262 HOW CROPS FEED. 
As to the mode in which the soil thus assimilates free 
nitrogen, several hypotheses have been offered. One is 
that of Schénbein, to the effect that in the act of evapora- 
tion free nitrogen and water combine, with formation of 
nitrite of ammonia. In a former paragraph, p. 79, we 
have given the results of Zabelin, which appear to render 
this theory inadmissible. 
A second and adequate explanation is, that free nitrogen 
existing in the cavities of the soil is directly oxidized to 
nitric acid by ozone, which is generated in the action of 
ordinary oxygen on organic matters, (in the same way as 
happens when ordinary oxygen acts on phosphorus, ) or is, 
perhaps, the result of electrical disturbance. 
Experiments by Lawes, Gilbert, and Pugh (PAil. 
Trans., 1861, II, 495), show indeed that organic matters 
in certain conditions of decay do not yield nitric acid 
under the influence of ozone. 
They caused air highly impregnated with ozone to pass 
daily for six months through moist mixtures of burned 
soil with relatively large quantities of saw-dust, starch, 
and bean meal, with and without lime—in all 10 mixtures 
—but in no case was any nitric acid produced. 
It would thus appear that ozone can form nitrates in 
the soil only when organic matters have passed into the 
comparatively stable condition of humus, 
That nitrogen is oxidized in the soil by ozone is highly 
probable, and in perfect analogy with what must happen 
in the atmosphere, and is demonstrated to occur in Schién- 
bein’s experiments with moistened phosphorus (p. 66, 
also Ann. der Chem. u. Pharm., 89, 287), as well as in 
,Zabelin’s investigations that have been already recounted. 
(See pp. 75-83.) 
he fact, established by Reichardt and Blumtritt, that 
humus condenses atmospheric nitrogen in its pores (p. 
167), doubtless aids the oxidation of this element. 
The third mode of accounting for the oxidation of 
