LIGHT 



343 



MONTHLY VARIATION OF BACTERIA IN A NORMAL AND POLLUTED 



STREAM. 



Sedimentation. Bacteria disappear more rapidly from still or 

 slow-flowing streams than from rapid-flowing streams, due to the 

 fact that the transporting power of a stream varies as the sixth 

 power of its velocity. A current moving six inches a second will 

 carry fine sand; one moving twelve inches a second will carry 

 gravel; four feet a second, stones of about two pounds' weight; 

 and thirty feet a second, blocks of three hundred and twenty tons. 



The sedimentation of bacteria themselves takes place very 

 slowly even in still water, for 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, being quite 

 within the limits of experimental error. In the natural streams 

 however, the bacteria are, to a great extent, attached to larger 

 solid particles, and upon these the action of gravity is more import- 

 ant. Sedimentation is one of the most important factors, according 

 to Jordan, in purifying waters. He states that "it is noteworthy 

 that all the instances recorded in the literature where a marked 

 bacterial purification has been observed are precisely those where 

 the conditions have been most favorable for sedimentation." 



Light. Light is one of the best germicides, for when it plays 

 upon the naked protoplasm of the bacterial cell it kills both vege- 

 tative and spore forms in a short time. Opinions vary, however, 

 as to the part played by light in destroying bacteria in natural 

 waters. Buchner found that plates containing B. tuberculosis 

 were sterilized in four and one-half hours at a depth of five feet, 

 but were unharmed at a depth of ten feet. Plates exposed at 

 various depths and containing various saprophytes gave the fol- 

 lowing counts after three hours: 



1 Rain or high water due to previous thaw. 



