THE COLON GROUP OF BACILLI 117 



only 474 or 58 per cent gave between 25 and 70 per 

 cent of gas in the closed arm with 25-40 per cent carbon 

 dioxide. 



Stamm (1906) and others have pointed out that the 

 ratio of carbon dioxide to hydrogen changes with 

 the age of the culture. At first the proportion of the 

 former to the latter is as two to one, and later, in the 

 same tube, the ratio is reversed. More recently, 

 Longley and Baton (1907), in one of the ablest and most 

 fruitful of recent contributions to water bacteriology, 

 have made it clear that neither of these quantitative 

 determinations is of importance if made in an ordinary 

 open tube. They show, first, that the total amount 

 of gas formed by B. coli varies widely, from 10 to 80 

 per cent, the mode of the curve being found, not at 50, 

 but at 35 per cent. Secondly, they show that the 

 proportion of carbon dioxide present is a function of 

 the total amount of gas. They find that when grown 

 in an atmosphere of CO2, B. coli produces a gas which 

 consists of about 3 parts of carbon dioxide to one of 

 hydrogen. Assuming that the gas originally formed 

 by B. coli has always about this composition, and that 

 the absorption of CO2 by the mediimi is the chief cause 

 of the differences observed in the gas which collects 

 in the closed arm, the gas ratio would vary directly 

 with the amount of total gas; the more rapidly gas is 

 formed, the greater the proportion of CO2 remaining 

 unabsorbed. Calculation on this basis gives a curve 

 very close to the observed data. 



These criticisms apply only to the fermentation 

 test made in an open tube and imcorrected for the 



