512 



Messrs. Carnelley and Haldane. 



The mere fact that the average number found in the sewer air (8*9) 

 was less than that in outside air at the same time (15*9) is itself 

 a strong argument in favour of the origin of most of the micro- 

 organisms from outside air. If the air takes up micro-organisms in 

 its course along a sewer, we should expect the number to increase 

 rather than diminish during its passage, whereas the opposite is the 

 case, doubtless from gradual settling of solid particles. This settling 

 is perhaps even greater than appears from our analyses, as it was not 

 practicable to take specimens of outside air at the gratings in the centre 

 of the roadway with the traffic proceeding as usual. At these points the 

 contamination of the air by solid particles of organic origin would of 

 course be at its maximum. 



It will be noticed that in the analyses made at Westminster the 

 numbers obtained for the sewer air close to the Clock Tower were 

 always larger than those for outside air (see Table, pp. 504 — 505). Not 

 much stress can, however, be laid on this fact, as a great part of the 

 air passing along the sewer at this point came from a side drain near 

 the Clock Tower, leading from a point where the outside air was much 

 more likely to be contaminated by dust from traffic than in the central 

 court, where the outside air analyses were made. The outside air de- 

 terminations at "Westminster apply strictly to the sewer determinations 

 near the Yictoria Tower and kitchen, as these determinations were 

 made just at the opening of the inlet grating ventilating this part of 

 the sewer. It will be seen that the micro-organisms inside the sewer 

 decreased in proportion to the decrease in those present in the outside 

 air. 



Another argument in favour of the origin of most of the micro- 

 organisms from outside may be derived from the fact that the 

 average proportion of moulds to bacteria was nearly the same in the 

 sewer air and corresponding outside air, 1 to 9 in the former and 1 to 

 8 in the latter. Were the micro-organisms in sewer air mostly de- 

 rived from a different source than outside air we should expect the 

 proportion to be different. Thus in two cases referred to below, in 

 which the micro-organisms were evidently derived from splashing in 

 the sewer, among 128 micro-organisms there were no moulds. In the 

 micro-organisms present in the air of naturally ventilated schools and 

 one-roomed houses, the average proportion present was found to be 

 1 : 132 and 1 : 49 respectively, as against 1 : 2"5 in the corresponding 

 outside air (' Phil. Trans.' 1887; B, p. 99). 



A final argument is that so far as a naked-eye examination of the 

 colonies allowed one to judge, the micro-organisms in the sewer air 

 we examined were, with perhaps one exception, similar to those in 

 outside air. The exception referred to was in the case of some very 

 rapidly liquefying colonies which occurred in several samples of sewer 

 air, collected at points where there was more or less splashing. 



