1 26 ADVENTURES IX RADIOISOTOPE RESEARCH 



This method, of course, requires long experimental times and a time of 

 3 months is needed for homogenizing a 5 mm layer even with silver 

 Avhich diffuses comparatively rapidly in lead. We have therefore often 

 used the following consideration for determining the solubility : 



(b) Disregarding the initial layer (lead-silver alloy) at first, the silver 

 concentration is determined at various positions after diffusion in the 

 originally pure lead and the diffusion constant is calculated from these 

 results. Now if the diffusion constant is known the corresponding con- 

 centration of silver can be calculated and this in turn yields the solubility 

 of silver in lead. 



Starting with a silver-lead alloy of 



10 atomic per cent 



1 atomic per cent 



0.5 atomic per cent 



we obtained the following values for the solubility of silver in lead 



0.15 atomic per cent 

 0.12 atomic per cent 

 0.13 atomic per cent 



Hence, starting with silver-lead alloys of various concentrations, from 

 which the same solubility values have been obtained, it may also be 

 concluded that the silver from the macroscopic grains of silver was supp- 

 lemented so rapidly that the original lead-silver alloy always remained 

 saturated. If a diffusion experiment is performed, commencing with a 

 0.1 atomic per cent silver-lead alloy, the same diffusion coefficient found 

 by the method described above is obtained both from the concentration 

 of the silver in the initially pure lead layers and from the concentration 

 decrease of the silver in the original silver-lead alloy. The method descri- 

 bed for determining the solubility is important in so far as there are at 

 present no other methods available for determining very low solubilities 

 of one metal in another^. 



Summary 



The diffusion of one metal in another sohd metal is in most cases an almost 

 unilateral process. For example, the velocity of diffusion of gold in lead is very 

 rapid hvit lead diffuses extremely slowly in gold. As the two allo;sing components 

 become increasingly similar, for example, in passing from gold-lead, silver-lead, 

 bismuth-lead, thalUum-lead, tin-lead to lead-lead, the one-sided nature of the 

 process gradually disappears. Diffusion measurements make it possible to determine 

 very low solubilities of one metal in another. The solubility of silver in lead at 

 285°C was thus found 1o be 0.13 atomic per ceni . 



^ In the litcratvH-c silver is stated to be insoluble in solid lead. 



