THE PROBLEM OF THE ISOTOPIC ELEMENTS 87 



We generally ascribe to each element a particular chemical character 

 which varies discontinuously from one element to another. In their mass 

 action silver atoms can replace only other silver atoms and not lead, 

 thallium or other atoms. Accordingly, in the Nernst formula for the 

 electrode potential of silver : 



RT , c 



f = In — 



nF C 



e can be changed only by the addition of silver ions and not by others. 

 If it is now found that atoms which, in spite of having different atomic 

 w<Mght, replace each other chemically and that c must be understood 

 as the total concentration of the isotopes, it then seems necessary to 

 define the concept of chemical individuality such that this does not 

 imply complete equality of the atoms involved but the mutual replace- 

 ability of the two atoms. The correlative of replaceability seems to be 

 equality of the nuclear charge numbers whose fundamental importance 

 becomes more and more prominent. 



Summary 



Experiments have been made to discover whether isotopic elements can replace 

 each other chemically ; the following electrochemical methods have been emplojcfl 

 for this purpose. 



(1) The electrolytic deposition of radium-E with and without the addition 

 of bismuth has been studied and it has been found that the decomposition potential 

 is displaced by the addition of bismuth in the sense and by the amount which 

 Avould be expected of the addition of the same (RaE) ions in accordance witli 

 Nernst's theory ; a study of the deposition of thorium-B with and without the 

 addition of lead yielded the same result. 



(2) It has been shown that the deposition of the very small amounts of ladio- 

 elements which precipitate below the decomposition potential is hindered by 

 the presence of isotopes (and only by these), and this likewise can be explained 

 only by replaceability. 



(3) Radium emanation has been allowed to disintegrate in quartz and the 

 radium-D formed has been deposited electrolytically as the peroxide on platinum 

 wires ; visible and at the same time electromotively active amounts (a few- 

 thousandths of a milUgram) have thus been prepared. The cell RaDOg I RaD(N03)2 I 



I KNO3 I KCl,Hg2Cl2,Hg showed the same electromotive force as a cell similarly- 

 made with lead peroxide, and furthermore the addition of lead ions to the RaD 

 solution changed this e. m. f. in the same way a corresponding addition of 

 RaD ions should change it according to Nernst's theory ; hence it is conducted 

 that the ionic concentration c in tlie Nernst formula 



RT , c 

 nF C 



must be understood as the sum of the isotopic ions. 



From Our study, therefore, the conclusion must be diawn that isotopic elements 

 are able to replace each other in their mass action. 



