392 Prof. E. Rutherford and Mr. F. Soddy on 



phenomenon, and not connected with the major part of the 

 activity, two hypotheses can be brought forward capable of 

 experimental test, and in accordance with the views advanced 

 on the nature of radioactivity, to account for the existence of 

 this part. First, if there was a second type of excited 

 -activity produced by ThX similar to that known, but with a 

 very slow rate of decay, it would account for the existence of 

 the non-separable activity. If this is true it will not be found 

 possible to free thorium from this activity by chemical means, 

 but the continuous removal of ThX over a very long period 

 would, as in the above case, cause its spontaneous decay. 



Secondly, if the change which gives rise to ThX produces 

 a second type of matter at the same time, i. e. if it is of the 

 type of a decomposition rather than a depolymerization, the 

 second type would also in all probability be radioactive, and 

 would cause the residual activity. On this view the second 

 type of matter should also be amenable to separation by 

 chemical means, although it is certain from the failure of the 

 methods already tried that it resembles thorium much more 

 closely than ThX. But until it is separated from the thorium 

 producing it, its activity will not decay spontaneously. Thus 

 what has already been shown to hold for ThX will be true 

 for the second constituent if methods are found to remove it 

 from the thorium. 



It has been shown (Soddy, loc. cit.) that uranium also 

 possesses a non-separable radioactivity extremely analogous 

 to that possessed by thorium, and whatever view is taken of 

 the one will in all probability hold also for the other. This 

 consideration makes the second hypothesis, that the residual 

 activity is caused by a second non-thorium type of matter 

 produced in the original change, the more probable of the 

 two. 



XI. 2 he Nature of the Radiations from Thorium and ThX, 

 From the view of radioactivity put forward it necessarily 

 follows that the total radioactivity of thorium is altered 

 neither in character nor amount by chemical treatment. 

 With regard to the first, the amount of activity, it has been 

 pointed out that the intensity of radiations emitted do not 

 furnish alone a measure of the activity. The absorption in 

 the mass of material must be considered also. The radiations 

 of thorium oxide are derived from a very dense powder; those 

 from ThX, on the other hand, have only to penetrate a very 

 thin film of material. The difficulty can be overcome to some 

 extent by taking for the comparison the radioactivity of a 

 thin film of a soluble thorium salt produced by evaj^orating 





