374 Pro!. J. C. McLennan on Diffusion of Actinium 



active deposit obtainable at any selected point should pass 

 through a maximum value if exposures were made as the 

 pressure was lowered. It is evident, too, from the interpre- 

 tation given that the pressure corresponding to the maximum 

 deposit at a selected point should be the lower the farther the 

 point under observation on the plates was taken from the 

 salt. 



In considering these experiments it can be readily seen 

 that the law observed by Kennedy, viz. 



jd 1 l T i = a constant, 



throws some additional light on the manner in which the 

 active deposit particles become positively charged. 



Let P denote the concentration of the emanation between 

 the plates and 



K x its coefficient of diffusion. 



Taking the axis of x along a line midway between the 



dP 



plates and perpendicular to the salt, it follows that — K x — 



is the rate at which the emanation crosses a square centimetre 

 taken perpendicular to the axis. 



d 2 P 

 From this it can be readily seen that K x ^— ^- represents 



the rate at which the emanation increases per c.c. at a 

 distance a from the salt. 



If now X : is the decay constant of the emanation, X 2 P will 

 denote the rate at which the emanation in each cubic centi- 

 metre is being transformed into the active deposit product, 

 and therefore for equilibrium we have 



72 p 



or gP X,P 



da* ~~ Kj ' 



or T . - V^-x V 



P = *i* +c 2 e % ^ x 



If, now, when a? = 0, P = P , and when 



x = infinity, P = 0, 

 we have C 2 = 0, 



and C^Pq, 



or 



P = IV Kl 



