THE APPLICATION OF RADIOACTIVE RECOIL IN DIFFUSION MEASUREMENTS 135 



Table 1. — Self-Diffusion of Pb Ions in PbCl, 



The curve of Fig. 1 can be represented by the equation 



D = 1.060x107 p-38.i2o/«r 



This yields 38,120 cal for the molecular heat of relaxation of lead ions 

 and the value 1.06 x 10-^ for the constant ^. The heat of relaxation of the 

 chloride ions in lead chloride is found to be 10,960 cal, from the conduct- 

 ance of PbCla- The large difference between the heats of relaxation 

 immediately renders intelligible the result of Tubandt, according to 

 w hich practically all the mobility in lead chloride is due to the chloride 

 ions. The lead ions require a much greater energy content than the 

 chloride ions to enable them to take part in place exchange processes. 

 The transport number of lead ions in lead chloride is found to be 10-^ 

 at 270"C. 



The investigation of self-diffusion in single crystalls of lead chloride, 

 in which the present experience suggests a smaller diffusion, will be 

 discussed later. 



