Kirchner — Bacteriological Examination of River Water. 293 



From the ta])le a number of interesting deductions can 

 be made, and it is interesting to note that during the passage 

 from the bear trap dam, II, to a point just below Ruby 

 St. bridge, V, a distance of 23,200 feet, about 45 per 

 cent, of the bacteria disappear. Tiie distance between the 

 points of collection V and VI is 14,800 feet, and the general 

 average for these two places shows that there was no material 

 change in the number of bacteria. 



The fall in the Drainage Canal (Des Plaines R.) from 

 Lockport to Joliet is about 30 feet, and the time required to 

 cover the distance is about two hours and a half. The water 

 flows rather swiftly over a rocky channel, and is in 

 most places but a few feet deep. After flowing over the 

 dam and spreading out over an extended area as a swift and 

 shallow stream, the water is thoroughly churned and agitated, 

 and these conditions afford excellent opportunity for aeration. 



Sedimentation and dilution are often prime factors in the 

 bacterial purification of streams, but these conditions do not 

 maintain in this instance. Whether the decrease in the num- 

 ber of bacteria is due to aeration and agitation, to the exhaus- 

 tion of the food supply, or to the toxins or poisonous prod- 

 ucts of the bacteria themselves, furnishes an interestino- but 

 difficult problem for solution. It would be interesting also 

 to know if water having few bacteria, under the same condi- 

 tions as above would show a similar decrease in the number 

 of bacteria. 



Have we an index of pollution of streams? A number of 

 standards have been set up as tests for the pollution of streams, 

 but none have at all times been reliable. For a time, the 

 finding of B. coli communis was regarded as an index of 

 sewage pollution, but the colon bacillus is very widely distri- 

 buted, and since it has been found in fish, the mere presence of 

 this organism in water may be of doubtful value. However, 

 quantitative tests for the presence of B. coli communis would 

 give us more satisfactory results. Experience has shown, 

 that the most reliable index of the quality of drinking-water 

 was to be found in carefully prepared health statistics of the 

 community, and we can learn much by a careful study of the 



