VENTILATION OF SEWERS AND DRAINS. VAz 
due to the fluctuation of flow, or moist from the sweating of the 
brick or concrete work, and that the microbes in the sewer air 
adhere to the damp surfaces and are thus prevented from floating 
about. ‘‘ This reasoning is strengthened by what is already known 
of the presence of microbes in the atmospheric air generally, for 
in dry dusty weather they are found in far larger numbers than 
in damp weather or after rainfall. 
In well made sewers the sewage is borne away from the houses 
in afresh and undecomposed condition, but in old defective sewers 
and even in moderately good ones, when the temperature is high 
and the quantity of water small, putrifactive bacteria undergo 
enormous multiplication, fermentive changes are set up in the 
sewage and gases are formed which bubble up and break upon the 
surface of the liquid. ‘There can be no doubt that given stagna- 
tion and putrefaction of sewage, sewer air will be found to be 
loaded with micro organisms of different kinds.” 
It is stated by authorities that the injurious effects of sewer air 
may be attributed to the organic matters with which it is so often 
laden. ‘This is especially the case where people are exposed to 
escapes of sewer air into houses for a long period, the inhaling of 
which may not at first lead to serious results, but long continued 
inhalation will tend to lower the system to such an extent as to 
offer slight resistence to attacks of acute disease. 
The experience gained from the men engaged in cleansing the 
city sewers show that it is only when they are employed in the 
old, defective, and unventilated sections they suffer, in the newer 
and better constructed portions no inconvenience is experienced. 
Ventilation.—The necessity for ventilation of sewers and drains 
is demonstrated by a study of the natural forces at work within 
them. Before discussing the remedies for the collection of noxious 
gases in sewers, it will be as well to consider for a time the 
different forces which are brought into action in the sewers by 
various causes. We must divest ourselves of the notion that air 
or gas of any kind can be as readily pushed aside and got rid of 
