l8 Elements of Water Bacteriology. 



But few studies have been made of the effect of light 

 on bacteria in flowing water. Jordan (Jordan, 1900) has 

 investigated several Illinois streams, and arrived at the 

 conclusion that in moderately turbid water, at least, the 

 Sim's rays are virtually without action. On the other 

 hand, Rapp has observed a considerable reduction of the 

 bacteria in the Isar at PuUach after the period of diurnal 

 insolation, as shown by the table on page 20. 



It is. uimecessary to dwell in detail upon the effect which 

 the lack of nutritive elements must exert upon 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 delicately the bacteria 

 respond to comparatively slight changes in their food 

 supply. Wheeler (1906) found that typhoid bacilli would 

 persist in almost undiminished 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 two to six weeks. 



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- 

 sistence of typhoid and colon bacilli in water, although 

 in nutrient media both forms may thrive imder anaerobic 

 conditions. 



