QUANTITATIVE EX A AGNATION OP WATER 31 



to gelatin count to be 1.7 to i.o for sewage, and 4.8 

 to 1.0 for sand filter effluent. With waters of still 

 better quality the ratio goes up higher, reaching a 

 maximum when the bacteria which increase and 

 multiply in water are most abundant. Miiller (1900) 

 foimd, for example, that water which normally showed 

 six times as many bacteria on NahrstoflF agar as on 

 gelatin might give a Nahrstoff-gelatin ratio of 20-30 

 after it had been standing for some time in the supply 

 pipes. The table below, taken from the valuable 

 paper by Gage and Phelps (1902), shows strikingly 

 the different Nahrstoff-agar ratios for waters of 



TABLE SHOWING PERCENTAGES OF BACTERIA DEVELOP- 

 ING ON REGULAR AG-\R .\ND NAHRSTOFF AGAR 

 FOR DIFFERENT CLASSES OF WATERS 



(Gage and Phelps, 1902) 



Regular Agar 



Ground water. . . 

 Filtered water. . . 

 Merrimac River . 

 Filtered sewage. . 

 Sewage 



Narhstoff Agar 



100 

 100 

 100 

 100 

 100 



