CONSTITUTION OF THE ATMOSPHERE 223 



About 75 per cent of common bituminous coal is carbon. The car- 

 bon of a ton (2,000 Ibs.) of such coal, united with oxygen from the 

 air (3 Ibs. of carbon unite with 8 Ibs. of oxygen), would make about 

 2 J4 tons of carbon dioxide, all of which goes into the atmosphere. 

 A ton of hard coal, which contains more carbon, would produce 

 still more carbon dioxide. If we knew the number of tons of 

 coal burned daily, we could calculate the amount of carbon dioxide 

 poured into the atmosphere daily, as a result of its burning. 

 Nearly a billion (1,000,000,000) tons of coal are mined each year, 

 and most of this is burned. When all other sources of carbon 

 dioxide are considered, it seems safe to say that carbon dioxide 

 is being supplied to the atmosphere at the rate of several billions 

 of tons per year. Yet the amount in the air does not increase 

 enough to be noted. It must be, therefore, that this gas is being 

 taken out of the atmosphere about as rapidly as it comes in. It is 

 taken from the air chiefly (1) by green plants, of which it is the 

 chief food, and (2) by union with mineral matter. 



It will be seen that some of the CO 2 is making a continuous 

 round of change. It is taken out of the air by plants, and its con- 

 stituents, or some of them, become a part of the plant. In this 

 process some of the oxygen is set free in the air. The carbon of 

 the plant is then burned, either in a fire or by decay, and the car- 

 bon dioxide thus produced passes back into the air, to be used 

 by plants again. Much carbon dioxide goes through this round 

 each year, for much vegetation grown during one growing season is 

 burned or partially decayed before the next. 



The supply and loss of carbon dioxide so nearly balance, that 

 no change in the amount of this gas in the air is noted from year to 

 year; but it seems quite possible that in the course of long periods 

 of time the supply may have exceeded the loss, or that the loss may 

 have exceeded the supply. 



Small as the amount of carbon dioxide is, it has an important 

 function besides supplying food to plants. The earth is constantly 

 radiating heat into space, somewhat as a hot stove radiates heat 

 into its surroundings, and carbon dioxide has the power of holding 

 much of this heat. It therefore serves as a blanket to hold in the 

 heat of the earth, and thin as the blanket is, it is more effective, 



