Excited Activity of Thorium. 



639 



the interchange oh' X l and \ 2 hi the equations, and therefore 

 the alteration of a constant, and it is only relative values that 

 Rutherford has obtained. It will be convenient to distinguish 

 the two stages as ''thorium A" and "thorium B," using the 

 same terminology as that applied by Rutherford to the excited 

 activity of radium *. 



The amount of separation effected in any given case can 

 easily be calculated by a comparison of the curve for the 

 exposed disk (which we may call «) with that for the control 

 disk, /3. The ratio of the activities of the two would remain 

 constant but for the diminution caused in one of them by the 

 discharge. As the activity observed is always due to the 

 second change (the first giving no rays), the fall in the ratio 

 of activities a. : /3 immediately after exposure gives the amount 

 of thorium B driven off from a. After three or four hours, 

 when the normal rate of decay is recovered, the activity of a 

 has become proportional to the amount of the slower-decaying 

 substance present, so that the ratio of a to /3 at this stage 

 gives the amount of thorium A driven off in the discharge- 

 tube. For example, the ratio of a to /3 immediately after 

 the exposure of a to the discharge was found on one occasion 

 to be '5 (taking the initial ratio as unity), and the constant 

 ratio obtained four or five hours later was *7, so that 50 per 

 cent, of thorium B was removed, and 30 per cent of thorium 

 A. "When the active disk was made cathode, and a large 

 proportion of the activity driven off, there was often very 

 little separation ; on one occasion 53 per cent, of A and 

 58 per cent, of B were removed, on another 90 per cent, of A, 

 and \)'2 per cent, of B. In other cases the separation 

 effected was generally greater ; and where the activity driven 

 off was also measured, it was found that the greater the pro- 

 portion of B to A in the active matter lost by the first disk, 

 the greater was the rate of decay of the separated activity. 



Percentage of the two active sub- 

 stances removed from the disk. 



Time of decay to 



half value of 



the separated 



activity. 



Thorium A. Thorium B. 



3 percent. 13 per cent. 

 1" „ 18 „ 

 28 „ 38 „ 

 90 „ 92 „ 



1 hour. 



1 hour 40 min. 

 3 hours. 

 11 hour.-:. 



* Since the above was written. Prof. Kutherford has himself made the 

 change suggested (Phil. Trans. A. 370). 



2 D 2 



