t of tin
oxid filled up the ends of the tubuli, thus preventing caries. We
believe that this bar to the progress of caries is set up more
frequently when tin is used than with any other metal under like
conditions.

CHAPTER V.

In some mouths tin does not discolor, but retains a clean, unpolished
tin color, yet when there is a sesquioxid of the metal formed, fillings
present a grayish appearance. In the same mouth some fillings will be
discolored, while others are not. As a general rule, proximal fillings
are most liable to show discoloration. Perhaps one reason is that on
occlusal and buccal surfaces they are subject to more friction from
mastication, movements of the cheeks, and the use of the brush.

We have seen a large number of fillings which were not blackened, yet
were saving the teeth perfectly, thus proving to a certainty that
blackening of tin in the tooth-cavity is not absolutely essential in
order to obtain its salvatory effects as a filling-material.

Where there is considerable decomposition of food which produces
sulfuretted hydrogen, the sulfid of tin may be formed on and around the
fillings; it is of a yellowish or brownish color, and as an antiseptic
is in such cases desirable. To offset the discoloration, we find that
the sulfid is insoluble, and fills the ends of the tubuli, thus lending
its aid in preventing further caries. A sulfid is a combination of
sulfur with a metal or other body. A tin solution acted on by
sulfuretted hydrogen (H_{2}S) produces a dark-brown precipitate (SnS),
stannous salt, which is soluble in ammonium sulfid (NH_{4})_{2}S_{2};
this being precipitated, gives (SnS_{2}) stannic salt, which is yellow.
Brown precipitates are formed by both hydrogen sulfid and ammonium
sulfid, in stannous solutions. Yellow precipitates are formed by
hydrogen sulfid and ammonium sulfid in stannic solutions. The yellow
shade is very seldom seen on tin fillings; the dark brown is more
common.

An oxid is a combination of oxygen with a metal or base destitute of an
acid. In oxidation the oxygen that enters into combination is not
sufficient to form an acid. The protoxid of tin (SnO) is black, and can
be obtained from chlorid of tin, or by _long_ exposure of tin to the
atmosphere. The oxygen in the saliva helps to blacken the tin, and the
metallic oxid penetrates the dentin more or less, acting as a
protection, because it is insoluble. Oxygen is the only element which
forms compounds wi