ed. It is, in fact, on the careful regulation of the two classes
of fermentation that the successful rotting of the manure depends. It
must further be remembered that, even with a certain amount of openness
in a manure-heap, anaerobic fermentation may take place. This is due to
the fact that the evolution of carbonic acid gas, in such a case, is so
great as to exclude the access of the atmospheric oxygen into the pores
of the heap.

3. The _dampness_ of the manure-heap is another important influence.
This, of course, will act in two ways. First, by lowering the
temperature. Where the manure-heap is found to be suffering from
"fire-fang," the common method in practice is to lower the temperature
by moistening the heap with water. Secondly, it acts as a retarder of
fermentation by limiting the supply of atmospheric oxygen, and thus
preventing, as we have just seen, aerobic fermentation.

4. The fourth chief influence in regulating fermentation of the
manure-heap is its _composition_, and more especially the amount of
nitrogen it contains in a soluble form. The rate at which fermentation
takes place in any organic substance may be said chiefly to depend on
the percentage of soluble nitrogenous matter it contains: the greater
this is in amount, the more quickly does fermentation go on. There are
always a number of soluble nitrogenous bodies in farmyard manure. These
are chiefly found in the urine, such as _urea_, _uric_ and _hippuric
acids_, and _ammonia_ salts.

_Products of Decomposition of Farmyard Manure._

The most important of the changes which take place in the rotting of
farmyard manure may be briefly enumerated as follows:--

1. The gradual conversion into gases of a large portion of the organic
elements in the manure. Of these gaseous products the most abundant is
_carbonic acid gas_ (CO_2). It is in this form that the carbonaceous
matter which constitutes the chief portion of the manure escapes into
the air. Carbon also escapes into the air, combined with hydrogen, in
the form of _carburetted hydrogen_ or _marsh-gas_ (CH_4), a product of
the decomposition of organic matter in the presence of a large quantity
of water. This gas is consequently found bubbling up through stagnant
water. Next to carbonic acid gas, _water_ (H_2O) is the most abundant
gaseous product of decomposition. The nitrogen present in the manure, in
different forms, is converted by the process of decomposition chiefly
into _ammonia_, whic