90 BACTERIA 



growth of vegetation (consistently with due purification) for 

 the amount of sewage supplied/* Like the former, there is 

 biological influence at work here, though in a less degree. 

 About one acre is required for every hundred persons in the 

 population. These two latter modes are much to be pre- 

 ferred to chemical treatment, yet on account of space and 

 management, as well as on account of the non-removal of 

 the *' sludge,*' their success has not been all that could 

 be desired. Until comparatively recent times the above 

 methods of treating sewage were the only ones available. 



In 1881 it appears that M. Louis Mouras, of Vesoul (Haute 

 Saone), published an account of a hermetically sealed, 

 inodorous, and automatically discharging cesspool, in which 

 sewage was anaerobically broken down by ** the mysterious 

 agents of fermentation." This is the first record we have 

 of the newly applied treatment of sewage by simply allow- 

 ing Nature to fulfil her function by means of bacteria. We 

 shall most easily arrive at an appreciation of the recent de- 

 velopments of the process in England by describing the 

 so- called septic tank and cultivation beds. 



The septic tank is a large underground vault of cemented 

 brick, having a capacity of thousands of gallons, according 

 to the population. That at Exeter has a capacity of 53,800 

 gallons, and takes the average sewage of 1500 inhabitants in 

 twenty-four hours. Near the entrance is a submerged wall, 

 seven feet from the entrance and twelve inches below the 

 surface of the liquid in the full tank. Within this are caught, 

 by gravity, gravel and such-like deposits. The remaining 

 solid matter of the sewage becomes deposited in the tank it- 

 self. Both in the sediment at the bottom of the tank and in 

 the thick scum on the surface the organic compounds are 

 broken down and made soluble. In the former position this is 

 accomplished by anaerobic bacteria, in the latter on the sur- 

 face by aerobic bacteria. It need hardly be added that these 

 are denitrifying and putrefactive bacteria, and that those at 



