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336 PLASTIC FILLING MATERIALS. —

Zinc oxychlorid as an ohtunding agent in tlie treatment of hyjxT-
sensitive dentin is of considerable value, and its use for tiiat jjurpose is
described in Chapter VII.
The use of zinc oxychlorid as a canal filling, and the mode of using
it, are discussed in Chapter XVII.
The powder of this cement is made of zinc oxid calcined and pow-
dered, to which have been added substances (borax, silica, etc.) which
affect its properties but little if at all.
T\\Q. fluid is made by dissolving pure zinc or its oxid in hydrochloric
acid to the point of saturation ; or, by making a solution of zinc chlorid
4 parts, water 3 parts, and filtering the solution.
The use and effects of zinc oxychlorid as a pulp capping are dis-
cussed in Chapter XVI.
Zinc Phosphate.—These cements are nominally a combination of
calcined zinc oxid with a syrupy solution of orthophosphoric acid :

3ZnO + 2H3PO, = Zn3(POj2 + 3H2O,

although their actual composition is more variable than that of any other
filling material. Both base and solvent commonly contain impurities
those of the base owing to lack of discrimination, or worse, in the source
of the oxid. Many of the impurities of the phosphoric acid are due
primarily to the well-known inconstancy of the acid itself, and others to
the mode of its manufacture.
Many of the specimens of powder are prepared from commercial
metallic zinc, and therefore contain the impurities of that metal.
Among the latter is arsenic, so that the presence of arsenic compounds
in inferior cement powders is by no means impossible, which may pos-
sibly explain in some cases the death of non-exposed pulps in teeth
which have been filled with zinc phosphate ; but as recent chemical in-
vestigation has shown that the arsenic when present in cement powders
is in the form of an insoluble zincarsenito, the danger of arsenical irrita-
tion of the pulp from tliat source would seem to be a remote one.
A common source of the glacial phosphoric (metaphosphoric) acid of
commerce is from sodium phosphate, variable quantities of which are
retained in the acid solution as acid sodium phosphate (dihydrogen
sodium phosphate). This substance is soluble in water, and must there-
fore greatly increase the solubility of any cement containing it.
To properly make pure specimens of zinc oxid and phosphoric acid
is a comparatively expensive operation—which will serve to explain the
seemingly high cost of fine specimens of cement, and incidentally serve
as a warning against the indiscriminate use of cheap cements.
Makixg the Powder.—A quantity of pure zinc oxid is luted in a