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The Collaborative International Dictionary of English v.0.48 (gcide)

iridium \i*rid"i*um\ ([-i]*r[i^]d"[i^]*[u^]m), 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 atomic number 77, of the same
group as platinum, which it much resembles, being
silver-white and indifferent to most corrosive agents, but
harder, more brittle, and with a higher melting temperature
(2410[deg] C, versus platinum 1772[deg] C). With the
exception of osmium, it is the heaviest substance known, its
specific gravity being 22.4. Symbol Ir. Atomic weight 192.22.
[1913 Webster PJC]

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.
[1913 Webster]