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338 PLASTIC FILLING MATERIALS.
of these varieties showed none of them to give the reaction of the pyro-
acid ; a few giving traces of the mota- acid.
Precipitates which form in cement flnids are probably metallic ])hos-
phates. The instability of cement fluids is notorious. Aside from the
known or probable contaminations which they may contain this insta-
bility is to be regarded as a distinctive feature of phosphoric acid.
The Cement.—To make the cement, successive portions of the oxid
are mechanically incorporated with the fluid until a stiif paste results.
In five minutes a ball made of the paste glazes, and rebounds when
dropped upon a hard surface. It breaks with a granular surface ; in
fifteen minutes it is cut with some difficulty. If the cement fluid con-
tain the acid sodium phosphate, an acid reaction may remain for hours
or days. The atmospheric conditions markedly modify the properties.
In warm, or hot and moist weather, the setting is more rapid and it
may be sudden. In cold weather it is delayed. The greater the dilu-
tion (the thinner the fluid), the more rapid the setting.
In its freshly mixed state zinc phosphate is adhesive, losing this
property in a great degree when set, if surrounded by moisture. It has
a higher rate of heat conductivity than zinc oxychlorid.
Uses.—Its legitimate field of usefulness is in situations and under
conditions where its advantageous properties may be utilized and its
disadvantages minimized. One of the principal facts to be borne in
mind is the solubility of the cement in lactic acid, M'hich is present
almost always about the necks of the teeth, in approxiraal spaces, and
along gingival margins. Its clinical use is therefore attended by the
greatest measure of success when placed at a distance from such situa-
tions—as, for example, in cavities opening upon the masticating sur-
faces of teeth, where its great hardness is an element of advantage.
Good specimens have been known to last for periods varying from
three to eight years. Dr. Henry Weston has cited cases where ah un-
usually good zinc phosphate filling has lasted for ten years.
As a filling material 'per se, zinc phosphate has but limited employ-
ment except for the teeth of children, and as a temporary filling in the
teeth of adults. Times and occasions will suggest themselves to every
operator where gold, amalgam, and gutta-percha are contraindicated as
filling materials ; in such cases zinc phosphate performs a useful ser-
vice. Its great field of usefulness—where, indeed, there is no substi-
tute for it— is in the filling of the greater portion of extensive cavities,
which are then filled and sealed with gold or amalgam, by an inlay, or it
may be by a partial crown. It is invaluable, and in most cases indispen-
sable, as the retaining medium of fixed bridge work and of many forms
of artificial crowns.
Prior to placing the zinc phosphate filling in a cavity, it is a wise