THE NITRIC ACID OF THE SOIL. 257 
either that absorbed by the so'l from the atmosphere, or 
that originating in the soil itself by the decay of nitrog- 
enous organic matters. Knop made an experiment with 
a sandy loam, as follows: The earth was exposed ina box 
to the vapor of ammonia for three days, was then mixed 
thoroughly, spread out thinly, moistened with pure water, 
and kept sheltered from rain until it became dry again. 
At the beginning of the experiment, 1,000,000 parts of 
the earth contained 52 parts of nitric acid. During its 
exposure to the air, while moist, the content of nitric acid 
in this earth increased to 591 parts in 1,000,000, or more 
than eleven times; and, as Knop asserts, this increase took 
pace at the expense of the ammonia which the earth had 
absorbed. The conversion of ammonia into nitric acid is 
an oxidation expressed by the statement 
2 NH xt #0 = NH NO, :+>H,0<* 
The oxygen may be either ozone, as already explained, 
or it may be furnished by a substance which exists in all 
soils and often to a considerable extent, viz., sesquioxide 
ofiron. This compound (Fe, O,) readily yields a portion 
of its oxygen to bodies which are inclined to oxidize, be- 
ing itself reduced thereby to protoxide (FeO) thus :— 
Fe, O, = 2 FeO0 + O. The protoxide in contact with the 
air quickly absorbs common oxygen, passing into sesqui- 
oxide again, and in this way iron operates as a carrier of 
atmospheric oxygen to bodies which cannot directly com- 
bine with the latter. The oxidizing action of sesquioxide 
of iron is proved to take place in many instances ; for ex- 
ample, a rope tied around a rusty iron bolt becomes “ rot- 
ten,” cotton and linen fabrics are destroyed by iron-stains, 
the head of an iron nail corrodes away the wood sur- 
rounding it, when exposed to the weather, and after suf- 
* The above equation represents but one-half of the ammonia as converted 
into nitric acid. In the soil the carbonates of lime, etc., would separate the 
nitric acid from the remaining ammonia and leave the latter in a condition to 
be oxidized, 
