396 Dr. G. von Hevesy on the 



and U'-f V _ o -i 9 



u+v -°' lw > 



and when y/e = 100 D' = D^ (1 + 0*002) . 



When hydrochloric acid is present with the body diffusing, 

 100 times in excess, the value of the diffusion coefficient found 

 differs from the theoretical value by less than 0'2 per cent. 

 In our experiment with RaCJ 2 y/c was greater than 10 5 , and 

 in the experiments carried out with the short-lived radio- 

 elements it was even greater than 10 15 . 



From equation (2) we deduce that, in the case of diffusion 

 of the cation in excess of the anion, e. g. the diffusion of 

 radium chloride in hydrochloric acid, the' diffusion constant 

 is given by the expression 



D = 0'02242 



u 



n 



(3) 



Equation (3) is a linear relation connecting the diffusion 

 coefficient of radium chloride, the velocity of the radium 

 ion, and the valency of radium. This equation is a general 

 one, and is free from any hypothetical assumptions. 



§ 4. Relation betiveen the Magnitude of the Diffusion 

 . Constant, and the Valency of the Cation. 



Since, when diffusion takes place in excess of the anion, 

 the diffusion constant depends only on the mobility and the 

 valency of the cation, and since, also, the mobility of the 

 cation varies only within somewhat narrow limits, the mag- 

 nitude of the diffusion constant can be obtained immediately 



«/ 



from the valency of the diffusing ion. The limits referred 

 to extend from the value of the mobility of sodium 44 to 

 that of caesium 6S. The mobility of lithium, however, is 

 exceptionally low, and this is due to the fact that the ion 

 is strongly hydrated. 



In Table II. are given the mobilities of cations so far as 

 they are known. 



Table II. 



Metal. 



Mobility. 



Metal. 



Mobility. 



Na 



436 

 45-9 

 466 

 47-2 

 47-4 

 51-4 

 515 

 54-0 



Ba 



551 



574 

 611 



644 

 647 

 66-6 

 676 



682 



M<r 



Ea 



Zn 



Pb 



Cu 



NH 4 



Cd 



K 



Ca 



Tl 



Sr 



Eb 



H 



Cre 



1 



1 



