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FILLING MATERIALS. 99

adaptation to the margins of cavities. Great effort has been
made to bring this form of gold into general use, but the gen-
eral judgment of the dental profession has seemed to be against
it, for foil has always maintained its position as the form of
gold most used.
Welding Properties of Gold.
Gold prepared in the form of foil or crystals welds
perfectly in the cold state when clean surfaces are
pressed into contact. It is the only metal which has this
property in a high degree. In order that the welding property
may be successfully used the surfaces of the gold must be
clean. As this property of gold is readily lost by improper
care, it is well to know intimately the conditions of the develop-
ment of it, and the loss of it.
All metals, except those known as the noble metals, oxidize
quite readily when exposed to the air ; that is, they attract the
oxygen of the air and unite with it to form a film of oxide upon
the surface. This prevents the contact of clean surfaces and
therefore prevents welding. Gold, silver, platinum and mercury
do not oxidize in this way, and it is for this reason that they are
called noble metals. Silver and platinum do, however, attract
oxygen to their surfaces in the form of a transparent film of con-
densed gas sufficient to prevent their surfaces from coming in
contact, which prevents them from welding cold. Gold does not
attract oxygen nor nitrogen to its surface and it is for this reason
that it can be welded cold. Gold does, however, attract to its
surface certain gases that are often present in our atmosphere in
such quantities as to prevent its surfaces from coming in con-
tact, and destroy, temporarily or permanently, its welding
properties. Some of these gases are such as will Ije removed
from the surface of the gold by volatilization when heat is
applied (by annealing;, rendering the surface again clean with
restoration of welding property. Other gases, notably those of
the sulphur and phosphorus groups, condense upon the surface
of gold and refuse to volatilize by heat, and in that case the
welding property of the gold is permanently destroyed. In
these cases it seems probable that compounds in the form of
fixed salts— non-evaporablc— are formed on the surface of the
gold.