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 fostered. 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:--