th all others, and is the type of electro-negative
bodies; it combines with all metals, therefore with tin, and in many
cases only the metal is discolored, and not the tooth. Steam boilers are
made tight by oxidation.

Where there is complete oxidation, the tooth is blackened to but a very
slight depth, and the oxid fills the ends of the tubuli, thus affording
an additional barrier to the entrance of caries. The filling itself will
prevent caries, but oxidation acts as an assistant.

"In the mouth, a suboxid is more likely to be formed than a protoxid,
but both are black; sulfur and oxygen are capable of acting on tin under
favorable circumstances, such as warmth, moisture, full contact,
condensation of elements, and their nascent conditions; the first three
are always present in the mouth. The protosulfuret of tin is black."
(Dr. George Watt.) Others give the color as bluish-gray, nearly black.

Experiments show that slight galvanic currents exist between fillings of
dissimilar metals in the mouth, and practical experience demonstrates
that these currents occasionally produce serious results.

Direct galvanic currents do not decompose normal teeth by true
electrolysis, but acids resulting from decomposition of food and fluids
react upon the lime constituents of the teeth and promote secondary
caries.

When two metals are so situated in the mouth that the mucous membrane
forms a connecting conductor and the fluids are capable of acting on
one metal, galvanic action is established sufficient to decompose any of
the binary compounds contained in these fluids; the liberated nitrogen
and hydrogen form ammonia, which being exposed to the action of oxygen
is decomposed and nitric oxid formed, resulting in nitric acid. We also
have in the mouth air, moisture, and decomposing nitrogenous food to
assist in the production of nitric acid.

"Galvanic action is more likely to develop hydrochloric acid, for the
chlorids of sodium and potassium are present in the normal saliva and
mucus, and when decomposed their chlorin unites with the hydrogen
derived from the water of the saliva." (Dr. George Watt.)

The fact should also be noted that both nitric and hydrochloric acids
are administered as medicine, and often assist in producing decay.

When there is a battery formed in a mouth containing tin fillings and
gold fillings, and the fluids of the mouth are the exciting media, tin
will be the positive element and gold the negative