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138 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

take the acid constituents to itself. This action is inconsiderable
as compared with the following, (2) because the electric current
generated between the two metals would liberate acid over the
whole surface of the zinc, by which the action upon the zinc
would be still much more increased. We see, then, if teeth were
made of zinc, what disastrous results would follow their filling
with gold.
But can we, without further question, assume that the same
takes place even to a limited extent, or to any extent whatever,
when a natural tooth is filled with gold or any other metal ?
In other words, do any two substances when brought into
contact assume a difference of potential, or can any two sub-
stances act as the generating plates of an electric element? An
article in the Co7n-espoi(dc)iz-BlattfurZahndrzte,triin&\atQd from
the Practitioner^ contains the statement that a galvanic battery
may be constructed with almost any two different substances
whatever. It is only necessary that one of them be more readily
acted upon chemically than the other.
A stick of wood and a piece of mica are two substances which
fulfill the above condition, but we would try in vain to form an
electric element by use of them ; we would equally fail with
sealing-wax and glass, or with gold and glass, even though both
substances might be entirely consumed in the attempt. But
what condition necessary to the production of an electric current
is wanting in the above cases ?
Conductivity.—When two chemically dissimilar substances
are made to touch one another, they do not assume a difference
of potential or become charged with electricity if one (or both)
of them is a non-conductor, and no electric current can be pro-
duced by the use of any two chemically dissimilar substances if
one (or both) of them is a non-conductor. Tooth-bone is a non-
conductor, and consequently cannot become electro-positive, or
be changed in potential by contact with gold, and cannot act as
the pole of a cell when immersed in an acid or salt solution.
I say that tooth-bone is a non-conductor, though the fluids
with which the tooth is permeated are conductors.
It is necessary that the difference be thoroughly understood. A
silk thread, for instance, is a perfect non-conductor, but by dipping