The blood thus becomes purified and reinvigorated, and at the same time is changed in color from purple to scarlet, from venous to arterial. It is now evident that if this interchange is to continue, the air in the cells must be constantly renewed, its oxygen restored, and its excess of carbon dioxid removed. Otherwise the process just described would be reversed, making the blood still more unfit to nourish the tissues, and more poisonous to them than before. 215. Change in the Air in Breathing. The air which we exhale during respiration differs in several important particulars from the air we inhale. Both contain chiefly the three gases, though in different quantities, as the following table shows. Oxygen. Nitrogen. Carbon Dioxid. Inspired air contains 20.81 79.15 .04 Expired air contains 16.03 79.58 4.38 That is, expired air contains about five per cent less oxygen and five per cent more carbon dioxid than inspired air. The temperature of expired air is variable, but generally is higher than that of inspired air, it having been in contact with the warm air passages. It is also loaded with aqueous vapor, imparted to it like the heat, not in the depth of the lungs, but in the upper air passages. Expired air contains, besides carbon dioxid, various impurities, many of an unknown nature, and all in small amounts. When the expired air is condensed in a cold receiver, the aqueous product is found to contain organic matter, which, from the presence of _micro-organisms_, introduced in the inspired air, is apt to putrefy rapidly. Some of these organic substances are probably poisonous, either so in themselves, as produced in some manner in the breathing apparatus, or poisonous as being the products of decomposition. For it is known that various animal substances give rise, by decomposition, to distinct poisonous products known as _ptomaines_. It is possible that some of the constituents of the expired air are of an allied nature. See under “Bacteria” (Chapter XIV). At all events, these substances have an injurious action, for an atmosphere containing simply one per cent of pure carbon dioxid has very little hurtful effect on the animal economy, but an atmosphere in which the carbon dioxid has been raised one per cent by breathing is highly injurious. The quantity of oxygen removed from the air by the breathing of an adult person at rest amounts daily to about 18 cubic feet. About the same amount of carbon dioxid is expelled, and this could be represented by a piece of pure charcoal weighing 9 ounces. The quantity of carbon dioxid, however, varies with the age, and is increased also by external cold and by exercise, and is affected by the kind of food. The amount of water, exhaled as vapor, varies from 6 to 20 ounces daily. The average daily quantity is about one-half a pint.