402 Dr. G. von Hevesy on the 



T)H 2^1 (). fi 9Q 



15 °- 9 ~ 2-0 x 0-452 ~ UbwJ 



0*57 

 ( 2 > D ' 6 °-° = ;5-21x 0-282 =0 ' G31 



d -=mS«o= - 634 - 



The mean value of Di 8 o = 0*GG4 sq. cm. per day, and 

 therefore the valency of ActX is 2. 



(c) Diffusion velocity of ThB and TJt C. 



In these experiments the active deposit of thorium was 

 dissolved in 1/100 normal HC1, the solution was allowed to 

 stand 10 hours in order to allow ThC to be formed in equi- 

 librium amount, and then placed in the diffusion apparatus 

 surmounted by a 1/100 normal HC1 solution. One c.c. of 

 the active solution contained 10~ 13 gm. ThB. Each layer 

 of active material after removal from the apparatus was 

 evaporated to dryness, and 10 hours later its activity was 

 measured. From the activities of the different layers, the 

 diffusion constant of ThB was calculated. The following 

 results were obtained : — 



^ u ° -i-02x 0-530 - 10b 

 ( 2 > D - = I^^7 =1 - 05 



,11 0-57 



~ 1-84x0*289 



I)IU= , ... ,' =1-07. 



Mean value Di 8 o = l*21 sq. cm. per day. 



The valency of ThB is therefore 1. 



A shorter diffusion apparatus was used in determining the 

 diffusion constant of ThC owing to the rapidity with which 

 this body decays. We had at our disposal the active deposit 

 from a preparation of mesothorium 1 which equalled in 

 activity 2 milligrams of radium, and also that from a pre- 

 paration of radiothorium equal to 4 milligrams of radium. 

 The duration of an experiment must not of course exceed 8 

 to 10 hours, for at the end of this time the ThC originally 



