532 THE POPULAR SCIENCE MONTHLY. 



per minute, and takes in over twenty cubic inclies of air at each inspi- 

 ration. 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 fur- 

 ther deteriorated by the exhalation of nearly as much carbonic-acid 

 gas (COj) 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 usu- 

 ally found in the school-room will here be enumerated. They are car- 

 bonic oxide (CO), carbonic-acid gas (COJ, ammonia (Nllg), sulphur 

 (S), sulphuretted hydrogen (HgS) 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 hap- 

 pily 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 (H^O), carbon monox- 

 ide (CO), and carbonic-acid gas (COJ, being produced. If a sufli- 

 cient 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 (CO^ + 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 sup- 

 ply of air is limited, the amount is increased, so that more or less car- 



