smooth the cut surfaces with fine emery or sand paper. Examine both kinds
of sections, noting arrangement and extent of dentine, enamel, and pulp.
Make drawings.

(_c_) Examine a decayed tooth. Which substance of the tooth appears to
decay most readily? Why is it necessary to cut away a part of the tooth
before filling?

(_d_) Test the effect of acids upon the teeth by leaving a tooth over
night in a mixture of one part hydrochloric acid to four parts water, and
by leaving a second tooth for a couple of days in strong vinegar. Examine
the teeth exposed to the action of acids, noting results.

*To show the Importance of Mastication.*—Fill two tumblers each half full
of water. Into one put a lump of rock salt. Into the other place an equal
amount of salt that has been finely pulverized. Which dissolves first and
why?

*To illustrate Acid and Alkaline Reactions.*—To a tumbler half full of
water add a teaspoonful of hydrochloric or other acid, as vinegar. To a
second tumbler half full of water add an equal amount of cooking soda.
Taste each liquid, noting the sour taste of the acid, and the alkaline
taste of the soda. Hold a piece of red litmus paper in the soda solution,
noting that it is turned blue. Then hold a piece of blue litmus paper in
the acid solution, noting that it is turned red. Add acid to the soda
solution, and soda to the acid solution, until the conditions are
reversed, testing with the red and blue litmus papers.

Hold, for a minute or longer, a narrow strip of red litmus paper in the
mouth, noting any change in the color of the paper. Repeat, using blue