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198 MECHANICAL DENTISTRY AND METALLURGY.

possessing a high degree of expansihihty, will upon cooling
be smaller than the plaster model from which the mold
was made. It is, therefore, an admirahle metal for mak-
ing dies for swaging metal plates. The fact that plaster
of paris expands in setting is well known, and the contrac-
tion of zinc upon cooling, in this connection, at least, com-
pensates for this expansion.
Change of Volume on Solidification.—The change of
volume when passing from the fluid to the solid state, above
referred to, is common with all metals with the exception
of bismuth, which expands. This change of volume is of
practical importance, as indicated in the making of dies
with zinc.
Galvanic Action.*—AVhen two dissimilar metals are im-
mersed in a fluid, the chemical action of which is more
energetic on one than on the other and then brought into
contact a feeble electric current is produced known as
galvanic action. This galvanic action is sometimes ex-
perienced when different metals are in contact in the
mouth. The amount of action is infinitesimal in healthy
states of saliva ; but in acid, or other unhealthy condition
a ver}^ efficient exciting fluid is in constant action.
Effects of Exposure in the Mouth.—Some metals, such
as silver, copper, and cadmium, are readily acted upon when
exposed in the mouth. This is due principally to the
presence of sulphuretted hydrogen, and the affinity these
metals have for sulphur. Gold and platinum are not acted
upon to any appreciable extent as they are practically non-
oxidizable and have no affinity for sulphur.
Welding.—This is the property possessed by metals when
in a plastic state (the stage between the molten and the
solid state), of being joined together by the cohesion of the
molecules, induced by hammering or other external force.

* Smith's Dental Metallurgy.