28o C.-E. A. WiNSLOW AND E. E. LOCHRIDGE 



sion by 40 minutes' exposure to a 0.005 normal solution of either 

 hydrochloric, sulphuric or benzoic acid. The colon bacillus will 

 endure exposure, under similar conditions, to solutions from two to 

 four times as strong. Ninety-nine per cent of the bacteria in a 

 suspension are killed by solutions of from one-half to two-thirds 

 this strength, the last few organisms being especially resistant. 



The mineral acids, hydrochloric and sulphuric, are fatal in con- 

 centrations at which they are highly dissociated. Their action runs 

 parallel, not to their normal strength, but to the number of free hydro- 

 gen ions per unit volume. With the two organisms tested, both the 

 99 per cent and the 100 per cent reductions were affected, at the same 

 concentration of dissociated hydrogen, whichever acid was used. 



The organic acids, acetic and benzoic, are fatal to the typhoid and 

 colon bacilli at a strength at which they are only slightly dissociated. 

 The effect here appears to be due to the whole-molecule and is specific 

 for each acid, acetic having only 10-20 per cent the toxicity of 

 benzoic. 



The presence of i per cent of peptone greatly diminishes the toxic 

 action of acids, the action being somewhat less marked with sulphuric 

 acid than with hydrochloric, and still weaker with the organic acids. 

 In the case of hydrochloric acid we find that the diminished toxicity 

 is accounted for bv decreased ionization. 



It is evident that the action of organic matter and other neutral sub- 

 stances in decreasing toxicity greatly complicates the study of disin- 

 fectant action. It will be necessary to bear this phenomenon in mind 

 in considering the composition of culture media, since the apparent 

 acidity, as determined by titration, may be quite different from the 

 effective acidity which influences living organisms. With the mineral 

 acids, any factor which affects dissociation, such as the presence of 

 neutral salts of the same anion, will change the effective acidity. 

 In considering the viability of disease germs in sewage and water, 

 it is evident that differences in dilution and the effect of inorganic 

 salts, organic matter, and suspended solids introduce such complex 

 factors that detailed studies of specific local conditions are desirable. 



