THE BACTERIA IN NATURAL WATERS. II 



and in salt as in fresh water the amount of bacterial life 

 decreases in general as one passes downward from the 

 surface and outward from the shore. 



The principal factors in the destruction of the bacteria 

 in water during storage 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 becoming attached 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 inves- 

 tigations by different workers seem to indicate that 

 sedimentation takes place slowly, and that the differ- 

 ence in numbers between the top layer and the bottom 

 layer of water in tall jars in laboratory experiments of 

 only 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, 

 that in natural streams bacteria are to a great extent 

 attached to larger solid particles upon which the action 

 of gravity is much more important. 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 



