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132 THE TREATMENT OF TEETH

again carefully rubbed with the lingers until it
becomes very plastic. The superfluous mercury is
then squeezed well out, and it is ready for insertion.
It is considered that the iron precipitate produces
more even and trustworthy results than the zinc
precipitate.
Copper amalgam gives no flow under pressure,
and consequently never curls up at the edges. It is
also considered to shrink very slightly, or not at all.
It can be placed in a tooth in a very plastic condi-
tion, and will set very hard. If a little soft dentine
is left in the cavity it will be permeated by the
copper salts, stained a dark colour, and any decay
that might otherwise have progressed underneath
the filling will be prevented. It will be seen there-
fore that it possesses certain valuable properties. Its
colour, although black, is not of much moment in
cavities that are not exposed to view. It will not
discolour a live tooth if all the soft dentine has
been removed, and the walls are formed of hard,
white dentine, but will turn a pulpless tooth very
dark. Its merits are, however, more than counter-
balanced by its defects. It wastes away on the
surface, and in some cases rots, or becomes dissolved
away, at the cervical margin. In fact, except that
it is somewhat more durable, it behaves in these
respects very like a cement filling. The general