Iridium (Lat. iris, rainbow), a metal so named from the colors exhibited by its solutions; symbol, Ir; chemical equivalent, 98.56. It was discovered by Descotils in 1803, and by Smithson Tennant in 1804. It occurs native and nearly pure, also associated with osmium, platinum, and rhodium, and in alloys of various proportions of these metals. An alloy of one fifth platinum and four fifths iridium has been met with in octahedral crystals whiter than platinum, and of specific gravity 22.66. When native platinum is dissolved in nitro-hydro-chloric acid, black scales remain behind, which are composed of iridium and osmium. These metals may then be separated by one of the methods in use, and the iridium is obtained in a gray metallic powder, resembling spongy platinum. It is very hard, white, and brittle, and may be melted on lime by the oxyhydrogen blowpipe, or by the heat of the voltaic current. When thus fused it has the specific gravity of 21.15. None of the acids attack the pure metal, but when alloyed with platinum it is readily dissolved by aqua regia. Iridium black, similar to platinum black, may be obtained by decomposing a solution of its sulphate by alcohol. If heated in a finely divided state in the open air, iridium absorbs oxygen; it is also oxidized by nitre and caustic potash.
Small grains of iridium containing a little platinum are picked out from the grains of the latter metal, and from their extreme hardness make excellent nibs for gold pens.