SOME REMARKS OX SEWERAGE SYSTEMS. 113 



vations upon the amount of COo and the number of micro- 

 organisms contained, firstly, in the air of various domestic 

 and public buildings, and secondly, in both ventilated and 

 unventilated sewers, choosing for his example of the latter 

 the sewers of Bristol. 



In the first place, he brings out the fact that, though sewer 

 air, in a ventilated sewer, contains about twice as much 

 COo per litre than does the outside air, it contains less 

 micro-organisms. vSecondly, that the air of a ventilated 

 sewer contains four or five times as many micro-organisms 

 as an unventilated sewer, and that the amount of CO^ in 

 the unventilated sewer was surprisingly small as compared 

 with what the subsoil air at the same depth probably 

 contained. 



From these observations he remarks: '' I think there is a 

 strong case from the sewer point of view against outside air. 

 It is evidently, as a rule, the outside air which contaminates 

 the sewer air with micro-organisms, and not the other way. 

 The results of these researches will perhaps tend to miti- 

 gate some of the terror with which we have come to regard 

 sewer air. Sewer air has commonly been supposed to be 

 ''loaded" with micro-organisms, whereas it turns out to be, 

 in reality, some of the freest air from micro-organisms that 

 can be found." 



We do not yet fully understand the functions of the 

 various micro-organisms found in air and water, so that we 

 cannot yet tell whether a large number of micro-organisms 

 per litre shows the air to have been injuriously polluted. 



The above remarks and experiments, however, seem to 

 jjoint in the direction of avoiding the free ventilation of 

 sewers by contact with currents of fresh air. Apparently 

 the erection of simple vents to prevent any undue pressure 

 of sewer gas— should such be found— ought to be quita 



I 



