on Radioactive Change. 511 



so separate themselves by the mere process of diffusion, giving 

 rise to the radioactive emanations which are produced by 

 compounds of thorium and radium. These emanations can be 

 condensed by cold and again volatilized ; although they do 

 not appear to possess positive chemical affinities, they are 

 frequently occluded by the substances producing them when 

 in the solid state, and are liberated by solution ; they diffuse 

 rapidly into the atmosphere and through porous partitions, 

 and in general exhibit the behaviour of inert gases of fairly 

 high molecular weight. In other cases again the new matter 

 is itself non-volatile, but is produced by the further change of 

 the gaseous emanation ; so that the latter acts as the inter- 

 mediary in the process of its separation from the radioactive 

 element. This is the case with the two different kinds of 

 excited activity produced on objects in the neighbourhood of 

 compounds of thorium and radium respectively, which in turn 

 possess well-defined and characteristic material properties. 

 For example, the thorium excited activity is volatilized at 

 a definite high temperature, and redeposited in the neigh- 

 bourhood, and can be dissolved in some reagents and not in 

 others. 



These various new bodies differ from ordinary matter, 

 therefore, only in one point, namely, that their quantity is far 

 below the limit that can be reached by the ordinary methods 

 of chemical and spectroscopic analysis. As an example that 

 this is no argument against their specific material existence,, 

 it may be mentioned that the same is true of radium itself as 

 it occurs in nature. No chemical or spectroscopic test is 

 sufficiently delicate to detect radium in pitchblende, and it is 

 not until the quantity present is increased many times by 

 concentration that the characteristic spectrum begins to make 

 its appearance. Mme. Curie and also Giesel have succeeded 

 in obtaining quite considerable quantities of pure radium 

 compounds by working up many tons of pitchblende, and the 

 results go to show that radium is in reality one of the best 

 defined and most characteristic of the chemical elements. 

 , So, also, the various new bodies, whose existence has been 

 discovered by the aid of their radioactivity, would no doubt, 

 like radium, be brought within the range of the older methods 

 of investigation if it were possible to increase the quantity ot 

 material employed indefinitely. 



§ 2. The Synchronism between the Change and the Radiation. 

 In the present paper the nature of the changes in which 

 these new bodies are produced remains to be considered. 

 The experimental evidence that has been accumulated is now 



