Kirchner — Bacteriological Examination of River Water. 293 
From the table a number of interesting deductions can 
be made, and it is interesting to note that during the passage 
from the bear trap dam, I, to a point just below Ruby 
St. bridge, V, a distance of 23,200 feet, about 45 per 
cent. of the bacteria disappear. The 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 aération. 
Sedimentation and dilution are often prime factors in the 
bacterial purification of streams, but these conditions do not 
maintain inthis instance. Whether the decrease in the num- 
ber of bacteria is due to aération and agitation, to the exhaus- 
tion of the food supply, or to the toxins or poisonous prod- 
ucts of the bacteria themselves, furnishes an interesting 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 JB. 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 
