SUMMARY 137 



smelters have been erected for the treatment of sulphide 

 ores, large volumes of sulphur dioxide are discharged into 

 the air during the roasting of the sulphides. This has a 

 very harmful effect on vegetation and has led to legislative 

 action in a number of states. Not only does the gas itself 

 harm vegetation, but its solution in rain water as sulphurous 

 acid is poisonous. In large cities where much soft coal is 

 burned sulphur dioxide is present in the air and this accounts 

 in part for the weak, sickly appearance of trees and grass 

 in such centers of industrv. 



114. Summary. —An important factor in the growth of 

 crops is the atmosphere, not only for valuable constituents 

 which are essential to plant growth, but also for constitu- 

 ents which affect the formation of soils. This atmosphere 

 surrounds the earth to a thickness of about 200 miles and 

 exerts a pressure of a little less than 15 pounds per square 

 inch, or about 45,000 pounds per acre. It is a mixture of 

 gases, vapor, and fine particles of solids, and is usually 

 colorless and odorless. 



One of the very important constituents of the air is 

 ivater vayor which occurs on the average to an extent of about 

 1.3 per cent, by volume. Where the amount increases 

 beyond the saturation point it condenses as fog, rain, or 

 snow. The water vapor serves not only to supply water 

 indirectly for plants, but also to regulate the temperature. 



The nitrogen of the atmosphere is the original source of 

 all nitrogenous compounds. It is removed from the air for 

 the most part by bacteria, and returned when organic 

 matter is burned. Oxygen is present in country air in larger 

 amounts than in city air, because of its evolution by plants, 

 on the one hand, and its absorption when animals breathe 

 and fuel is burned, on the other hand. The amount of 

 carbon dioxide in country and city is just the reverse of that 

 of oxygen. Growing plants use large quantities of this gas, 

 while breathing animals and the combustion of fuel add 

 considerable quantities to the air. The total amount present 

 in the atmosphere, however, is very small. Compounds of 

 nitrogen occur to a very slight extent and are valuable as 

 fertilizers as far as they go. Bacteria are always present in 



