Burning and life.—When a piece of the flesh of any animal has been dried it may easily be set on fire. The burning is caused by the union of the constituents of the flesh with some of the oxygen of the air to form various gases. One of these gases is carbon dioxide, formed by the combination of the carbon of the flesh with oxygen. Carbon is present in all the parts of animals and plants, as is evident from the separation of charcoal (an impure form of carbon) in the first stages of burning; the carbon dioxide gas which is formed may easily be recognised by the milkiness which it produces in clear lime-water. The liberation of heat, and the formation of carbon dioxide, which always accompany the burning of animal and plant tissues, are worthy of very careful attention. Fig. 168.—Experiment to prove that expired air contains carbon dioxide. It is well known that the body of a living animal such as a rabbit or a man is always warm; and the experiment (Fig. 168) of passing through clear lime-water the air breathed out from the lungs shows, by the milkiness produced, that the animal is also constantly producing carbon dioxide during its life. Is life, then, always accompanied by a peculiar form of burning, in which the living substance of the body is the combustible material? It seems so, and the experiments of physiologists tend to confirm this view. The necessity for breathing.—The energy which enables a muscle to contract is derived from the oxidation—the slow burning—of part of its substance, just as the energy which enables a steam-engine to move is derived from the burning of fuel in the boiler fires. The fires soon go out, and the engine stops, unless fresh fuel is added from time to [Pg 242] time and a plentiful supply of air is available. Similarly, a muscle loses its power of contracting, a gland that of secreting, the brain that of thinking, unless the waste matters resulting from previous activities are cleared away and replaced by fresh food and fresh oxygen. Wherever vital action is taking place, whether in a contracting muscle, a secreting gland, or a thinking brain, there is continual consumption of oxygen and continual production of waste material, chiefly carbon dioxide. In the higher animals the renewal of oxygen and the removal of waste material are performed by the blood. Blood-vessels are to the body what rivers and canals are to a country: they act as highways for the transport of material. We may perhaps carry the analogy a little farther and find in the red corpuscles of the blood the rough equivalents of boats or canal-barges, for they carry with them tiny loads of oxygen. As the blood creeps along the narrow channels in an active tissue the red corpuscles relinquish their oxygen, and the fluid portion of the blood takes up carbon dioxide. The blood continues its course, and sooner or later arrives at a place where it can obtain a fresh supply of oxygen and get rid of its surplus