304 THE AMERICAN MONTHLY [Oct. 



ble distance from the pumping station run dry when the 

 latter is in operation. Such contamination from human 

 sources may originate typhoid and diarrlieal disorders. 

 If, on the other hand, the source of supply is remote 

 from human habitation there may be malarial contamina- 

 tion. Indeed the natural habitat of malaria is in new and 

 undrained countries and virgin soil. In view of this dis- 

 tribution of the disease it is surprising that well-drained 

 cities, having perfect sewers, should yield a certain per- 

 centage of malarial fevers until the source of their water- 

 supply is noted, it being in such cases, as a rule, some 

 pond or stream in whose vicinity these diseases are pre- 

 valent. Shallow wells in alluvial soil also may yield 

 malarial infection. It is said that since the substitution 

 of deeper artesian borings for such wells there has been 

 a notable decrease of malarial diseases in some parts of 

 the Southern States of North America. 



In many localities it is difficult, if not impossible, to 

 secure an adequate supply of water free from the forms 

 of contamination to which reference has been made. This 

 necessitates some system of purification. 



It has been discovered recently that there is an antag- 

 onism between disease germs and what are known as 

 nitrifying organisms, which produce nitrates and nitrites 

 in the soil. Advantage has been taken of this to insti- 

 tute an intermittent process of filtration. Water con- 

 taining the bacteria that it is desired to destroy is allowed 

 to run into a filter composed of sand, containing an 

 abundance of nitrifying organisms, and instead of being 

 drawn off immediately is allowed to stand for a sufficient 

 length of time to permit the destruction of the disease 

 germs by their natural foes. 



Such filtration as that just described is but the perfec- 

 ting of natural processes. Alternation of rainfall and 

 dry weather operates substantially on the same plan, 

 tending to purify the ground water in the sc>il from infec- 



