dentin there is enough moisture to oxidize the tin, by
reason of the current set up by the gold." (Dr. S. B. Palmer.)

Electricity generated by heat is called thermo-electricity. If a cavity
with continuous walls is half filled with tin and completed with gold,
or half filled with silver and completed with gold, and the junctions of
the metal are at 20-1/2 deg. C. and 19-1/2 deg. C., if the electrical action
between the tin and gold be 1.1, the action between the silver and gold
will be 1.8, thus showing the action in silver and gold to be nearly
two-thirds more than in the tin and gold, a deduction which favors the
tin and gold.

Rubbing two different substances together is a common method of
producing an electric charge. Is there not more electricity generated
during mastication on metal fillings than when the jaws are at rest?
Friction brings into close contact numerous particles of two bodies, and
perhaps the electrical action going on more or less all the time through
gold fillings (especially when other metals are in the mouth) accounts
for a powdered condition of the dentin which is sometimes found under
cohesive gold fillings, but not under tin.

CHAPTER VI.

White caries, the most formidable variety known, may be produced by
nitric acid, and in these cases all the components of the tooth are
acted upon and disintegrated as far as the action extends. In proximal
cavities attacked by this kind of caries, separate freely on the lingual
side, and fill with tin. When such fillings have been removed the dentin
has been found somewhat discolored and greatly solidified as compared to
its former condition; this solidification or calcification is more
frequent under tin than gold, which is partly due to the tin as a poor
conductor of heat. Nature will not restore the lost part, but will do
the next best thing--solidify the dentin. In some cases, under tin, the
pulp gradually recedes, and the pulp-cavity is obliterated by secondary
dentin. In other cases the pulps had partly calcified under tin. It has
been known for years that tin would be tolerated in large cavities very
near the pulp without causing any trouble, and one reason for this is
its low conducting power. Attention is called to the fact that gold is
nearly four times as good a conductor of heat as tin, and more than six
times as good a conductor of electricity. Where tin fillings are
subject to a large amount of attrition, they wear away sooner or l