\I*rid"i*um\, n. [NL., fr. L. iris, iridis, the rainbow.
So called from the iridescence of some of its solutions. See
{Iris}.] (Chem.)
A rare metallic element, of the same group as platinum, which
it much resembles, being silver-white, but harder, and
brittle, and indifferent to most corrosive agents. With the
exception of osmium, it is the heaviest substance known, its
specific gravity being 22.4. Symbol Ir. Atomic weight 192.5.
Note: Iridium usually occurs as a native alloy with osmium
      (iridosmine or osmiridium), which may occur alone or
      with platinum. Iridium, as an alloy with platinum, is
      used in bushing the vents of heavy ordnance. It is also
      used for the points of gold pens, and in a finely
      powdered condition (iridium black), for painting
      porcelain black.