The liberation of energy by oxidation, or burning, at a low tern-perature and in the presence of moisture, is such a fundamental fact in physiology, and its essential agreement with ordinary combustion so difficult to grasp by most pupils, who naturally associate burning with a high temperature and luminosity, that it is worth while to illustrate these facts by a few simple experiments.

1. Buy a coil of magnesium wire, which can be obtained at small cost. Rub it clean with fine emery paper : cut it in two, apply a lighted match to one half and show how it is rapidly consumed with the evolution of light and heat, leaving behind only a white powder, magnesia, which is oxidized magnesium. Put the other half away in a bottle with a few drops of water. After a day or two its surface will be covered by a layer of magnesia; if this be scraped off another will succeed it; and so on. This experiment shows that oxidation may occur rapidly at a high temperature in a short time, or slowly at a low temperature in a long time, but the ultimate product, in each case, is the same.

2. In relation to a subsequent paragraph (p. 132) the magnesia obtained by burning the wire in the air, may be kept, and attempts made to ignite it; this will serve to show the uselessness of oxidized substances to the body, as sources of energy : they cannot be any more oxidized, and the best thing to do is to get rid of them.

3. Get a bundle of iron wire : rub it bright with emery or sand paper. Place some in a warm dry bottle by the stove or fireplace. Put the rest in a bottle containing a little water. Next day the first specimen will be found bright, and the second covered with rust. This shows that oxidation may sometimes occur better in the presence of water than in its absence ; and serves as a text for pointing out how oxidations occur in the moist tissues of the body.