In the process of mineralisation two stages may be distinguished, viz. the first or disintegration stage, and the second or oxidation stage, i.e. the organic substances are first split up and afterwards converted into inorganic ones; and frequently these processes are taking place side by side and not after each other. [Sidenote: The splitting up of organic substances is frequently carried out in the presence of air.] It has been maintained—probably with a view to justifying the necessity of a septic tank—that the preliminary process of splitting up is best carried out in the absence of oxygen, but sufficient proof does not appear to have been advanced in support of this statement, and in some cases at any rate it is evidently carried out quite satisfactorily in the presence of air. Concerning the presence or absence of oxygen, Fischer observes as follows:— “The effects of the presence of oxygen are somewhat better understood. If air have free access, putrefaction (decomposition) may go on without any odour at all, the evil-smelling gases (NH{3} and SH{2}, for example) being oxidised at once to form nitrates and sulphates. Aerobic bacteria, too, such as the nitre and sulphur bacteria, bring about this mineralisation of organic nitrogen. Moreover, when air is circulating freely, there is no accumulation of intermediate products such as skatol or indol. It occurs on the surface of manure heaps, on the outer surfaces of carcases, and in well ventilated soil. “In anaerobic decomposition (putrefaction proper), as in anaerobic fermentation, the organic molecules are at first only partly disintegrated, intermediate products such as leucine, tyrosine, skatol and indol being formed. In the absence of air these accumulate, and hence it is that putrefaction going on in the mud of ponds and ditches, or inside carcases, is accompanied by such evil odours. “Although the details of the process vary considerably, according to the presence or absence of air, the ultimate products of decomposition and putrefaction are in both cases the same: namely, free nitrogen, free hydrogen, ammonia, methane, carbonic acid and sulphuretted hydrogen. These are also the end-results of the disintegration of the human body. “After the organic nitrogen of decomposing substances has been converted into ammonia, and to a small extent into free nitrogen, the latter can at once be utilised by the root-nodule organisms and other bacteria in the soil, but the ammonia must undergo two further changes and combine with a base to form a nitric salt before it is available for plant life. These two changes are brought about by bacteria, which convert the ammonia first into nitrous and then into nitric acid; this process has been called ‘nitrification.’”