BACTERIA IN WATER. 17! 



teria 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. Such a reduction is not simple dilution. 



3. The action of sunlight is known to be injurious to bac- 

 teria, and it has been thought that this may be one of the 

 factors destroying the bacteria in the streams. But its action 

 in muddy streams must be very slight. 



4. The deleterious action of other living organisms in the 

 water may have its influence. Microscopic animals certainly 

 destroy great quantities of bacteria, actually feeding upon 

 them. That many microscopic animals do feed upon bacteria 

 can easily be demonstrated with the microscope and these 

 animals may be one of the efficient means of the self-purification 

 of streams. Possibly these animals may also use up the food 

 which the bacteria need. 



5. Sedimentation is certainly a factor. It is well known 

 that bacteria generally are heavier than water and will slowly 

 sink to the bottom. In slowly flowing streams sedimentation 

 probably plays an important part in the purification of the water 

 from sewage bacteria. 



6. ExJiaustion of the Food Supply. Bacteria certainly need 

 food and some species need large amounts of food. The food 

 in the water is of course used up either by themselves or by 

 other organisms, and finally becomes insufficient to support 

 bacterial life. 



Whether these factors explain the purification of streams, or 

 whether there is some other yet unexplained factor it is hardly 

 possible to say. But certain it is that sewage -polluted streams 

 become freed from most of their bacteria after flowing a few 

 miles. Whether such water is safe for drinking is at least 

 doubtful. 



