Manchester Memoirs, Vol. xlviii. (1904), No. 8. 3 



■the element uranium and of certain others. An insignifi- 

 cant proportion of the radiation, the so-called 7 rays, 

 resembles the type discovered by Rontgen, and are 

 probably X rays of very great penetrating power, which 

 result at the moment of the expulsion of the /3 rays. 

 The latter have been shown by Becquerel to consist of 

 very high- velocity cathode-rays, and Kaufmann has 

 detected in the radiations of radium cathode-rays travel- 

 ling with a speed of 95 per cent, of that of light. But 

 although experimentally very obvious the j3 rays are, like 

 the y rays, of secondary importance. The fundamentally 

 new feature in the phenomenon of radio-activity is the 

 "type of radiation known as the a rays. Rutherford* last 

 year proved that these, like the j3 rays, were deviated 

 by a magnetic field, but in the opposite direction, indi- 

 cating that the a particle is positively charged. The 

 ■deviation is extremely small, even in the most intense 

 magnetic field it is possible to obtain, and the radius of 

 ■curvature of the ray in a field of given strength is about a 

 thousand times greater for the a ray than for the cathode- 

 ray. If we accept the view that the charge on the 

 •a particle is the atomic or ionic charge, Rutherford 

 showed by measuring the extent of the deviation that its 

 mass is of the same order as that of the hydrogen atom, 

 and its velocity is about one-tenth of that of light. Thus 

 the radio-active substances spontaneously expel into 

 space particles of atomic dimensions with a velocity of 

 twenty thousand miles a second. 



The foregoing is a brief review of our knowledge with 

 regard to the new kinds of radiation. We have seen that 

 the X rays and probably the 7 rays are ether disturbances, 

 while the cathode-rays, and the a and j3 rays are cor- 

 puscular in character. Yet the resemblance of the two 

 * Phil. Mag.^ 1903, s,6, v. 5, p. 177. 



