44 



Elements of Water Bacteriology. 



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. 



501-1000 



1001-10,000 



10,001-50,000 



50,001-100,000 



100,001-500,000 



500,001-1,000,000 



1,000,001-5,000,000 



NUMBERS OF BACTERIA FROM 



1-50 shall be recorded to the nearest unit 

 51-100 " " " " Sl 



101-250 " " " " " " 10 



251-500 " " " " " " 25 



5 



" " " 100 



" " " 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 labora- 

 tory 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 rotating the tube until 

 the medium has solidified in a thin layer on the inner wall. 

 Other bacteriologists have devised ingenious field kits for 

 adapting the plate method to this purpose. The oppor- 

 tunity for air infection in work done outside a proper 



