Experiment 50.  Boil the milk as before, and allow it to cool; then add rennet.  No coagulation will probably take place.  It is more difficult to coagulate boiled milk with rennet than unboiled milk.

Experiment 51.  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 obtained from the blood.  The blood is thus continually being impoverished by having its nourishment drained away.  How, then, is the efficiency of the blood maintained?  The answer is that while the ultimate purpose of the food is for the repair of the waste, its immediate destination is the blood.[19]