THE 



IN NATURAL WATERS. 13 



Many investigations conducted since the pioneer re- 

 searches of Downes and Blunt (Downes and Blunt, 1877) 

 have confirmed the results reported by them, viz., direct 

 sunlight is fatal to most bacteria in the vegetative state 

 and even to spores if the exposure be sufficiently long, 

 while diffused light is harmful in a lesser degree. Opin- 

 ions vary as to the degree to which light is active in 

 destroying the bacteria in natural water. Buchner 

 (Buchner, 1893) found by experiment that the bacteri- 

 cidal 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 sun- 

 light was passed vertically through 60 cm. of drain-water 

 the lower layers contained nearly as many bacteria after 

 three 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. 



Although it is hard to estimate the exact importance 

 of each factor, the general phenomena of the self-purifi- 

 cation of streams are easy to comprehend. A small 



