Living matter contains hydrogen, oxygen, sulphur, chlorine, iodine, fluorine, nitrogen, phosphorus, carbon, silicon, potassium, sodium, calcium, magnesium, and iron. The removal of one of these elements causes the death of the body. They must be arranged in a definite, prescribed order to constitute cellular protoplasm, and any disarrangement of this order causes intoxication, disease, or death. Hydrogen is a constant product in the putrefaction of animal matter, of animal food, and is present in the intestinal tract. Oxygen is found dissolved in water and loosely combined in blood as oxyhemoglobin. All the elements, except fluorine, combine with oxygen, forming oxides, and the process is called oxidation. The production of heat and all vital motion depend on oxidation, decomposition of matter. In the nuclei of cells there is a so-called "oxygen-carrier," a nucleo-proteid, which contains iron, and this appears to be the chief oxidizing agent in the body. Chlorine, which in hydrochloric acid is essential to digestion, is ingested as chloride, and leaves the body chiefly through the urine and sweat. Iodine is a necessary part of the thyroid gland, an indispensable vital organ. Fluorine is found in all cells. Nitrogen goes into the body combined in proteids; and phosphorus, combined in the alkalies and alkaline earths of the foods. Carbon occurs in all cells and leaves them through the lungs as carbon dioxide. The amount of energy set in action in the body in the decomposition of any food is equal to the energy that had been expended in the synthesis of that food from its organic elements, and the liberated energy set free in the body appears as heat, work, and nervous impulse. In a plant the chlorophyl and the sun's rays combine water and the carbon dioxide of the air into sugar and free oxygen. This sugar is changed in a plant into starch, cellulose, and fat, and also, when combined with some nitrogen, into proteid. An animal eats this plant, which contains starch, cellulose, fat, and proteid, and it either adds these ingredients to its own substance or oxidizes them so as to prevent the destruction of its own substance. These are the ends of all food. Broadly speaking, plants synthesize elements; animals analyze them, reduce them into simpler bodies.