232 



reach of the river in which the bacterial action and attendant 

 decay of the sewage ensues, are lengthened as the temperature 

 falls, and we find in consequence an increase in the bacteria in 

 the river water passing Havana which approximates fortyfold. 

 The pulse in bacteria due to the sewage of Peoria which is 

 found at Pekin during the summer, reaches Havana also, thirty- 

 two miles below, as temperatures fall in the autumn. 



The averages of the number of colonies of bacteria found 

 in the canal and river water at points from Bridgeport to Graf- 

 ton during the period of analyses given in the first table are to 

 be found in the following table, also taken from Jordan ('00). 



Chlorine and Bacteria — Des Plaines and Illinois Rivers, 

 Bridgeport to Grafton. 



Collecting Stations 



Distance from 



Bridgeport, 



in miles 



Chlorine (pts. 

 per million) 



Number of col- 

 onies per cm.^ 



Bridgeport . . 

 Lockport . . . 



ioliet 

 /lorris 



Ottawa 



La Salle 



Henry 



Averyville . . 

 Wesley City 



Pekin 



Havana 



Beardstown. 

 Kampsville . 

 Grafton 



29 

 33 

 57 

 81 



95 

 123 



159 

 165 



175 

 199 

 231 

 288 

 319 



119. 2 



117.4 

 104.8 

 68.1 

 58.5 

 46. 1 

 44.2 

 40.9 

 40.9 



38.4 

 36.2 



29-3 

 22.9 



18.3 



1,245,000 



650,000 



486,000 



439,000 



27,400 



16,300 



11,200 



3,660 



758,000 



492,600 



16,800 



14,000 



4,800 



10,200 



The chlorine content at the several points also appears in 

 the table, and exhibits a steady decline from Chicago to the 

 Mississippi River with a brief pause at Peoria. This decline 

 expresses approximately the dilution which on an average the 

 sewage undergoes during the low-water period. The flood 

 season of the spring was not included in the period of analysis. 

 From Ottawa to the mouth of the Hlinois, as indicated by the 

 chlorine, this dilution is about one third. 



The decrease in number of bacteria, while it may not coin- 

 cide strictly with the completion of processes of decay still 



