*Summary.*—Materials called foods are introduced into the body for rebuilding the tissues, supplying energy, and aiding in its general work. Only a few classes of substances, viz., proteids, carbohydrates, fats, water, and some mineral compounds have all the qualities of foods and are suitable for introduction into the body. Substances known as drugs, which may be used as medicines in disease, should be avoided in health. Before foods can be passed into the body proper, they must be converted into the liquid form, or dissolved. In this process, known as digestion, water is the solvent; and certain chemical agents, called enzymes, convert the insoluble nutrients into substances that are soluble in water. *Exercises.*—1. How does oxidation at the cells make necessary the introduction of new materials into the body? 2. What different purposes are served by the foods? 3. What is a nutrient? Name the important classes. 4. What are food materials? From what sources are they obtained? 5. Name the different kinds of proteids; the different kinds of carbohydrates. Why are proteids called nitrogenous foods and fats and carbohydrates non-nitrogenous foods? 6. Show why life cannot be carried on without proteids; without water. 7. What per cents of proteid, fat, and carbohydrate are found in wheat flour, oatmeal, rice, butter, potatoes, round beef, eggs, and peanuts? 8. State the objection to a meal consisting of beef, eggs, beans, bread, and butter; to one consisting of potatoes, rice, bread, and butter. Which is the more objectionable of these meals and why? 9. State the general plan of digestion. 10. Show that digestion is not a simple process like that of dissolving salt in water. PRACTICAL WORK *Elements supplied by the Foods.*—The following brief study will enable the pupil to identify most of the elements present in the body and which have, therefore, to be supplied by the foods. _Carbon._—Examine pieces of charred wood, coke, or coal, and also the "lead" in lead pencils. Show that the charred wood and the coal will burn. Recall experiment (page 114) showing that carbon in burning forms carbon dioxide. _Hydrogen._—Fill a test tube one third full of strong hydrochloric acid and drop into it several small scraps of zinc. The gas which is evolved is hydrogen. When the hydrogen is coming off rapidly, bring a lighted splinter to the mouth of the tube. The gas should burn. Hold a cold piece of glass over the flame and observe the deposit of moisture. Hydrogen in burning forms water. Extinguish the flame by covering the top of the tube with a piece of cardboard. Now let the escaping gas collect in a tumbler inverted over the tube. After holding the tumbler in this position for two or three minutes, remove and, keeping inverted, thrust a lighted splinter into it. (The gas should either burn or explode.) What does this experiment show relative to the weight of hydrogen as compared with that of air?