently obtained in the form of a spongy gray mass, or a gray powder; but by heating either of these to whiteness, and subjecting them to compression, the metal may be brought into the form of a compact, lustrous mass re sembling steel. In this form it is harder than iron, and somewhat malleable when hot, though brittle when cold. At ordinary temperatures it has a specific gravity of about 22.4, being among the heaviest substances known. Its
specific heat is about 0.0323 at ordinary tern-. peratures, and its coefficient of linear expansion (Fahrenheit) is about 0.0000039. Its melting point is very high, the estimates given by various authorities ranging from F. to 4.500° F. Compact iridium, after being strongly heated: is insoluble in all acids, and is not affected by air or moisture. The metal has the symbol Ir. and an atomic weight of 193.1 if 0-16, It forms numerous salts, but these are of no practical importance. The name °iridium° (from Greek, urainbow,°) refers to the varied colors exhibited by the salts of this element, as they pass from one state of oxida tion to another. Metallic iridium, either alone or alloyed with other metals of the platinum group, is used in the arts to a limited extent, where a hard, non-corrosible metal is needed. The standard meter at Paris is a 10 per cent alloy of iridium with platinum. In particular, it is used for the tips of gold and stylographic pens, for the construction of standards of length, and for standard electrical resistance coils. Chemically it is related to osmium and platinum and more distantly with rhodium, ruthenium and palladium.