It is estimated that the lungs of a man from thirty-five to forty years old will contain about 225 cubic inches of air; it is less before that age, and rails to a little less than 200 cubic inches at sixty years of age. The capacity is smaller in women, and varies also according to the individual. It is only possible to obtain approximate experimental results, as the lungs are not completely emptied at each expiration, and the cells always retain a quantity of air, and this quantity is greater in proportion as the respiration is calm and shallow.
It is clear from the foregoing, that the air which is expired has neither the same volume nor the same proportion of constituent elements as the air which is inspired. In fact, an adult man absorbs by respiration from 450 to 550 grains of oxygen in an hour. He exhales in the same time 632 grains of carbonic acid; a less quantity of nitrogen, amounting to about a hundredth of the oxygen absorbed; and lastly, about 9720 grains of water in the form of vapour. This exhalation of water by the lungs constitutes the pulmonary perspiration, a function analogous to the perspiration of the skin. The expired air, as already stated, is deprived of a portion of its oxygen, and is charged with carbonic acid gas. The proportion of this gas is about 4 parts in 100. From 350 to 400 cubic feet of air are taken into the lungs in 24 hours, and rapidly changed, and the gravest consequences result from placing a man under conditions in which the air cannot be renewed. During the English war in India in the last century, one hundred and forty-six prisoners were shut up in a room scarcely large enough to hold them, into which the air could only enter by two narrow windows; and at the end of eight hours only twenty-three remained alive, and these were in a most deplorable condition. Percy relates that after the battle of Austerlitz, three hundred Russian prisoners were confined in a cavern, and two hundred and sixty of these unfortunates perished in a few hours from asphyxia.