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180 HISTOKY OF DENTAL SUEGEEY

portions ot !-i\ty-five silver and thirty-five tin, wlien freshly cut, would make
a stable amalgam that would neither contract nor expand. He found, how-
ever, that fresli cut tilings required more mercury, were harsher to work
(amalgamated with greater difficulty), hardened more (|uickly, and would
shrink less or expand more than if the filings had been standing for some time
at ordiiuiry teHij)cratures. The ageing or amiealing, as we now call it, is greatly
hastened hy heat. It is accomplislied in fifteen minutes at the tem])erature of
boiling water. If the cutting nuichine is run fast enough to heat the alloy
sufficiently it is sometimes annealed in cutting. If filings are kept as cold as
17° F. annealing does not occur at all. The proportions required for a stable
amalgam (that will not shrink or expand) after full annealing were found to
be about 'J'21'o per cent, silver and 27]'^ per cent. tin. A larger proportion of
silver would make an amalgam that would exjiand and a larger proportion of
tin made one that would contract. Most of the anuilgams in general use at the
time, having ap])i-oxiniately forty per cent, silver and sixty per cent, tin, (with
some modifications by the addition of other metals), would shrink enough to
drop out of the steel tubes. Variations in the proportion of mercury used,
while having important inlfuence upon otlier qualities of the amalgam, had
little or no ell'ect upon expansion or contraction. The published account
of these experiments and their results, and the demonstrations of then)
at numerous dental society clinics, at some of which lillings were made
in the steel tubes and tested from a great nund)er (jf different alloys
brought by the members for- the purpose from the supplies they were
using in their offices, soon convinced the reading and thinking men of
the profession that a radical change was needed in the amalgam alloys in
general use. Many of the manufacturers sent representatives to attend a
laboratory course given by Dr. Black for instruction in the scientific principles
and facts, and the details of manufacture and testing which must be followed
in the making of alloys that could be trusted to accomplish what we iiave a
right to expect from amalgam fillings by reason of the newly acquired knowl-
edge of the material. In a short time a nund)er of new alloys were put upon
the market wliicli, if properly used, will make amalgam fillings that will not
shrink.
Another characteristic of amalgam, determined anil illustrated by Dr.
Black in the same series of experiments, is what may lie termed "flow.'" It is
the disposition to move continuously under a given pressure and is due to the
tin in the alloy. It is ilhi'^trated in this way. If a suuill rulie of amalgam
having a large projiortion of tin be ]n\t under heavy ]iressure rapidly in-