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THE DECAY OF THE TEETH. 141

sistance was much greater; it could not, however, he definitely
determined on account of the polarization.
It may he easily shown by the following simple experiment
that the resistance of the enamel is much greater than that of the
dentine. Place one pole of a battery consisting of four Siemen's
cells upon the gums and the other upon the enamel of a sound
tooth, not the slightest sensation will be produced ; if, however,
we place the second end upon the dentine or upon a metallic
filling in contact with the dentine, we will at once experience a
very unpleasant sensation.
There can be no doubt that there are electric currents in the
mouth whenever there are teeth with metallic fillings. These
currents do not, however, exist in such a manner between the
filling and the tooth-substance that the latter could in any way be
compared with the generating plate of a galvanic cell ; they owe
their existence alone to the heterogeneity of the metallic fillings.
Electric currents will l>e produced on the surface of every
filHng, even of a pure gold one, when the filling does not have
the same density in all parts. They flow from the denser to the
less dense points of the surface. If these currents were even
strong enough and durable enough to be considered as anything
more than infinitesimal, they still could have no injurious eft'ect
upon the tooth deserving of mention, since it could rarely happen
that a great excess of them would be directed toward the margin
of the filling.
A\nien two fillings of difterent materials, in the same tooth or
in approximating teeth touch each other, a current is produced,
which flows from the more oxidizable (electro-positive) metal
through the liquids of the mouth and tooth to the less oxidizable
(electro-negative). These currents, by electrolytically decom-
posing the liquids of the mouth, may produce injurious effects
upon the teeth. Through them acid will be set free on the sur-
face of the electro-positive metal, which may attack the tooth at
the margin of the filling. This action, however, is very slight
at best, and appears to cease altogether as soon as the surface of
the positive pole becomes oxidized; in short, we have not found
in practice that such a process has any injurious eft'ect upon the
teeth.