378 Prof. J. C. McLennan on Diffusion of Actinium 

 on resubstituting for y that the product of aq is given by 



«?=jj«-< (5) 



where q is the rate at which the emanation leaves the salt, 

 and a. is the proportionality factor between the concentration 

 of the emanation and the concent ration of the active deposit 

 particles produced from it, which are or become positively 

 charged. 



It is evident, then, from (5) that the law discovered by 

 Kennedy for the distribution of the active deposit leads to 

 the conclusion that either one or other of the magnitudes 

 a and q, or perhaps both, is a function of the pressure. 



Some interesting information, which could be used in 

 testing the result given in (5), might readily be obtained by 

 confining the actinium salt and the emanation liberated from 

 it at different pressures in a small chamber constructed in 

 such a way that a negatively charged electrode inserted 

 in it would have access to all the emanation during an 

 exposure. 



Such an arrangement, however, would possess one defect. 

 According to the conclusions we have reached, the active 

 deposit particles which get positively charged would grow 

 fewer and fewer as the pressure in the chamber was lowered, 

 and the quantity of the deposit which would reach the 

 negatively charged electrode through the agency of the 

 directive action of its field would then become less and less 

 with decreasing pressures. On the other hand, as the pres- 

 sure was decreased the active deposit particles, which w r ere 

 uncharged, would reach the bounding surface of the chamber, 

 including the surface of the negatively charged electrode, 

 either by projection or diffusion in ever-increasing amounts; 

 and so, unless the negatively charged electrode had a surface 

 exceedingly small in comparison with that of the walls of 

 the chamber, the deposits obtained on it at different pressures 

 could not be taken as measures of the number of active 

 deposit particles present which carried a positive charge. 



Some information might also be obtained regarding a and 

 q separately by arranging suitable experiments with an active 

 emanation possessing a longer life than that from actinium — 

 for example, that from radium. 



If a quantity of radium salt were confined in a chamber 

 to which a second was attached which could be cooled down 

 with liquid air, the emanation expelled by the salt in a given 

 time could be collected in the second chamber. This could 

 be done for a series of different pressures, care being taken 



