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L XXXIV. Ionization by Charged Particles. 

 By Norman ( "ampbell, Sc.D* 



1. fTlHE following numbers denote papers which will be 

 JL frequently quoted : — 



(1) Lenard, Ann. d. Phys. v. p. 149 (1902). 



(2) „ „ „ xii. p. 449 (1903). 



(3) Fiichtbauer, Phys. Zeit. vii. p. 748 (1906). 



(4) v. Baeyer, „ „ x. p. 176 (1909). 



(5) Gehrts, Ann. d. Phys. xxxvi. p. 995 (1911). 



(6) Kossel, „ „ xxxvii. p. 393 (1912). 



(7) Bloch, „ „ xxxviii. p. 559 (1912). 



(8) Bumstead, Phil. Mag. xxii. p. 907 (1911). 



(9) „ and McGougan, Phil. Mag. xxiv. p. 462 (1912). 



(10) Campbell, Phil. Mag. xxii. p. 276 (1911). 



(11) „ „ „ xxiv. p. 527 (1912). 



(12) „ „ „ xxiv. p. 783 (1912). 



(13) J. J. Thomson, Phil. Mag. xxiii. p. 449 (1912). 



(14) Bohr, Phil. Mag. xxv. p. 10 (1913). 



(15) Ramsauer, Jahrb. d. Radioakt. ix. p. 515 (1912). 



2. When a rays, positive rays, or ft rays fall on a metal 

 plate they cause the emission of electrons moving with 

 speeds of less than 30 volts. These electrons have been 

 termed 8 rays when they have been produced by a rays, 

 and secondary cathode rays when they have been produceoV 

 by positive or ft rays ; in this paper they will always be 

 termed 8 rays. It is generally believed that similar 

 secondary rays are emitted from the molecules of a gas 

 acted on by the same primary rays, and that the mechanism 

 of the liberation of the delta rays is precisely the same as 

 that of the ionization of a gas. The evidence for this belief 

 is based on the experiments of (1), (2), (4), (5), (6), (7), (8), 

 (9), which show that the power of a and ft rays to excite 

 8 rays varies in the same way as their power to cause 

 ionization in a gas. 



Bumstead and McGougan (9) have produced evidence to 

 show that the a rays excite also a secondary ft radiation 

 with velocities from 40 to 2000 volts or more, and suggest 

 that the $ rays are liberated by the action of these secondary 

 rays and not by the a rays themselves. The matter will be 

 discussed further in § 11 3 but most of the considerations here 

 presented are independent of the correctness of this view. 



3. The experiments of Fiichtbauer have shown that the 

 velocity with which the 8 rays are emitted is the same, 

 whether they are excited by positive rays or by cathode rays, 

 and whatever is the speed of those rays, at least if that speed 



* Communicated bv the Author, 



