100 



ADVENTURES IN RADIOISOTOPE RESEARCH 



less since the viscosity of solid lead is very much greater than that of 

 liquid mercury^. The viscosity of mercury is 0.016 at 18°C whereas the 

 value for solid lead, according to Kurnakow and Zbmaczny^, is 3 x lO^-; 

 the velocity of diffusion of lead in solid lead calculated from these 

 figures is 2 X 10~^^ cm^ hr-^^. The minuteness of this diffusion velocity is 

 best brought out by means of the following analysis : Consider a diffu- 

 sion cylinder consisting of four equal parts, each part being surrounded 

 by four molecular layers with an assumed thickness of 0.8 x 10~' cm 

 and with the lead atoms of the lowest four molecular layers labelled. 

 After 1 hr there will be less than one per thousand of the labelled lead 

 atoms in the uppermost part of the diffusion cylinder, in the third part 

 only 1.6 per cent. The kinetic exchange during 1 min can extend only 

 to the uppermost molecular layer and to a small extent to the second 

 and third layers. 



Table 2 records the amounts of lead which have exchanged bet- 

 ween a 0.001 N Pb(N03)2 solution and a lead rod in various times. 



Table 2 



Time 

 (sec) 



Amount of lead 



exchanged 



(am) 



Number of molecular 



layers 1 cm^ in area 



which the exohan(?ed 



lead can cover 



Table 3 contains the results of such experiments in which a lead rod 

 dipped into 10 cm^ of a 0.1 N solution of Pb(N0.j)2. 



Table 3 



•■'How fur such an oxtrapolalion is permissible for the sohd state will slioitly 

 be discussed on the basis of experiments. 



* Kurnakow and Zemaczny, Jh. Radioakt. 11, 25 (1914). 



