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62 MECHANICAL DENTISTRY AND METALLURGY.
spaces, are then carefully packed with rubber, when it is
vulcanized and finished in the usual way.
The old method of " under-cutting " in repairing rubber
plates is unnecessary, a perfect union being obtained in
such cases if the surfaces of contact are freshly cut, abso-
lutely clean, and properly roughened.

CELLULOID AS A DENTAL BASE.

In taking up the study of celluloid. Dr. Richardson in his
treatise upon " Mechanical Dentistry " says : That celluloid
possesses many important qualities which commend its em-
ployment as a base in preference to rubber can hardly be
Cjuestioned. It is more in harmony with the soft tissues
of the mouth, more cohesive in texture, approximates more
nearly the natural gum color, contains far less vermilion
pigment in its composition, and is less objectionable by
reason of the comparative cleanliness accompanying its
manipulation.
The chief objection urged against celluloid as a base is
its low power of transmitting caloric, but it is believed to
be less objectionable in this respect than rubber. Both
are poor conductors, and the soft tissues of the mouth in
contact with either sufifer, in some degree, as a consequence
of this property.
Celluloid, as at present produced and when properly
manipulated, does not, in any appreciable degree, undergo
change of form after molding by warping either in or out
of the mouth, as was formerly the case, nor, it is believed,
does it absorb the oral secretions. It loses somewhat the
freshness and clearness of its original pink color after
having been in use for some time, in many cases in a very
marked degree.
Though not bearing so perfect a resemblance to the com-