142 Elements of Water Bacteriology. 



directly to the dextrose-broth, tubes and incubated for 

 twenty-four hours at 37 degrees. 



On measurement of the gas, if the results above given 

 are obtained, the reaction is considered typical. If the 

 amount of gas is between 10 and 25 per cent or more than 

 70 per cent, or the percentage of carbon dioxide is greater 

 than 40, the reaction is considered atypical. If no gas 

 forms, or less than 10 per cent, the test is called 

 negative. 



In recent years, Irons (Irons, 1901) was perhaps the 

 first to call attention to the value of this method, stat- 

 ing that "when the dextrose tube yields approximately 

 33 per cent of CO2, Bacillus coli communis is almost 

 invariably present." In the next year the reliability of 

 the fermentation test as an indication of B. coli was 

 worked out by Gage (Gage, 1902) as given in the follow- 

 ing table: 



I C.C. 100 ex. 



Number of samples tested 



Number giving preliminary fermentation . 

 Per cent of latter proved to contain coli . 



S172 



1036 



70 



137s 



474 



71 



Whipple (Whipple, 1903) examined a large number 

 of surface-water supplies by this "presumptive test" 

 and obtained striking results, shoMm in the following 

 table. The waters are arranged in six groups according 

 to the results of sanitary inspection. Group I including 

 waters collected from almost uninhabitated watersheds, 



