THE BACTERIA IX NATURAL WATERS 23 



part at least, due to the greater persistence of t^'phoid 

 bacilli at low temperatures. 



Relation between Time of Storage and Self-piirifica- 

 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; and in a comprehensive study of the 

 self-purification of the Potomac River, the investigators 

 of the U. S. Public Health Service have found that the 

 time relation is close enough to be stated mathematically. 



Prof. Phelps expresses this in the formula log — = kt^ 



where t is the time and tii and W2 the respective mmi- 

 bers of bacteria; k, of course, will vary with the tem- 

 perature and the other conditions discussed above. 



The absolute time required to make water safe will 

 vary widely with this value k. 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 

 Lfike 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 



