f 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
litmus paper. What effect, if any, has the saliva upon the color of the
papers? Has the mouth an acid or an alkaline reaction?

*To show the Action of Saliva on Starch.*--1 (Optional). Prepare starch
paste by mixing half a teaspoonful of starch in half a pint of water and
heating the mixture to boiling. Place some of this in a test tube and thin
it by adding more water. Then add a small drop of iodine solution (page
136) to the solution of starch. It should turn a deep blue color. This is
the test for starch.

Now collect from the mouth, in a clean test tube, two or three
teaspoonfuls of saliva. Add portions of this to small amounts of fresh
starch solution in two test tubes. Let the tubes stand for five or ten
minutes surrounded by water having about the temperature of the body. Test
for changes that have occurred as follows:

(_a_) To one tube add a little of the iodine solution. If it does not turn
blue, it shows that the starch has been converted into some other
substance by the saliva, (_b_) To the other tube add a few drops of a very
dilute solution of copper sulphate. Then add sodium (or potassium)
hydroxide, a few drops at a time, until the precipitate which first forms
dissolves and turns a deep blue. Then gradually heat the upper portion of
the liquid to boiling. If it turns an orange or yellowish red color, the
presence of a form of sugar (maltose or dextrose) is proved. See page 136.

2. Hold some powdered starch in the mouth until it completely dissolves
and observe that it gradually acquires a sweetish taste. This shows the
change of starch into sugar.

*To illustrate the Action of the Gastric Juice.*--Add to a tumbler two
thirds full of water as much scale pepsin (obtained from a drug store) as
will stay on the end of the large blade of a penknife. Then add enough
hydrochloric acid to give a slightly sour taste. Place in the artificial
gastric juice thus prepared some boiled white of egg which has b