408 Dr. G. von Hevesy on the 



bromide has fallen to ^ . The radium bromide of concen- 



c w 



tration ~ not present in / x must be in the cathodic branch l 2 . 



If we assume, as Kohlrausch has done, that as soon as the 

 field is applied the ions move ail together as a column, the 



path traversed by the ions in the time t will be ■— = -? . If 



after time t there is still radium bromide of concentration 



€ 



~ present in /i, the displacement of the ion column is 



- — — I and the mobility of the cation U is ^, where 



V is the potential gradient. It was by this method that 

 G. Hoffman * determined the mobility of the ThX ion. The 

 disadvantage of it is that the precipitation of a portion of 

 the dissolved active substance may easily take place, and 

 this makes results uncertain. We tried this method but 

 were unable to obtain consistent results with it. In the 

 surface method, on the other hand, the displacement of the 

 end of the ion column is measured. In this case also the ions 

 move in a column, but the distance traversed by the whole 

 column is much greater, and by this method there is no 

 difficulty in obtaining consistent results. So far when this 

 method has been used the displacement of the ions has been 

 observed optically. The apparatus shown in fig. 2 enables 

 the displacement of the ions to be measured without, if 

 necessary, using optical means. The apparatus is filled, for 

 example, with a solution of copper sulphate. Copper elec- 

 trodes are inserted at A and at B. The tap G is closed, and 

 then the taps 1 and 2. This enables the copper sulphate in 

 li to flow out through a and b without disturbing the re- 

 mainder of the system. Copper sulphate of the same strength 

 as the last, but containing now some active body, say 

 RaF(S0 4 ) 2 , is drawn in through «, and the taps 1 and 2 

 turned as they had been before. After the field has been 

 applied for a definite time, the three-way tap 3 is turned 

 so that the tubes to the right of the tap are shut off from 

 the rest of the apparatus, and at the same time tube / 3 is 

 open to the air. This enables the contents of / 3 to flow out 

 through c. This amount of solution is tested for activity. 

 The time t is varied until it is just possible to trace the 

 activity of the solution in /•>. The mobility of the radio- 

 cation is then calculated from the formula 



* Phys. Zeit. vol. viii. p. 553 (1907^ 



