IRIDZIIM (Neo-Lat.. from Gk. ipes, iris, rain bow; so called on account of its iridescence). A metallic element discovered in 1S03 by Smithson Tennant. It is found in platinum ores; as iridos mine, a native alloy of iridium and osmium in varying proportions: and in the form of native alloys with the other platinum metals. The principal sources of these ores are various dis tricts in the Ural Mountains, and ,ut the Pacific Coast of the United States. The preparation of metallic iridium involves its separation from the accompanying metals by a long and corn plieated process.
Iridium (symbol Ir; atomic weight, 193.5) is a white steel-like metal, brittle when cold, and somewhat malleable at a white heat. In its ordinary, compact form, iridium is insoluble not only in the several mineral acids, but, also in aqua regia ; the latter reagent. however. dissolves iridium powder, which may also be converted into the oxide Irft, by heating in the air. Iridium has a specific gravity of 22.4. and it melts at 2300° C. It combines with oxygen to form an iridium sesquioxide and an iridium dioxide, which in turn give rise to iridious and iridic salts. The metal itself alloys with copper, gold, and lead, as well as with other platinum metals, and the alloy consisting cf nine parts of plati num with one part of iridium is extremely hard, as elastic as steel, perfectly unalterable in the air. and capable of taking a high polish. It
is of this alloy that the standard meter kept in Paris was made. Iridium is also used for the manufacture of standard weights. knife edges of balances, and for other articles which it is desired to preserve from the influence of the atmosphere. It has been used in the United States to tip the edges of gold and other pens so as to produce a very hard surface. William L. Dudley of Cincinnati invented a process, in ISSO, for the electrolytic deposition of this material. and his process is now success fully used for coating surgical instruments and other articles requiring an exceedingly hard sur face.
IR'rDOSINIINE (from iridium + osmium). A native alloy of iridium and osmium that crystallizes in the hexagonal system. It is tin to light steel-gray in color, and has a metallic lustre. The proportions of iridium and osmium in it vary, and other platinum metals, !such as platinum, rhodium. and ruthenium. are frequently present. The mineral occurs with platinum minerals in the Ural Mountnins, in the auriferous drift; and sands in New South \\ ales, and in northern California.