amount of warmth, air, and moisture, as well as the presence of certain
food-constituents, especially nitrogenous bodies, were necessary.

Subsequent researches by Pasteur and others have conclusively
demonstrated that the micro-organic life instrumental in effecting the
putrefaction or decay of organic matter of any kind, may be divided into
two great classes:--

1. Those which require a plentiful supply of oxygen for their
development, and which, when bereft of oxygen, die--known as _aerobies_.

2. Those which, on the contrary, develop in the complete absence of
oxygen, and which, when exposed to oxygen, die--known as _anaerobies_.

In the fermentation of the manure-heap, therefore, we must conceive of
the two classes of organisms as the active agents. In the interior
portion of the manure-heap, where the supply of oxygen is necessarily
limited, the fermentation going on there is effected by means of the
anaerobic organism--_i.e._, the organism which does not require oxygen;
while on the surface portion, which is exposed to the air, the aerobic
(or oxygen-requiring) organism is similarly active. Gradually, as decay
progresses, the aerobic organisms increase in number. It is through
their instrumentality that the final products of decomposition are
largely produced. The functions of the anaerobic organisms may be, on
the contrary, regarded as largely preparatory in their nature. By
breaking up the complex organic substances in the manure into new and
simpler forms, they advance the process of putrefaction through the
initial stages; and when this is accomplished, they die and give place
to the aerobic, which, as we have just seen, effect the final
transformation of the organic matter into such simple substances as
_water_ and _carbonic acid gas_.

The conditions influencing the fermentation of farmyard manure may be
summed up as follows:[163]--

1. _Temperature._--The higher the temperature the more rapidly will the
manure decay.

2. _Openness to the Air._--Of course it will be seen that the effect of
exposing the manure to the action of the air is to induce the
development of the aerobic type of organism, and thus to promote more
rapid fermentation. If, on the other hand, the manure be impacted, the
slower but more regular fermentation, due to the anaerobic type of
organism, will be chiefly promoted. It must be remembered that in the
proper rotting of farmyard manure both kinds of fermentation should be
foster