644 Mr. G. Shearer on the 



2. An expression for the critical field as a function of 

 pressure and radius of point, deduced on certain assumptions 

 by Edmunds, is shown to be in reasonable agreement with 

 the experimental values given in this and a previous paper. 



3. Further measurements of the field at a point under 

 varying conditions of discharge have been made; the effect 

 of supplying the point with ions of a given sign from an 

 external source is briefly discussed in the light of the theory 

 of collisions. 



LXVI. The Ionization of Hydrogen by X-Rays. By George 

 Shearer, ALA., JB.Sc, Vans Dunlop Scholar, University of 

 Edinburgh *. 



IN many ways hydrogen occupies a somewhat anomalous 

 position among the elements. As, from a dynamical 

 point of view, the hydrogen atom is in all probability the 

 simplest — consisting of a central positively charged nucleus 

 around which there rotates a single electron f — it might be 

 expected that a study of this gas would lead to conclusions 

 of the greatest theoretical interest. 



The work discussed in this paper was an attempt to measure 

 the ionization produced in hydrogen by X-rays, and was 

 suggested by the fact that, although many previous workers 

 had made measurements of the relative ionizations of hydro- 

 gen and air, the results obtained differed widely from one 

 another, while some of them exhibited certain features which 

 showed the advisability of a redetermination of the ratio of 

 the ionization of hydrogen to that of air. 



Historical. 



Among the earliest determinations of the relative ionization 

 of air and hydrogen are those given in Table I. 



These values were obtained before the importance of the 

 corpuscular radiation excited by X-rays was fully realized. 

 The rays in passing through the chamber set free from the 

 walls and electrodes fast moving electrons which ionize the 

 gas. On the other hand, some of the electrons emitted from 

 the gas itself become absorbed in the walls and electrodes 

 before they have contributed their full share to the ioniza- 

 tion. These two effects will result in a net gain or loss of 

 ionization depending upon the gas in the chamber. In the 



* Communicated by Prof. C. G. Barkla, F.R.S. 

 t Bohr, Phil. Mag. [6] xxvi. p. 1 (1913). 



