The Atmosphere. 31 



acre. This supply of carbon dioxide is assimilated from air taken 

 in through the leaf pores or stomata. When united with water 

 brought from the roots, it forms the basal compounds of the 

 plant. The removal of this gas by plants is offset by its return 

 from processes of combustion, fermentation, and animal respira- 

 tion so that there is maintained a nearly constant proportion in 

 the air. When produced by the decay of humus-forming mate- 

 rial, it dissolves in the soil water and becomes a leading factor 

 in liberating plant food from the mineral compounds of the soil. 



Ozone. This gas bears the relation to oxygen of 3 2 

 where 2 is the molecular symbol for oxygen. It is one-half more 

 concentrated than oxygen and as a consequence is much more 

 active. Ozone occurs in the air as a result of the action of 

 electrical discharges upon oxygen. It acts as an antiseptic by at- 

 tacking and destroying bacterial matter. Because of its great 

 activity, it is rapidly exhausted and never amounts to more than 

 a trace in the atmosphere. 



Nitric oxide. Traces of this gas accumulate in the wake of 

 electrical storms. It is a compound of one part of nitrogen with 

 one part of oxygen (14 parts of nitrogen with 16 parts of oxy- 

 gen by weight), the formation of which is induced by electrical 

 discharges. Nitric oxide readily unites with oxygen and water 

 to form nitric acid and washes to the soil in the rain. Knop 

 found ordinary rain water at Leipsig, Germany, to contain 56 

 pounds of nitric acid in 10,000,000 pounds of water, while rain 

 which fell during a thunder shower contained 98 pounds in 10,- 

 000,000. Nitric acid brought to the earth in this way is not free 

 but combined with ammonia in the air. Reaching the soil as 

 ammonium nitrate it is directly available to the plant. 



Ammonia. This gas accumulates in traces in the air as a re- 

 sult of the decay of organic nitrogenous compounds. It is pro- 

 duced in considerable amounts by the rapid fermentation of 

 manures and in such cases may be detected by its pungent odor. 

 It dissolves readily in water and washes to the soil in rains, gen- 

 erally in combination with nitric acid. In this form its nitrogen 



