Effect of Acids on Typhoid and Colon Bacilli ^75 



amount, probably playing the principal part. The same thing is 

 true of benzoic acid. Here the molecule is more highly toxic, 

 producing a 99 per cent reduction at 0.0097 normality and a 100 per 

 cent reduction at 0.0199 normality, with about i per cent dissocia- 

 tion. As in the case of the mineral acids B. typhi is more sensitive 

 than B. coli, showing 100 per cent reduction with benzoic acid at a 

 strength of 0.0057 normal. 



It appears, then, that the toxicity of these organic acids is due not 

 mainly to hydrogen ions, but to the action of the undissociated 

 molecule, varying widely, as might be expected, with the acid 

 employed. A comparison of the corresponding toxic normal 



TABLE 9. 



Toxicity of Organic and Mineral Acids for B. coli and B. typhi. Strength in Normality 



Producing 99 Per Cent and 100 Per Cent Reduction. 



strength, made in Table 9, shows that benzoic acid is almost as 

 toxic as the mineral acids, the effect being due in one case to the 

 whole molecule, and in the other case to hydrogen ions. Acetic 

 acid, on the other hand, has only 10-20 per cent as high a disinfect- 

 ant action. 



5. THE DIMINISHED TOXICITY OF ACIDS IM THE PRESENCE 



OF PEPTONE. 



Having fixed with some precision the killing-point for the various 

 acids studied, when acting in tap water, we next desired to determine 

 what would occur in the presence of organic matter. .\ series of 

 experiments was carried out, parallel to those reported above, except 

 that a I per cent solution of Witte's peptone was used instead of tap 

 water. The results with the mineral acids, presented in Tables 10-12, 

 showed that the toxicity of the acid is profoundly modified by the 

 presence of organic matter. 



