*To illustrate the General Movement of Oxygen through the Body.*—Into a glass tube, six inches in length and open at both ends, place several small lumps of charcoal (Fig. 59). Fit into one end of this tube, by means of a stopper, a smaller glass tube which is bent at right angles and which is made to pass through a close-fitting stopper to the bottom of a small bottle. Another small tube is fitted into a second hole in this stopper, but terminating near the top of the bottle, and to this is connected a rubber tube about eighteen inches in length. The arrangement is now such that by sucking air from the top of the bottle, it is made to enter at the distant end of the tube containing the charcoal. After filling the bottle one third full of limewater, heat the tube containing the charcoal until it begins to glow. Then suck the air through the apparatus (as in smoking, without drawing it into the lungs), observing what happens both in the tube and in the bottle. What are the proofs that the oxygen, in passing through the tube, unites with the carbon, forms carbon dioxide, and liberates energy? Compare the changes which the oxygen undergoes while passing through the tube with the changes which it undergoes in passing through the body. CHAPTER IX - FOODS AND THE THEORY OF DIGESTION The body is constantly in need of new material. Oxidation, as shown in the preceding chapter, rapidly destroys substances at the cells, and these have to be replaced. Upon this renewal depends the supply of energy. Moreover, there is found to be an actual breaking down of the living material, or protoplasm, in the body. While this does not destroy the cells, as is sometimes erroneously stated, it reduces the quantity of the protoplasm and makes necessary a process of repair, or rebuilding, of the tissues. This also requires new material. Finally, substances, such as water and common salt, are required for the aid which they render in the general work of the body. Since these are constantly being lost in one way or another, they also must be replaced. These different needs of the body for new materials are supplied through *The Foods.*—Foods are substances that, on being taken into the healthy body, are of assistance in carrying on its work. This definition properly includes oxygen, but the term is usually limited to substances introduced through the digestive organs. As suggested above, foods serve at least three purposes: 1. They, with oxygen, supply the body with energy. 2. They provide materials for rebuilding the tissues. 3. They supply materials that aid directly or indirectly in the general work of the body.