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244 MFXHANICAL DENTISTRY AND METALLURGY. )

Only chemically pure mercury should Ije employed in the
preparation of dental amalgams. This can now be obtained
Method of Refining.—The method most frequently em- I
from reliable dealers.

ployed for the purification of mercury is distillation. The
mercury is placed in a retort and is covered with about one-
fifth its weight of powdered cinnabar. The cinnabar suffers
decomposition during distillation of the mercury, and the
mercury it contains is also distilled over, while the impure
metals are largely converted at the same time into sulphids,
remaining with the other impurities in the retort. To
further purify the mercury, it is taken from the receiver
and bathed for several hours in dilute nitric acid with gentle
heat. The acid dissolves out impurities and any traces
of more readily oxidisable metals.

AMALGAMS.
Amalgam is the name given to an alloy of mercury and
one or more other metals.
Dental Amalgams.—The constituents of amalgams em-
ployed in dentistry are usually silver, tin, gold, and platinum
in varying proportions, with mercury. Zinc, copper, and
other base metals are sometimes employed, according to
many different formulae.
Properties Desirable in Dental Amalgams.—An alloy
for dental amalgam should possess the qualities of (
i
strength to withstand the force of mastication; (2) strength
and sharpness of edge; (3) being capable of retaining its
shape and (4) as far as possible, free from discoloration.
It is also evident that an amalgam liable to expand to any
great extent or contract is not to be relied upon as a filling
material.
Discoloration of Amalgam Fillings,—The discoloration of