Test fresh milk with red litmus paper; it should turn
the paper pale blue, showing that it is slightly alkaline. Place aside for
a day or two, and then test with blue litmus paper; it will be found to be
acid. This is due to the fact that lactose undergoes the lactic acid
fermentation. The lactose is converted into lactic acid by means of a
special ferment.

Experiment 52. Evaporate a small quantity of milk to dryness in an
open dish. After the dry residue is obtained, continue to apply heat;
observe that it chars and gives off pungent gases. Raise the temperature
until it is red hot; allow the dish then to cool; a fine white ash will be
left behind. This represents the _inorganic matter_ of the milk.

Experiments with the Sugars.

Experiment 53. Cane sugar is familiar as cooking and table sugar. The
little white grains found with raisins are grape sugar, or glucose. Milk
sugar is readily obtained of the druggist. Prepare a solution of the
various sugars by dissolving a small quantity of each in water. Heat each
solution with sulphuric acid, and it is seen to darken or char slowly.

Experiment 54. Place some Fehling solution (which can be readily
obtained at the drug store as a solution, or tablets may be bought which
answer the same purpose) in a test tube, and boil. If no yellow
discoloration takes place, it is in good condition. Add a few drops of the
grape sugar solution and boil, when the mixture suddenly turns to an
opaque yellow or red color.

Experiment 55. Repeat same experiment with milk sugar.

Chapter VI.

Digestion.

128. The Purpose of Digestion. As we have learned, our bodies are
subject to continual waste, due both to the wear and tear of their
substance, and to the consumption of material for the production of their
heat and energy. The waste occurs in no one part alone, but in all the
tissues.

Now, the blood comes into direct contact with every one of these tissues.
The ultimate cells which form the tissues are constantly being bathed by
the myriads of minute blood-vessels which bring to the cells the raw
material needed for their continued renewal. These cells are able to
select from the nutritive fluid whatever they require to repair their
waste, and to provide for their renewed activity. At the same time, the
blood, as it bathes the tissues, sweeps into its current and bears away
the products of waste.

Thus the waste occurs in the tissues and the means of repair are obtaine