SKJ.F-BIFFUSION IN SOLID METALS 115 



1)0 Ibund. It was, therefore, concluded that Ihe sell'-dif'fLusion in solid 

 lead is, even at this high temperature, less Ihan lO""* cm^ day~i. 



To increase the sensitiveness of the method, we prepared in the present 

 work two thin foils, one of ordinary lead. Hie oilier wilh lead containing 

 thorium B in homogeneous mixture, and pressed these together in 

 vacuo. The thickness of the inactive foil was chosen slightly greater than 

 the range of the a-particles to be measured ; therefore no scintillations 

 originating from the radioactive lead could be observed wdien investigat- 

 ing the inactive foil. But, on heating the aggregate of the foils, a diffusion 

 of the active lead into the inactive one took place and the a-particles 

 due to the diffused atoms or their successive products of disintegration 

 produced scintillations on the observing screen. By comparing the number 

 of these scintillations with the number of scintillations produced by 

 the active foil at the beginning of the experiment, the rate of self- 

 diffusion in lead was determined. The following values were found : 



The diffusion rate 2° below the melting point is thus 10,000 times smal- 

 ler than in molten lead. 



When investigating the diffusion of two very similar metals like silver 

 and gold, or thallium and lead, into each other, we can expect to find 

 conditions not very far removed from those encountered in the case of 

 self-diffusion. By using a foil of thallium and one of active lead it was 

 found that the coefficient of diffusion of lead in thallium amounts at 285°, 

 i.e. 15° under the melting point of the latter, to 2 x 10-^ cm^ day-^. 



On the other hand, when investigating the diffusion of two not simi- 

 lar metals into each other, much more intricate conditions were to be 

 expected. We determined the rate of diffusion of polonium, which is 

 the highest homologue of sulphur, into both lead foils and single crystals. 

 The coefficient was found about the same both in the foil and crystal 

 (at 310° Z) = 1.3 X 10-5 eni2 clay-i). In this connexion it may be men- 

 tioned that, in discussing the discrepancy between the values of the 

 period of decay of polonium found by different investigators, Mme. Cu- 

 rie has put forward the explanation, that during the long time of 

 observation, the polonium in some cases diffused into the metal from 

 the surface of which it was collected. Recently, Maracineanu (C i?. 

 176. 1879. (1923)), working in Mme. Curie's laboratory, has obtained 

 evidence that the apparent period of polonium is appreciably shorter 

 if the lead on which it is collected is heated for a while. 



8* 



