96 ADVENTURES IN RADIOISOTOPE RESEARCH 



in piiro 10-3N HNO3 1.99 



in 10-3N Pb(N03)2 + IQ-^N HNO3 1.49 



which is still a significant difference. 



The conclusion to be drawn from these experiments is that the velocity 

 of exchange between the solid phase and its saturated solution is already 

 commensurate with the velocity of dissolution for a molecular layer, 

 but, that when the experimental time is short an effect on the velocity 

 of dissolution due to the presence of isotopic ions in the solution can 

 l)e detected. 



The following experiment seemed to be of interest in connection w ith 

 those described above: 200 mgm of Pb(N03)o labelled w^ith ThB was 

 added to a solution of PbClg (200 mgm), a portion of the PbClg was then 

 allowed to crystallize out and the distribution of the different kinds 

 of lead atoms between the chloride and nitrate was studied. After a few 

 minutes required for performing the manipulations it was shown that 

 there was a completely uniform distribution of all the lead atoms, 

 within the limit of error amounting to 1 per cent. 



Z. KlemeisSiewicz^ has recently performed similar experiments, 

 lie studied the distribution of ThB and also RaB between a lead amal- 

 gam and a mercuric nitrate solution and found a completely uniform 

 distribution ; the accuracy of his experiments was greater, with an error 

 of Yo per cent. 



Summary 



The velocity of dissolution of molecular (infinitely thin) lajers shows quali- 

 tatively the same behaviour as that of finite layers. The velocity of dissolution 

 of lead and bismuth isotopes in nitric acid increases with acid concentration, 

 with lowering of the viscosity of the solution and with the sohibility of the 

 suVjstance involved. 



The presence of lead ions in the solution lowers the velocity of dissolution 

 of the lead isotope ThB without affecting that of the bismuth isotope ThC. 



The velocity of exchange between the molecules of solid lead chloride and a 

 saturated lead chloride solution can be determined by labelling the lead chloride 

 with ThB ; in the case of a finite layer it is vanishingly small compared with 

 the velocity of dissolution but in the case of a molecular layer the two properties 

 are commensurable . 



^Z. Klemensiewicz, C, B. Accd. ScL, Paris 158, 1889 (1914) 



