12 ELEMENTS OF WATER BACTERIOLOGY. 



necessity for summoning another cause "to explain the 

 diminution in numbers of bacteria," and he further adds: 

 11 It is noteworthy that all the instances recorded in the 

 literature where a marked bacterial purification has been 

 observed are precisely those where the conditions have 

 been most favorable for sedimentation." 



Little is known as to the share of other organisms in 

 hastening the decrease of bacteria in stored water. Doubt- 

 less predatory Protozoa play some part here, and cer- 

 tain bacteriologists have believed that the toxic waste 

 products of some species of bacteria materially check 

 the development of other forms. Horrocks (Horrocks, 

 1901), Garre (Garre, 1887), Zagari (Zagari, 1887), 

 and Freudenreich (Freudenreich, 1888) have shown 

 that an " antagonism" exists when bacteria are grown 

 in artificial culture media such that the substratum 

 which has supported the growth of one form may be 

 rendered antiseptic to another. It is difficult, however, 

 to believe that any poisons are produced of such enor- 

 mous power as to cause this effect in a stream or a 

 lake, and there is no evidence in support of such a 

 view. 



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 bacilli in ice or cool water over 40 per cent will 

 perish in three hours and 98 per cent and upwards in two 

 weeks. 



