116 Bacteria in Relation to Country Life 



underdrained. On heavy, ill-drained soils, the sewage 

 became foul and was purified but little. Moreover, even 

 the lands adapted to sewage-farming could not be 

 forced beyond a certain point in purifying the sewage 

 applied. A proper supply of air was apparently neces- 

 sary for the rapid and thorough purification of the liquid 

 wastes, and the most satisfactory results were obtained 

 when the land was worked intermittently, that is, allowed 

 periods of rest and aeration. 



With the discovery of the biological nature of 

 nitrification, and the further demonstration by Schlosing 

 and Miintz in France, and Warrington in England, that 

 the nitrifying bacteria need a plentiful supply of air 

 for their development, new light was thrown on the 

 efficacy of intermittent filtration. 



The system of intermittent filtration is based pre- 

 eminently on the action of aerobic bacteria. The Mouras 

 scavenger and the septic tank depend for their efficiency 

 on the work of anaerobic organisms. The earlier instal- 

 lations of bacterial filters relied largely on the activities 

 of the former class of organisms. The most satisfactory 

 results were secured when the sewage had been previously 

 subjected to screening and chemical precipitation. When 

 .this preliminary treatment was left out the filters showed 

 a marked reduction in capacity, due to the deposition 

 of solid materials around the grains of the filter. In 

 other words, the total pore-space between the solid 

 particles of the filter was reduced on account of the 

 accumulation of materials which the aerobic bacteria 

 were evidently unable to destroy. On further examina- 

 tion, these substances were found to consist of woody 



