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20 MECITANICAl, DENTISTRY AND METALLURGY.

carljonate of soda; the surface will be converted into car-
bonate of lime, which will render it hard and durable. Or
the model may be boiled in a strong solution of alum. Or
add to the plaster from five to six per cent, of powdered
alum, or the same amount of ammonium chlorid, before
mixing- with water.
To retard the setting of plaster, mix with it before
adding the water, about three per cent, of powdered althsea
root. But where for any reason it is desirable to retard the
setting for a half hour longer, about eight per cent, of
althcca root should be added. This not only retards the
setting of the plaster but enables it to be sawed, filed or
turned.
The Separating Fluids are very easily made and are in-
expensive, h'or the soap solution take of castile soap one
ounce, and dissolve in a pint of hot water. This solution
should then be kq)t bottled. In preparing shellac varnish,
take of pure gum shellac one ounce and alcohol about a
half pint; this should be digested over a moderate heat
until thoroughly dissolved, when it should be kept securely
bottled.
One of the most convenient methods for keeping such
a solution free from dust and other foreign substances, and
at the same time keep it air-tight, so that the preparation
will not become too thick by evaporation of the alcohol, is
to use the preparation cup known as the " clover leaf," which
is shown in Fig. 7.
Taking the Bite and Antagonizing the Models.—A cor-
rect articulation is of as great importance as any other fea-
ture in the construction of an artiticial denture, for no
matter how perfect the impression and model, or how thor-
oughly and artistically the plate may be finished, if the
articulation is not correct the entire piece of work is prac-
tically a failure. It is in taking the bite for full cases,