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HISTOBY OF DENTAL SURGERY 179

moisture as is the' case witli natural teeth in the uioutli, a very hrief ex-
posure to the air before inserting tlie test fillings being sufficient to cause
The
enough contraction to spoil tlie reliability of the test. first step in Dr.
Black's investigation was to build a micrometer to measure to one ten-
thousandth of an inch, the greatest difficulties being to '"so arrange and inter-
mingle the metals entering into the construction as to eliminate errors from
temperature changes and to arrange to accurately remove and replace the
tubes containing test fillings."' A third machine had to be built before these
results were fully accomplished. The second step was the purchase of a
binocular microscope to corroborate by eyesight tlie readings of the micrometer,
it being considered important to show the worthlessness of most amalga/ns in
general use so plainly that every dentist looking at them would be con-
vinced. Dr. Black says:
"'The next problem was to find out what the manufacturers knew
about amalgams, for there were many secrets in the trade. * * *
In one way or another I obtained pretty much all of these secrets and
have never given one of them away to this day. In value they did not
amount to two good liills of beans.' I also obtained all of the principal
formulas of the amalgams in the market for the piirpose of studying them."
Dr. Black also obtained from the manufacturers ingots of their alloys for
the purpose of studying the success or failure in compounding the alloys, as to
whether they were true alloys or mixtures in part, and whether uniform in all
parts of the ingot. "A mixture of metals is not an alloy and has not the effect
of an alloy in amalgam mahing." For making alloys Dr. Black finally used the
closed electric crucible and melted and mixed in hydrogen. He procured
cylindrical steel blocks or tubes about one inch in diameter and the same in
length, into which holes were drilled five-eighths of an inch in diameter and of
uniform depth, into v.hich the test fillings were packed, having a small flat
steel disc imbedded in the surface for the needle of the micrometer to rest
against. This instrument measured the expansion or contraction in the depth
of the fillings in the steel cylinders. The expansion or contraction of the cir-
cumference was easily observed with the microscope. (The contraction more
readily and accurately than the expansion). A little practice enabled the ob-
server with the microscope to anticipate the readings of the measuring instru-
ment and vice versa. Having a stable material in whicli to pack amalgam and
a trustworthy means of determining the exact amount of cliange in volume that
might take place after it was packed, a series of experiments was made with al-
loys of silver and tin, which determined the fact that an alloy in about the pro-