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FILLING TEETH 119

it is reduced, and that in course of time, owing
principally to increase of atmospheric temperature
produced by the heat of summer, or the artijficial
heating of rooms during cold weather, these filings or
shavings become gradually untempered or annealed.
In order to prevent this variation in the behaviour
of amalgam fillings, Dr. Black set himself to work
to produce alloys that would give good results in
the untempered state, by annealmg the filings or
shavings. This is usually accomplished by en-
closing them in a bottle or flask, and placing the
flask in boiling water for a certain time. From
fifteen to eighteen minutes is sufiicient to thoroughly
anneal an amalc^am, althous^h doubtless the time
will vary with different alloys, and must be deter-
mined by experiment. It is stated that it takes
two years for an amalgam alloy to become com-
pletely annealed by time ageing. It is also stated
that the older the alloy is after a certain age, or the
longer it is annealed after a certain annealing, the
weaker it will be (Dr. Black).
Many amalgam alloys, when used freshly cut and
without annealing, are much more difficult to mix
with mercury, and the production of a sufficiently
plastic mass to admit of comfortable and satisfac-
tory manipulation, is not only more difficult, but
demands a large proportion of mercury. The same