146 BACTERIA IN WATER 
Illinois River into a drainage canal for the great amount of sew- 
age 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 pollution 
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 condi- 
tions. The following factors have been advanced as explaining it: 
The dilution of the water by tributary streams. This doubt- 
less 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, where the number of bacteria in the river finally becomes 
no greater than the number in the tributary streams. 
The action of sunlight is known to be injurious to bacteria, and 
it has been thought that this may be one of the factors destroying 
the bacteria in streams. But its action in muddy streams must be 
very slight. 
Other living organisms in the water have a deleterious action. 
Microscopic animals certainly destroy great quantities of bacteria, 
actually feeding upon them, and they may be one of the efficient 
means of the self-purification of streams. 
It is well known that bacteria are generally heavier than water 
and that they will slowly sink to the bottom. In slowly flowing 
streams sedimentation probably plays an important part. 
The food in the water is of course used up either by bacteria or 
some other organisms, and finally becomes insufficient to support 
bacteria life. 
