The Atmosphere which we breathe consists of 79 volumes of nitrogen with 21 of oxygen, containing in every 10,000 volumes four or five of carbonic acid. It further contains minute quantities of accidental gaseous impurities, and has constantly numbers of exceedingly small particles of organic matter floating about in it, including germs of different kinds of mould. The degree of moisture varies at different times, but the amount which it is capable of dissolving increases rapidly with rise of temperature.

The air exhaled in expiration differs from what has been breathed, in temperature, moisture, and the quantity of carbonic acid and oxygen which it contains.

The temperature of the exhaled air is approached to that of the blood, varying usually from 97° to above 99°F., according to the rapidity of the respiration and the temperature of the surrounding air. When respiration is slow, the air has longer time to become assimilated in temperature to the interior of the body; and when the surrounding temperature is low, a longer contact is required to approach it to blood heat.

The air exhaled is always nearly saturated with moisture, however dry it may have been when taken in; and therefore the maximum of water is removed from the body by this channel during exercise in air which is cold and dry; for then the respiration is active, and the air admitted and warmed within the chest requires most moisture for its saturation. The average amount of water thus removed has been calculated to be from nine ounces to more than a pound in twenty-four hours.