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220 MECHANICAL DENTISTRY AND METALLURGY.

and elasticity of the plate are objectionable. The alloy
should be dissolved in nitro-miiriatic or hydrochloric acid,
called aqua rcgia. The best proportions for a(|na regia
are three parts of hydrochloric acid to one of nitric. If the
acids are at all good, four ounces of the aqua regia will be
an abundance for an ounce of the alloy. The advantage of
using the acids in the proportion of three to one, instead of
two to one, as directed in most of the text-books, is, that
when the solution is completed there is but little, if any,
excess of nitric acid. If the acids be ' chemically pure,'
four parts of the hydrochloric to one of the nitric produces
still better results.
" By this process the metals are all converted into
chlorids ; and, as the chlorid of silver is insoluble, and has
a greater specific gravity than the liquid, it is found as a
grayish-white powder at the bottom of the vessel. The
chlorids of the other metals, being soluble, remain in solu-
tion. By washing and pouring off, allowing the chlorid
of silver time to settle to the bottom, the solution may be
entirely separated from it.
" is now
The object to precipitate the gold while the
others remain in solution. This precipitation may be
effected by any one of several different agents, but we will
mention only the protosulphate of iron.
" This salt is the common green copperas of the shops,
and, as it is always cheap and readily obtained, we need
look no further. It should be dissolved in clean rain-water,
and the solution should be filtered, and allowed to settle
until perfectly clear. Then it is to be added gradually to
the gold solution as long as a precipitate is formed, and
even longer, as an excess will the better insure the precipita-
tion of all the gold. The gold thus jjrecipitated is a brown
powder, having none of the appearance of gold in its ordi-
nary state. The solution should now be filtered, or the gold