18 



ELEMENTS OF WATER BACTERIOLOGY 



intestinal bacteria and soil bacteria in waters of ordinary 

 purity. Comparative studies of culture media, to be 

 quoted in the succeeding chapter, will show how del- 

 icately the bacteria respond to comparatively slight 

 changes in their food-supply. Wheeler (1906) found 

 that tj^hoid bacilH would persist in almost undimin- 

 ished numbers in sterilized water from a polluted well 

 containing considerable organic matter and kept in 

 the dark at 20 degrees, while in purer water or in the 

 light they died out in from 2 to 6 weeks. 



EXAMINATIONS OF THE ISAR AT PULLACH 



(Rapp, 1903) 

 (A) Carried out September 26, 1898, no rain having fallen for three weeks 



(5) Carried out November 28, i8g8, no rain having fallen for some time 



Whipple and Mayer (1906) have called attention to 

 another important factor in the general problem. They 

 find that the presence of oxygen is essential to the per- 



