PRESUMPTIVE TESTS FOR B. COLI. 95 



from 25 to 70 per cent of the capacity of the closed arm 



of the fermentation- tube. Of this gas approximately 



one-third is carbon dioxide and two-thirds hydrogen, 



H 2 

 that is, as the gas formula is generally expressed, 7^=r= -• 



In testing a water by this method a series of samples 

 of the water, in suitable dilution, .001, .01, .1, 1.0, or 10 c.c, 

 is added directly to the dextrose-broth tubes and incu- 

 bated for twenty-four hours at 37 . 



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 if 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. It is recognized that the distinction is not 

 absolute, since even pure cultures of B. coli sometimes 

 fail to give typical reactions, and on the other hand, an 

 atypical organism sometimes gives a satisfactory positive 

 test. There can be little question, however, as to its 

 value to the sanitary engineer. 



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

 first to call attention to the value of this method, stating 

 that "when the dextrose tube yields approximately 

 33 per cent of C0 2 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. 



