88 BACTERIA 



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 con- 

 tained in sewer air and those contained in sewage. Indeed, 

 there is a marked difference which forms a contrast as strik- 

 ing as it is at first sight unexpected. The organisms isol- 

 ated from sewer air are those commonly present in the open 

 air. Micrococci and moulds predominate, whereas in sew- 

 age 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 ne- 

 cessary to remark upon one distinguishing" feature. A very 

 large number of sewage bacteria are decomposing and denitri- 



