History ------- (L. iris, rainbow) Tennant discovered iridium in 1803 in the residue left when crude platinum is dissolved by aqua regia. The name _iridium_ is appropriate because its salts are highly colored. Properties ---------- Iridium, a metal of the platinum family, is white, similar to [platinum](/research/charts/periodic-table-of-elements/platinum), but with a slight yellowish cast. Because iridium is very hard and brittle, it is hard to machine, form, or work. It is the most corrosion-resistant metal known, and was used in making the standard meter bar of Paris, which is a 90 percent platinum and 10 percent iridium alloy. This meter bar was replaced in 1960 as a fundamental unit of length (see under Krypton). Iridium is not attacked by any of the acids nor by aqua regia, but is attacked by molten salts, such as NaCl and NaCN. The specific gravity of iridium is only very slightly lower than osmium, which is generally credited as the heaviest known element. Calculations of the densities of iridium and osmium from the space lattices give values of 22.65 and 22.61 g/cm^3, respectively. These values may be more reliable than actual physical measurements. At present, therefore, we know that either iridium or osmium is the densest known element, but the data do not yet allow selection between the two. Sources ------- Iridium occurs uncombined in nature with platinum and other metals of this family in alluvial deposits. It is recovered as a by-product from the nickel mining industry. Uses ---- Although its principal use is as a hardening agent for platinum, iridium is also used to make crucibles and devices requiring high temperatures. It is also used for electrical contacts. The element forms an alloy with [osmium](/research/charts/periodic-table-of-elements/osmium) which is used for tipping pens and compass bearings.