Valency of the Radioelements. 397 



The mean value of the mobility of a cation is 55*7. The 



mean value of the diffusion constant calculated from this 



1*25 

 value is cm. 2 per day. For monovalent ions, therefore, 



the most probable diffusion coefficient is 1*25, for divalent 

 ions it is 0*63 and so on. The mobility of sodium, and 

 especially that of lithium, is exceptionally low. The dif- 

 fusion coefficient of sodium is 098 when the NaCl diffuses 

 in excess of chlorine ions. Sodium is therefore monovalent. 

 The diffusion constant of lithium, however, is 0*75. Lithium 

 may therefore be either mono- or divalent. This element 

 has a very low atomic weight, and it is not to be expected 

 that this uncertainty regarding its valency will apply to 

 elements of high atomic weight like the radioelements. It 

 is therefore legitimate to calculate the number of charges 

 borne by the cation from the velocity with which a salt 

 diffuses in excess of its anion. 



There is another method of determining the valency, and 

 that consists in determining the mobility of the ion, and 

 calculating n from the equation 



rc = 0-02242^. 



Both these methods have been used in this research and 

 both have yielded similar results. The validity of the method 

 of obtaining the valency of the diffusing cation from its 

 diffusion constant in excess of the anion has therefore been 

 confirmed. 



§ 5. Analogy between Diffusion of Ions in Gases and 



in Liquids. 

 Equation (1) holds also for diffusion of gaseous ions. If 

 k be the mobility of the gaseous cation and k' that of the 

 anion , then 



p__ 0-04483 *y 



n(k + k') 

 The experiments of Townsend * and others have shown that 

 this equation holds as accurately for gas ions as it does for 

 liquid ions. The well-known equation of Townsend con- 

 necting the diffusion coefficient of a gas ion with its mobility 



JN£ n 



(II being the pressure of the gas, N the number of molecules 



* Phys. Zeits. vol. i. p. 313 (1900). Nernst, Theoret. Chem. 6 edit, 

 p. 404. 



Phil. Mag. S. 6. Vol. 25. No. 147. March 1913. 2 E 



