2 SODDY, Evolution of Matter by Radio-active Elements. 



electrical discharge in vacuo, have been shown to be due 

 to the flight of rapidly moving corpuscles carrying a 

 negative charge. The proof rests upon their property of 

 being deviated by a magnet. As the conclusions of a 

 series of researches which are quite beyond the scope of 

 the present subject, Professor J. J. Thomson and his 

 colleagues have shown (i) that the charge carried by the 

 cathode-ray corpuscle is the atomic charge, or that carried 

 by an univalent ion in electrolysis, (2) that the mass of the 

 corpuscle is only about one-thousandth of the mass of 

 the lightest atom known to chemists, (3) that the velocity 

 of the cathode-ray corpuscle is ordinarily about one-tenth 

 that of light, but varies within certain fairly wide limits. 

 Lenard found that if a Crookes' tube is provided with a 

 window of thin aluminium foil in the path of the cathode- 

 rays, the latter are able to penetrate the window and can 

 be investigated outside the tube. The corpuscles travel- 

 ling with low velocity are stopped by the metal foil and 

 those penetrating were found to possess velocities about 

 one-third as great as that of light. 



The X rays of Rontgen were the next discovery, and 

 these, according to accepted doctrines, are new examples 

 of ethereal waves. They result whenever the cathode-ray 

 corpuscle suffers change of velocity, as for example, when 

 it strikes an obstacle. A sudden electro-magnetic pulse 

 is propagated in all directions from the corpuscle and gives 

 rise to the X rays. These differ from ordinary light 

 mainly in the irregular and sudden character of the 

 individual pulses. 



The rays we are most nearly concerned with w;ere 

 discovered by Becquerel in 1896, during an investigation 

 of the fluorescence of certain compounds of uranium. We 

 know now that the new radiation is not connected with 

 fluorescence, but is a specific and unalterable property of 



