THE CONTAMINATION AND PURIFICATION OF STREAMS. 135 
is safe, provided it is free from snow ice and has been stored at least 
two or three months. 
II. THE CONTAMINATION AND PURIFICATION 
OF STREAMS. 
The sewage contamination of streams has been increasing year 
by year, until many a stream that was clear and limpid thirty years 
ago is now a vile collection of filth. Until some other means of 
disposing of sewage is generally adopted, this pollution must continue 
to increase. There has been a widely held belief that running water 
purifies itself, and that these streams rapidly become free from 
their pollution. This is partly correct and partly erroneous. A 
stream does not purify itself by running, but there is always a 
tendency for water to become pure, and in time sewage contamina- 
tion quite disappears from water, whether running or stagnant. 
The best studied example of this is in the Chicago Drainage Canal. 
Recently the city of Chicago converted the Illinois River into a 
drainage canal for the great amount of sewage of that city. This 
river is a small one and flows very slowly. It finally empties into 
the Mississippi River, after flowing some 300 miles. It empties 
a few miles above the point where St. Louis takes its water-supply, 
and naturally it excited considerable alarm in the latter city. A 
careful examination of the bacteria in the river shows that there is 
a constant decrease in numbers as the distance from Chicago is 
increased, and when it finally empties into the Mississippi, all of 
the bacterial contamination from Chicago has disappeared. In 
this flow the river has purified itself of sewage bacteria. In other 
examples when the polution is less a flow of even ten miles largely 
purifies the water. 
Evidently the phenomenon is practically identical with the 
bacterial purification of sewage, modified by the different conditions. 
The following factors have been advanced as explaining it: 
The dilution of the water by tributary streams. This doubtless 
accounts, in part, for the decrease in number of bacteria per c.c., 
but it cannot be a very important factor in cases such as shown, 
