468 M. E. COLLETT 



concentration in Paramoecium. In many experiments upon 

 other material benzoic has been found very effective, and simi- 

 lar specific differences in susceptibility have been observed 

 which suggest differences in lipoid content (Harvey, '14; Crozier, 

 '16; Haas, '16; Loeb, '13). 



Peters and Burres ('09) conclude from their experiments upon 

 the toxicity of Cu salts to P. aurelia that toxic effect is due to 

 injury to an essential enzyme and not to direct chemical injury 

 of the protoplasm. If the normal metabolic processes of the 

 cell are interrupted, as they would be by the failure of an im- 

 portant enzyme, it is obvious that the chemical and physical 

 balance of the whole cell would be affected. Possibly something 

 of the sort may be involved in the toxic action of acids upon the 

 cilia of Paramoecium, Euplotes, etc., and may account for the 

 specific differences involved. This explanation is of course 

 purely hypothetical at present. There are many other possible 

 factors in toxicity, but conclusions are difficult and uncertain 

 the moment one ventures beyond very simiple and obvious 

 comparisons. 



SUMMARY 



It is found that the relative toxicity of a series of acids varies 

 decidedly with the concentration, and therefore it is unwise to 

 base conclusions as to mode of action upon results obtained 

 with only one concentration. The fact that power of penetra- 

 tion also varies greatly with concentration makes it probable 

 that the same is true of many of the physiological effects of 

 acids. There are also great differences in the effects of acids 

 upon different species: even organisms so closely related as the 

 infusoria used in these experiments show characteristic differ- 

 ences. When tested in the same solution, one species may be 

 two, four, or even twenty times as resistant as another; and in 

 addition the order of toxicity of the series of acids is somewhat 

 different for each species. Another factor in determining rela- 

 tive toxicity is temperature. Ordinarily toxicity increases with 

 increase in temperature and decreases with decrease in tem- 

 perature, but the degrees of influence exerted by temperature 



