SELK-niKKlSIOX IX SOLID LEAD 



111 



latter temperature being somewhat higher and thus more favourable 

 to diffusion. At this latter temperature, which is not very far removed 

 from the melting point, the self-diffusion is still extraordinarily slow 

 and should be incomparably slower, for example, at room temperature. 

 When relating this result to the velocity of self-diffusion in other metals 

 it must be borne in mind that lead is one of the softest metals and that 

 self-diffusion should presumably prove to be much slower in the harder 

 metals . 



Quantitative data on the diffusion in solid metals have been provided 

 only by Roberts-Austen, but metallurgy is plentifully supplied with 

 qualitative experiences which point out the relatively rapid diffusion 

 of alloying solid materials, of which the rapid interpenetration of iron 

 and carbon! supplies at 250 °C the best-known example. Thus there 

 exists a very considerable difference between the diffusion of two solid 

 metals into each other and the self- diffusion in a solid metal, in complete 

 contrast to the diffusion in the liquid media. Thus, we have obtained- 

 a value for the self-diffusion constant of molten lead which is only 

 slightly different from the constant for gold in lead. 



The main reason why self-diffusion in lead is so much slower than 

 the diffusion of gold into lead appears to be that the gold diffusing into 

 the lead loosens up the crystal structure and in this way facilitates its 

 own transmission. It is found that the introduction of an impurity into 

 the crystalline structure can have exactly the same effect as a rise of 

 temperature in facilitating the place exchange of the ions (atoms, 

 molecules). 



However, it is not stated absolutely that all other metals diffuse more 

 easily into lead than do its own atoms ; w^e attempted to allow simultane- 

 ous diffusion of the lead isotope radium-D and polonium (Avhich is 

 a homologue of tellurium) into lead, but no positive result was obtained. 



Diffusion experiments in solid bodies claim so much interest, for this 

 and other reasons, because information can be obtained from the results 

 concerning the magnitude of the resistance opposing the displacement 

 of individual atoms in the crystal structure. Diffusion experiments of 



iM. A. CoLSON, Ann. Chim. et Phys. 17, 221 (1846). 

 2 J. Groh and G. Hevesy, Ann. Fhys. 63, 85 (1920). 



