_I.--Chemical Needs of Life_ Animals, unlike plants, require highly organised atoms for nutriment; they can subsist only upon parts of an organism. All parts of the animal body are produced from the fluid circulating within its organism. A destruction of the animal body is constantly proceeding, every motion is the result of a transformation of its structure; every thought, every sensation is accompanied by a change in the composition of the substance of the brain. Food is applied either in the increase of the mass of a structure (nutrition) or in the replacement of a structure wasted (reproduction). Equally important is the continual absorption of oxygen from the atmosphere. All vital activity results from the mutual action of the oxygen of the atmosphere and the elements of food. According to Lavoisier, an adult man takes into his system every year 827 lb. of oxygen, and yet he does not increase in weight. What, then, becomes of this oxygen?--for no part of it is again expired as oxygen. The carbon and hydrogen of certain parts of the body have entered into combination with the oxygen introduced through the lungs and through the skin, and have been given out in the form of carbonic acid and the vapour of water. Now, an adult inspires 32-1/2 oz. of oxygen daily; this will convert the carbon of 24 lb. of blood (80 per cent. water) into carbonic acid. He must, therefore, take as much nutriment as will supply the daily loss. And, in fact, it is found that he does so; for the average amount of carbon in the daily food of an adult man is 14 oz., which requires 37 oz. of oxygen for its conversion into carbonic acid. The amount of food necessary for the support of the animal body must be in direct ratio to the quantity of oxygen taken into the system. A bird deprived of food dies on the third day; while a serpent, which inspires a mere trace of oxygen, can live without food for three months. The number of respirations is less in a state of rest than in exercise, and the amount of food necessary in both conditions must vary also. The capacity of the chest being a constant quantity, we inspire the same volume of air whether at the pole or at the equator; but the weight of air, and consequently of oxygen, varies with the temperature. Thus, an adult man takes into the system daily 46,000 cubic inches of oxygen, which, if the temperature be 77 deg. F., weighs 32-1/2 oz., but when the temperature sinks to freezing-point will weigh 35 oz. It is obvious, also, that in an equal number of respirations we consume more oxygen at the level of the sea than on a mountain. The quantity of oxygen inspired and carbonic acid expired must, therefore, vary with the height of the barometer. In our climate the difference between summer and winter in the carbon expired, and therefore necessary for food, is as much as one-eighth. _II.--The Cause of Animal Heat_