It is now more than thirty years since Pasteur showed that the fermentation which ensued on keeping a sample of urine was due to the action of a minute organism, for the propagation of which a certain 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.