BACTERIA 95 



to the lower temperature of the water in winter which favors 

 the continuance of bacterial vitality. In highly-polluted rivers the 

 processes of decomposition are retarded by cold weather ; in con- 

 sequence, bacteria together with their food substances travel for a 

 greater distance down stream in winter than in summer. This 

 condition has been shown to exist, for example, in the Illinois 

 River which is heavily polluted with Chicago sewage. 



Besides these important seasonal fluctuations, daily and hourly 

 changes may be noticed, depending upon the amount of rainfall, 

 the velocity of the current, the direction and force of the wind and 

 perhaps the germicidal action of sunlight. For these reasons, it is 

 necessary, in order to interpret correctly the sanitary significance 

 of the bacterial content of any body of surface water, to make re- 

 peated examinations under a variety of circumstances and with 

 particular attention to the effect of modifying conditions. In the 

 case of ground waters (wells, springs, etc.), the number of bacteria is 

 less affected by changes in external conditions, but here also great 

 caution is necessary in drawing conclusions from a limited number 

 of observations. 



The following table gives some conception of the number of 

 bacteria that may be found by the gelatin plate method in various 

 bodies of water. Great variations occur and any such tabulation 

 can have only an approximate value. 



Per cubic centimeter 



Sewages or sewage-polluted waters 100,000 to 1,500,000 



Rivers not highly polluted 1,000 to 10,000 



Lakes and ponds not highly polluted 100 to 1,000 



Pure spring waters 5 to 50 



The enormous number of bacteria which such figures show to be 

 present in all natural bodies of water comprises many different 

 kinds. There is no special and characteristic class of " water 

 bacteria," but germs from the air, from the soil, from decomposing 

 animal and plant substances and from the healthy and diseased 

 tissues of animals and plants may at times find their way into 

 water. The bacterial flora of a given stream or pond is therefore 



