Experiment 49. Take one or two teaspoonfuls of fresh milk in a test tube; heat it, and add a small quantity of extract of rennet. Note that the whole mass curdles in a few minutes, so that the tube can be inverted without the curd falling out. Soon the curd shrinks, and squeezes out a clear, slightly yellowish fluid, the whey.
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.