COMPOSITION OF THE AIR 135 



city air contains 0.05 to 0.07 per cent., and the carbon dioxide 

 in the air of crowded auditoriums may rise to 0.5 per cent. 

 At night the amount of carbon dioxide is greater than during 

 the day, due to the inactivity of plants in the dark. Since 

 one man exhales about 550 liters of carbon dioxide daily, 

 and a ton of coal gives off in burning about 1,500,000 liters 

 of carbon dioxide, it is easy to account for the higher pro- 

 portion of carbon dioxide in the air of cities. In the country 

 where there are many plants and a large area of leaf surface 

 absorbing carbon dioxide, the amount is naturally less (see 

 Frontispiece). An acre of corn, for example, at the height 

 of the growing season would absorb about 10,000 liters of 

 carbon dioxide per day, and it has been estimated that an 

 acre of forest uses up about 6000 liters per day. In addition 

 to these compensatory changes in the amount of carbon 

 dioxide in the air there are volumes poured into the air by 

 some volcanoes and other openings in the earth. The 

 decay of organic matter causes evolution of carbon dioxide; 

 the weathering of rocks on the other hand uses up some 

 carbon dioxide (Section 134). The amounts of carbon 

 dioxide absorbed do not balance the amounts given off into 

 the air. The ocean apparently acts as a regulator of the 

 amount. When there is any increase in the percentage of 

 carbon dioxide in the air this naturally increases the pressure 

 of carbon dioxide on the surface of the water and some is 

 dissolved and changed to bicarbonate of calcium. On the 

 other hand a diminution in the amount causes a lowering 

 in the pressure and some bicarbonate of calcium changes to 

 carbonate again and releases carbon dioxide. In this way 

 there is maintained a fairly uniform amount of carbon 

 dioxide in the air. 



(d) Argon and Hydrogen. — Argon is one of the so-called 

 rare elements. It has no agricultural bearing, and is so 

 inert that it will unite with no other element. In fact its 

 name means that it will not work. In addition there are 

 several other rare gases existing in much smaller amounts, 

 but none of them is of any importance. Hydrogen, also, 

 although present in the air in fairly constant quantities, 

 is of no agricultural value and need not be considered. 



