QUANTITATIVE EXAMINATION OF WATER 49 



box with top and bottom of glass 7 inches square and 

 ij inches deep, through which cold water is allowed to 

 circulate. 



Expression of Quantitative Results. It is customary 

 in determining numbers to make plates in duplicate, 

 thereby affording a check upon one's own work. Owing 

 to the lack of precision in the method, the limit of 

 experimental error is a wide one. It should be possible 

 for careful manipulators to obtain results within 10 

 per cent of each other, but a closer agreement than this 

 is hardly to be expected. It has been suggested by the 

 committee of the American Public Health Association 

 that the following mode of expressing results be adopted 

 in order to avoid the appearance of a degree of accuracy 

 which the methods do not warrant. 



NUMBERS OF BACTERIA FROM 



1-50 shall be recorded to the 



51-100 



101-250 



251-500 



501-1000 



1001-10,000 



10,001-50,000 



50,001-100,000 



100,001-500,000 



500,001-1 ,000,000 



i ,000,001-5,000,000 



nearest unit 



5 

 10 



25 

 50 



IOO 



500 



1,000 

 10,000 

 50,000 



100,000 



The determination of numbers of bacteria in water 

 in the field has frequently been attempted. Since 

 the laboratory method of "plating out" is difficult 

 to use in field work, the Esmarch tube process has often 

 been employed. This consists in introducing into a 

 tube of melted gelatin or agar i c.c. of the water and then 



