136 BACTERIA IN WATER. 
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. 
Although these factors do not wholly explain the purification 
of streams, it is certain that sewage-polluted streams are in time 
freed from most of their bacteria. Commonly, however, such streams 
continue to receive contamination all the way to their mouth, and 
never again become fit for drinking purposes, unless the water is 
subsequently purified by filtering or otherwise. 
