ATMOSPHERIC AIR AS THE FOOD OF PLANTS, 85 
is twice as rich in nitrogen as the stubble of ripened grain. 
This is a result of the fact that the clover is cut when in 
active growth, while the grain is harvested after the 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, which 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 when grain 
crops are raised upon it. 
Formation of Nitric Acid from Ammonia,—Ammonia 
(carbonate of ammonia) under the influence of ozone is 
converted into nitrate of ammonia, (Baumert, Houzeau). 
The reaction is such that one-half of the ammonia is oxid- 
ized to nitric acid, which unites with the residue and with 
water, as illustrated by the equation : 
pa OO ONE NOD HO 
In this manner, nitrate 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, perhaps, 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 
