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PHOSPHORIC ACID. 231

is obtained by the direct union of its constituents, which takes
place when phosphorus is burned in perfectly dry oxygen gas.
Thus procured it is in the form of a white amorphous powder,
extremely deliquescent, volatilizable at a red heat, and assumes,
when it cools, after fusion, a vitreous appearance.
Glacial Phosphoric Acid^ or monohydrated phosphoric acid, con-
centrated phosphoric acid, is readily obtained from calcined bones,
by first heating them with sulphuric acid, which produces an
insoluble superphosphate of lime ; then dissolving out the latter
salt, and saturating it with carbonate of ammonia, which gener-
ates phosphate of ammonia in solution, and finally obtaining the
phosphate of ammonia by evaporating it to dryness, and then
igniting it in a platinum crucible. The ammonia and all of the
water, except one equivalent for each equivalent of the acid, are
driven off, and the glacial phosphoric acid remains, the formula
of which is HO,?©^, and contains 11.2 per cent, of water. It
is a white, transparent fusible; solid, generally in the form of
sticks, inodorous and sour to the taste. It slowly deliquesces,
and is sparingly soluble in water, but freely soluble in alcohol.
Dental Uses.—Glacial phosphoric acid and white oxide of zinc
formed into an anhydrate, give the plastic material for filling
teeth, known as oxyphosphate of zinc. The powder or solid
portion of this preparation is prepared by packing pure oxide of
zinc in a clay crucible and subjecting it to almost a white heat
for two hours, when it will have been reduced in bulk fifty per
cent. It is then pulverized in a mortar to an almost impalpable
powder. The burning of the oxide of zinc colors it to a light
yellow, and it is now in a condition for use, and may be kept in
a covered vessel for any length of time.
The liquid portion of this filling material being glacial phos-
phoric acid, is prepared by dissolving the acid in pure water until
a saturated solution is obtained, when it is reduced by boiling in
a glass vessel until it is of the consistency of glycerine, in which
operation it loses one-third in bulk. It is now ready for use, and
must be kept in a close glass-stoppered bottle.
Formulae for Fletcher's and Weston's preparations of oxyphos-
phate of zinc filling materials :