A bluish-white, hard, crystalline metallic
element, very heavy and infusible, symbol Os,
atomic number 76, relative atomic mass 190.2.
It is used for lamp filaments, with iridium
to form a very hard alloy suitable for
pen-nibs and fine machine bearings, and as a
catalyst. It is found in platinum-bearing
river sands, and with iridium in osmiridium.
Heated in air, it gives off a pungently
irritating poisonous vapour. It was
discovered in 1803 by British chemist
Smithson Tennant (1761-1815).