QUANTITATIVE BACTERIOLOGICAL EXAMINATION. 33 



When it is possible to do so, all the colonies on the 

 plate should be counted. When they exceed 400 or 500 

 it is often easier and fully as accurate to count a frac- 

 tional part of the plate and estimate the total number 

 therefrom. This should not be done, however, except 

 in case of necessity. 



It is customary in determining numbers to make plates 

 in duplicate, thereby affording a check upon one's own 

 work and enhancing the value of the results through 

 the greater accuracy obtained. Owing to the lack of 

 precision in the method, the limit of experimental error is 

 a wide one. It should be possible,, however,, for careful 

 manipulators to obtain results within 10 per cent of eacfy 

 other, and a closer agreement than this is hardly to be 

 expected. It has been suggested by the committee of 

 the American Public Health Association to adopt the fol- 

 lowing mode of expressing results. 



NUMBERS OF BACTERIA FROM 



1-50 shall be recorded to the nearest unit 



51-100 



101-250 



251-500 



501-1,000 



1,001-10,000 



10,001-50,000 



50^001-100,000 



100,001-500,000 



500,001-1,000,000 



1,000,001-5,000,000 



The determination of numbers of bacteria in water in 

 the field has frequently been attempted. Since the labora- 



