Oxygen is necessary to cause combustion of fats, starches and sugars, just as it is necessary to cause combustion of carbon in wood, or coal. The heat from “burning” wood is produced by the oxygen of the air uniting with hydrogen and carbon, forming carbon-dioxid (carbonic acid gas) and water. The light in the burning of wood is caused by the rapid combustion of the carbonic acid gas. The same combustion occurs within the body continuously, though more slowly, hence no light is produced. The carbon in the body is liberated and brought into contact with more oxygen in the blood through exercise and full breathing, just as a fire is fanned to flame by bringing more oxygen into contact with the fire, by means of a draught of air. Keep all air away from a fire and it “goes out,” or ceases to unite with the oxygen, and no heat is produced; keep all air from within the body, by cessation of breathing, and it also becomes cold. A room in which the air is impure, containing insufficient oxygen, is heated with difficulty,—the body which is not constantly supplied with pure air generates very little body heat. The effect of oxygen in the creation of heat is practically demonstrated by repeatedly filling the lungs with air while out in the cold. The body will become quickly warmed on the coldest day by this practice. Deep breathing aids digestion and assimilation, not alone because of the regular exercise given to the pancreas, the spleen, the stomach, and the liver by the correct movement of the diaphragm, but because of the latent heat which the oxygen liberates within the digestive organs and out among the tissues. While the chemical action of food creates activity within, this activity is materially aided by exercise, and oxygen is imperative, as shown above. Exercise and oxygen are also necessary for chemical action in tearing down waste and in putting raw material into condition to be appropriated to the body needs. _Two glasses of water in the morning and fifteen minutes’ brisk exercise of well selected movements, to start a forceful circulation and to surge the water through the vital organs, are a daily necessity if one is to keep clean and strong within. It is as important to cleanse the body within as without. It is the method employed by all men and women who would retain strong vital forces to a ripe old age. They fully enjoy the mere_ LIVING. * * * * * [Sidenote: “Tired” or Disturbed Balance] Since the condition of the body so materially affects the digestion, absorption, and metabolism of food, as well as the elimination of waste, it is not amiss to discuss it here. The habit of eating when too tired and then at once going to work, so that the blood is called from the stomach, is almost sure to result in indigestion. The average person is tired because the brain and nerves are more active than the muscles and is rested by muscular exercise, or change of work.