History

Smithson Tennant, a British scientist, is the man credited with the discovery of Iridium.  He was working with platinum and trying to improve upon its fabrication.  Platinum was known to dissolve in aqua regia, a combination of nitric acid and hydrochloric acid which can dissolve gold and platinum meaning it is a very corrosive solution.  After the platinum dissolved, a black residue was left over.  This was thought to be graphite.  In 1803, Tennant began to study the left over material and this lead to the discovery of both Iridium and Osmium.

Louis Nicolas Vauguelin

At the same time that Tennant was studying the black residue, another scientist was also getting close to the discovery of Iridium.  The French scientist, Louis Nicolas Vauquelin, was also studying the black residue and proclaimed it a new element.  It could be argued that Vauquelin did in fact discover the element first, but Tennant's better understanding of it and his other finding of Osmium gave him the credit for discovering Iridium.  Tennant also gave Iridium its name.  Iridium comes from the Latin root Iris meaning rainbow.  He gave it this name from "from the striking variety of colors which it gives while dissolving in marine acid".

Kilogram (10% Iridium, 90% Platinum)

Some other interesting history dealing with Iridium is that it along with platinum were used in 1889 to form some mass and length units.  A 90% platinum, 10% iridium alloy was used to make the standard meter bar and kilogram mass.  These units were then kept by the International Bureau of Weights and Measures located in France.  The meter bar was replaced in 1960 but the alloy of platinum and iridium remains the first kilogram unit that we still use today.