90 A. P. Mathews 



it is clear that the ordinary figures given for the solution tension are 

 not strictly applicable to the physiological conditions. We introduce 

 into the salt solution, not a plate of metal, but a particle of proto- 

 plasm, and we wish to know what is the tendency of any ion in that 

 solution to give up its charge in whole or in part to the protoplasm. 

 The solution tension is not therefore a proper measure of the par- 

 ticular property of the ion we seek. 



In previous work, in order to get comparable results, I had to 

 adopt arbitrarily the solution tension of the metals in the normal ionic 

 solutions of their salts, although there was no reason at all why this 

 value, rather than the value in any other concentration, should have 

 been taken. 



It is clear that what is most important in the ion in determining 

 its physiological action, and its chemical action as well, is not the 

 difference of voltage between a plate of metal and any solution of its 

 salts, but rather the difference in pressure between a single ion and a 

 single atom of the metal. That is, it is the inherent tendency of any 

 ion in any concentration to change into an atom of its metal. This 

 last property has been called the ionic potential. The method of com- 

 puting it is as follows : 



Nernst' has shown that the formula which expresses the amount 

 of work necessary to compress a gas from volume i to volume 2 is of 

 very general applicability, and also expresses the amount of work 

 necessary to transform one gram atom of a metal into one gram ion 

 at any concentration. This formula is as follows: 



v 

 Amount of work — L = RTln— . 



V2 



In this formula R is the gas constant; T, the absolute temperature; 

 v^ and-y^ the gas volumes ; and the logarithm is the natural logarithm 

 This formula may also be expressed using pressures instead of vol- 

 umes, or: 



L = RTln^ . 

 Pi 



In this formula p^ is what is known as the solution pressure of the 

 metal, and p^ the osmotic pressure of the ions of the metal in the solu- 

 tion. Instead of p^ we may write P. By taking R and T in absolute 



' Nernst, Theoretische Chemie, 1903, 4te Aufl. 



