Page 163 - My FlipBook

THE DECAY OF THE TEETH. 137

with different materials. After these pieces had been exposed
to the action of an acid for a week, he found that they had de-
creased in weight in the following gradation :
The piece filled with gold . . . 0.06
" " " " araalgam . . 0.04
" " " " tin . . . 0.03
" " " " gutta-percha . . 0.01
" " " " wax . . . 0.01
" " " " oxychloride of zinc 0.00
From these results Chase concluded that gold is the worst of
all filling-materials, and laid down his notorious proposition,
" Every tooth filled with a metal is a galvanic battery which
begins its work as soon as the surrounding liquid has an acid
reaction." Practitioners of repute actually discarded gold as a
filling-material on the strength of this unfounded statement.
It is necessary to grasp two fundamental facts in order to
understand the arguments upon which the electrical theory is
based.
1. When two heterogeneous metals (in general two chemically
dissimilar conductors) touch one another, a difierence of potential
is produced between the two conductors, charging one of them
(electro-positive) with positive electricity, and the other (electro-
negative) with negative electricity in exactly equal amounts. In
this state, the former, the electro-positive, has an increased affinity
for electro-negative or acid bodies. The charge of electricity so
developed, as well as the increased chemical activity resulting
from it, is, however, very small.
2. In any cell of a galvanic battery acid is set free at the
positive pole and alkali at the negative, and so long as the cur-
rent continues to flow, the positive pole (when not a noble
metal) continues to be acted upon by the acid so liberated.
Therefore, if we were to take a piece of zinc or a zinc model
of a tooth and make a gold filling in it or attach a piece of
gold to it, the zinc would be more rapidly attacked when im-
mersed in any electrolyte than when no other metal was in con-
tact with it, and for two reasons : (1) because the whole surface
of the zinc would become electro-positive by contact with gold,
and would therefore more rapidly decompose a solution and