The Volume Of Carbonic Acid contained in the expired air forms usually about 4 1/2 per cent of its bulk. The proportion is, however, variable. When respiration is rapid, the percentage of carbonic acid in each breath is diminished, while the total amount exhaled in a given time is increased. When the same air is breathed several times, as happens in crowded rooms, and with deficient ventilation, the percentage of carbonic acid in the expired air is increased. But this accumulation of carbonic acid in the air furnishes an impediment to breathing, independent of the exhaustion of the oxygen; for however often the same air be passed through the lungs, it never contains more than 10 per cent, carbonic acid. In an artificial atmosphere, animals have lived till the percentage of carbonic acid reached 12 and even 18; but when they are placed at once in an atmosphere containing that amount, they are immediately suffocated, no matter how great the amount of oxygen present. It is plain, therefore, that carbonic acid inhaled is actively deleterious, and differs altogether from the nitrogen contained in the atmosphere; nitrogen being simply negative, acting as a diluent of the oxygen, incapable of taking the place of that substance., but in no way interfering with its action. Animals live without discomfort in an atmosphere in which hydrogen is substituted for nitrogen.

The amount of carbonic acid exhaled in a given time goes on increasing in males till thirty years of age, while from forty, or sooner, it diminishes as age advances.

In females the amount ceases to increase at puberty, and remains stationary till the cessation of reproductive activity, when it again increases for a time. The amount of carbonic acid exhaled from the lungs of an adult man may be estimated as sufficient to yield about nine ounces avoirdupois of carbon in twenty-four hours.* But it varies according to a great variety of circumstances, being increased by cold, by food, and most of all by exercise; while warmth, fasting, rest, and sleep diminish it.