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224 THE TECHNICAIi PBOCEDUEES IN FILLING TEETH.

Physical Properties of Filling Materials and the

Correlation of Forces Concerned.

Filling Materials.

THE filling materials at present in use for permanent opera-
tions are gold, wliicli holds first i^lace, and amalgam, wliicli
holds second place in value. Added to these, a few good men
would reckon tin as holding an important place, but the general
judgment of the dental profession would seem to consign tin
to a very obscure position as a filling material, as, nowadays,
very few make much use of it. Eecently inlays of porcelain and
of gold are assuming prominence as filling materials and are
being pushed to the front by many skillful operators.
The filling materials for temporary operations are the
cements, esi^ecially the oxj^ihosphate of zinc cements, and the
different preparations of gutta-percha.
The qualities most desired in a filling material for perma-
nent operations are indestructibility in the fluids of the mouth,
adai^tability to the walls of cavities, freedom from shrinkage
or expansion after having been made into fillings, resistance to
attrition and sustaining power against the force of mastication.
The qualities of secondary importance are color, or appear-
ance, non-conductivity of thermal impressions, and convenience
of manipulation.
Gold. Of these qualities, gold seems to possess those most
essential in much the greatest degree. It is perfectly indestruc-
it is very perfectly adaptable to
tible in the fluids of the mouth ;
it is free from objectionable shrinkage or
the walls of cavities ;
expansion; its resistance to attrition is good and it sustains
the force of mastication better than amalgam.
The greatest intrinsic worth of gold as a filling material
lies in the fact that it may be adapted to cavity walls with great
force, and is capable of immediately and permanently sustain-
ing that forcible ada]:)tation. Dentin is one of the most elastic
sujjstances known. It is this elasticity that renders it so pre-
eminent for billiard balls. Its range of elasticity is short as