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210 PRELIMINARY PREPARATION OF CAVITIES, ETC.

arrest its action by sufficient irrigation to remove all traces of the salt
from the cavity.- Its active coagulating power renders it a valuable
agent in excessive dentinal sensitivity where there is not close ])roximity
of the pulp, and its safety is ensured by the fiicility with which any re-
mains of the salt may be taken up with watei-.
Unless employed in excess and too long continued the action of the
zinc chlorid does not pass beyond the zone of the exalted tissue, which,
as we are aware, generally is of limited depth. The cessation of the
pain produced by it indicates the time for its removal, when usually
the dentin will be found to be insensitive. There are instances, how-
ever, when no apparent effect is produced, which can only be satisfac-
torily explained on the ground that the vital resistance of the tissue is
sufficient to overcome the coagulative power of the zinc salt.
In general, zinc chlorid must be regarded as an entirely safe agent
if used with discretion. It is more applicable to shallow cavities which
are so situated, or are of such form, as to require much formative cutting
at the margins of the cavities, as in buccal and labial surfaces and in the
very superficial cavities of incisors and bicuspids. A warning, however,
should be presented that as the pulp cornua of incisors frequently pro-
ject near the surface, particularly in the young subject, considerable care
is here required in any but shallow cavities of decay. If it were used
in excess and its action extended there would always be danger, as
its energies would not cease until the affinities of the whole amount
were satisfied. In deep cavities the effect, particularly in soft teeth,
would eventuate in the ultimate devitalization of the pulp. It fol-
lows, therefore, that it would be improper to seal up any quantity of
this substance in a cavity.
The action of zinc chlorid is terminated when the excess is removed
and the cavity irrigated with water. The affinity it has for water
quickly removes the excess and soon deprives the tissue of the remain-
ing portion.
When cavities are deep and it is found necessary to resort to this
agent the surface of the deeper parts should be protected by an insoluble
coating, after which the margins, where the sensitivity is acute, may be
acted upon without detriment. Here it is requisite to remove the deep
caries, desiccate the surface and make a coating with a varnish. For
this purpose red gutta-percha rubbed in chloroform is applicable, since
it may be deftly applied to any given part and when the chloroform has
escaped is protective.
To properly apply zinc chlorid it is highly important to isolate the
tooth by means of rubber dam to protect the gum and to prevent the
entrance of moisture. Its affinities for water are so great that even
the vapor of the mouth dilutes it so much as to lessen its power. The