264 HOW CROPS FEED. 
containing pumice, carbonate of lime, and urea, was desti- 
tute of nitrates. 
Experiments 2, 4, and 5, demonstrate that the concourse 
of nitrogen gas, a porous body, and an alkali-carbonate, 
is insufficient to produce nitrates. Experiment No. 10 
shows that the highly nitrogenous substance, urea,* dif- 
fused throughout an extremely porous medium and expos- 
ed to the action of the air in moist contact with carbonate 
of lime, does not suffer nitrification. In the brick (ves- 
sels Nos. 3 and 6), something was obviously present, 
which determined the oxidation of free atmospheric ni- 
trogen. Cloez took the brick fresh from the kiln where 
it was burned, and assured himself that it included at 
the beginning of the experiment, no nitrogen in organic 
combination and no nitrates of any kind. Cloez believes 
the brick to have contained some oxidable mineral sub- 
stance, probably sulphide of iron. The Gentilly clay, 
used in making the brick, as well as some iron-cinder, 
added to it in the manufacture, furnished the elements of 
this compound. 
The slight nitrification that occurred in the vessels 
Nos. 7 and 8, containing washed chaik and burned soil, 
likewise points to the oxidizing action of some mineral 
matter. In vessel No. 9, the simply washed soil, which 
was thus freed from nitrates before the trial began, un- 
derwent a decided nitrification in remarkable contrast to 
the same soil calcined (No. 8). The influence of humus 
is thus brought out in a striking manner. 
It may be that apocrenic acid, which readily yields 
oxygen to oxidable matters, is an important agent in 
* Urea (COH4 N.) contains in 100 parts: 
Carbon, 20.00 
Hydrogen, 6.67 
Nitrogen, 46.67 
Oxygen, 26.66 
100.00 
