Cool and weigh as metallic platinum.
IRIDIUM
Occurs in nature alloyed with osmium as osmiridium or iridosmine, which
is "rather abundant in the auriferous beach sands of Northern
California" (Dana). It occurs in bright metallic scales, which do not
alloy with lead, and are insoluble in aqua regia. Iridium also occurs in
most platinum ores, and forms as much as two per cent. of some
commercial platinum. In chemical properties it resembles platinum, but
the ammonic irido-chloride has a dark red colour, and on ignition leaves
metallic iridium, which does not dissolve in aqua regia diluted with
four or five times its volume of water and heated to a temperature of
40° or 50° C.
The other metals of the platinum group are Palladium, Rhodium, Osmium,
and Ruthenium. They differ from gold, platinum, and iridium by the
insolubility of their sulphides in a solution of sodium sulphide.
Palladium is distinguished by the insolubility of its iodide; and Osmium
by the volatility of its oxide on boiling with nitric acid.
MERCURY.
Mercury occurs native and, occasionally, alloyed with gold or silver in
natural amalgams; but its chief ore is the sulphide, cinnabar. It is
comparatively rare, being mined for only in a few districts. It is
chiefly used in the extraction of gold and silver from their ores
(amalgamation); for silvering mirrors, &c.
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