Whenever decomposition occurs in organic substances there is a reduction of compound bodies, and in such cases the putrefying substances obtain their decomposing and denitrifying bacteria from the air. The chief conditions requisite for bringing about a loss of nitrogen by denitrification are enumerated by Professor Warington as follows: (1) the specific micro-organism; (2) the presence of a nitrate and suitable organic matter; (3) such a condition as to aëration that the supply of atmospheric oxygen shall not be in excess relatively to the supply of organic matter; (4) the usual essential conditions of bacterial growth. "Of these," he says, "the supply of organic matter is by far the most important in determining the extent to which denitrification will take place." The necessarily somewhat unstable condition facilitates its being split up by means of bacteria. The bacteria in their turn are ready to seize upon any products of animal life which will serve as their food. Thus, by reducing complex bodies to simple ones, these denitrifying organisms act as the necessary link to connect again the excretions of the animal body, or after death the animal body itself, with the soil. In a book of this nature it has been deemed advisable not to enter into minute description of all the species of bacteria mentioned. Some of the chief are described more or less fully. We cannot, however, do more than name several of the chief organisms concerned in reducing and breaking down compounds. As we shall find in the bacteria of nitrification, so also here, the _entire_ process is rarely, if ever, performed by one species. There is indeed a remarkable division of labour, not only between decomposition bacteria and denitrification bacteria, but between different species of the same group. _Bacillus fluorescens non-liquefaciens_, _Mycoderma ureæ_, and some of the staphylococci break down nitrates (denitrification), and also decompose other compound bodies. Amongst the group of putrefactive bacteria found in soil may be named _B. coli_, _B. mycoides_, _B. mesentericus_, _B. liquidus_, _B. prodigiosus_, _B. ramosus_, _B. vermicularis_, _B. liquefaciens_, and many members in the great family of _Proteus_. Some perform their function in soil, others in water, and others, again, in dead animal bodies. Dr. Buchanan Young, to whose researches in soil we have referred, has pointed out that in the upper reaches of burial soil, where these bacteria are most largely present, there is as a result no excess of organic carbon and nitrogen. Even in the lower layers of such soil it is rapidly broken down. [Illustration: MICROCOCCUS FROM SOIL]