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THE CEMENTS. 131

firmly held between the filling and the proximating tooth.
Any attempt to pull this out is liable to injure the filling. In
this case straighten out the ends of the matrix to the
lingual and to the buccal and reapply the separator with
the matrix between the claw^s. Then tighten the sepa-
rator sufficiently to loosen the matrix and remove it.
Now, while the separator is in place, with the finishing knives
trim the proximate surface to form and remove all overlaps about
the margins. Then burnish lightly, remove the separator and
the rubber dam. Be careful to note the occlusion, and if
a cusp of an opposing tooth strikes the filling, trim it
down so that there may be no danger of injury to the fill-
ing from that cause.
Trimming to form as described is very important, because it
can be done at this time while the amalgam is still but partially
hardened, with ease, and very quickly. If we wait until the
amalgam is fully hard it is a tedious, difficult operation.
The patient may now be dismissed for one, two or three
days, as may be mutually convenient, or until the amalgam is
fully hard. Then the whole filling must be polished. This
polishing is practically the same, is done with the same
instruments as a gold filling, and must be done with the same
care. In proximate fillings it is very often necessary to re-
apply the separator to properly finish the proximate surface
and the contact point.
The Cements.
The cements at present available for use in filling operations
are the oxy-chloride of zinc and the oxy-phosphate of zinc. The
composition of these you have already learned in the chemical
laboratory. Formerly the oxy-chloride of zinc cement was much
used, but since the introduction of the oxy-phosphate it has fallen
into disuse on account of the very irritating qualities of its fluid
portion. The phosphate of zinc cement is very much less irrita-
ting and is generally preferred for this reason. The cements are
received from the dealer in two separate bottles, the one contain-
ing the powdered oxide of zinc and the other the fluid. In use
a small portion of the powder is placed upon a porcelain or glass
slab, and a drop or two of fluid is placed beside it. The two are
then drawn together and thoroughly mixed by rubbing them