328 RESPIRATION 



is gained to the sewer. The air is saturated with moisture, and 

 may be somewhat warm if much warm water flows into the sewer. 

 Chemically speaking, however, the air is very little contaminated. 

 Even in the sewers of Bristol, where ventilating shafts were re- 

 duced to a minimum, I found only about 0.2 per cent of CO 2 . On 

 determining the number of bacteria in the air we found that 

 fewer were present in the sewer air than outside, but of much the 

 same kinds. In sewers which were well ventilated there were far 

 more than in badly ventilated sewers; and it was evident that 

 nearly all the bacteria came from the outside through the venti- 

 lators. Where there was much splashing, however, a few were 

 thrown into the air. These results, which have been confirmed by 

 other investigators, are just what might be expected. Particulate 

 matter is not given off from moist surfaces apart from mechani- 

 cally acting causes, and any bacteria or other particles driven 

 into suspension in the air of a sewer will tend to fall back again. 

 It is conceivable that infection might be carried by sewer air ; but 

 innumerable other paths of infection are much more probable. 



Although ordinary sewer air is chemically very pure, and not 

 even a trace of H 2 S can be found, accidents to sewermen from 

 foul air are not very uncommon ; and there is no doubt that most 

 of these accidents are due to H 2 S. I investigated one case of this 

 kind where five men had lost their lives at a manhole the last 

 four in attempts at rescue. 24 All the symptoms described, includ- 

 ing irritation of the eyes, were those of H 2 S poisoning ; and though 

 the air was not poisonous when I descended, a little H 2 S was 

 present. When some of the sewage was put into a large bottle and 

 shaken up, H 2 S was found to be present, and a mouse lowered 

 into the bottle showed severe symptoms of H 2 S poisoning. These 

 symptoms were absent when lead acetate was added before shak- 

 ing, or when caustic soda was added. 



It is only when sewage stagnates or deposits solid matter that 

 H 2 S is formed. Any cause that stirs this sewage, or liberates H 2 S 

 from it, may make the air dangerous. About 0.2 per cent will kill 

 an animal within a minute or two ; and o. I per cent will rapidly 

 disable it. H 2 S is thus a good deal more poisonous than CO, and 

 far quicker in its action. 



Another source of danger is lighting gas from leaky street 

 mains. Lighting gas is frequently met with in sewers, and I have 

 several times smelt it in sewers. In one recent case which I in- 

 vestigated two men were killed by CO poisoning from lighting 



*' Lancet, Jan. 25, 1896. 



