QUANTITATIVE BACTERIOLOGICAL EXAMINATION. 29 



smaller number will not give average figures, and if more 

 than 200 colonies are present on a plate many bacteria 

 will 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 rota- 

 tion. 



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 small 

 residue of medium remaining in the tube which will 

 retain varying numbers of bacteria and thus interfere 

 with the accuracy of the count. Before pouring the 

 medium into the plate the mouth of the tube should 

 be flamed to remove any possibility of contamina- 

 tion. 



The exact composition of the medium used is, of course, 

 of prime importance in controlling the number of bac- 

 teria which will develop. The reaction of the medium 

 was found as early as 1891 to be important, for Reinsch 

 (Reinsch, 1891) showed in that year that the addition 

 of one one-hundredth of a gram of sodium carbonate to 

 the liter increased sixfold the number of bacteria develop- 

 ing. Fuller (Fuller, 1895) and Sedgwick and one of 

 ourselves (Sedgwick and Prescott, 1895), working inde- 

 pendently, established the fact that an optimum reac- 

 tion existed for most water bacteria and that a deviation 

 either way decreased the number of colonies developing. 



