The Air of Sewers. 



511 



length of time during which the air is present in the sewer, and there- 

 fore with increase in the carbonic acid. But it has been shown above 

 (p. 509) that the very opposite is the case, the micro-organisms be- 

 coming less numerous with increase of carbonic acid. We have also 

 classified the whole of our observations according to whether they 

 were made at points where there was a strong, moderately strong, or 

 very feeble draught. Here again it will be seen that the results are 

 against the theory that the micro-organisms come from the sewer 

 itself. 





Carbonic 

 acid. 



Organic 

 matter. 



Micro- 

 organisms. 





6- 6 



7- 5 

 9-4 



5-7 

 8-8 

 8-1 



9-9 

 8-9 

 6-7 







In excess of outside air. 



Carbonic 

 acid. 



Organic 

 matter. 



Micro- 

 organisms. 





2-6 

 39 

 6'0 



3 5 

 6-6 

 5-5 



- 23 



- 9-2 

 -14-3 



A similar result was obtained from our observations at West- 

 minster, where we made six observations before, and six after the 

 improvements in the ventilation referred to above. 





Carbonic 

 acid. 



Organic 

 matter. 



Micro- 

 organisms. 





7-8 

 6-2 



110 

 2-7 



7 



10-3 



The only other source for the micro-organisms of sewer air is con- 

 tamination from the outside air. The same arguments which have 

 just been applied against the sewer itself being a source of micro- 

 organisms may be urged in favour of their origin from outside air. 



