430 Prof. W. H. Bragg on tlie Properties and 



It appears to me to be a first deduction from such a 

 comparison that in all cases the bulk of the ionization which 

 the rays effect is of the same character, and consists in the 

 displacement of slow-moving electrons, or 8 rays, from the 

 atoms of the gas or other substance which they traverse. 

 Let us consider the various rays in turn. 



In the case of the cathode rays this principle has been 

 clearly established by Lenard in the course of his long series 

 of beautiful experiments. He has shown that cathode rays 

 of the most varied speeds, impinging on bodies of various 

 kinds, or traversing different gases, cause the liberation of 

 slow-speed electrons from the atoms of the solid or gas. The 

 speed of the electrons is in every case lhat due to the fall 

 through less than ten volts. This is in no way a contradic- 

 tion of the fact that cathode rays of high speed are also 

 liberated from a solid surface struck by primary cathode 

 rays ; or from the atoms of a gas through which the primary 

 rays pass. But, Avhether these high-speed secondary rays 

 are scattered primary rays, or are true secondary rays, they 

 must in their turn produce electrons of slow speed in the gas 

 through which they pass ; and so, directly or indirectly, by 

 primary or secondary or tertiary or rays still more trans- 

 formed, eventually the great majority of the electrons set 

 free in the ionization-chamber of ordinary experiment are of 

 the slow-speed type. 



In the case of the a rays there is abundant evidence that 

 their impact on, or emergence from, solid surfaces causes the 

 ejection of slow-speed electrons (J. J. Thomson, Cambridge 

 Phil. Soc. Trans., February 1905 ; Rutherford, < Nature/ 

 March 2, 1905 ; Logeman, Proc. Roy. Soc, September 1906). 

 Now, it is generally characteristic of all these electric radia- 

 tions that they are concerned with the individual atoms and 

 molecules, and that they do not recognize any difference 

 between the atom in the solid and the atom in the gaseous 

 condition. Consequently, there is every reason to suppose 

 that the heavy ionization caused by an a particle in traversing 

 a gas consists in the production of the same slow-speed 

 electrons as are set free from a solid, and indeed no trace of 

 faster-moving electrons has ever been found. The slow- 

 speed electrons originated by a rays have been called 8 rays, 

 and the term may be applied to all such slow-speed electrons 

 as we are now considering. 



Again, it has been shown by Fuchtbauer (Phi/s. Zeit., 

 Nov. 1, 1906) that h rays are emitted from a metal surface 

 struck by canal rays ; and here also there is every reason to 

 suppose that gas molecules struck by such rays emit the 



