Valency of the RacUoelements. 401 



ActX ) diffuses in practically infinite excess of the anion 

 CI'. The acidity of the solution prevents also any possible 

 precipitation of these substances which might easily take 

 place if the solutions were neutral. The ThX was prepared 

 by the usual method. Some thorium nitrate was dissolved 

 in water, from this solution the thorium was precipitated as 

 hj-droxide by ammonia, aud the filtrate which contained the 

 ThX evaporated to dryness. The solution used contained 

 initially 10" 12 gm. ThX per c.c. 1 c.c. of this solution was 

 diluted to 10 c.c. and evaporated to dryness on a watch-glass. 

 This was done with 10 c.c. of each of the layers on com- 

 pletion of diffusion. About six days later when ThX, ThB, 

 and ThC were formed in equilibrium amounts, the activities 

 of the watch-glasses were determined, and from these 

 measurements the quantities of ThX present were obtained. 



The following are the values of the diffusion constant 

 obtained for ThX : — 



« D ^=r9o^Mo=°- 600 



D5o ' 2= 1-92 x 0-492 = °' 604 

 (2) ^=2-^^=0-599 



D*. = ^ U °/,^ =0-602. 



0-57 



2*32 x : 409 



Mean value of D 1S o = 0*659 sq. cm. per day. 

 The valency of ThX is therefore 2. 



G. Hoffman* investigated the diffusion velocity of 

 ThX(N0 3 ) 2 in water by Weber's method, which consists in 

 determining the change in concentration with time at the 

 free surface of a system consisting of two layers of liquids 

 of different concentration. In his experiment Hoffman 

 determined the ThX by means of the emanation it produced, 

 by passing a current of air over the free surface. He found 

 Dio° =0-504, from which by calculation we get Di S o =0*608 

 sq. cm. per day. 



The diffusion velocity of ActX was determined by the 

 same methods as used for ThX, and the following results 

 obtained. Each c.c. of the solution used contained 1*5 x 10 ~ 12 

 gm. ActX. 



* Ann. der Phjs. vol. xxi. p. 239 (1906). 



