644 Miss M. S. Leslie : Comparison of the Coefficients oj 



long-period substances thorium B or actinium B. Indeed, 

 ultimately it was only the amounts of thorium B and actinium 

 B which were found, since, when the measurements were 

 made, sufficient time had passed since the removal of the 

 plates to allow all other products brought from the diffusion 

 vessel to disappear. Wellisch and Bronson *, in a recent 

 paper on the distribution of the deposit of radium in an 

 electric field, have shown that with a difference of potential 

 of 160 volts not more than 80 per cent, radium deposit 

 reaches the cathode, and even with 4000 volts only 91 per 

 cent, was obtained. This, it is true, was across a distance of 

 28 mm. and not 2 mm. as in the present experiments. 



With an increase in the amount of emanation there will 

 be an increase in the ionization. Hence, more reaction 

 between the ions and the active deposits resulting in a 

 greater value of D may reasonably be expected. This would 

 account for the high values obtained with radiothorium as 

 source of emanation. Wellisch and Bronson, in the paper 

 just referred to, state that only with fields of a few volts 

 does any change in the amount of deposit collected appear 

 to follow a change in the degree of ionization. One might 

 assume, however, that a neutral particle could collide with 

 an ion and become charged again. In the experiments 

 referred to, the final result would be the same as if the 

 particle had remained charged all the time. This would not 

 be the case, however, in the diffusion experiments, where it 

 is important that the active deposit should attach itself to 

 the plate at the level at which it is formed. 



Evidently, it is exceedingly difficult to say when the 

 thorium and actinium emanations may be justly regarded as 

 comparable with one another. This may perhaps have been 

 approximately the case when amounts of emanation which 

 were equally active were employed, that is when the value 

 of D for thorium emanation was "111, and for actinium 

 emanation *107. If such an assumption were made, the 

 results would indicate that the molecular weights are very 

 close to one another. More reliance, however, could be 

 placed on this conclusion if the periods of all the thorium 

 products bore the same ratio to one another as those of the 

 actinium products. The diffusion of the active deposits 

 might then be expected to produce proportionately the same 

 effect in each case. 



With regard to the absolute value of the coefficient of 

 diffusion, it may be concluded that the smaller values found 

 are more nearly correct than the larger ones. The smallest 



* E. M. Wellisch and H. L. Bronson, Phil. Mag. May 1912 p. 714. 



