1887.] 



The Air of Sewer, 



395 



the sewers examined was in a much better condition than might have 

 been expected. (2.) That the carbonic acid was about twice, and the 

 organic matter rather over three times as great as in outside air at 

 the same time, whereas the number of micro-organisms was less. 

 (3.) That in reference to the quantity of the three constituents 

 named the air of the sewers was in a very much better condition 

 than that of naturally ventilated schools, and that with the notable 

 exception of organic matter it had likewise the advantage of mechani- 

 cally ventilated schools (cf. paper by the authors and Dr. Anderson 

 in 'Phil. Trans,' 1887). (4.) That the sewer air contained a much 

 smaller number of micro-organisms than the air of any class of 

 house, and that the carbonic acid was rather greater than in the air 

 of houses of four rooms and upwards, but less than in two- and one- 

 roomed houses. As regards organic matter, however, the sewer air 

 was only slightly better than the air of one-roomed houses, and much 

 worse than that of other classes of houses. (The data for all the 

 classes of houses refer to sleeping rooms when occupied during the 

 night.) 



The amount of carbonic acid found by the authors was much less 

 than that noted by earlier observers, showing that the sewers they 

 examined were much better ventilated than those previously investi- 

 gated. 



On taking the average of a comparatively large number of analyses 

 it was found that the quantity of organic matter in sewer air increased 

 with the carbonic acid, whereas the micro-organisms on the whole 

 decreased with increase of the other constituents. 



With regard to the sources of the several impurities in sewer air the 

 following conclusions are drawn : — (1.) The carbonic acid in excess of 

 outside air may be partly due to diffusion from the neighbouring soil, 

 but its chief source is probably the oxidation of the organic matter in 

 the sewage and in the air of the sewer. (2.) The organic matter in 

 excess of outside air is most probably wholly or for the most part 

 gaseous, and is of course derived from the sewage itself. (3.) The 

 micro-organisms in sewer air come entirely, or nearly so, from the 

 outside, and are not derived, or only in relatively small numbers, from 

 the sewer itself. This is proved by the following facts : — First, the 

 average number of micro-organisms in sewer air was less than in out- 

 side air at the same time — viz., about 9 in the former to 16 in the latter. 

 Second, the number increased with the efficiency of the ventilation. 

 Third, the average proportion of moulds to bacteria in sewer air was 

 almost exactly the same as in outside air at the same time, whereas 

 one would expect the proportion to be very different were the outside 

 air not the source from which they were derived, seeing that such 

 a difference has been proved to exist in the air of houses, schools, &c. 

 Fourth, the naked eye appearance of the colonies from sewer air is 



