TOXICITY OF ACIDS TO INFUSORIA 



73 



fatty acids tested. Here the molecule must be capable of toxic 

 action. In the case of formic acid this would ordinarily be insig- 

 nificant on account of the almost complete dissociation of the acid, 

 but with other members of the series (acetic to caproic) it is im- 

 portant and accounts very largely for their great toxicity as com- 

 pared with HCl of the same pH. With benzoic, although the 

 salt is somewhat toxic to Paramecium, the toxicity of the acid- 

 salt mixture is too great to be due to the action of the anion or 

 to the pH of the mixture, and must therefore be due in part to 

 the acid molecule. 



TABLE 3 



The anion of citric acid is only slightly toxic to Paramecium, 

 the molecule not at all. The same is true of succinic and as 

 regards both organisms. With lactic and tartaric the length of 

 Ufe is about the same as in HCl of the same pH. 



It is possible that what seems to be the toxic action of the mole- 

 cule may instead be due to increased sensitiveness of the organism 

 to the salt when in the presence of acid. The simpler and more 

 probable explanation, however, is the one developed here. 



^ SUMMARY 



1. When ionization of these organic acids is depressed by the 

 addition of HCl, the toxicity of the mixture is at first unchanged, 

 but with further addition of HCl is increased. 



