y that
knowledge, will largely depend our success in preserving teeth.

It is a good material for filling many cavities in the temporary teeth,
and children will bear having it used, because it can be placed quickly,
and but little force is required to condense one or two layers of No. 10
foil. The dentin in young teeth has a large proportion of organic
material, for which reason, if caries takes place, many believe it is
hastened by thermal changes. Gold fillings in such teeth might prevent
complete calcification, on account of the gold being so good a
conductor; but if tin is used, there is much more probability of
calcification taking place, because of its low conductivity and its
therapeutic influence. It does not change its shape after being packed
into a cavity. Under tin, teeth are calcified and saved by the deposit
of lime-salts from the contents of the dentinal tubuli. This is termed
progressive calcification.

Like other organs of the human body, the teeth are more or less subject
to constitutional change. The condition in which we find tooth-structure
which needs repairing or restoring should be a sure indicator to us in
choosing a filling-material. Up to the age of fourteen, and sometimes
later, we find many teeth which are quite chalky. In some mouths also,
at this period, the fluids are in such a condition that oxychlorid and
oxyphosphate do not last long; for some reason amalgam soon fails, while
gutta-percha is quickly worn out on an occlusal surface. In all such
cases we recommend tin, even in the anterior teeth, for as the patient
advances in years the tooth-structure usually becomes more dense, so
that, if desirable, the fillings can be removed, and good saving
operations can be made with gold. By treating cases in this manner very
little, if any, tooth-structure is lost.

The teeth of the inhabitants of Mexico and Guatemala are characteristic
of their nervous and nervo-lymphatic temperaments; children ten years of
age often have twenty-eight permanent teeth, and they are generally soft
or chalky, but our dentists there report good success in saving them
with tin.

In filling this class of teeth, we should be very careful not to use
force enough to injure the cavity-margin, for if this occurs, a leaky
filling will probably be the result. Still, we have seen some cases
where _slight_ imperfections at the margin, which occurred at the time
of the filling or afterward, did no harm, because the deposi