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PHYSICAL PEOPERTIES OF FILLING MATERIALS. 227

In secondary qualities its color is bad, so bad, indeed, that
it should never be used in the front teeth on this account. Its
conductivity of thermal changes is nearly equal to that of gold.
Its working qualities render it much more convenient than gold
in very large and difficult cavities. It can be placed much
(luicker, the same expenditure of force is required, but much less
time. Perfection of adaptation is less certain.
Porcelain and gold inlays will be discussed later.

FORMS OF gold.
Gold is prepared in the form of foil for filling teeth. This
foil is prepared in various thicknesses, which are rated on the
basis of the number of grains in the sheet four inches square.
If a sheet of this size contains two grains, it is called No. 2; if
three grains, No. 3 ; if four grains. No. 4, and so on up to what
are known as the heavy foils, such as Nos. 30, 60, 120, etc. Now-
adays any thickness of gold foil above No. 10 would be regarded
as heavy foil. Such numbers as 60 or 120, which were so popular
some years ago, have practically gone out of use in dentistry.
Gold is also prepared for use in filling teeth in the crystalline
form. These crystals are put up in various kinds of masses,
with about as many names as there are makers. These forms
possess certain desirable qualities, especially convenience of
manipulation. In this respect, however, it is liable to be very
decei^tive. In any of these forms it is much less readily kept in
good working condition than foil, and for this reason is not so
certain in its working jiroperties. It often fails to weld per-
fectly and especially it is, except for the most skilled manipula-
tion, very unreliable in adaptation to the margins of cavities.
Great effort has been made to bring this form of gold into gen-
eral use, but the general judgment of the dental profession has
seemed to be against it, for foil has always maintained its posi-
tion as the form of gold most used.

WELDING PROPERTIES OF GOLD.
Gold prepared in the form of foil or crystals welds per-
fectly 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 inti-
mately the conditions of the development of it, and the loss of it.