Pharmacodynamics of Salts and Drugs 87 



In the action of salts on protoplasm we have to deal, then, with a 

 transfer of energy from the ions to the protoplasm, or vice versa. 

 From the general principles of physics we conclude that the physiolo- 

 gical action of any salt solution must be a function of its energy content. 

 The question arises how this energy content is to be measured. 



It has been shown that much, if not all, of the action of salt solu- 

 tions is due to the ions present. We must, therefore, measure the 

 energy content of the ions. The total energy of the ion is composed 

 of two factors, the free or available energy and the bound energy. It 

 is only the free energy, or that which can be transferred to or from 

 the ion, which is of importance in this connection. 



1. The potential factor 0} the free energy. — The interchange of 

 energy between the salt solution and the protoplasm must depend 

 on the relative potentials of the two systems, since whether any sub- 

 stance can transfer energy to another depends, not on the total 

 amount of energy in the two substances or systems, but on the poten- 

 tial of the energy in the two cases. The action of any salt solution is 

 then determined by its available energy, and by the available energy 

 in any salt is meant the product of the difference of potential between 

 the protoplasm and the salt multiplied into the amount of energy 

 transferred from one to the other before the potential is equalized. 

 If the protoplasm and the ion have energy at the same potential, the 

 difference in potential will be zero, the available energ)' is hence zero, 

 the work done is zero, and the ion should produce no direct effect due 

 to its energy content on protoplasm, though it might affect it cata- 

 lytically in the manner indicated. 



2. Total free energy. — The total free or available energ)' of any ion 

 is composed of two factors, the potential energy and the kinetic energy. 

 The kinetic energy, or energy of motion, will be equal to h MV. As 

 I do not know the actual velocity of ionic movement when the poten- 

 tial gradient is unknown, I am unable to determine the kinetic energy. 

 It is, in any case, generally small when compared to the potential 

 energy, although not negligible when the latter factor approaches zero. 

 That is, if the potential of two ions and the protoplasm are about the 

 same, these ions may have different actions owing to differences in their 

 kinetic energy, i. c., their ionic masses and velocities. In this paper, 

 however, I shall consider only the potential cnerg>' factor. 



