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SELECTION AND ARRANGEMENT OF TKETII. 43

Fig. 19, or where the hps are of suificient length to entirely
conceal the gums.
When gum teeth are used every care should he taken to
so grind the joints that their surfaces will come in perfect
contact, and not, as is so commonly practiced, make
\^-shaped spaces. As each block is thus ground, it should
be secured to the trial plate, and so on until all the teeth
are in position. The wax plate should then Ije neatly
shaped to resemble the finished plate, when it is ready to
l)e tried in the mouth. Then after making alterations,
if necessary, the plate should be replaced upon the cast and
secured by passing a hot spatula around its edge, which will
prevent the plaster in flasking from running between the
plate and the cast.
The practice of packing ill-fitted joints in order to make
them impervious to the rubber and to hide poor workman-
ship, is a very poor expedient, as in a short time these ma-
terials will yield to the action of the fluids of the mouth,
when the joints will become receptacles for the secretions,
and soft particles of food. There is nothing that will so
surely and eft'ectively exclude the rubber as accurate joint-
ing and careful packing of the rubber.

FLASKING, VULCANIZING, AND FINISHING
THE RUBBER BASE.

Before entering upon a study of the manipulation of
rubber as a dental base, we will consider the composition
of the different rubbers used for this purpose. The bases
of these compounds is caoutchouc, commonly called India
rubber or gum elastic*
* Caoutchouc is a milky, concrete juice, obtained principally from the
Siphonia elastica, or Siphonia Cahuchu, a South American tree. It is
obtained by tapping the tree, and is of a yellowish-white color, but