Qtcantitative Bacteriological Examination. 27 



sewage, and 4.8 to i.o for sand filter efiSuent. With waters 

 of still better quality the ratio goes higher, reaching a 

 maximum when the bacteria which increase and multiply 

 in pure water are most abimdant. Miiller (1900) found, for 

 example, that water which normally showed six times as 

 many bacteria on NahrstoflE agar as on gelatin might give 

 a Nahrsto£f-gelatin ratio of 20-30 after it had been stand- 

 ing 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-gelatin ratios for 

 waters of various grades of purity, 



TABLE SHOWING PERCENTAGES OF BACTERIA DEVEL- 

 OPING ON REGULAR AGAR AND ON NAHRSTOFF 

 AGAR FOR DIFFERENT CLASSES OF WATERS. 



NAHRSTOFF AGAR. 



