SKJ,1'-I)IK1 rsiON IN SOLID LKAl) 1 [;} 



on account of llu> mixing at the boundary; this activity, found even 

 l)efore the dil'l'usit)n, could, however, be subtracted from that found after 

 diffusion and the mixing at Ihe boundary could be taken into account 

 in this way. Yet the correction described would onlv acquire importance 

 in the case of an experiment giving a positive result ; since no activity 

 noticeably in excess of the natural decay was found in layers III and IV, 

 it was unimportant. 



In order to be abl(> to measure accurately the a-activit}- of tli(> in- 

 dividual sections we have also used here, as in the experiments which 

 served for determining the self-diffusion in molten lead, rolled sheets 

 of lead and measured the activity of the disk thus obtained in the a- 

 electroscope. The total length of filament amounted to 16 --20 mm. 



The a-activity of polonium indicated by the electroscope is a measure 

 of the amount of the lead isotope (RaD) present only if the RaD and 

 Po exist in radioactive equilibrium. The amount of RaD which diffused 

 in the first four months harl come to more than 80 per cent radioactive 

 equilibrium when 14 months had elapsed; that which diffused in the 

 second 4 months had reached over 50 per cent after the same time, when 

 the measurements were made. The absence of any appreciable activity 

 in the layers III and IV made it possible to determine the diffusion 

 constants of both lead in lead and polonium in lead at 280 °C as being 

 less than 0.0001 cm^/day, without awaiting exactly the establishment o\' 

 the radioactive equilibrium between RaD and Po. It is intended, how- 

 ever, to follow this process further during the next year and thus to be 

 able to extend the observations beyond the determined limits of the 

 diffusion constants mentioned above. We are also concerned in work- 

 ing out other types of method which permit the determination of very 

 much smaller diffusion constants than those mentioned. 



We may mention here that tw^elve lead filaments have been prepared 

 as described above, and have been introduced separately into evacuated 

 glass tubes and heated in an electric resistance furnace for 400 days. 

 The furnace temperature, which varied between 270 and 290°C', was 

 followed constantly with a quartz thermometer, since our experience 

 with continuously heated glass thermometers in similar experiments 

 has been unsatisfactory. 



Summary 



The self- diffusion velocity of solid lead has been d(>t<Mniined by following the 

 diffusion of the lead isotope radium- D in solid lead at 280° (' for more than 1 year. 

 The diffusion constant, even at this temperature which is only 4()'' helow the 

 melting point, is shown to be still less than 0.0001 cm^/day. Self -diffusion in l(^ad 

 thus takes place at least two-hundred times more slowly than the diffusion of 

 gold in solid lead at the same tempeiatuie. 



8 He 



vesy 



