86 HOW CROPS FEED. 



soil. Here, or at one moment, oxidation prevails; there, 

 or at another moment, reduction preponderates. It is 

 only as one or another of the results of this incessant ac- 

 tion is withdrawn from the spliere of change, that we can 

 give it permanence and identify it. The quantities we 

 measure are hut resultants of forces that oppose each oth- 

 er. The idea of rest or permanence is as foreign to the 

 chemistry of the atmospliere as to its visible phenomena. 



Nitric Acid in tlie Atmospliere. — The occurrence of ni- 

 tric acid or nitrate of aunnunia in the atmosphere has been 

 abundantly demonstrated in late years (1854-6) by Cloez, 

 Boussingault, De Luca, and Kletzinsky, who found that 

 when lai-ge volumes of air are made to bubble through 

 solutions of potash, or to stream over fragments of brick 

 or pumice which have been soaked in potash or carbonate 

 of potash, these absorbents gradually acquire a small 

 amount of nitric acid. In the experiments of Cloez and 

 De Luca, the air was first Avashed of its ammonia by con- 

 tact with sulphuric acid. Their results prove, therefore, 

 that the nitric acid was formed independently of ammonia, 

 though it doubtless exists in the air in combination with 

 this base. 



Proportion of Nitric Acid in Rain-water, etc.— In at- 

 mospheric waters, nitric acid is found much more abund- 

 antly than in the air itself, for the reason that a small bulk 

 of rain, etc., washes an immense volume of air. 



Many observers, among the first, Liebig, have found ni- 

 trates in rain-water, especially in the rain of thunder- 

 storms. The investigations of Boussingault, made in 

 18.56-8, have amply confirmed Barral's observation that 

 nitric acid (in combination) is almost invariably present in 

 rain, dew, fog, hail, and snow. Boussingault, {Agrotiomie, 

 etc., 11, 325) determined the quantity of nitric acid in 134 

 rains, 31 snows, 8 dews, and 7 fogs. In only 16 instances 

 out of these 180 was the amount of nitric acid too small 



