﻿GIO Dr. G. von Heresy on the 



using a large current density, the mixture of B and G 

 deposited in the cathode is similar to that obtained by the 

 first method on zinc (e ( H g )= — l'l volt). 



In this way it is clearly shown that the composition o£ the 

 mixture of B and C obtained on an electrode is purely a 

 function of the potential difference. 



6. Previous work on Electrolysis of Solutions containing an 

 Active Deposit, the explanation of v. Lercli's method of 

 separating Radium C. 



Dorn was the first to carry out an electrolysis of radium 

 solutions*. In 1905 v. Lerch f electrolysed an acid solution 

 of thorium active deposit. A year later Miss Brooks i elec- 

 trolysed the HC1 solution of the active deposit of actinium, 

 and Pegram § a solution of thorium nitrate. 



V. Lerch found later || that by dipping a nickel wire into 

 a HC1 solution of radium active deposit, extremely pure 

 radium C could be obtained on it. 



This convenient method of preparing pure radium C is 

 known as v. Lerch's method, and became of great practical 

 importance in later work. 



The method may be explained thus : — 



(1) The potential difference Nickel/Electrolyte is ab- 

 normally negative and is equal to that at the hydrogen 

 electrode. This is due to the considerable resistance of the 

 nickel ions — as contrasted with the behaviour of all other 

 ions — in giving up their charge. On account of this phe- 

 nomenon of Passivity the potential difference Nickel/Elec- 

 trolyte is well suited for bringing about the deposition of 

 radium 0. The radium thus deposited is not at first pure ; 

 the purification is due to the action of the acid. 



(2) Deposition of a substance on an electrode and solution 

 therefrom in an acid are contrary phenomena. Radium B 

 is deposited on nickel much less easily than radium ; it is 

 also much more easily soluble in the hot acid than radium 0. 

 Radium C has, therefore, a double chance of being deposited 

 pure. 



(3) If nickel be dipped into neutral solutions pure radium 

 C is never deposited on it. The composition of the mixture 

 of B and C varies with the passivity of the metal. 



More than thirty experiments, for the most part with 

 thorium B and 0, have been carried out. The C content 



* Abh. der nat. Ges. Halle (1900\ t Ann. d. Phys. xii. p. 745 (1903). 

 % Phil. Mag. Sept. 1904. § Phys. Rev. xvii. p. 427 (1903). 



II Wien. Ber. cxvi. lla, Dec. 1907. 



