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NATURE AND PROPERTIES OF AMALGAM. 309

bring about the change, was found to have no influence. After exhaus-
tive and conclusive experiments it was ascertained that the change was
due to a molecular alteration of the cut alloy, through a process of an-
"
nealing or " tempering — /. e. heat was the agent producing the change.
The degrees of heat applied ranged from 130° to 212° F.
It was found that the amount of time during which an alloy was
subjected to the action of heat governed the extent of tempering ; for
•example, alloy subjected to a temperature of 130° for a given period,
had the amount of amalgam expansion reduced a given amount ; if
the heat were maintained for a longer period the expansion was corre-
spondingly decreased. Each formula has its zero point beyond which
tempering has no eifect.
In general terms, it was found that alloys in amalgams which
expanded in hardening had the extent of expansion reduced by anneal-
ing ; those which contracted had the contraction increased.
Alloys which were without alteration of volume unannealed, shrank
when annealed.
The following tables will show the extent of change produced by
annealing. It will be noted that the alloy of 72.5 silver, 27.5 tin, ex-
hibits the minimum contraction after annealing. It will also be observed
that less mercury is required to effect amalgamation in the annealed
alloy.^ Amalgams made from annealed alloys have both their flow and
crushing stress slightly increased.

I. Exhibit of Unmodified Silver-Tin Alloys:

^For a full exhibit of this stupendous work of Dr. Black's, the reader is referred
to his contributions in the Dental Cosmos for 1895 and 1896.
2 Black, Dental Cosmos, 1896, p. 982.