THE BACTERIA IN NATURAL WATERS 23 



polluted water occur in cold weather and this is, in 

 part at least, due to the greater persistence of typhoid 

 bacilli at low temperatures. 



Relation between Time of Storage and Self-purifica- 

 tion. It is obvious that the efficiency of all the agencies 

 which tend to decrease the number of bacteria in sur- 

 face waters will increase with the prolongation of the 

 period for which they act. Time is the great measure 

 of self -purification. 



The longer the storage the greater the improvement, 

 and after a certain period even a fairly polluted water 

 may be safe and potable. The absolute time necessary 

 to produce this result varies of course according to many 

 conditions. Food supply, light, temperature and the 

 activity of other living forms vary widely and in depos- 

 ited material conditions are different from those which 

 obtain in the water itself. Jordan, Russell and Zeit 

 (1904), in an important series of experiments, added 

 typhoid bacilli to the unsterilized waters of Lake 

 Michigan, the Chicago River and Drainage Canal and 

 the Illinois River, in collodion sacs suspended in the 

 respective bodies of water. From the relatively pure 

 Lake Michigan water the specific organisms could be 

 isolated for at least a week, but in the polluted waters 

 of the rivers and the Drainage Canal they were not 

 found after 3 days except in a single instance. Russell 

 and Fuller, (1906) confirmed these general results, 

 finding that typhoid bacilli would live for 10 days in 

 the unsterilized water of Lake Mendota, while they 

 could be isolated only after 5 days when immersed 

 in sewage. Other observers record much greater 



