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BASIC ZINC CEMENTS. 337
sand crucible and kept at the highest forge-heat for hours. AThen cool
the crucible is broken away and the vitreous mass of yellowish zinc oxid
is reduced to a powder which will pass through a fine bolting cloth.
This powder is placed in tightly stoppered bottles, for if exposed to the
air it absorbs carbon dioxid and a portion of it is converted into the
hvdrated carbonate of zinc. This change may be noted in old powders
by the effervescence due to the disengagement of carbonic oxid when
phosphoric acid is added to them. Numerous substances have been
added to the basal powder with the object of lessening the disintegra-
tion, I. €. chemical solution, when used as a dental cement. Usually
these additions are the oxids of other metals. The oxid of magnesium
added to the powders causes the cement to set more rapidly ; the oxid of
aluminum increases the rapidity of setting and makes a finer-grained
cement, the central texture of which is, however, inferior. Cements of
zinc oxid and phosphoric acid alone are apparently less soluble in lactic
acid than when the oxids of aluminum and magnesium are added.
Various other substances have been added which do not enter into
chemical combination with the phosphoric acid, in the hope of confer-
ring greater durability on the cement, but as yet but few of them have
been shoAvn to possess any value.
The Fluid.—Phosphoric acid in its pure state is formed by hydrating
phosphorus pentoxid :

PA + 3H20=2H3PO,.

Much of the phosphoric acid used for cements is made by hydrating
the glacial (metaphosphoric) acid, HPO3. The acid dissolves readily
in water, being deliquescent when pure. Difficulty of solution is-
therefore an indication of impurity of the glacial acid. It requires a
definite degree of heat to bring about the chemical hydration of the
acid. At a temperature of 210° F. the union occurs, which is attended
by the evolution of heat, the glacial acid being transformed into ortho-
phosphoric acid. These acids are all hygroscopic. They will even ab-
stract water from sulfuric acid.
Impurities.—The commercial glacial acid is commonly, or as a rule,
impure, containing variable amounts of sodium and magnesium phos-
phates. These salts, particularly the dihydrogen (acid) sodium phos-
phate, are permanently soluble in the phosphoric acid, and therefore
give no evidence of their presence by the formation of precipitates.
They are also soluble in water, which fact has a direct bearing upon the
durability of cements made with the impure acid.
It has been stated by writers that the acids of cement were occasion-
ally the meta- and pyrophosphoric. A test of some of them said to be