Page:Popular Science Monthly Volume 19.djvu/548

532 per minute, and takes in over twenty cubic inches of air at each inspiration. Boys and girls inspire somewhat less than twenty cubic inches, but breathe more rapidly than an adult—say twenty-five times per minute. In five minutes each will breathe over a cubic foot of air, and in a two-hours session nearly twenty-five cubic feet: so that, in a school of forty pupils, one thousand cubic feet will be inhaled every two hours. This is under, rather than above, the average.

Oxygen to the amount of nearly five per cent, of the quantity inhaled disappears at every breath, being absorbed by the blood—or twenty cubic inches per minute, for each individual—representing a total of fifty cubic feet for a school of forty pupils during a two-hours session. But, in addition to the consumption of oxygen, the air is further deteriorated by the exhalation of nearly as much carbonic-acid gas (CO2) as there is oxygen consumed—say forty-five cubic feet in two hours, about one fortieth of the total amount produced being thrown off by the cutaneous surface of the body. Each cubic foot of carbonic-acid gas contains nearly half an ounce of pure carbon, or twenty-three ounces in all: so that, by breathing, forty mouths—like veritable little chimneys—puff out in two hours an amount equal to about a pound and a half of solid carbon. This is injurious in two ways, each of which will be examined in the proper place.

The air occasionally contains many impurities, but only those usually found in the school-room will here be enumerated. They are carbonic oxide (CO), carbonic-acid gas (CO2), ammonia (NH3), sulphur (S), sulphuretted hydrogen (H3S) all in the gaseous form; to which must be added aqueous vapor, organic matters, inorganic matters, epithelial cells, and animal exhalations.

The most toxic of all these is undoubtedly carbon monoxide (CO). It is a product of the incomplete combustion of carbon (C), but happily it is not usually found in the school-room in any large amount. A fire is the result of the chemical combination of the carbon of coal, or other combustible, with the oxygen (O) of the air; the atoms of the gas rush into combination with those of the carbon, and the arrested motion is transformed into heat—aqueous vapor (H2O), carbon monoxide (CO), and carbonic-acid gas (CO2) being produced. If a sufficient supply of air has free access to the lower portions of the fire, carbonic-acid gas is directly formed; but this in its passage upward through the central portion of the fire, where the temperature is higher, takes up another atom of carbon (CO2 $$+$$ C $$=$$ CO $$+$$ CO) and becomes carbon monoxide, or carbonic oxide, as it is commonly called. This carbonic oxide, on reaching the upper surface of the fire, takes up an additional atom of oxygen from the air, and, burning with a bluish flame, becomes carbonic-acid gas once more, and makes its escape by the chimney. But usually a portion of the carbonic oxide fails to take up the additional atom of oxygen; and, when the supply of air is limited, the amount is increased, so that more or less