38 Prof. J. N. Blasted and Prof. G. Hevesy on 



mercury molecules between the surface-layer and the 

 interior of the liquid is also rapid ; for it is clear that a 

 slow exchange of molecules would lead to an accumulation 

 of the heavier isotope in the surface-layer, thus frustrating 

 the separation. In so far as the exchange is only accom-* 

 plished by diffusion, the time required can be approximately 

 calculated. We can, namely, determine approximately the 

 velocity with which mercury diffuses in mercury (self- 

 diffusion) from the known diffusion-rate of lead in mercury*. 

 The latter f at the temperature of 100° equals about 

 3 . 10 ~ 5 cm. 2 /sec. _1 , and as the mean displacement square 

 of the molecule per sec. depends on the diffusion-constant 

 (D) as in the equation 



t- = 2D, 

 then the mean displacement (t) of the mercury molecules in 

 the liquid mercury is about 5 . 10~ 3 cm./sec. -1 . It follows 

 from this calculation that if not more than 5.10 -3 c. cm. 

 per cm. 2 surface evaporates during the time-unit, no dis- 

 turbing accumulation of the heavier isotope in the surface- 

 layer takes place. 



(b) The experimental results. 



As initial material, 2700 c. cm. of the purest mercury was 

 used. This was submitted, in portions of about 300 c. cm., 

 to an "ideal distillation 3} in the large apparatus (fig. 1), 

 and the process continued until about a fourth of the 

 mercury was distilled over. A total of 2062 c. cm. residual 

 and 642 c. cm. distilled substance, which we will denote 

 by Rj and D l5 was acquired. 



In order to obtain heavy mercury, R : was submitted to a 

 similar treatment as the initial material, and 1602 c. cm. 

 residual (R 2 ) and 160 c. cm. distillate (B^D^ acquired. 

 By proceeding in the same manner, continuously decreasing 

 remainder volumes (R 3 , R. 4 , . . .) of continuously increasing- 

 density were obtained. Beginning from R 10 , the volume 

 of the mercury became too small to be treated in the large 

 apparatus ; so first the middle-sized, and from R 14 onwards 

 the small apparatus was used. 



The progression of the experiment in the case of the 

 residual mercury is shown in Table I., which contains the 

 densities found with the density of ordinary mercury as unity. 



As seen from the table, the density of the It-fractions 

 increases gradually as the residual volume decreases, and 

 ultimately in the case of the heaviest mercury becomes 

 J %o larger than the density of the normal substance. 



* Compare Groh & Heresy. Ann cl. PIn/s. lxiii. p. 92 (1920). 

 t M. Knudsen, Ann. d. Pllysik, (4) xxix. p. 179 (1909). 



