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MATEEIALS FOR FILLING. 87

pressure ; but, in the mode in which it is now pre-
pared, it will weld readily and thoroughly. There
are certain requisites essential to this welding pro-
perty of gold: if it is in the form of foil, it should not
present a smooth, planished surface ; it must be an-
nealed after hammering, in order that its ultimate par-
ticles may be in the best condition for cohering : it
must be entirely free from all deposits of foreign sub-
stances ; and it must be kept from exposure to the
atmosphere.
Gold is a good conductor of heat ; and this is the

chief objection to it as a material for filling. As to
sensitive teeth, this is a very serious objection, in
some cases necessitating the employment of non-
conducting materials with it, and in others preclu-
ding its use altogether. The color of gold, however,
is seldom an objection to its use; though it some-
times renders it unsuitable for fillings in the front

teeth. But this objection has been already adverted
to.
Various preparations of Gold.—And first, of the
manufacture of gold foil. For this purpose, pure
gold is used; for procuring which, various methods
are employed. But the most common of these are
insufficient for the production of gold absolutely pure.
It is, however, deemed irrelevant here to detail the
process by which this end is attained : it is enough to