124 Bacteria in Relation to Country Life 



slight excess of acid in their medium, may be injured or 

 entirely destroyed. Injury may also be caused by other 

 substances not necessarily acid. Thus, the ammoniacal 

 spent liquors from gas works and coke ovens, when 

 forming 1 per cent of the total volume of sewage, ap- 

 parently do not prevent or retard the purification pro- 

 cesses in the bacterial beds. When, however, the spent 

 liquors form as much as 3 per cent of the entire 

 volume of sewage, the injury to the bacteria is at once 

 apparent. 



The working capacity of bacterial filters. — There are 

 natural variations in the working capacity of filters of 

 different construction and location. . Apart from the 

 thorough digestion in the septic tank and the skilful 

 construction of the filters, the high efficiency in some 

 cases may be ascribed to the favorable temperatures 

 that prevail throughout the year. 



