98 



ADVENTURES IN RADIOISOTOPE RESEARCH 



A different result is obtained, however, if a study is made of the ex- 

 change, not between a finite layer of lead chloride and its saturated 

 solution but between a molecular film of lead chloride, which can easily 

 l)e prepared by a radiochemical method, and the saturated solution. 

 It is then seen that the percentage of exchanged molecules is very con- 

 siderable and the velocity of exchange becomes commensurable with 

 the velocity of dissolution of the molecular layer. 



This result can be expected from kinetic considerations ; a rapid 

 exchange can take place, in general, only in the superficial layers. 



A lead rod, 4 cm long and 5 mm in diameter, was immersed for 1 min 

 in 10 cm^ of a lead nitrate solution labelled with a known amount of ThB, 

 and then the quantity of ThB deposited on the lead surface was deter- 

 mined. This gives the number of lead ions originally in the solution 

 which have thus been transferred to the lead surface. The first column 

 of Table 1 records the normality of the Pb(N03)2, the second the number 

 of lead ions per thousand originally in the solution and then occurring 

 on the surface of the lead, the third the amounts of lead in grammes, 

 the fourth the amount, expressed as a fraction, of that required, accord- 

 ing to MtJLLER and Koenigsberger^, to indicate the potential of lead 

 peroxide. On the basis of the Loschmidt number the mass of a unimole- 

 cular layer of PbOg is calculated^ as 3.2 x 10"'^ gm. According to the 

 measurements of Koenigsbeeger and Mijller^ twice this mass is re- 

 quired for optical detection and eight times to impart the PbOg poten- 

 tial to an area 1 cm^ in extent. 



Table 1 



According to our ideas on the process of the galvanic production 

 of current, lead will either go into solution or will be deposited, when a 

 lead rod is immersed in a solution of lead nitrate, according as the con- 

 centration of lead nitrate is on one or the other side of the limit at which 



1 cf . W. J. MuLLER and J. Koenigsberger, Phys. Z. 6, 849 (1905). 



2 J. Koenigsberger and W. J. Muller, Phys. Z. 12, 606 (1911). 



