§2 ADVENTURES IN RADIOTSOTOPE RESEARCH 



(b) Deposition below the Decomposition Potential 



As was shown some time ago^, a small quantity of any radioelement 

 deposits even below the decomposition potential and can be measured 

 with the aid of sensitive methods which are now available. Thus, for 

 example, 4 parts per 1000 of RaE are deposited at about —0.17 V in 

 24 hr on an electrode 1 cm^ in area when the stirring is thorough ; this 

 deposition is not affected by the presence of foreign ions, apart from 

 bismuth, in the solution. If the solution is made 0.01 N in Bi ions the 

 deposition of RaE no longer takes place. At a higher concentration 

 the percentage deposition should naturally be much smaller ; 4 parts 

 per 1000 of a 0.01 N bismuth nitrate solution would indeed amount to a 

 few milligrams and thus would form a visible cover which cannot exist 

 below the decomposition potential. This specific effect of bismuth ions 

 on the deposition of RaE ions cannot be interpreted in any way other 

 than by replaceability of these isotopes. 



We found similar results for ThB, irrespective of whether it was de- 

 posited as metal or peroxide"^. For example, at 1 V i.e. below the deposi- 

 tion potential, 5 per cent deposited and the deposition was in no way 

 affected by the presence of thahium or other ions near to lead. In 10^^ N 

 lead nitrate solution, the deposit was already less than I/2 P^rt per 1000 

 and in 10"^ N the fraction deposited was no longer detectable. Clearly 

 in this instance also, increased deposition occurs because of the high 

 concentration, but the positions of most of the ThB atoms are taken 

 by lead atoms, depending upon the concentration ratio of the two. 



(c) Measurement of a RaD Peroxide Cell 



Concerning the question of isotopy of the elements we are mostly limited 

 to indirect methods Uke those described above, since no single instance 

 is known in which both of two isotopes exist pure and in visible amounts. 

 Visible amounts can be made available only from relatively long-Hved 

 elements ; when recovered from minerals they are always contaminated 

 with isotopes, e.g. uranium-II with uranium-I, ionium with thorium, 

 mesothorium with radium, and so on. Radium-D, which occupies a posi- 

 tion midway between the long- and short-lived, is always mixed with 

 about ten million times the amount of lead when obtained from pitch- 

 blende ; the considerable quantity of radium emanation available to 

 us, however, gave us the opportunity to ol)tain directly visible amounts 

 of RaD, completely free from lead because of its formation from 

 emanation allowed to disintegrate in carefully purified quartz vessels. 



iG. Hevesy, Phil. Mag. 23, (528 (1912), 



2 The anodic doposition of ThB at strongly positive potentials was explained 

 by the formation of (ThB)02 (F. Paneth and G. Hevesy, Si/?. Ber. Akad. Wi^-.s. 

 Wien 122, 1027 [19131). 



