THE HEAT Ol' KELAXATION OK ITIK LEAD LATTICE 117 



ol self'-dilTusion in lead are structure sensitive. The method used lias 

 been described in detail on a previous occasion^ TIk^ lead isotope ThH 

 is condensed on a lead surface and 1h(> ionization caused by the a-radia- 

 tion of the radioisotope (or its decay products) is measured before and 

 after the course of dilfusion. The deeper the thorium-B penetrates by 

 diffusion into the lead, the greater is the absorption of the a-radiation 

 and the smaller will be the ionization arising from it. The calculation 

 of the velocity of diffusion of thorium-B in lead, which is the same as 

 the velocity of self-diffusion of lead, is executed by means ol' a formula 

 developed by R. Furth'^, which correlates the ionization values before 

 and after dilfusion. the range of the u- radiation in lead and the lime. 

 In a more sensitive modification of the method which has also been 

 described the recoil yield, instead of the ionization due to the a-radia- 

 tion, is measured before and after diffusion. 



Both "Kahlbaum" lead and lead of very high purity, which was kindly 

 made available by the Akkumulatoren-Fabrik A. G., Jiagen, West- 

 phalia, were used in the experiments. The lead was freed from its content 

 of gases by prolonged fusion in a vacuum and was purified from oxide 

 content by passing it through a capillary system. The lead single crystals 

 were prepared by the method of Ivyropoulos and were characterized, 

 as compared with crystalline lead, by their remarkable resistance to 

 oxidation by the air. All the experiments described below were carried 

 out with complete exclusion of air either in an atmosphere of nitrogen 

 or in a vacuum, and the small tubes containing the single crystals were 

 broken in the evacuated apparatus. All the results recorded in Table 1 

 can be represented by the equation : 



or Z) = 5.76 X 1()5q-uo25'T 



log Z) = 5.76 — 04343 (14025/T) 



or by a straight line (Fig. 1). Thus Q amounts to 14025 E = 27870 

 cal/mole and A = 5.76 x 10^ and, within the limits of experimental 

 error, there is no difference lietween the behaviour of the single crystals 

 and the crystalline material. 



An investigation was then made as to whether destruction of the 

 texture at the surface of the single crystal by a milling machine, with 

 the specimen necessarily being exposed to the air for a short time, has 

 a measurable effect on the self-diffusion constant. No marked effect 

 on the velocity of diffusion due to this manipulation could be found. 

 Table 2 shows the results of these measurements. The values thus 

 obtained can also be represented by the above equation and by the 

 straight line applying to the imworked material. 



1 G. HKVE.SY and W. Seith, Z. Phi/s. 56, 790 (1929): Jhid. 57, 809 (1929). 



2 R. FtJRTH, Handb. d. pht/.s: u. techn. Mechanik 7, 687 (1930). 



