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252 FILLING MATERIALS.

Till is also a poorer thermal conductor than gold, and this is an
important consideration wlicn thermal irritation is to be avoided, and is
of great value in deep cavities approachino; dani^crously near to tiie pul}).
The objections to this metal are its color when exposed to view and
its softness, which greatly lessens its value in positions where it may be
subject to severe attrition.
Its most important use is for the temporary teeth of children, Avhere
it may be easily inserted and readily condensed, and rapid progress in
its introduction may be made, producing good results.
Except when freshly prepared, tin is not cohesive, a quality which
cannot be restored by heat, as may be done with gold.

AMALGAMS.
Tlieir Composition.—The essential metals which enter into the com-
position of the dental amalgams are silver, tin, and mercury. To
these are added various metals in varying proportions to modify the
" setting," the color, and the affinity for sulfur compounds. For these
purposes gold is used to influence the rate of chemical combination, and
it also affects the color. Zinc and copper are added in order to modify
the shade and also to lessen the affinity for sulfur.
The effect of various proportions of the metals entering into the
formulas upon the working qualities of an amalgam is extremely puz-
zling ; slight differences in proportions causing widely varying results.
The order in which the metals are introduced into the crucible and
the degree of heat to which the mass is subjected in the fusing process
also affect the working qualities.
The Proportion of the Ingredients.—Valuable tables have been given
by Dr. Black which indicate that a nearly definite ratio betw^een the
silver and tin should be maintained. This ratio is found to be approxi-
mately as follows—Silver 65, Tin 35—when only these two metals are
used to make the alloy. Whatever addition of a modifying metal is
introduced should be of small quantity and at an equivalent reduction
of the percentage of the tin.
The ingot of the alloy should be finely divided either by filing or by
thin shavings made by turning them off in a lathe. When the commi-
nution of the alloy is made immediately before using, amalgamation is
more easily effected than when the filings are kept for any considerable
time, unless in the latter case there is a disproportion of tin or gold.
This has been attributed to oxidation of the particles taking place, which
would retard the amalgamation. Silver not being an oxidizable metal
under ordinary conditions, the cause of the tardy combination with
mercury is to be found in the attachment of sulfids to the surface, and
also to the influences of occluded gases, which also tend to retard