THE BACTEEIA IN NATURAL WATERS 17 



the exposure be sufficiently long, while diffused light 

 is harmful in a less degree. Opinions vary as to the 

 degree to which light is active in destroying the bacteria 

 in natural waters. Buchner (Buchner, 1893) found 

 by experiment that the bactericidal power of light 

 extends to a depth of about three meters before it 

 becomes imperceptible. On the other hand, Procaccini 

 (Procaccini, 1893) found that when sunlight was passed 

 vertically through 60 cm. of drain- water the lower 

 layers contained nearly as many bacteria after 3 hours' 

 treatment as before the exposure. The middle and 

 upper portions showed a great falling off in numbers, 

 however. 



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 sun's rays are virtually without action. On 

 the other hand, Rapp has observed a considerable 

 reduction of the bacteria in the Isar at Pullach after the 

 period of diurnal insolation, as shown by the table on 

 the following page. Clemesha (191 2 a ) attributes very 

 great importance to the action of light in the self- 

 purification which takes place in Indian lakes and 

 rivers; his opinion is apparently not based on com- 

 parative experiments including and excluding this 

 factor, but chiefly on the greater numbers of intestinal 

 bacteria at the bottom as compared with the superficial 

 layers of water. 



It is unnecessary to dwell in detail upon the effect 

 which the lack of nutritive elements must exert upon 



