88 A. P. MATHEWS 



3. The potential energy of ions. What is the measure of the poten- 

 tial energy of any ion ? The potential energy must be the difference 

 in the energy content of the ion or atom in different conditions. If 

 any substance has any available potential energy, it necessarily means 

 that it is capable of existing in two conditions which differ in their 

 energy content, and that it gives up energy in passing from one con- 

 dition to the other. That ions and atoms do exist in such different 

 conditions is well known. Thus the chemical differences between 

 atomic and ionic sodium, and between ferric, ferrous, and metallic 

 iron, are due to differences in the energy content of the atoms in differ- 

 ent conditions. The available potential energy of the sodium atom 

 is very much greater than that of the sodium ion, as is indicated by 

 the fact that when the atom becomes an ion, a large amount of heat 

 is set free. 



The potential energy of the ion must be sharply distinguished from 

 the ionic potential. The potential energy is in its turn composed of 

 a capacity and an intensity factor; the capacity factor being repre- 

 sented by the amount of electricity transferred; the intensity fac- 

 tor, by the tendency of the ion or atom to change its state ; in other 

 words, by its stability or ionic potential. The potential energy of 

 any ion must be measured hence by the ionic potential multiplied 

 into the capacity. The capacity factor falls out of account if equiva- 

 lent solutions are compared, since in that case each equivalent has 

 the same quantity of electricity in it, and the differences between the 

 actions of ions are hence due to differences in ionic potential. The 

 question now comes down to the determination of the ionic potential. 



4. The determination of the ionic potential. In my earlier papers 

 it was not clear to me how this ionic potential could be determined, 

 so I used instead, as a rough measure of it, the solution tension of the 

 metal. I assumed that the solution tension would be the reciprocal 

 of the ionic potential. Inasmuch as the solution tension varies with 

 the concentration of the salt, I used arbitrarily the solution tension 

 of the metals in normal ionic solution. 



Inasmuch as the determination of the ionic potential depends on 

 the determination of the solution tension, it is necessary to under- 

 stand the latter term, and as this may not be familiar to all physiol- 

 ogists, the following explanation is given: 



