22 T. P. ANDERSON STUART. 



favourable conditions, the organisms multiply rapidly. The abso- 

 lute number, however, is no criterion of the evil they may do : 

 the only way to estimate this is to ascertain what manner of 

 organisms they are. For instance, an hundred pigeons would be 

 less dangerous to their fellow birds than a solitary hawk. The 

 whole question then depends, not on the quantity but on the 

 quality, not on the number but on the nature of the organisms. 

 In illustration of this I may mention that Haldane states, that 

 the number of microbes he found in the air of a room in which a 

 common door-mat had been shaken, compared very unfavourably 

 with the number found in any sewer air ! 



The nature of the organisms may, in a measure, be inferred by 

 ascertaining where they came from. It is obvious that they may 

 come from the air that enters the sewer, or from the sewage. 

 These organisms are distributed throughout all the lower regions 

 of the external atmosphere, but, heavier than air, they are con- 

 tinually tending to subside on to the earth's surface, and so the 

 nearer the earth's surface, the greater the number in a given 

 volume of the outer air. But the air which enters the sewers is 

 precisely this lowest layer of the outer air, containing the most 

 organisms. What wonder, then, that sewer air contains at least 

 as many organisms as the worst air outside, and, moreover, the 

 same kind of organisms as in that outside air. It is what one 

 would a priori expect, and what has been proved by experiment 

 to be the case ; for it has been shown in the case of the sewers of 

 London, Bristol and Dundee, that the numbers and kinds of 

 organisms observed in the sewer air were not those observed in 

 the sewage, but those observed in the outside air at the same 

 time. 



Sometimes, however, the numbers in the sewer air are greater 

 than in the outside air, sometimes they are fewer. These excep- 

 tions, as it were, only prove the rule ; for it is clear that in very 

 dry weather the air containing the organisms, when it enters the 

 sewer, becomes moister and it may be warmer, and thus more 

 favourable to the organisms. It may also be that volatile product* 



