ANNIVERSARY ADDRESS. 23 



of decomposition saturate the now moist particles to which the 

 organisms cling, so that they may have a food supply while yet 

 floating in the sewer air. In these circumstances, then, it is not 

 difficult to understand an increase in number of the organisms in 

 the air after entering the sewer, altogether apart from any contri- 

 butions from the walls of the sewer or from the sewage. Then, 

 again, the organisms tend to subside within the sewer just as they 

 did outside, so that when the conditions already referred to as 

 favourable to their increase are not dominant, the longer the air 

 resides in the sewer, the more the organisms have fallen into the 

 sewage. Thus, when the ventilation is not enough to obliterate 

 the effects of the subsidence, and apart from other conditions, 

 the air, when it enters the sewer, may actually lose many of its 

 microbes there. 



As to the other possible source of the organisms, the sewage 

 itself, it follows for what has already been said, that it is the less 

 important. For the following reasons sewer air ought not to contain 

 so very many more organisms than the ordinary outer air. Experi- 

 ments have conclusively shown that, when organisms are contained 

 in a liquid, the only probable ways in which they can find their 

 way into the air above that liquid, are splashing of the liquid and 

 the bursting of bubbles of gas. In properly constructed sewers 

 there is not necessarily any splashing, nor are there many gas 

 bubbles, for the sewage is disposed of before it has time to decom- 

 pose so as to produce much gas, and even if there were gas, . the 

 diminutions of barometric pressure leading to disengagement of 

 the gas are not so very frequent. In the case of sewage, the deposits 

 on the walls may dry, become pulverulent and get carried away 

 by air draughts, but drying does not often take place in sewers, 

 and when it does, the mere drying kills many organisms, and the 

 air draughts are not always there at the same time. Taking into 

 account all that has just been said against the sewage origin of 

 the microbes in the sewer air, and all already said in favour of 

 their origin mainly from the outer air, the greater importance 

 of this latter source is obvious. In a word far and away most of 



