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GUTTA-PERCHA. 135

that it can be cut smoothly without dragging. After trimming
it may be burnished Hghdy, and if necessary smoothed with
polishing strips.
After the filling is completed it should be protected from
moisture for some minutes, the longer the better, as the cement
will become harder. It seems that when moisture is admitted
early a part of the phosphoric acid is dissolved out of the cement,
robbing it of a part that is necessary for the most complete hard-
ening of the mass.
These general principles should be observed in all uses of
cements. Softer masses must be used in setting crowns and some
other operations requiring a plasticity approaching a fluid condi-
tion, but it should be remembered that this renders the cement
less strong and more soluble, and is to be avoided whenever the
conditions of the case in hand will allow.
Gutta-percha.
Gutta-percha is used for various purposes in connection with
filling operations. The best form for this purpose is the ordinary
base-plate gutta-percha. Besides this, however, a multitude of
makes for the purpose of filling teeth are found in the market.
None of these are superior to the ordinary base-plate, and the
greater number of them are very inferior. Many of them seem
to be mixtures of gutta-percha and wax that soften very readily
by heat. These are especially to be avoided.
Gutta-percha is the best material we have for root filling, for
sealing dressings in root-canals, and for most of the temporary
stoppings used in connection with treatment cases. Under
certain conditions it is also an excellent material for more or less
permanent fillings in cavities in the teeth.

Filling with Qutta-percha.
In filling cavities in teeth with gutta-percha the cavity should
be prepared almost as for gold or amalgam, but generally con-
venience points for starting may be dispensed with. The cavity
should, however, be made as strongly retentive as possible.
When otherwise ready for filling, every portion of the cavity walls
should be moistened with eucalyptol or oil of cajuput. These
oils take strongly to the cavity walls and also dissolve slightly the
surface of the gutta-percha as it comes in contact with it. The