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232 THE TECHNICAL. PROCEDURES IN FILLING TEETH.

The Specific Gravity and Eesistance to Crushing Stress of
Pure Gold in Varying Physical Conditions
AND in Fillings.

The specific gravity of substances is an expression of their
weight as compared with the weight of an equal bulk of pure
water. As water itself, and also the substances compared, are
subject to contraction and expansion by temperature changes, it
is necessary to establish a definite temperature at which this
comparison shall be made. In this particular, there have been
differences in the practice of scientific men. At i^resent the
most general preference is 39 degrees, or its equivalent centi-
grade, which is the maximum density of water. This, however,
is a very difficult temperature to maintain in doing such delicate
work, and many prefer a temperature of 60 degrees F. The spe-
cific gravity is found by weighing the object, such as a mass of
gold, first in the air and then weighing it suspended in water.
If a mass of gold is weighed in air, and the weight is found
to be 722 milligrams, and then weighed suspended in water,
and the difference is found to be 38 milligrams, this difference
is the exact weight of the bulk of water displaced by the bulk of
gold suspended in it. As a cubic centimeter of water weighs one
gram, the bulk of water displaced equals 38 thousandths of a
cubic centimeter and exactly equals the bulk of gold. If we
divide the weight of gold (722 milligrams) by the weight of the
bulk of water displaced (38 milligrams), (722-^38=19) we find
the gold nineteen times heavier than water. Then the specific
gravity is said to be 19 for that nugget of gold. This example
is close to the usual specific gravity of pure gold when cast, as
in an ingot. Pure gold when annealed is a soft metal. AVlien
laid upon a slab of dentin cut from a human tooth, it may be
mashed or spread by the force applied to it without injury to
the dentin. By hammering, gold is made harder than when in
the annealed state. It may again be reduced to its original state
of softness by annealing, i. e., by heating to full redness. There-
fore, by careful manipulation we are able to make use of gold
in any degree of hardness between these two extremes of hard-
ness and softness. Gold may be slightly condensed and its spe-
cific gravity increased by hammering or rolling, as in making
gold plate. This makes the gold very hard, but, when it is
annealed, it again becomes soft without losing its increased
density.