930 ADVENTURES IN RADIOISOTOPE RESEARCH 



The existence of such an interchange was demonstrated^^\ While, in 

 the case of lead peroxide, the interchange was found to take place 

 between the outermost layer of molecules of the geometrically calcu- 

 lated electrode surface, in the case of the lead electrodes numerous 

 layers of molecules were found to participate in the interchange process. 

 This finding is to be interpreted as a result of local currents due to varia- 

 tions in the structure of the metallic surface. An interchange between 

 atoms of a lead foil and the lead ions present in a solution was found 

 to occur very rapidly, while the ions adsorbed on colloidal lead par- 

 ticles were found to interchange at a slow rate only.^*^ 



In contrast to metallic surfaces, Paneth^^^ found that in the case 

 of salt crystal surfaces the interchange was restricted to the uppermost 

 molecular layer of the crystal. On this observation he based an important 

 method for the determination of the surface areas of crystalline pow- 

 ders(*^>. 



When lead sulphate is shaken with its saturated solutions, a constant 

 kinetic exchange occurs between the molecules of lead sulphate in the 

 solution and those on the surface of the solid. If the solution contains 

 marked molecules, after equilibrium has been attained the numerical 

 ratio of marked molecules on the surface to those in solution is identical 

 to that of total molecules on the surface to total molecules in the 

 solution. As the distribution of the labelled molecules is determined by 

 means of radioactive measurements, and as the lead sulphate content 

 of the saturated solution is evaluated by the usual methods of analytical 

 chemistry, the amount of lead sulphate present in the uppermost mole- 

 cular layer can be computed. When the weight is known of a uni- 

 molecular layer of lead sulphate of 1 cm^ area, the surface of the crystal 

 powder can be calculated from the above data. 



Among the numerous applications of radioactive indicators by Paneth 

 I wish to emphasize the importance of his discovery of the existence 

 of bismuth hydride^'^' and lead hydride^*\ After he gained experience 

 regarding the best method of preparation and the stability of radioactive 

 bismuth hydride and lead hydride, he succeeded in preparing these 

 compounds from inactive bismuth and inactive lead, respectively. 



SELF-DIFFUSION 



The conception of the diffusion of a substance into itself, self-diffusion, 

 was introduced by Maxwell. No further use was made of this concept 

 until fifty years later, when the method of radioactive (isotopic) indi- 

 cators was introduced. The possiblity of measuring self-diffusion by 

 following the rate of penetration of the lead isotopes ThB or RaD into 

 lead soon suggested the measurement of the self-diffusion in liquid and 



