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192 PATHOLOGY OF THE HAED TISSUES OF THE TEETH.
relation to decay, some persons seem to have the thought that
if we exclude microorganisms we necessarily prevent recurrence
of decay. This is wrong. In conditions of susceptibility to den-
tal caries, it is necessary that fillings be absolutely water-tight.
A full conception of what that means is not very easy to obtain.
A fairly clear conception of the size of microorganisms has been
obtained in the bacteriological laboratory. Roughly speaking, a
couple of thousand of them can be laid in a straight row across
the head of a pin and not fall off, and yet they are large enough
so that by the use of the microscope and microscopic methods
we can see them, handle them, count them and measure their
size. We can not see the molecules of water; they are so infi-
nitely smaller than microorganisms that we gain no real con-
ception of their size; we can not see them, count them nor
measure them ; we have no means of getting at the size of a
molecule of water, or a molecule of acid, or a molecule of alcohol.
They are infinitely small. Yet we must make our fillings so
perfect that a molecule of water will not go in between the walls
of the cavity and the filling material; that a molecule of acid
will not go in ; because, if microorganisms happen to lie on the
margins of our fillings and form acid, the filling must prevent
that acid from seeping in. It must not only simply prevent
microorganisms from going in, but must also exclude the acid,
for if the acid goes in it will soon make room, by the solution of
the calcium salts, for the microorganisms. It will be seen from
this again why we should lay our enamel margins in regions least
susceptible to decay. But, do as we may, the margins of the
filling are the vulnerable lines, and these we must devise means
of making tight enough so that acid may not seep in between the
filling and the margins of the cavity.
If the walls of a cavity are wet, we can not remove the last
trace of water by any pressure we can bring to bear upon gold
or amalgam placed against them. Capillary attraction is suffi-
cient to resist all effort to remove the last trace of the film ; after-
ward, there will be, in accordance with physical laws, an exchange
of fluids in this space whenever a different fluid or solution comes
in contact with its margin. An action occurs similar to dialysis,
which has been explained in considering caries of dentin and of
enamel. In this way a solution of any acid upon the surface of
the tooth will be exchanged for the water in any such film.
The removal of water from the walls of cavities must be
done (1) by absorbents; (2) by evaporation; (3) by absorbents
followed by evaporation, and, finally (4) by freshly shaving or