14 Elements of Watet Bacteriology. 



The decrease in numbers which takes place when a 

 surface water is stored in a pond or reservoir, indicates 

 that the forces which tend to produce bacterial self- 

 purification are important ones. It is necessary to con- 

 sider in somewhat more detail just what these forces are, in 

 order to gauge their potency in any particular instance. 



Chief of them appear to be sedimentation, the activity 

 of other micro-organisms, light, temperature, and food- 

 supply, and perhaps more obscure conditions such as 

 osmotic pressure. 



The subsidence of bacteria either by virtue of their own 

 specific gravity or as the result of their attachment to 

 particles of suspended matter is unquestionably partly, 

 if not largely, responsible for changes in the number of 

 bacteria in the upper layers of water at rest or in very 

 sluggish streams. The results of numerous investigations 

 by different workers seem to indicate that sedimentation 

 of the bacteria themselves takes place slowly, and that 

 the difference in numbers between the top layer and the 

 bottom layer of water in tall jars in laboratory experi- 

 ments of a few days' duration is very slight or quite within 

 the limits of experimental error (Tiemann and Gartner, 

 1889). Different species may, of course, be differently 

 affected (Scheurlen, 1891). It must be remembered, 

 however, that in natural streams bacteria are to a great 

 extent attached to larger solid particles upon which the 

 action of gravity is more important. Spitta (1903) 

 found that from one-fifth to one-half of the bacteria in 



