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



XII. Summary of Results. 



The foregoing experimental results may be briefly sum- 

 marized. The major part of the radioactivity of thorium — 

 ordinarily about 54 per cent. — is due to a non -thorium type 

 of matter, ThX, possessing distinct chemical properties, which 

 is temporarily radioactive, its activity falling to half value in 

 about four days. The constant radioactivity of thorium is 

 maintained by the production of this material at a constant 

 rate. Both the rate of production of the new material and 

 the rate of decay of its activity appear to be independent of 

 the physical and chemical condition of the system. 



The ThX further possesses the property of exciting radio- 

 activity on surrounding inactive matter, and about 21 per 

 cent, of the total activity under ordinary circumstances is 

 derived from this source. Its rate of decay and other con- 

 siderations make it appear probable that it is the same as the 

 excited radioactivity produced by the thorium emanation, 

 which is in turn produced by ThX. There is evidence that, 

 if from any cause the emanation is prevented from escaping 

 in the radioactive state, the energy of its radiation goes to 

 augment the proportion of excited radioactivity in the 

 compound. 



Thorium can be freed by suitable means from both ThX 

 and the excited radioactivity which the latter produces, and 

 then possesses an activity about 25 per cent, of its original 

 value, below which it has not been reduced. This residual 

 radiation consists entirely of rays non-deviable by the 

 magnetic field, whereas the other two components comprise 

 both deviable and non-deviable radiation. Most probably 

 this residual activity is caused by a second non-thorium type 

 of matter produced in the same change as ThX, and it should 

 therefore prove possible to separate it by chemical methods. 



XIII. General theoretical Considerations. 



Turning from the experimental results to their theoretical 

 interpretation, it is necessary to first consider the generally 

 accepted view of the nature of radioactivity. It is well 

 established that this property is the function of the atom and 

 not of the molecule. Uranium and thorium, to take the 

 most definite cases, possess the property in whatever mole- 

 cular condition they occur, and the former also in the 

 elementary state. So far as the radioactivity of different 

 compounds of different density and states of division can be 

 compared together, the intensity of the radiation appears to 

 depend only on the quantity of active element present. It 



