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 résults of this incessant ac- 
tion is withdrawn from the sphere of change, that we can 
give it permanence and identify it. The quantities we 
measure are but resultants of forces that oppose each oth- 
er. The idea of rest or permanence is as foreign to the 
chemistry of the atmosphere as to its visible phenomena. 
Nitric Acid in the Atmosphere,—The occurrence of ni- 
tric acid or nitrate of ammonia in the atmosphere has been 
abundantly demonstrated in late years (1854-6) by Cloez, 
Boussingault, De Luca, and Kletzinsky, who found that 
when large volumes of air are made to bubble through 
solutions of potash, or to strgam 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 washed 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, Licbig, have found ni- 
trates in rain-water, especially in the rain of thunder- 
storms. The investigations of Boussingault, made in 
1856-8, have amply confirmed Barral’s observation that 
nitric acid (in combination) is almost invariably present in 
rain, dew, fog, hail, and snow. Boussingault, (Agronomie, 
etc., 11, 825) 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 
