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GUTTA-PERCHA AND GOLD— VAEIOVS KINDS OF GOLD. 361

percha or cement is cocoa butter. A small porcelain druggist's jar
into Avhicli it has been melted is convenient to have on the operating
table. Plastic fillings will rarely stick to instruments that have been
rubbed on cocoa butter. If a shaving of it is placed on a completed
cement filling it will instantly melt and flow over the entire surface,
preventing the disagreeable taste when the dam is removed, and will
keep it from contact with the saliva for some time.

Gutta-percha and Gold.
For many years it has been the habit of some good operators to fill
the interior of large cavities with gutta-percha, covering it with gold.
Although this may not be objectionable practice in some cases, it cer-
tainly cannot be recommended for general use. The jH'incipal objection
to it is the danger of frail walls being fractured by the subsequent
expansion of the gutta-percha. So many instances have been noticed
where fracture has followed this combination that the fact seems well
demonstrated that this danger exists. Again, there is no need of com-
bining these two materials when zinc phosphate, which is so much
better dian gutta-percha for this purpose, is available and does not pos-
sess the dangerous quality of expansion attributed to gutta-percha.

Gutta-percha and Amalgam.
What has been said in regard to gutta-percha and gold will apply
equally well to gutta-percha and amalgam. Rarely, if ever, can this
combination be used to so good advantage as can zinc phosphate and
amalgam.
Various Kinds of Gold in Combination.
(A) The So-called Plastic or Crystal Mat Gold, with Other
Porms of Gold.—Within a few years, preparations of gold other
than that known as foil, or foil made into cylinders, ropes, and so
forth, have been introduced and have become of great value in the
filling of teeth.
These golds are commonly known as " plastic gold." The term is,
however, misapplied. The granular quality of these gold preparations,
i. e. lack of fiber, is what gives them their peculiar and, for certain
purposes, very valuable working qualities. To understand this charac-
teristic, take a ])iece of White's " crystal mat gold " and place it upon
a piece of blotting pa])er, then press the point of a medium-sized gold
packer upon the centre. It will be observed that when the pressure is
applied the gold is not inclined to curl up, but rests in its flat posi-
tion, and the instrument has cut a clean track in the gold, condensing
only that which is directly under the point. The gold being without