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236 THE TECHNICAL PROCEDURES IN FILLING TEETH.

for children are genei'ally of less specific gravity than those
made for adults. When these are excluded, the difference is
reduced to about forty (40) hundredths of a volume, or slightly
more than two per cent. Therefore, these experimental fillings
represent very closely that which is done in the mouth. Really,
we can never have a test of the lightest fillings made in the mouth
for the reason that they go to pieces, instead of being lost as one
piece.
The fillings made in the mouth represent the same peculiar-
ities regarding adaptation to walls of cavities as has been noted
in exiDerimental fillings. In this examination, the fact that a
considerable number of practitioners never grasp the ideas
necessary to close adaptation of fillings to cavity walls comes
out strongly. They fail in this, even though they make a filling
that is very resistant to crushing stress. Other men make fill-
ings with close adaptation to walls of cavities very uniformly
even when the total densitj^ is low and the resistance to crushing
very much less. It therefore becomes apparent that resistance
to crushing stress is not the best test of gold fillings, provided,
of course, that reasonable strength is attained. Much hammer-
ing of gold will make a hard mass, but no amount of hammering
of gold that is available in the mouth will make close adaptation
to cavity walls unless the laying of the gold, stepping of the
plugger point on the gold and the direction of force are correlated
to this end. Some men seem to get this intuitively, even when
they are unable to explain in words how they do it. Their fill-
ings, whether experimental or practical, show this peculiarity.
Their fillings are regularly wedged between the cavity walls.
Other men seem never to grasp this in the manipulative sense.
Their fillings are loosely and imperfectly packed against the
cavity walls, no matter how resistant to stress the mass of the
filling may be. Such fillings are comparatively easy to move in
the cavity by heavy stress, because the cavity walls have no suffi-
cient sustaining grasp upon them. Final failure from leakage is
their characteristic. Notwithstanding this, a certain minimum
density must be obtained in order for a gold filling to stand and
serve its purpose. The density actually required varies greatly
in different cases, but in all it must be sufficient to prevent
al)sorption of fiuids into the filling itself. This is pretty certain
to occur in any filling with a specific gravity of less than 15,
or any portion of which may be below that density. This is a
matter of easy experiment when we have the filling fresh from
the mouth. Clean its surface quickly with ether and dry it. Heat