QUANTITATIVE EXAAIINATION OF WATER 41 



for the count. A much smaller number will not give 

 average figures, and if more than 200 colonies are present 

 on a plate many bacteria wiU be checked by the waste 

 products of those which first develop and the count 

 obtained will be too low. After the addition of the 

 diluted sample and the nutrient medium, their thorough 

 mixture in an even layer on the bottom of the plate 

 is obtained by careful tipping and rotation. 



It was formerly customary to mix the water with the 

 gelatin in the tube before pouring into the plate, but 

 this method is objectionable because there is always 

 a residuum of medium remaining in the tube which 

 will retain varjdng numbers of bacteria and thus 

 interfere with the accuracy of the count. Before pour- 

 ing the medium into the plate the mouth of the tube 

 should be flamed to remove any possibility of con- 

 tamination. 



The usual method of determining the number of 

 bacteria in water for sanitary purposes in Germany, 

 England and the United States has always been by 

 the use of gelatin plates with a 2-day incubation period 

 at 20 degrees. The 1905 Standard Methods Report 

 of the American Public Health Association Committee 

 recommended this procedure, which has been universally 

 adopted. The 191 2 Report, however, suggests the use 

 of agar with a i-day period at 37 degrees, as yielding 

 quicker results and indicating the presence of bacteria 

 more nearly related to pathogenic t^-pes. The com- 

 parative value of the two methods has been well dis- 

 cussed by Whipple (1913). The use of gelatin is not 

 only more time-consuming, but requires the use of a 



