The Bacteria in Natural Waters. 17 



Temperature has a direct relation to bacterial life, and 

 the number of parasitic bacteria at least may be quickly 

 lessened by the action of cold. Sedgwick and one of 

 us (Sedgwick and Winslow, 1902) have shown that of 

 typhoid or colon bacilli in ice or cool water, over 40 per 

 cent will perish in three hours and 98 per cent and up- 

 wards in two weeks. This diminution is not of course 

 due to cold alone, but at high temperature the decrease 

 is much less marked. At Harrisburg, Pa. (1907), an 

 actual increase of colon bacilli was observed in a small 

 reservoir during a warm period when its temperature 

 approximated blood heat. 



Many investigations conducted since the pioneer re- 

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

 have confirmed the results reported by them, which showed 

 that 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 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 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 exposure. The middle 

 and upper portions showed a great falling off in numbers, 

 however. 



