QUANTITATIVE BACTERIOLOGICAL EXAMINATION. 21 



Table showing Percentages of Bacteria Developing on Regu- 

 lar Agar and on Nahrstoff Agar for Different Classes of 

 Waters. (Gage and Phelps, 1902.) 



regular agar. 



Class of Water. 



Days' Count. 



Ground-water . . 

 Filtered water. . . 

 Merrimac River. 

 Filtered sewage. . 

 Sewage 



NAHRSTOFF AGAR. 



Ground-water. . . 

 Filtered water. . . 

 Merrimac River. 

 Filtered sewage. . 

 Sewage 



6 



7 



9 



19 



46 



100 

 100 

 100 

 100 

 100 



The procedure for the quantitative determination of 

 bacteria in water consists, in brief, in mixing a definite 

 amount of a suitably collected specimen of the water with 

 a sterile solidifiable culture medium and allowing it to 

 develop for a sufficiently long time to permit reproduction 

 of the bacteria and the formation of visible colonies which 

 may be counted. The process is divided naturally into 

 four stages — sampling, plating, incubating, and counting 

 — which will now be discussed. 



Sampling. — All samples of water for bacteriological 

 examination should be collected in clean, sterile bottles 

 with wide mouths and glass stoppers, preferably of the flat 



