﻿Electrochemistry of Radioactive Bodies. G39 



though it had a different potential from that which it really 

 has, bat in the case of copper they are well marked. 



The indefinite potential difference Cu/H 2 0, when the 

 copper is dipped in water exactly a minute, is the same as 

 that between cadmium and cadmium sulphate, i.e., 0*7 volt 

 more negative than the definite potential of copper, for B 

 and are deposited on the copper in the same ratio as on 

 cadmium. 



Again, silver dipping into a solution containing CI' ions 

 has the same potential as the definite potential of copper, 

 and when it dips into a KCN solution, it shows the potential 

 of cadmium. It is, therefore, obvious that such expressions 

 as "deposited on copper, not deposited on silver/' &c, are 

 quite indefinite, and it is from this indeflniteness that contra- 

 dictory assertions regarding the deposition of radioactive 

 bodies on certain metals have arisen. 



5. Further evidence that the Potential of the Electrode determines 

 the composition of the mixture of B and C. 



If an acid solution containing B and C in equilibrium be 

 electrolysed with a very small current density (a fraction of 

 a milliampere per sq. cm.), with platinum electrodes, i.e., with 

 a voltage very much below the decomposition voltage of 

 water, very pure radium C is obtained*. 



The fact that the decomposition voltage has not been 

 reached means simply that there is a smaller potential 

 difference at the cathode than there is at the normal hydrogen 

 electrode. The further we can keep below the potential 

 difference at the hydrogen electrode the purer is the C 

 obtained. If, next, the potential difference between the 

 electrodes exceeds the decomposition voltage of water, and 

 therefore at the cathode the potential difference is equal to 

 that at the hydrogen electrode, a mixture of B and C is 

 obtained of the same composition as was obtained with a 

 potential difference of e (Hg) = — 0*4 volt by the first method. 



If the electrolysis be carried out in a neutral, or better. 

 in an alkali solution, a more negative potential difference 

 is obtained at the hydrogen electrode (e(H g )= — 1*1 volt) 

 owing to the smaller concentration of H' ions m the solution. 

 The mixture of B and C obtained in ihis case on the elec- 

 trode is that obtained on cadmium in the first set of experi- 

 ments (e(Hg)=— 0'7 volt), the B being in excess of the (J. 

 If, finally, the decomposition voltage of water is exceeded by 



* For thorium C, v. Lerch (Wien. Bet: cxiv. \\a (1905); for 

 actinium 0, Miss Brooks, Phil. Mag-, vol. viii. p. S73 (1904). 



