DIFFUSION IX METALS 



123 



stoadily decreasing diffusion velocity. The margin l)etween the diffusion 

 velocity of the added element and that of lead thus becomes steadily 

 smaller and the unilateral fliffusion becomes gradually less apparent. 

 Considering now an alloy of ordinary and radioactive lead, both con- 

 stituents will exhibit the same diffusion velocity. This is an example of 

 self-diffusion and therefore a complete mutual replaceability in the 



220 2^0 260 2S0 300 320 



Fig. 1 . Diffusion in Blcilogieiungen — Diffusion in lead allo,>s 

 Schmclzpunkt des Blcis — Melting point of lead 



diffusion process. The gradual decrease of diffusion velocities of metals 

 in lead in the sequence, Au — Pb, Pb — Pb, and also the step- wise increase 

 of the heat of relaxation (heat of activation), can be seen in Fig. 1 . The 

 latter quantity is also recorded in Table 1. 



The marked preferential diffusion of one component is also encountered 

 in salt-like compounds but there is no mutual replaceability of the 

 components which, indeed, is a characteristic of metallic systems. The 

 silver in silver chloride can only change places with silver; it is other- 

 wise in metallic alloys. Considering the place-exchange processes in a 

 saturated silver-lead phase, the readily detachable silver atom will leave 

 its place with great frequency in unit time. The silver atoms intermit- 

 tently seek out the lead atoms and occasionally also replace other silver 



