24 T. P. ANDERSON STUART. 



the microbes in sewer air are derived from the external atmosphere, 

 not from the sewage. 



Now, if we bear in mind that we live every day and all day in 

 the most microbe laden lowest stratum of the outer air, or what 

 is worse, in closed spaces, close rooms etc., in the air of which the 

 microbes are usually more numerous than in the air outside, we at 

 once must admit that, apart from their numbers, the nature of 

 the microbes, thus common to the outer air and the sewer air, can 

 not be so poisonous after all. As a matter of fact there are 

 microbes and microbes, harmless or friendly, and injurious or 

 unfriendly, those that never prey upon the healthy animal body, 

 but always on dead or non-living organic matter, those that never 

 are found except as parasites preying upon the living, and inter- 

 mediate kinds that vary according to the conditions. The vast 

 majority of the microbes in ordinary air and in sewer-air belong 

 to the group of harmless or friendlies, and the questions to be 

 answered are, whether there are any of the second kind, the patho- 

 genic or disease-producing organisms, and if there are any, what 

 is their nature and how many are there of them ? Ln this last 

 connection the number is of importance, because the healthy 

 animal body has means of combating the attacks of disease-pro- 

 ducing organisms, a struggle ensues between the body and the 

 microbes — if the latter be few they are overcome and destroyed, 

 if too many they overcome the body's natural protectors (phago- 

 cytes ?), and so obtain a footing, grow, multiply and produce their 

 specific disease. These three questions constitute the problem of 

 the sewer air, and the answers are by no means easy. It is not 

 enough to say that any particular organism belongs to this or that 

 group, for to a single group belong both harmless and harmful 

 individual sorts. The answers to these three questions can only 

 be given by a laborious separation of the different kinds, and an 

 investigation of the nature and properties of the separate kinds, 

 a task which so far as I know, has never yet been fully undertaken, 

 and certainly has not as yet been accomplished. 



