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DENTINAL ANESTHESIA BY CHEMICAL AGENTS. 211
form in which it is best to employ it is the saturated deliquesced salt,
which is taken from a bottle containing the salt in excess. The fluid
is introduced on a ])ledget of cotton and is permitted to remain until
the pain occasioned by it has ceased. It will be found that there are
two periods of pain : the first from its irritation of the fibrils in the
bottom layer of the caries, and then again when it has reached the
zone of exalted dentin a little beneath this ultimate layer of decay.
It follows, if the caries has all been previously removed and the
sensitive tissue interdicts further cutting, that but one period of pain
is encountered. The cutting should therefore be deferred until after
the second period of pain has passed. The disregard of this considera-
tion has sometimes cast discredit upon the efficiency of this sovereign
remedy.
It is requisite that the chlorid be chemically pure, and the fused
form is preferable to the crystals of the shops.
The PAIN attending the application is sometimes extreme for a mo-
ment. This can be moderated by air-drying the cavity and dressing it
with carbolic acid, which does not seem to prevent the action of the
chlorid.
To avoid the loss of time which may be occasioned by the slow
action it is advisable, after securing the dam at the neck of the tooth
by a ligature, to very tightly tie the free portion of the rubber a short
distance from the tooth with a strong ligature, and after cutting away
the excess of rubber some other service may be rendered. When the
pain has ceased the case may be proceeded with, or the excess of chlorid
may be thoroughly washed out and the cavity temporarily closed until
a subsequent time.
Another method of securing the action of zinc chlorid is to make a
paste of zinc oxychlorid and fill the cavity with it. Even after crys-
tallization of the paste takes place' it contains a slight excess of the
chlorid, which slowly acts upon the hypersensitive tissue. This method,
however, is not adapted to deep cavities, and care must be exercised con-
cerning its use in teeth of inferior grade.
Zinc chlorid is an extremely valuable remedy when the previously
stated that the chief danger of its use consists in the liability of the
coagulant and esciiarotic action reaching the pulp in deep cavities.
This danger is further enhanced when the teeth are soft, as in this con-
dition the penetration is liable to be greater than would be the case with
dense dentin. The same caution must be observed when the structure
is incomplete, as it is in the teeth of young subjects. Even here, as
•extreme sensitiveness is always found at the peripheral limits of the
