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SILVER. 225
liul if il is siuldenh- chilled 1)_\- immersing' it in cnld water,
it instantly resnnies its ori^'inal luster. (3) When a drnj)
of the tincture of iodin is let fall upon its surface and
evaporated over the i^ame of a lam]), a black spot remains,
which does not occur with platinum. Palladium melts at
about the heat recpiired to fuse malleable iron, and is the
most fusible of the platinoid metals. ]t is soluljle in nitric
acid, but its best solvent is nitro-hydrochloric acid.
Palladium, being very costly, and possessing no properties
that specially recommend it for dental use, is but little em-
ployed in prosthetic practice.
Iridium ( Symbol, Ir. Atomic weight, 193), though gen-
erall}' found associated with platinum, osmium, and other
allied metals, sometimes occurs native and nearly pure. It
is very refractory when exposed to high temperatures, and
can only be fused by the oxy-hydrogen blowpipe or by the
heat of the voltaic current.
The extreme hardness and consequent rigidity of iridium
renders it in its unalloyed state practically unfit for base
plates, on account of the great difficulty of swaging it into
proper form.
SILVER.

Symbol, Ag (Argentum). Atomic weight, 108.
Occurrence.—Silver is found first, as native silver, that
is, in the metallic state, usually in flat masses; sometimes
these deposits are of considerable size, weighing several
hundred pounds. Silver is frequently found in combina-
tion with sulphur and chlorin, and in most of the ores of
lead, particularly that of lead sulphid (galena). It also
occurs in this country mixed with native copper. The most
common ores from which silver is extracted are those re-
sulting from its combination with sulphur as sulphids.
General Properties of Silver.—Pure silver, when plan-