ATMOSPHEltlC AIU AS THE FOOD OF PLANTS. 85 



is twice as rich iu nitrogen as the stubble of ripened grain. 

 This is a result of the fact that tlie clover is cut when in 

 active growth, while the grain is harvested after tlie roots, 

 stems, and leaves, have been exhausted of their own juices 

 to meet the demands of the seed. 



Whatever may be the value of our explanations, the 

 fact is not to be denied that the soil is enriched in nitrogen 

 by the culture of large-leaved plants, whicli are harvested 

 while in active growth, and leave a considerable propor- 

 tion of roots, leaves, or stubble, on the field. On the other 

 hand, the field is impoverished in nitrogen Avhen grain 

 crops are raised upon it. 



Formation of IVUric Acid from Ammonia. — Ammonia 

 (carbonate of ammonia) under tlie influence of ozone is 

 converted into nitrate of ammonia, (Bauraert, Ilouzeau). 

 The reaction is such tluxt one-half of the ammonia is oxid- 

 ized to nitric acid, which unites with the residue and with 

 water, as illustrated by the equation : 



2NH3 + 40 = NH,, NO3 + H,0 



In this manner, niti'ate of ammonia may originate in the 

 atmosphere, since, as already shown, ammonia and ozone 

 are both present there. 



Oxidation and Reduction in the Atmosphere,— The 



fact that ammonia and organic matters on the one hand, 

 and ozone, nitrous and nitric acids on the other, are pres- 

 ent, and, perhaps, constantly present in the air, involves at 

 first thought a contradiction, for these two classes of sub- 

 stances are in a sense incompatible with each other. 

 Organic matters, ammonia, and nitrous acid, are converted 

 by ozone into nitric acid. On the contrary, certain or- 

 ganic matters reduce ozone to ordinary oxygen, or destroy 

 it altogether, and reduce nitric and nitrous acids to am- 

 monia, or, peihaps, to free nitrogen. The truth is that 

 the substances named are being perpetually composed and 

 decomposed in the atmosphere, and at the surface of the 



