578 Prof. Rutherford and Mr. Soddy 



sufficiently complete to enable a general theory of the nature 

 of the process to be established with a considerable degree of 

 certainty and definiteness. It soon became apparent from 

 this evidence that a much more intimate connexion exists 

 between the radioactivity and the changes that maintain it 

 than is expressed in the idea of the production of active 

 matter. It will be recalled that all cases of radioactive 

 change that have been studied can be resolved into the 

 production by one substance of one other (disregarding 

 for the present the expelled rays). When several changes 

 occur together these are not simultaneous but successive. 

 Thus thorium produces thorium X, the thorium X produces 

 the thorium emanation, and the latter produces the excited 

 activity. Xow the radioactivity of each of these substances 

 can be shown to be connected, not with the change in which 

 it was itself produced, but with the change in which it in turn 

 produces the next new type. Thus after thorium X has been 

 separated from the thorium producing it, the radiations of the 

 thorium X are proportional to the amount of emanation that 

 it produces, and both the radioactivity and the emanating 

 power of thorium X decay according to the same law and at 

 the same rate. In the next stage the emanation goes on to 

 produce the excited activity. The activity of the emanation 

 falls to half-value in one minute, and the amount of excited 

 activity produced by it on the negative electrode in an 

 electric field falls off in like ratio. These results are fully 

 borne out in the case of radium. The activity of the radium 

 emanation decays to half-value in four days, and so also does 

 its power of producing the excited activity. 



Hence it is not possible to regard radioactivity as a 

 consequence of changes that have already taken place. The 

 rays emitted must be an accompaniment of the change of the 

 radiating system into the one next produced. 



Non-separable activity. — This point of view at once accounts 

 for the existence of a constant radioactivity, non-separable by 

 chemical processes, in each of the three radio-elements. This 

 non-separable activity consists of the radiations that accompany 

 the primary change of the radio-element itself into the first 

 new product that is produced. Thus in thorium about 25 

 per cent, of the a radiation accompanies the first change of 

 the thorium into thorium X. In uranium the whole of the 

 a radiation is non-separable and accompanies the change of 

 the uranium into uranium X. 



Several important consequences follow from the conclusion 

 that the radiations accompany the change. A body that is 

 radioactive must ipso facto be changing, and hence it is not 



