PHARMACODYNAMICS or SALTS AND DRUGS 83 



What enables any ion to act at all? What makes a mercury ion, 

 for example, so enormously more toxic than a calcium or magnesium 

 ion ? The answer to this question, as I shall now proceed to show, 

 is, that the mercury ion has an enormously greater ionic potential than 

 the calcium ion. 



IONIC POTENTIAL AND PHYSIOLOGICAL ACTION. 



In an earlier paper 1 the term "ionic potential" was suggested to 

 designate the tendency of any ion or atom to change its electrical state. 

 Bodlander has used the term " Haf tintensitat " to designate the same 

 factor, and he and Abegg have presented evidence to show that the ionic 

 potential is one of the chief factors in determining chemical affinity. 



The idea that this property of the ions of the salt might be of 

 importance in determining their physiological action was first suggested 

 by my colleague, Dr. J. Stieglitz, at the meeting of the American 

 Physiological Society in Chicago in December, 1901. At that time 

 the importance and real bearing of the suggestion were not appre- 

 ciated by me, but about a year or so later I was much struck by the 

 fact that the arrangement of the metals according to their solution 

 tensions, as given by Nernst, was practically the same as an arrange- 

 ment in the order of their toxic actions. Stieglitz 's suggestion 

 appeared to me in a new light, and I set to work to get additional 

 evidence that it is the ionic potential which chiefly determines the 

 physiological action of ions. In 1904 I published results showing 

 the remarkable parallelism between toxicity and ionic potential in the 

 action of salts on the eggs of Fundulus heteroclitus. In that paper 

 I showed that valence and ionic velocity factors to which main 

 importance had been attached by Hardy, Loeb, Pauli, Posternak, and 

 which have been recently emphasized by Robertson 2 are unim- 

 portant when compared with the importance of the ionic potential as 

 a determining factor of toxicity. I showed also that the phenomena 

 of stimulation of the motor nerve by salts demonstrate the same rela- 

 tionship over again, and in the clearest and most decisive manner. 

 Inasmuch as I had already interpreted the phenomena of chemical 

 stimulation of motor nerves to mean that the nerve impulse was due 

 to a progressive coagulation of the colloids of the nerve, it was a 



1 MATHEWS, Amer. Jour. Physiol., 1904, n, p. 456. 



ROBERTSON, Trans. Roy. Soc. of South Australia, 1905, 29, p. n. 



