^12 ADVENTURES I\ RADIOISOTOPE RESEARCH 



the Roberts- Austen type, however, are not suitable for obtaining 

 the desired information on this point. If it were required to decide upon 

 the slowness of place exchange in solid lead from the Roberts-Austen 

 data a completely erroneous result would be given, whereas the appli- 

 cation of radioactive indicators, i.e. the measurement of the velocity of 

 diffusion of a lead isotope in lead yields the required information. 



EXPERIMENTAL METHOD 



The layers of active and inactive lead were joined together by the 

 method already describedi. The inactive lead was melted in a vacuum 

 in one limb of the Y-shaped hard glass tube and, after it had solidified, the 

 fused active lead contained in the other limb was poured on, thereby pro- 

 ducing a cohesive metallic cylinder. While the active material used in the 

 determination of the velocity of self- diffusion in molten lead was ordinary 

 lead labelled with ThB, this procedure was no longer admissible in the 

 present instance because the ThB decays with a half-life of 10.6 hr and 

 the time of experiment amounted to more than 1 year. Joachimsthal 

 lead, a mixture of ordinary lead, uranium-lead and RaD, has therefore 

 been chosen as the active material. Of these three lead isotopes only 

 the RaD is active and this only to such a small extent that its radiation 

 is not suitable for determining the amount of RaD present; the a- 

 rays of its daughter product, polonium, however, serve as a convenient 

 means for determining the RaD. 



Another point in which the experimental method followed here 

 differed from that used for diffusion in the liquid was that, after joining 

 together the layers of active and inactive lead, the boundary surfaces 

 were fused together by means of a finely pointed flame in order to obtain 

 complete contact between the two kinds of lead, this being clearly of 

 great importance for uninterrupted diffusion. A completely cohesive 

 column was thus obtained but of course mixing of the sharp boundaries 

 of the active and inactive lead was unavoidable. In order to take account 

 of this fact, we proceeded as follows : 



The column of lead, moulded in the manner described, was cut into 

 two vertical sections with a toothed saw and one of these strips was 

 sealed in an evacuated glass tube which was then placed into an electrical 

 resistance furnace. After the experiment the strip was sectioned at three 

 places marked with India ink and was thus separated into four equal 

 parts. The second vertical strip had already been cut at the correspond- 

 ing places before the experiment and was used as a control. If the active 

 layer of lead is denoted by I, then layer II likewise showed some activity 



1 J. Groh and G. Hevesy, Ann. Pliys. 63, 85 (1920). 



