Valency of the Radioelements. 409 



The value o£ the mobility of the H ion of HC1, obtained by 

 this method, phenolphthalein being used as an indicator, was 

 0*0027 cm. per second. The value found by Lodge under 

 the same conditions, but with a different arrangement, was 

 0*0026; that calculated from Kohlrausch's formula is 0*0028 

 cm. per second. Whetham * has shown that in the range of 

 potential gradients employed in these experiments the velocity 

 of the ions is strictly proportional to the potential gradient. 



(a) The velocity of the ions of Radium. 



In this experiment the indifferent electrolyte used was a 

 1/50 normal HC1 solution. Palladium electrodes were used 

 in order to obviate disturbances due to the generation of gas. 

 When I was 10*6 cm. and V, 2*12 volts, the values of t 

 found in four different experiments were 9180, 9270, 9140, 

 and 9220 seconds respectively at 16°*2. From these results 

 the mobility of the radium ion at 18° is calculated to be 

 56'6 cm. per sec. When V was 1*49 volts the value was 

 55*9, and when V was 3*30 volts it was 56*2. The potential 

 gradient was measured by a compensation method. 2*2 per 

 cent, per degree was taken as the temperature coefficient of 

 the mobility, a value which has been obtained in previous in- 

 vestigations of ion mobility. As mentioned above, the value 

 obtained by Kohlrausch and Henning from the conductivity 

 of the radium bromide was 57*3, a value similar to that 

 obtained by us directly. 



We prefer to accept Kohlrausch's as the more correct 

 value, and all our results are calculated on this value. Any 

 alteration in the value obtained by Kohlrausch will therefore 

 necessitate a corresponding change in the values given in 

 this paper. 



By inserting the values D = 0*667, and ?;= 57*3 in the 

 equation 



a 



D = 0*02232-, 

 n 



we obtain n for radium 1*91. 



This value of the valency is in good agreement with the 

 value calculated from the diffusion coefficient alone. It is 

 of course also the value to be expected from the chemical 

 behaviour of radium. 



(b) Ion mobilities of ThX, ActX, RaC, ThB, ThC, RaD r 

 RaE, and RaF. 



The ion mobilities of these radio-ions were also determined 

 by the method described above. The results are tabulated 



* Proc. Roy. Soc. vol. Hi. p. 289 (1892). 



