THE CONTAMINATION AND PURIFICATION OF STREAMS 147 
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 pur poses, unless the 
water is subsequently purified by filtering or otherwise. 
