EFFECT OF ACIDS ON TYPHOID AND COLON BACILLI 273 



The critical points derived in these tests are brought together in 

 Table 5. They show that the typhoid bacillus is a little less than half 

 as resistant as the colon bacillus to dilute acids, and that the toxicity 

 of these acids depends, not on their normal strength of acid or on the 

 kind of acid used, but on the number of dissociated hydrogen ions. 

 Between 7.4 and 7.7 parts of dissociated hydrogen effects a 99 per cent 

 reduction of the colon bacillus, and between n.8 and 12.6 parts, a 

 100 per cent reduction. For the typhoid bacillus the corresponding 

 figures are 2.5-3.0 parts and 3.9-4.9 parts. Since at the dilutions 

 used the hydrochloric acid was over 96 per cent dissociated, its 

 effect must have been almost entirely ionic; and since the sul- 

 phuric acid at 75 per cent dissociation showed only the toxicity which 

 would have been expected from its dissociated hydrogen, it appears 

 that in this case too the undissociated molecule exerts no appreciable 

 influence. The anions have been shown to be neutral in the experi- 

 ments of other observers. It is evident, then, that the toxicity of these 

 acids at high dilution is a function of the dissociated hydrogen. 



TABLE 5. 

 DISINFECTANT ACTION OF MINERAL ACIDS IN TAP WATER. 



4. THE DISINFECTANT ACTION OF ACETIC ACID AND BENZOIC 

 ACID UPON B. TV PHI AND B. COLI. 



We next desired to study examples of the incompletely dissociated 

 organic acids. Acetic and benzoic acids were selected as types, 

 and the experiments were carried out as before. The results ob- 

 tained with benzoic acid are probably somewhat inaccurate on 

 account of the difficulty of securing complete solution. The results 

 are shown in Tables 6-8. 



An inspection of these tables shows a marked difference from the 

 results obtained with the mineral acids. With B. coli in acetic acid the 

 99 per cent reduction is reached at a strength of 0.0812 normal, and 



