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144 FILLING MATERIALS JgM^

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spicuous as possible. Those individuals who do not possess
this attribute of correct appreciation of the esthetic should be
educated in that direction by a high sense of the artistic

displayed on the part of the ethical dentist.
3. Adaptability to Cavity Walls.—For filling teeth, mate-
rials should be selected which can be most perfectly adapted
to the walls of the cavity and maintain that adaptation, in
order that the cavity may be perfectly and permanently

sealed against the entrance of moisture.
4. Crushing Resistance.—During the act of mastication,
the teeth are subjected to a crushing stress ranging from 75
to 250 pounds pressure, depending on the object incised or

chewed, the teeth performing the work and the strength of
the muscles of mastication. This being the case, the filling
must possess the necessary resistance to strain and wear as well
as density, hardness, and tenacity, to withstand this stress.
The greater amount of the latter qualities it contains, the

more ^^ edge strength^ ^ it possesses when built out over long
enamel bevels.
5. Freedom from Molecular Change.—All matter is com-
posed of atoms united together in definite proportions to form
molecules. These molecules possess certain characteristics,
and have certain definite relationships to each other, as far as

their shape, size, closeness of adaptation and position is con-
cerned. In some metals, the molecules of which they are
composed have a much greater tendency to change their
relationship than others under certain physical conditions,

such as constant pressure, age, heat and cold. Filling mate-
rials, under the conditions to which they are subjected in the
mouth, should be especially free from this tendency to molec-
ular change.
6. Non-conductivity.—The dentinal fibrillae, when their
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ends are exposed, through loss of the enamel covering, conduct