TOXICITY OF ACIDS TO INFUSORIA 93 



the cell-body, behaves like a lipoid containing traces of an organic 

 acid and an organic base. In my experiments the phenomena of 

 acid-induced hydration and coagulation suggest the presence of 

 some protein at the surface of the cell, while the penetrating 

 power of xylol and of such acids as butyric, caproic, benzoic, 

 etc., indicates in addition a lipoid-like substance as an important 

 component of the surface film. The facts observed in the antago- 

 nism of the acids by salts are consistent with Clowes' tentative 

 identification of the surface lipoid as a soap, for the Na-soap would 

 protect against lipoid-soluble substances and the Ca-soap against 

 water-soluble substances. There may also be Na- or Ca-pro- 

 tein salts of unlike solubility, resembling the Na- and Ca-gela- 

 tin salts which Loeb^" has found to differ widely in hydration. 

 My experiments, however, taken by themselves, are insufficient 

 to establish the exact nature of the surface film or to explain the 

 specific differences observed. 



