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MATERIALS FOR FILLING. 89

1850, by Dr. S. A. Main, of New York. Their prep-
arations, however, were simply precipitates, and
nothing more. But in 1853, Dr. A. J. Watts, of
Utica, New York, obtained letters patent for his prep-
aration of gold for filling teeth. This preparation
was at first denominated sponge gold, but after some
modification, received its present name. There are
numerous formulas by which preparations of crystal
gold may be made ; but so far as we are acquainted

with them, they are all embraced in two general
methods : the one, to obtain simply a precipitate of
the metal, adaptable to the filling of teeth ; and the
other, to combine this precipitate with mercury,
and obtain a definite crystallization. For the prepa-
ration of the sponge or crystal gold, the absolutely
pure metal is required. This is dissolved in nitro-
muriatic acid, the gold being added till the solution

is saturated. Various materials may be used to pre-
cipitate it, the most common of which are sulphate of
iron and oxalic acid, the latter on some accounts
being preferable. The character of the precipitate
will be determined, in a great degree, by the manner
in which the precipitant is added : if slowly, the pre-
cipitate will be more fibrous, or structural.
A preparation may be made by introducing the

precipitant gradually, and then carefully washing the
precipitate, and heating almost to redness. For per-