644 On the Excited Activity of Thorium. 



explanation does not appear to apply. The electrolysis of 

 active hydrochloric acid solutions with electromotive forces 

 too small to decompose the water, gave an active deposit 

 which decayed according to an exponential law, and fell to 

 half value in about 4^ hours. This would therefore appear 

 to be a third stage in the changes gone through by the 

 active deposit, though other investigations have given no 

 evidence of such a substance. 



Pegram*, by electrolytic methods, has also obtained pro- 

 ducts with various rates of decay, but these again are never 

 exponential, and the substances are probably in all cases 

 mixtures. 



Conclusions. 



The conclusions drawn from the above investigations may 

 be summarized as follows: — 



The two stages in the disintegration of thorium-excited 

 activity, whose existence has been already indicated by theo- 

 retical considerations, are always present simultaneously on 

 a surface which has been exposed to thorium emanation. 

 The constituent with the slower rate of decay (half value in 

 11 hours) is the one first formed, and may be called thorium 

 A. The thorium B formed from it, which is responsible for 

 the radiation emitted, is the substance whose rate of decay is 

 the more rapid one (half in 55 minutes). 



These two substances show differences in their properties 

 by means of which they may be separated. They are both 

 partly volatilized on exposure to the cathode-ray discharge, 

 thorium B being more volatile than thorium A under these 

 circumstances. The cause of the volatilization is an electrical 

 one, being partly a direct action of the cathode rays, and 

 partly a sputtering of the active matter, somewhat similar to 

 that observed with metallic cathodes. 



Under the influence of heat, thorium A is more volatile 

 than thorium B. The former constituent can be almost 

 entirely removed by keeping an active wire for a few minutes 

 at a dull red heat. Nearly pure thorium B is then left on 

 the wire, the activity of which decays to half value in about 

 55 minutes. 



In conclusion, I wish to express my best thanks to Prof. 

 J. J. Thomson for his very kind interest and valuable advice 

 during the course of these experiments, which were carried 

 out at the Cavendish Laboratory. 



* Phys. Review, Dec. 1903. 



