114 Prof. Rutheri'ord on Excited Radioactivity and 



reduced to the point below which it can no longer ionize the 

 gas. Strutt has put forward the view that the a rays were 

 positively charged bodies since the j3 rays emitted from the 

 same body carried a negative charge. This view has also been 

 advanced in a recent paper by Sir W. (Jrookes. 



The evidence in favour of the projection nature of the 

 a rays is so far all indirect in character, and is briefly 

 summarized below: — 



(1) The absorption of the a. rays in matter (like the ft rays 

 which we know are projected particles) is approximately 

 proportional to the density of the material. It has been 

 shown that the absorption of uranium, thorium, and radium 

 rays is roughly proportional to the density for air and for 

 aluminium. 



(2) The absorption of the a rays by a given thickness of 

 matter increases rapidly with the thickness traversed. 



I have found that this is a general property of the a, radiations 

 not only for the radioactive elements proper, but for the 

 radiations from the emanation and excited bodies. This is to 

 be expected if the rays consist of projected particles, but is 

 difficult to explain if the radiations are aether-waves similar to 

 Rontgen rays. 



(3) In the case of the emanations we have direct evidence 

 that a negatively charged particle * is projected. The radiation 

 from the emanation is due to these projected particles which 

 ionize the gas in their path. This satisfactorily explains the 

 experimental observation that the amount of excited activity 

 is directly proportional to the amount of radiation from 

 the emanation. It also serves to explain the fact especially 

 noticeable in the experiments on the radium emanation, that 

 some of the carriers of excited activity have sufficient initial 

 velocity to move against the electric field. This velocity is 

 due to the recoil consequent upon the projection of the charged 

 body. 



If these rays are due to projected charged particles they 

 should possess the properties of the a. rays of deflectability by 

 a magnetic and electric field. 



No deviation of the a. rays has so far been detected in a 

 strong magnetic field, but the experiments have not yet been 



* I was at first inclined to suppose that the particle expelled from the 

 emanation was a negative electron, since it is known that both thorium 

 and radium compounds and bodies excited by them, emit some deviable 

 rays. I have, however, made a close examination of the radiation from 

 the emanation by the electrical method, but was unable to detect the 

 presence of any penetrating deviable rays. If such deviable rays are 

 present, they certainly exist in far less proportion compared with the 

 a, rays than in the other radioactive substances. 



