72 



ELEMENTS OF WATER BACTERIOLOGY 



counts under 25 at 37, and 7 of them gave counts 

 over 100, the highest figure being 425. 



Whipple (1913) gives some figures for the raw water 

 at the Wilmington, Del., filter plant (page 71) which 

 bring out the seasonal variation very clearly. 



Another special case in which the ratio between the 

 20 and the 37 count fails to be significant is that 

 of a water which has been treated with bleaching 

 powder. Most of the bacteria which survive chlorine 

 treatment are of course spore formers, many of them 

 belonging to the hay bacillus group, and it happens 

 that most of these spore formers can grow at body 

 temperature. Thus it is common to get counts as high 

 at 37 as at 20 with such waters, although the absolute 

 numbers are generally small. This point is illustrated 

 in the two tables below, showing the results of experi- 

 mental treatment of Merrimac River water at the 

 Lawrence Experiment Station and of swimming pool 

 water at the University of Wisconsin. 



COMPARATIVE EFFECTS OF CHLORINE DISINFECTION 

 UPON 20 AND 37 COUNTS, MERRIMAC RIVER 

 WATER, AT LAWRENCE, MASS. 



(CLARK AND GAGE, 1909) 



