The bacteria required for the putrefaction of organic matter are among the most widely distributed of all the micro-organisms. They are always found in the air, except on mountain tops, in deserts, and over the ocean. They are very numerous in surface waters, such as streams and ponds, and their relative number everywhere increases as the amount of organic matter increases, so that the greater the need for them the greater is their number. It has been found that the great majority of these bacteria require air for their energetic development, and this fact is most important when it comes to the practical construction of a piece of apparatus for making use of these bacteria. It has also been found that, for several reasons, these bacteria work most effectively in the soil and can take care of a larger quantity of organic matter there than elsewhere. This is partly because in the surface layers of the soil, particularly where that soil has been cultivated, a great number of the particular bacteria involved in decay are always to be found. Pure, clean sand from the desert contains almost none of these beneficent bacteria. Rich garden soil is fairly teeming with them, so that, curiously, the more organic matter and the more bacteria present in any soil, the more active that soil will be in taking care of other organic matter. Then, again, the soil particles, particularly in sandy soil, are so separated as to allow between them a certain and appreciable amount of air, and by means of this air the activity of the bacteria is made continuous and the products of their activity utilized. Without such an admission of air, the bacteria are choked and diminish rapidly in numbers. There is, however, a definite degree of purification and a certain quantity of organic matter which can be taken care of by the bacteria incident to any particular soil. Up to that quantity purification proceeds more or less satisfactorily according to the intelligence shown in feeding the bacteria in such a way as suits their convenience. If, however, that quantity be exceeded, all purification stops, the bacteria are apparently discouraged, and no further improvements can be expected. A fine-grained soil will not be so useful as a coarse-grained soil because the former does not allow sufficient air in the interstices of its soil particles. Another practical reason for not making use of soils of fine grains is that such soils can absorb only a small amount of liquid because of the mechanical construction of the material. On the other hand, soils whose grains are too coarse are undesirable because their mechanical construction is such that the liquids containing organic matter in solution pass through so rapidly that time enough is not given for bacterial action.