138 HOW SROPS FEED. | 
matter, and the more soluble substances and albuminoids 
it contains, the more rapidly does it decay or humify. 
It has been shown by a chemical examination of what 
escapes in the form of gas, as well as of what remains as 
humus, that the carbon of wood oxidizes more slowly 
than its hydrogen, so that humus is relatively richer in 
carbon than the vegetable matters from which it origin- 
ates. With imperfect access of air, carbon and hydrogen 
are to some extent disengaged in union with each other, 
as marsh gas (CH,). Carbonic oxide gas (CQ) is proba- 
bly also produced in minute quantity. The nitrogen of 
the vegetable matter is to a considerable extent liberated 
in the free gascous state; a portion of it unites to hydro- 
gen, forming ammonia (NH,), which remains in the de- 
caying mass; still another portion remains in the humus 
in combination, not as ammonia, but as an ingredient of 
the ill-defined acid bodies which constitute the bulk of 
humus; finally, some of the nitrogen may be oxidized to 
nitric acid. 
Chemical Nature of Humus.—In a subsequent chapter, 
(p. 224,) the composition of humus will be explained at 
length. Here we may simply mention that, under the in- - 
fluence of alkalies and ammonia, it yields one or more 
bodies having acid characters, called humic and ulmic 
(also geic) acids. Further, by oxidation it gives rise to 
crenic and apocrenic acids. The former are faintly acid 
in their properties; the latter are more distinctly char- 
acterized acids. 
Influence of Humus on the Minerals of the Soil.— 
a. Disintegration of the mineral matters of soils is aided 
by the presence of organic substances in a decaying state, in 
so far as the latter, from their hygroscopic quality, main- 
tain the surface of the soil in a constant state of moisture. 
6. Organic matters furnish copious supplies of carbonte 
acid, the action of which has already been considered 
