Welliseh — Experiments on Active Deposit of Radium. 301 



If we take the ratios a ■ : o : c as 9 : 20 : 180 then we have from 

 equations (10) and (11) D = '0415 ; 1c = '064. 



With a : b : e = 12 : 20 : 180 we deduce D = -029. 

 With a : I : c = 9 : 24 : 216 we deduce D = '061. 



The value D = *0445 cm2 sec -1 represents the most probable 

 value for the coefficient of diffusion of uncharged atoms of 

 radium active deposit in dry air at a pressure of one atmosphere 

 and at room temperature. The other values for D are to be 

 regarded merely as extremes between which the true values 

 of I) appear to lie. 



The value obtained for D is smaller than the values which 

 have been obtained experimentally for the diffusion of radium 

 emanation in dry air. In the case of the emanation the values 

 obtained are not very different from *10. The value of D for 

 the active deposit is approximately the same as that found by 

 Townsend for the diffusion of a gaseous ion in air. 



The Experiments of Eckmann. 



7. In the Jahrbuch der Kadioaktivitat (1912) Gerhard Eck- 

 mann describes the results of a series of experiments bearing 

 on the diffusion and distribution of the active deposit particles 

 of radium. These experiments cover in many respects the 

 same ground as those here described ; this fact was not 

 recognized till the present series was completed. Eckmann's 

 work may be for convenience summarized under the following 

 heads : 



(1) distribution of the deposit in electric fields : 



(2) investigation with regard to aggregates : 



(3) determination of the coefficient of diffusion (D). 



With regard to (1) he comes to the conclusion that about 98 

 per cent of the particles are positively charged, the remainder 

 being negatively charged. This is of course opposed to the 

 results of the present investigation. Eckmann assumes that 

 the total deposit is the sum of the deposits obtained on a 

 central electrode when it is first raised to a high positive and 

 then to a high negative potential. The present experiments 

 show that this assumption is incorrect, the 2 per cent which is 

 obtained in the latter instance being due to the diffusion to the 

 central electrode of a small fraction of the neutral particles. 

 Eckmann ascribes the deposit on the anode to negatively 

 charged particles and accordingly neglects a considerable 

 amount of deposit which settles on the case of his vessel. 



Eckmann's work with regard to aggregates anticipates to a 

 large extent that described in the present paper. He shows 



