20 ELEMENTS OF WATER BACTERIOLOGY 



at Pittsburgh the antiseptic effect of acid wastes was 

 strikingly shown. (Engineering News, 1912.) 



Although it is hard to estimate the exact importance 

 of each factor, the general phenomena of the self- 

 purification of streams are easy to comprehend. A 

 small brook, immediately after the entrance of polluting 

 material from the surface of the ground, contains many 

 bacteria from a diversity of sources. Gradually those 

 organisms adapted to life in the earth or in the bodies 

 of plants and animals die out, and the forms for which 

 water furnishes ideal conditions survive and multiply. 

 It is no single agent which brings this about, but that 

 complex of little-understood conditions which we call 

 the environment. If any one thing is of prime impor- 

 tance it is probably the food-supply, for only certain 

 bacteria are able to multiply in the presence of the 

 small amount of organic matter present in ordinary 

 potable waters. As Jordan (Jordan, 1900) has said: 

 " In the causes connected with the insufficiency or 

 unsuitability of the food-supply is to be found, I believe, 

 the main reason for the bacterial self-purification of 

 streams." 



Effect of Temperature upon Bacteria in Water. 

 The effect of temperature upon the survival of bac- 

 teria in water varies according to this primary con- 

 dition of food-supply which has just been discussed. 

 When bacteria are in a medium in which they are able 

 to grow and multiply, warmth, within reasonable limits 

 of course, favors their development, At times this 

 may be true even of certain intestinal bacteria in water. 

 Thus at Harrisburg, Pa., a series of B. coli examinations 



