total will belong to putrefactive species. We may, however, make a provisional list of normal sewage bacteria[18] as follows: 1. _Bacillus coli communis_ and all its varieties and allies. 2. _Proteus vulgaris_ and the various protean species. 3. _B. enteritidis sporogenes_ (Klein). 4. Liquefying bacteria, _e. g._, _Bacillus fluorescens liquefaciens_, _B. subtilis_, _B. mesentericus_. 5. Non-liquefying bacteria. 6. Sarcinæ, yeasts, and moulds. [Illustration: PROTEUS VULGARIS] [Illustration: B. ENTERITIDIS SPOROGENES] We have not included in the above inventory any pathogenic bacteria. Doubtless such species (_e. g._, typhoid[19]) not infrequently find their way into sewage. But they are not normal habitants, and though they struggle for survival, the keenness of competition among the dense crowds of saprophytes makes existence almost impossible for them. Nor can they expect much sympathy from us in the difficulties of life which fortunately confront them in sewage. Of those we have named as normally present it is unnecessary to speak in detail, with the exception of the newly discovered anaërobe, _Bacillus enteritidis sporogenes_ of Klein.[20] This bacillus is credited to be a causal agent in diarrhœa, and has been isolated by Dr. Klein from the intestinal contents of children suffering from severe diarrhœa, and from adults having cholera nostras. It has been readily detected in sewage from various localities, and also in sewage effluents, after sedimentation, precipitation, and filtration. Its biological characters are shortly as follows: It is in thickness somewhat like the bacillus of symptomatic anthrax, thicker and shorter than the bacillus of malignant œdema, and standing therefore between the latter and anthrax itself. It is motile and possesses flagella, but has no threads. It readily forms spores, which develop as a rule near the ends of the rods and are thicker than the bacilli. It is stained by Gram's method. In various media (particularly milk) it produces gas rapidly. It is an anaërobe, and is cultivated in Buchner's tubes. A recent epidemic of diarrhœa affecting 144 patients in St. Bartholomew's Hospital was traced to milk in which _B. enteritidis_ was present. _Sewer Air._ Though not of material importance as regards bacterial treatment of sewage, this subject calls for some remark. For long it has been known that air polluted by sewage emanations is capable of giving rise to various degrees of ill-health. These chiefly affect two parts of the body; one is the throat, and the other is the alimentary canal. Irritation and inflammation may be set up in both by sewer air. Such conditions are in all probability produced by a lowering of the resistance and vitality of the tissues, and not by either a conveyance of bacteria in sewer air or any stimulating effect upon bacteria exercised by sewer air. What evidence we have is against such factors. (See p. 105.)