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CAVITY LINING IN RESPECT TO PROXIMITY OF THE PULP. 257

employed in the condensation of gold, and as a desideratum to avoid
the great amount of time required to fill large cavities with this metal.
It also here imparts in some instances much strength to frail margins.
In the cavities which early form upon the occlusal surfaces of the
permanent molar teeth of children it is of great value, as here it is kept
clean by the friction of mastication, and being under easy observation
can be renewed when this is required. When the child reaches the age
to have permanent operations the margins may be shaped for the reten-
tion of gold, and in this case the principal part of the cement should
be allowed to remain.
Zinc phosphate is of questionable use in pulp chambers as not hav-
ing antiseptic properties, and being porous it becomes after several
years quite offensive. For the same reason it is inadmissible for canal
fillings. Furthermore, for this jiurpose it is questionable, on account
of its adhesiveness, whether it is capable of being thoroughly introduced
into root canals. All things considered, it is for these purposes greatly
inferior to oxychlorid of zinc.
Gutta-percha.—Compounds of gutta-percha with oxid of zinc form
a useful substance for temporary fillings, acceptable for teeth of low
grade at points not subject to attrition. Its preservative properties are
very considerable, and were it not subject to surface degeneration would
in the situations noted be a nearly permanent material.
The requirements for its successful use are that it be not heated
liigher than 212° F., that the cavity be quite dry, that it be intro-
duced in small pieces, and be kept under continual condensation until
cooled to prevent shrinkage. (See Chapter XIII.)

Cavity Lining in Respect to Proximity of the Pulp.
As caries approaches the pulp it reaches a period when the proximity^
of this organ is so close as to require much care to avoid irritation and
probable congestion. Under these circumstances it is necessary to
avoid thermal conduction and to exclude chemical influences. After
disinfection of the dentin some substance the ingrediency of which
is non-irritating and non-conduetinir should be selected to overlay the
pulp wall of the cavity. Here choice must be made between gutta-
percha and either of the classes of mineral cements.
When the use of gold is preferable for the external portion of the
filling, it is required that the foundation be sufficiently solid to with-
stand the force to be applied to the gold. Hence one of the cements is
here necessary. Previous to the placement of the cement, should the
pulp be near, the surface should be covered with a thin solution of one
of the resins to prevent the influence of the fluid element of the cement
from producing irritation. Copal ether varnish, a solution of hard
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