﻿Ionization by Positive Rays, 783 



If we take the mean observed value of the first line 

 6708*12, which corresponds to the oscillation-frequency 

 14903*26 in vacuo, and, applying the Bydberg-Schuster 

 rule, add 28579*71, Hicks's mean value for the convergence- 

 frequency of the subordinate series, we obtain 43482*97 

 for the convergence-frequency of the principal series. 



LXXXII. Ionization by Positive Rays. 

 By Norman Campbell*. 



1. rf^HE chief facts concerning the ionization of gases by 

 JL electrons seem to be now thoroughly established, but 

 comparatively little is known concerning ionization by the 

 impact of atoms except when the atoms are those constituting 

 a-rays. The first direct attack on this problem is described 

 in the recent papers of v. Bahr and Franck f and of 

 Pawlow t, who come to the surprising conclusion that the 

 positively charged particles from hot platinum and hot 

 phosphates can ionize gases when their energy is less than 

 that required by electrons. This result is of the greatest 

 importance for theories of the structure of the atom, for it 

 would seem to indicate that the " ionization potential " for 

 electrons has not the fundamental significance which has 

 often been attributed to it ; it appears that farther experi- 

 ments, especially on the variation of the ionizing power of 

 positive particles with their speed, energy or charge, are 

 desirable. 



It is not easy to conduct such quantitative experiments 

 on a gas, into which rays of such low speed as are here con- 

 sidered have practically no power of penetration. But recent 

 work has tended to show § that the liberation of electrons T 

 which takes place when ionizing rays of any kind fall on a 

 metal surface, varies with the properties of those rays in 

 precisely the same manner as the ionization which the rays 

 cause in a gas. Indeed it is highly probable that, when the 

 surface of the metal has been recently polished, the electrons 

 are actually liberated from a layer of gas on the surface of 

 the metal. Experiments on such metal surfaces are free 

 from many of the difficulties which attend experiments on 

 ionization in a gas which has any appreciable volume. 



* Communicated by the Author. 



t E. v. Bahr and J. Franck, Deutsch. P/iys. Gesell. Vtrh. xvi. 1. p. 57 

 (1914). 



X W. Pawlow, Roy. Soc. Proc. A. vol. xc. p. S9S (1914). 



§ See, e.g., N". Campbell, Phil. Mag. June 1918, p. 803, and March 

 1915, p. 3G9. 



