# Page:Popular Science Monthly Volume 19.djvu/548

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 ${\displaystyle +}$ C ${\displaystyle =}$ CO ${\displaystyle +}$ 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 car-