﻿Electrochemistry of Radioactive Bodies. 643 



With thorium B and C, measurements of the activity 

 curve cannot of course be completed so quickly as with 

 actinium B and C, but on account of the slow variations o£ 

 activity o£ the former, the experiments yield more accurate 

 results. 



Radium B and C is not at all suitable for these measure- 

 ments, owing partly to the small difference in the periods of 

 B and C (26'8 mins. and 19*5 mins. respectively), and partly 

 because B and come into equilibrium with each other only 

 after a certain time has elapsed owing to the presence of 

 radium A. With thorium and actinium bodies, equilibrium 

 is immediately attained because of the excessively short 

 periods of thorium A and actinium A. 



The suitability of this method of measuring single potential 

 differences has been tested by determining the potential of 

 tantalum, which has hitherto been unknown. A plate of 

 tantalum kindly supplied to me by Dr. W\ v. Bolton, was 

 immersed in a vessel containing normal potassium chloride 

 for an hour ; a solution of ActB^ CI^ + ActC^ Cl^ 2 was then 

 added to the chloride solution and the tantalum removed two 

 minutes later. 



The composition of the actinium products on the plate, 

 calculated from the decay of its a. activity, was practically 

 the same as that deposited on a copper plate which has been 

 dipped into a Ay N. CuS0 4 solution. The potential differ- 

 ence Tantalum/KCl (normal) is, therefore, — O03 Y with 

 respect to the calomel electrode. 



Under identical conditions Mr. Slade and the author, 

 measuring the potential by means of the usual electrical 

 compensation method, found the value — 0*04 V. 



The agreement is remarkably gnod considering that the 

 system under consideration is not " definite. '* 



In another experiment the potential difference Ta/Ta s O-, 

 solution gave by the radioactive methpd 0*00 V, and by the 

 electrical method also O'OO Y, 



9. Change of Potential Difference between a Metal and a 

 Solution with a change in the Ionic Concentration. 



If a metal be dipped in pure water, then since the latter 

 contains no ions of the former, the potential difference 

 between the two is really much greater than the potential 

 difference between the same metal and a solution of one of 

 its salts. With time it approaches a final value asymptotic- 

 ally. In other words, the potential difference changes as 

 more and more of the ions of the metal arc produced in the 



2 U 2 



