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196 MFXriANICAL DENTISTRY AND METALLURGY.
Tenacity.— 1'he property in \irtue of which metals resist
attempts to pull their particles asunder. This embraces
adhesion and cohesion.
Adhesion is the force which unites molecules of dilTerent
kinds.
Cohesion is the force which unites molecules of the same
kind.
Toughness.—The toughness of a metal is the property
of resisting- the separation of their molecules after the limit
of elasticity has been passed.
Hardness.—By this term we refer to the resistance offered
by the molecules of substances to their separation by the
penetrating action of another substance.
Brittleness.—The property in virtue of which the mole-
cules of certain l)odies are easily broken apart.
Crystalline Character.—A substance is said to be crystal-
line when the molecules arrange themselves in a definite
and regular manner. These forms are crystals and when
this action has taken place the substance is said to be
crystallized.
Elasticity.—The power certain bodies possess of resum-
ing their original form or volume size after the removal
of an external force, wdiich has changed that form or
volume.
Conductivity.—The property in virtue of which metals,
to a greater or less degree, transmit or conduct the electric
current or the impressions of heat and cold.
Fusibility.—The property in virtue of which metals upon
the application of certain degrees of heat pass from a solid
to the liquid state. On account of the difficulty experienced
in determining high temperatures, only those that fuse or
melt at temperatures below 2000 F. can be ascertained with
absolute accuracy. Those above this point must be taken
only as an approximation.