410 



Dr. G. von Hevesy on the 



in column 3 of Table IV. In column 2 are placed the 

 values of the diffusion constants, and in column 4 the calcu- 

 lated values of the valency. 



Table IV. 





D in sq. cm. per day. 



u in 10"' cm. per sec. 



». 



Ra 



0-667 



0-659 



0-664 



1-21 



0-625 



0625 



0651 



0-646 



0-760 



57'3 

 58-0 

 561 

 55-4 

 54-0 

 54-5 

 61-9 

 61-9 

 68-8 



1-91 

 1-96 

 1-88 

 111 

 1-93 

 1-95 

 211 

 2-13 

 202 



TbX 



A.ctX 



ThB 



ThC 



EaC 



RaD 



RaE 



RaF 





As is to be expected from the fact that they have similar 

 ■electrochemical properties, ThC, RaC, and ActO have very 

 similar ion mobilities. 



The ions o£ ThB, however, move more rapidly than those 

 of ThC, and in this way it is possible to separate ThB from 

 ThC. It is impossible by this method to separate RaD 

 from lead, as these bodies have very similar ion mobilities, 

 namely, Ol'O and 61*9 respectively. RaF may be easily 

 separated from both RaE and RaD on account of its greater 

 mobility. 



§ 8. Relation between the mobility of the Ions and 

 their 'position in the Periodic Table. 



Bredig * was the first to show that the mobility of an ion 

 in an infinitely dilute solution is a function of its weight. 

 The mobility of the ions of elements belonging to any par- 

 ticular group in the periodic system increases as the atomic 

 weight of the element increases. By means of this rule, it 

 is possible to indicate the position of the elements ThB, 

 RaB, and ActB in the periodic system. These elements are 

 monovalent, and therefore belong either to the group of 

 alkaline metals or to the group containing Ag, monovalent 

 Tig, and monovalent Tl. 



From Table V., however, it is seen that if ThB belonged 

 to the alkaline metal group, it would possess a greater 

 mobility than that of caesium, namely 6S"2. Its mobility 

 is, however, less than this amount. ThB is therefore most 

 likely a member of the second group, and since it has a 

 greater atomic weight than silver, and therefore is more 



* Zeit. phys. Chem. vol. xiii. p. 191 (1904). 



