The Air of- Sewers. 



513 



These possibly came from the sewer itself, as we have not observed 

 in outside air or in buildings any colonies which liquefied the jelly 

 as rapidly or so extensively. 



The conclusion thus arrived at as to the source of most of the 

 micro-organisms present in sewer air is, perhaps, at first sight, 

 contrary to what one might have expected. It is in agreement 

 with the fact that the state of cleanliness or nlthiness of a sewer 

 seems to have no perceptible effect on the number of micro-organ- 

 isms present in the air of the sewer. Thus two observations on 

 two different days and at two different points of the dirtiest sewer 

 we examined, gave only 2 J and 12 micro-organisms respectively, 

 as compared with an average of 4 J and 9 in other and cleaner 

 sewers on the same days. Our conclusion is also in agreement witli 

 what is known as to the distribution of bacteria in air. Nageli 

 ('Die JSTiederen Pilze,' pp. 109, 111) has shown that liquids or damp 

 substances do not, with ordinary air currents, give off micro- 

 organisms to the surrounding air. He even found that air drawn 

 through gravel which had been saturated with filth and then dried, 

 gave off no micro-organisms (p. 169). Miquel ('Comptes Rendus,' 

 vol. 91, p. 64) states that the vapour of water rising from the soil, from 

 rivers, or from masses in full putrefaction, is free from germs ; that 

 the gases evolved from decaying substances, and the air passed over 

 putrid meat are free from germs, provided that the putrefying sub- 

 stance is as moist as soil taken 03 metre from the surface. The 

 experiments of Professor Frankland (' Roy. Soc. Proc.,' vol. 25, 1877, 

 p. 542) also point to the improbability of micro-organisms being 

 disseminated in air by such agitation of a liquid as that produced by 

 the flow of sewage along a sewer. On the other hand, it is well 

 known that the micro-organisms already present in air are always 

 tending to sink to the ground. On this fact Hesse's method depends. 

 Hence air in its passage along a sewer will presumably tend to 

 gradually deposit its micro-organisms, especially if the air-current is 

 slow. 



In order further to elucidate this point, and in particular with 

 regard to the drain pipes leading into houses, we made some experi- 

 ments with an artificial drain-pipe. Through the side of a wooden 

 box, AB, there was passed the end of a piece of glass tubing, CD, 

 5 feet long and 1% inches in diameter, and open at both ends. In the 

 opposite side of the box there was a hole, by means of which the air 

 inside the box could be connected with the entrance to a Hesse's tube, 

 and the micro-organisms thus determined. Through the roof of the 

 box there passed a chimney, in which a draught was maintained by 

 means of a small flame, F, kept burning at the bottom. This, of 

 course, caused a corresponding draught through the long tube and into 

 the box. A constant stream of water was kept running along the 



