JACQUES LOEB 369 



the p. D. increasing with the concentration of the salt.' MacDonald^ 

 had observed a similar phenomenon; namely, the increase in p.d. 

 between nerve and surrounding salt solution with increasing dilution. 

 Donnan's theory was not known to us and we were not able to give 

 an explanation of the depressing effect of salt on the p.d. 



We next searched for those substances in the cortex of an apple or 

 leaf which might be responsible for these pecuUar concentration effects 

 on the p. D. When the p. d. between solid gels of gelatin and of coagu- 

 lated egg albumin and water was investigated no potential differences 

 were observed,^ to the great surprise and disappointment of the 

 writer who had hoped that the investigations of the p. d. might lead 

 to an explanation of the antagonistic ion effects in which he was then 

 interested. It is possible that the negative results with protein were 

 due to the fact that the measurements were accidentally made near 

 the isoelectric point. On the other hand, it was found that there 

 exists a p.d. at the boundary of lipoids (lecithin dissolved in guaiacol) 

 which is depressed by the addition of salts, and the more the higher 

 the concentration of the salt.^ 



This analogy between lipoids and living cells gave us the impression 

 that the proteins had no share in the potential differences observed 

 between living tissues or living cells and watery solutions. The 

 experiments recorded in this paper leave no doubt that this conclusion 

 was wrong; any ion in a cell or on its surface which cannot diffuse 

 into the surrounding watery solution (no matter whether the ion is a 

 protein or a fatty acid or some complicated lipoid or a complicated 

 carbohydrate or even a crystalloid) can or must give rise to a p.d. 

 which is depressed when a diffusible salt is added to the surrounding 

 watery solution. 



The idea that lipoids are the substances responsible for the p.d. of 

 tissues led Beutner to an extensive and most interesting investigation 

 of the P.D. at the boundary of water-immiscible substances and water.^° 

 He found always a depressing effect of the addition of salt. Beutner 



' Loeb, J., and Beutner, R., Biochem. Z., 1912, xli, 1. 

 8 MacDonald, J. S., Proc. Ray. Soc, 1900, l.xvii, 310. 

 »Loeb, J., and Beutner, R., Biochem. Z., 1913, li, 288; 1914, Hx, 195. 

 '• Beutner, R., Die Entstehung elektrischer Strome in lebenden Geweben, 

 Stuttgart, 1920. 



