298 Prof. E. Rutherford on Slow 



Rise of the a, Ray Activity of the Active Deposit. 



It was shown in the previous paper that the a ray activity 

 of the active deposit increased for the first 60 days at a 

 nearly uniform rate. Observations of the a ray activity of a 

 platinum plate, coated with the active deposit, have been con- 

 tinued for a period of nine months. The results are shown 

 in the table below. 



Time in a ray activity 



days, in arbitrary units. 



y 0-127 



43 0'57 



135 1-44 



163 1-65 



196 1-85 



290 2-17 



The results are shown diagrammatically in fig. 4. The 

 activity is still increasing, but not nearly so rapid as at first. 

 Observations have also been made on an active deposit 15 

 months old. The activity is still increasing, but slowly, and 

 is obviously tending towards a maximum value. 



The curve of fig. 4, as far as observations have gone, is very 

 similar in shape to the usual recovery curve, in which half 

 the final activity is reached in 143 days. Such a result is to 

 be expected since radium F is transformed far more rapidly 

 than radium D. 



From a comparison of the experimental curve with the 

 theory discussed below, it can be deduced that the activity 

 should finally reach the value 2 '90. The activity after 290 

 days has thus reached 75 per cent, of the maximum value. 



The full explanation of the rise of the a. ray aciivity involves 

 the theory of three successive changes, since the substance 

 initially deposited, radium D, changes into radium E, and 

 radium E into radium F, and the latter substance alone gives 

 out a rays. Since, however, radium E (half transformed in 

 six days) has a very short period compared with radium F 

 (half transformed in 143 days), for the purpose of calculation 

 the intermediate change may be neglected, and radium D 

 may be supposed to change at once into radium F. 



If Xj, X 3 are the constants of change of radium D and 

 radium F respectively, then the number q of atoms of radium 

 F present at any time t is given by 



/V 3 — Ax 



where n is the number of atoms of D originally deposited. 



