MATILDA M. BROOKS 183 



A comparison of the two curves shows that the rate of production 

 of CO2 by both organisms is a function of the pH value of the medium. 

 Below pH 5.0 Bacillus suhtilis is very sensitive to the effects of acid, 

 while Bacillus butyricus is only gradually affected as the concentra- 

 tion of the hydrogen ions increases. It is not possible to state, on 



Rate of CO^ production 



lOOyt 



507 



9 pH 



Fig. 3. Rate of production of CO2 (expressed as per cent of the normal) 

 B. butyricus, Curve A , and of B. subtilis, Curve B, as affected by the hydrogen 

 ion concentrations. 



The normal rate represents the maximum production. of CO2. This was 

 obtained at pH 7.0 in the case of B. butyricus and at pH 6.8 in the case of 

 B. subtilis. It was measured by noting the number of seconds required to 

 change the pH value of the indicator tube from 7.8 to 7.6. The time required 

 depended upon the amount of bacterial suspension used. Each point on the 

 curve represents an average of several experiments. Probable error of the mean 

 less than 2 per cent except in the last two points of the alkaline range, where it is 

 less than 3 per cent. 



the basis of these experiments, whether or not this is due, as some 

 investigators believe, to existence of a waxy envelope in the case of 

 the acid-fast bacteria, and to the absence of such a covering in the 

 case of the non-acid fast bacteria. 



It was found that when a small amount of acid was added to sus- 

 pensions of either Bacillus hutyricus or Bacillus subtilis the addition 



