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228 MECHANICAL DENTISTRY AND METALLURGY.

ounces of water) and pieces of clean wrought-iron i)ut in it,
wlicn a Cdpions evolution of hydrogen follows, which, unit-
ing with the chlorin of the argentic chkjrid, liberates me-
tallic silver. The latter should not be disturbed until the
last particle of it is thus reduced, when it will be found to
be a spongy mass. The undissolved iron sIkjuUI now be
carefully removed, the ferrous and ferric chlorid carefully
decanted, and the silver washed in hot water containing
about one-tenth its bulk of hydrochloric acid. This is re-
peated several times, and finally the silver is again thor-
oughly w^ashed with pure hot water. The silver, after dry-
ing, is then ready for melting, and if care has been observed
in the process it will be found to be of a fineness of 999.7
parts in 1000, the 0.3 of impurity present being due to
traces of iron. Tlie chlorids may be acidulated with sul-
phuric acid, and reduced wdth zinc instead of iron.''
Reduction of Silver to the Required Forms for Dental
Purposes.—Owing to the very soft and flexible nature of
sih-er in its pure state, it is usual, when converting it into
plate or other forms for use, to employ an alloy of the metal.
Hence silver coins, which are made harder by the copper
they contain, are generally selected for the purpose. The
tendency of silver to tarnish in the mouth when alloyed
with copper may be diminished by boiling the finished piece
in a solution of cream of tartar and chlorid of soda, or
common salt, or by scrubbing it with aqua ammonia, which
removes the superficial particles of copper and exposes a
surface of fine silver. When platinum is introduced as the
sole alloying component, the purity of the silver is not only
preserved, but the allo\- is less easily acted on chemically.
Formulas for Silver Solders.—Silver solders are usually
composed of silver, copper, and zinc in variable proportions.
Alloys formed from the following formulas are such as are
o;enerally employed in soldering- silver plate derived froni