104 ADVEXTURES IX RADIOISOTOPE EESEAECH 



ordinaiy and the othei' from radioactive lead, it is possible to distinguish 

 any atom of lead in the one substance from any atom of lead in the other 

 since they have distinct properties. By dissolving the two compounds 

 and after a certain time separating them again a simple measurement 

 of radioactivity will show whether each atom of lead is still in the same 

 kind of molecule as before the experiment or whether an exchange of 

 atoms has taken place. 



It has previously been demonstrated^ that Avhen equimolecular 

 quantities of inactive lead chloride and active lead nitrate are dissolved 

 and the latter subsequently reciystallized half of the active lead atoms 

 originally present in the nitrate molecules transfer to the lead chloride. 

 The originally inactive lead chloride was proved to be half as radioactive 

 after the experiment as the lead nitrate was before. 



The same result was obtained with the following combinations : Lead 

 nitrate (active) and lead chloride in pyridine ; lead formate (active) and 

 lead acetate in water ; lead acetate (active) and lead tetra-acetate in 

 glacial acetic acid ; and lead tetra-acetate (active) and lead acetate in 

 glacial acetic acid. 



In contrast it was found that there is no exchange of lead atoms when 

 the lead is firmly bound to carbon. The behaviour of organically bound 

 lead is illustrated by the following examples : Lead chloride (active) 

 and tetraphenyl lead dissolved in pyridine ; lead acetate (active) and 

 tetraphenyl lead in amyl alcohol ; and lead nitrate (active) with diphenyl 

 lead nitrate in dilute ethyl alcohol. 



The original activity or inactivity of the dissolved substances in these 

 instances was not modified by the experiment. The conclusion to be 

 drawn from these findings is that exchange of atoms does not take place 

 even if the lead is undissociated in only one of the two compounds. 

 Exchange is even less likely when this type of binding of the lead exists 

 in both components and particularly when these components are che- 

 mically identical, in other words, when they are molecules of the same 

 substance. There will be no exchange of lead atoms between two mole- 

 cules of tetraphenyl lead. The results up-to-date suggest, therefore, that 

 an intermolecular exchange of atoms (at least in the time required to 

 perforin chemical operations) is connected with the existence of an 

 electrolytic dissociation. 



The existing experimental data are inadequate to prove whether two 

 similar atoms of the same molecule are able to exchange in a measurable 

 time, although it can be assumed probable that the opportunities for 

 positional exchange within one molecule are similar to those arising 

 between two neighbouring molecules. We intend to attack this problem 

 more closely by introducing a radioactive and an inactive lead atom into 



iG. Hevesy and K. Rona, Phys. Cfiem. 89, 303 (1915). 



