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LEAKAGE.

tooth readily, the chances are that one of these invisible leaks will
occur. Therefore, in such mouths the holes should be cut so small
that they just escape tearing when stretched over the teeth. In this
manner the tooth is hugged so tightly that the contractile action of
the rubber compels it to reach as small a diameter as possible, which
is found up under the gun>-margin, because of the conical shape of
roots. Of course in a dry mouth any tendency to leakage caused by
a loosely-binding dam may be, and usually is, overcome by the liga-
ture. In the case above described, where there was room for
barely
the silk above the border of the the itself
cavity, ligature operated
the of the cause of while it in no
against discovery leakage, way pre-
vented the excessive moisture from under it, and
creeping immediately
upon the gold.
Another cause of leakage is seen in Fig. 56, which shows the dam
over two teeth whose crowns are omitted in order that the points of
leakage, a, may be more readily seen. Here the fault has not been
FIG. 57.

that the holes were made too large, but insufficient space was left be-
tween them, and the stretching necessary to place the dam over two
teeth has produced a space next to each, as pictured. Of course this
may be avoided by proper spacing. In the next diagram, however,
Fig. 57, is shown a similar difficulty arising from a different cause.
Here a section through the teeth near the gum-line discloses a distinct
concavity along the approximal surfaces. The dam stretched over
such a tooth must follow a straight line from the points b, b, so that a
space is unavoidably left. The question arises, Can the dam be placed
here so as to avoid leakage ? Moisture cannot be kept out by the
a roll of cotton
dam alone, but, the condition being recognized, dipped
in sandarac varnish should be packed along the leak ; then a ligature
should be knotted so as to present three or four knots, which should
be placed over the cotton and the silk then firmly tied around the
neck of the tooth. In a few moments all leakage ceases.
Another occasional source of leakage is where, in placing the dam
so as to include a molar, the edge is not forced between the teeth at