TOXICITY OF ACIDS TO INFUSORIA 71 



Euplotes : Citrate > salicylate > benzoate = tartrate > 

 succinate > formate — valerianate = lactate. Here as in many 

 other physiological reactions {Hober, True) the order of toxicity 

 is: benzoate > salicylate > tartrate > formate-valerianate > 

 chloride. In spite of the general similarity between the order for 

 Paramecium and that for Euplotes, there are several important 

 differences. Euplotes, so strikingly resistant to pH, falls to the 

 level of Paramecium in resistance to concentrated solutions of 

 several of the salts, e.g., chloride, formate, butyrate, benzoate, 

 salicylate, and citrate. 



Although all of these salts are fairly toxic at concentrations of 

 0.5 M and above, very few of them remain §o at lower concentra- 

 tions. At 0.01 M only salicylate, citrate, benzoate, and tartrate 

 kill Euplotes within sixty minutes. The extraordinary toxicity 

 of Na-citrate must be due to the anion rather than to the cation, 

 for Na-citrate 0.01 M is more toxic to Euplotes than NaCl of 

 five times its concentration. At still lower concentrations, such 

 as would be present in acids of 0.001 N or less, only three anions 

 would still be significantly toxic — salicylate and citrate to Eu- 

 plotes, sahcylate to Paramecium. Consequently, we cannot 

 explain the extraordinary toxicity of the monobasic fatty acids 

 and cyclic acids as due to the toxic action of their anions, except 

 perhaps to some extent in the case of salicylic. 



In order to test the point further and to secure additional evi- 

 dence as to the activity of the molecule, observations were made 

 with mixtures of each acid with its sodium salt (table 2). 



With salicylic in the case of Paramecium and with salicylic, 

 benzoic, and citric in the case of Euplotes the salt-acid mixture 

 is more toxic than the acid alone, even though the salt is rela- 

 tively non-toxic. Since ionization is depressed in the salt-acid 

 mixture and the number of acid molecules is consequently in- 

 creased, the greater toxicity of the salt-acid mixture must be due 

 to the activity of the molecule. It is noteworthy that these three 

 acids are among the most toxic of those studied. With these 

 three exceptions, the addition of 0.01 M Na-salt decreases the 

 toxicity of all the acids tested. With higher concentrations of 

 salt there is generally a further decrease in toxicity, which, how- 



