Isolation of the Colon Bacillus. 105 



light of more recent knowledge is the requirement that 

 dextrose broth shall be fermented "with the formation of 

 about 50 per cent of gas, of )vhich about one-third (CO2) 

 is absorbed by a two per cent solution of sodium hydrate." 

 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 laterj 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. They 

 show, first, that the total amount of gas formed by B. coli 

 varies w idely, 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 medium 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. Calcula- 

 tion on this basis igives a curve very close to the observed 

 data, and the conclusion of Longley and Baton that the 



