296 Prof. E. Rutherford on Slow 



Decay of the Activity of Radium F. 



The product radium F is deposited on a bismuth plate 

 from a solution of the active deposit. The surface of 

 such a plate, which has been left for some hours in the 

 solution, becomes strongly active. The activity consists only 

 of a. rays, the /3 rays being altogether absent. In this way 

 the a ray product is almost completely removed. Radium I) 

 is not deposited on the plate, for the solution, deprived in this 

 way of radium F, at once commences to grow a new supply 

 of radium F. 



The solution of the active deposit was obtained in the 

 following way : — The emanation from 30 milligrams of radium 

 bromide was condensed in a glass tube, which was then 

 sealed. After standing for a month the tube was opened, and 

 dilute sulphuric acid introduced. This almost completely 

 dissolved the active deposit on the glass. The solution 

 was evaporated to dryness in a glass vessel, and allowed to 

 stand undisturbed for nine months. During this time a 

 large quantity of radium F was produced by the primary 

 active substance radium D. The deposit was then again 

 dissolved in sulphuric acid, and three bismuth disks were 

 successively introduced into the solution. These three disks, 

 coated with radium F, were then placed aside, and the a-ray 

 activity examined at intervals over a period of about 

 9 months. The activity of each disk was found to decay 

 according to an exponential law with the time. The results 

 are shown graphically in fig. 3, where the abscissae represent 

 days, and the ordinates the logarithm of the activity in 

 arbitrary units. It is seen that the points lie nearly on a 

 straight line, showing that the activity decreases according 

 to an exponential law. The curves of the activity of the 

 three disks are marked radium F (1.), radium F (II.), 

 radium F (III.)- F (I.) was found to decay to half value 

 in 150 days, F (II.) in 136 days, F (III.) in 142 days. 

 The mean value is 143 days. 



We may thus conclude that the activity of radium F decays 

 to half value in about 143 days. 



The initial rate of decay of two of the curves was somewhat 

 slower than the final value. This may possibly be due to the 

 fact that a small quantity of radium J) was also deposited on 

 the bismuth plate. This w^ould produce more radium F, so 

 that the apparent rate of decay would be somewhat less than 

 the true rate of decay of radium F alone. 



The observations of the rate of decay of the bismuth disks 

 were made with a specially designed electroscope, which was 

 standardized at the time of each observation with the aid of a 

 constant sample of uranium. 



