14 ELEMENTS OF WATER BACTERIOLOGY 



Chief of them appear to be sedimentation, the activ- 

 ity 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 experiments 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 canal water may 

 be attached to gross particles, as evidenced by their 

 sedimentation in a few hours. Jordan (Jordan, 1900) 

 is firmly of the opinion that in the lower part of the 

 Illinois River, where there is a fall of but 30 feet in 

 225 miles, the influences summed up by the term 

 sedimentation are sufficiently powerful to obviate the 

 necessity for summoning another cause " to explain 

 the diminution n\ numbers of bacteria," and he further 



