934 ADVENTURES IX RADIOISOTOPE RESEARCH 



diffusion rate of Pb"*"^ . This shows that, in contrast to the case of PbClg, 

 the conductivity in Pbig at high temperatures is due almost exclusively 

 to the transference of charges by Pb+"^. With decreasing temperature 

 the role of Pb++ decreases and the transport number of I~ increases 

 accordingly. At 260°, only 40% of the conductivity is due to Pb^"*" 

 and 60 per dent to the I~ ; at 155° the share of the former is only 0.4 

 per cent. In the case of lead iodide, Tubandt's beautiful method for 

 the determination of transport numbers could also be applied. The values 

 obtained agreed well with those found by the measurement of self- 

 diffusion. 



Formerly, the self- diffusion rates of only lead, bismuth and a few 

 other elements could be determined. These elements have natural radio- 

 active isotopes. The discovery of artificial radioactivity greatly enlarged 

 the possibilities for the determination of self-diffusion rates. By making 

 use of the radioactive bromine isotope, we can determine the self-dif- 

 fusion rate of B~ in AgBr just as we determined that of Pb+"^ in PbCla. 

 Working at the Institute for Theoretical Physics at the University of 

 Copenhagen, H. A. C. McKay determined the self-diffusion in gold. 

 By the action of neutrons on gold a radioactive gold isotope, having 

 the atomic number 198, can be produced. Neutrons having an energy 

 of about 4 V are strongly absorbed in gold. A thin gold sheet exposed 

 to the action of such neutrons will be more strongly activated on the 

 side first struck by the neutron beam than on the opposite side. When the 

 activated film is heated, the difference in the concentrations of the 

 active gold atoms will decrease and, from the decrease of the activity 

 difference shown by the two faces of the foil, the rate of self-diffusion 

 in gold can be calculated^^'^\ 



SVANTE ARRHENIUS' THEORY OF ELECTROLYTIC DISSOCIATION 



If we dissolve sodium chloride and the equivalent amount of sodium 

 bromide in water and then separate the two salts by crystallisation, 

 it would have been expected in the time prior to Arrhenius that the 

 ions would retain their original partners, the same applying to the bromide 

 ions. According to Arrhenius, however, each chloride ion has the same 

 chance of associating with a sodium atom originally bound to chloride as 

 with one initially associated with bromide. The correctness of the much 

 debated views of Arrhenius was shown in different ways ; the most 

 direct proof, however, was provided through the application of isotopic 

 indicators^^"\ When equivalent amounts of PbClg and labelled Pb(N03).2 

 (or vice versa) were dissolved and the two compounds were separated 

 by crystallization, the labelled lead ions were found to be equally dis- 

 tributed between chloride and nitrate ions. 



