80 



the great majority of the electrons set free in the ionisation- 

 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, Cam- 

 bridge Phil. Soc, Trans., February, 1905; Rutherford, "Na- 

 ture," March 2, 1905 ; Logeman, Proc. Roy. Soc, Septem- 

 ber, 1906.) Now, it is generally characteristic of all these 

 electric radiations that they are concerned with the indi- 

 vidual atoms and molecules, and that they do not recognise 

 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 ionisation 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 elec- 

 trons as we are now considering. 



Again, it has been shown by Fuchtbauer (Phys. Zeit., 

 November 1, 1906) that 8 rays are emitted from a metal sur- 

 face struck by canal rays, and here also there is every rea- 

 son to suppose that gas molecules struck by such rays emit 

 the same 8 particles. The same author has shown by a direct 

 comparison that the velocity cf these particles is the same as 

 that of the 8 rays displaced by cathode rays, i.e., about 

 3'3 X 10^ cm. /sec, or the velocity due to about 20 volts, a 

 velocity only slightly larger than that found by Lenard. 



As regards fS and y rays, it is true that it has not 

 been definitely proved that most of the ionisation which they 

 cause is of the 8 type. But this may be inferred from well- 

 known experiments, such as those of Durack (Phil. 

 Mag., May, 1903), or McClelland (Trans. Roy. Dub. Soc, 

 February, 1906). When a pencil of /5 radiation is allowed 

 to cross an ionisation-chamber normally, and fall upon the 

 opposite wall, it gives rise to a secondary ionisation, less in 

 quantity but not much less in speed than the primary. A 

 tertiary radiation is caused by the secondary rays if they im- 

 pinge on the walls of the chamber, and there will doubtless 

 be still further derivations. But it appears that the quan- 

 tity of the derived radiations dies away much more quickly 

 than the speed. Thus the chamber is crossed and re-crossed 

 (a few times) by electrons of high speed, able to traverse an 

 average path of about 100 cm. in air at atmospheric pressure. 

 If the chamber is first exhausted and air gradually admitted, 

 it is found that the number of ions produced by the /3 rays is 

 proportional to the pressure. The paths of the [3 rays will not 



