Several series of investigations have been made into the bacteriology of sewer air, amongst others by Uffelmann, Haldane, Laws, and Andrewes. From their labours we may formulate four simple conclusions: 1. The air of sewers contains very few micro-organisms indeed, sometimes not more than two organisms per litre (Haldane), and generally fewer than the outside air (Laws and Andrewes). 2. There is no relationship between the microbes contained in sewer air and those contained in sewage. Indeed, there is a marked difference which forms a contrast as striking as it is at first sight unexpected. The organisms isolated from sewer air are those commonly present in the open air. Micrococci and moulds predominate, whereas in sewage bacilli are most numerous. Liquefying bacteria, too, which are common in sewage, are extremely rare in sewer air. _Bacillus coli communis_, which occurs in sewage from 20,000 to 200,000 per cc., is altogether absent from sewer air. 3. Pathogenic organisms and those nearly allied to them are found in sewage, but absent in sewer air. Uffelmann isolated the _Staphylococcus pyogenes aureus_ (one of the organisms of suppuration), but such a species is exceptional in sewer air. Hence, though sewer air is popularly held responsible for conveying diphtheria and all sorts of other virulent bacteria, there is up to the present no evidence of a substantial nature in support of such views. Sewer air neither conducts pathogenic organisms nor stimulates the virulence of such. 4. Lastly, only when there is splashing in the sewage, or when bubbles are bursting (Frankland), is it possible for sewage to part with its contained bacteria to the air of the sewer. Whilst we cannot here enter more fully into an account of the bacteria found in sewage or of their functions, it is necessary to remark upon one distinguishing feature. A very large number of sewage bacteria are _decomposing_ and _denitrifying_, that is to say, breakers down, by means of putrefaction, of organic compounds. The knowledge of this fact has recently been applied, in conjunction with oxidation, to the biological treatment of sewage. As this illustrates in a marked degree some of the facts we have dwelt upon in considering the bacteriology of soil, and as it is likely that the future will witness a still wider application of these same facts, it will be necessary to refer in some detail to the matter.