Manchester Memoirs^ Vol. xlviii. (1904), No. 8. 



THE WILDE LECTURE. 



VIII. The Evolution of Matter as revealed by the 

 Radio-Active Elements. 



By Frederick Soddy, M.A. 



Delivered February 23rd, 1^04. 



Radio - activity may be defined as the property 

 possessed by certain elements of emitting a peculiar 

 kind of radiation continuously and without any external 

 stimulus. A radiation is an influence transmitted 

 through space radially from its source, and the first 

 known example was of course that of light. Newton 

 conceived the radiation of light to be due to the emission 

 from the radiating object of tiny particles or corpuscles, 

 which travel in straight lines through space radially in all 

 directions. To-day we know that light is to be explained 

 by the undulatory or wave motion of the luminiferous 

 medium. The radiations from the radio-active elements, 

 on the other hand, were at first thought to be waves, but 

 are now recognised as the realisation of the Newtonian 

 conception of light. That is, the radiations are caused 

 by the radial flight of corpuscles or small particles, 

 and each radiant particle carries with it an electric 

 charge. The term " radiation " has therefore come to 

 designate with equal propriety two fundamentally dis- 

 tinct phenomena. The past decade will probably be 

 remembered for its discoveries of new kinds of radiations, 

 and these have not been confined to the one variety. 

 The " Radiant Matter " of Sir William Crookes, or to give 

 the phenomenon the name by which it is now more 

 generally known, the cathode-rays, produced by an 



March i6th, igo4. 



