580 Prof. Rutherford and Mr. Soddy 



for the belief that the a rays generally are projections and 

 that the mass of the particle is of the same order as that of 

 the hydrogen atom, and very large compared with the mass 

 of the projected particle which constitutes the /3 or easily 

 deviable ray from the same element. 



"With regard to the part played in radioactivity by the two 

 types of radiation, there can be no doubt that the a rays are 

 by far the more important. In all cases they represent over 



99 per cent, of the energy radiated *, and although the 

 j3 rays on account of their penetrating power and marked 

 photographic action have been more often studied, they are 

 comparatively of much less significance. 



It has been shown that the non-separable activity of all 

 three radio-elements, the activity of the two emanations, and 

 the first stage of the excited activity of radium, comprise 

 only a rays. It is not until the processes near completion in 

 so far as their progress can be experimentally traced that 

 the /3 or cathode-ray makes its appearance |. 



In light of this evidence there is every reason to suppose, 

 not merely that the expulsion of a charged particle accom- 

 panies the change, but that this expulsion actually is the 

 change. 



§ 4. The Law of Radioactive Change. • 



The view that the radiation from an active substance 

 accompanies the change gives a very definite physical 

 meaning to the law of decay of radioactivity. In all cases 

 where one of the radioactive products has been separated, 

 and its activity examined independently of the active sub- 

 stance which gives rise to it, or which it in turn produces, it 

 has been found that ,the activity under all conditions inves- 

 tigated falls off in a geometrical progression with the time. 

 This is expressed by -the equation 



la 



where I is the initial ionization current due to the radiations, 

 It that after the time t, and X is a constant. Each ray or 



* In the paper in which this is deduced (Phil. Mag. Sept. 1902, 

 p. 329) there is an obvious slip of calculation. The number should be 



100 instead of 1000. 



t In addition to the a and /3 rays the radio- elements also give out a 

 third type of radiation which is extremely penetrating. Thorium as well 

 as radium (Rutherford, Phys. Zeit. 1902) gives out these penetrating 

 rays, and it has since been found that uranium possesses the same 

 property. These rays have not yet been sufficiently examined to make 

 any discussion possible of the part they play in radioactive processes. 



