IRIDIUM (symbol Ir.; atomic weight 193.1), one of the metals of the platinum group, discovered in 1802 by Smithson Tennant during the examination of the residue left when platinum ores are dissolved in aqua regia; the element occurs in platinum ores in the form of alloys of platinum and iridium, and of osmium and iridium.
In this process the osmiridium is fused with zinc and the excess of zinc evaporated; the residue is then ignited with barium nitrate, extracted with water and boiled with nitric acid.
The double chloride is fused with nitre, the melt extracted with water and the residue fused with lead, the excess of lead being finally removed by solution in nitric acid and aqua regia.
When quenched as far as possible by rotation of a nicol prism, it exhibits a residue of a more intense blue colour; and further it is found that the direction of the most nearly complete polarization becomes inclined to the direction of the primary rays.
Under it the cost of the necessary land was to be found as to one-third by the state and as to the residue locally, but this arrangement proved unworkable and was abandoned in 1845, when it was settled that the state should provide the land and construct the earthworks and stations, the various companies which obtained concessions being left to make the permanent way, provide rolling stock and work the lines for certain periods.