Secondary Radiation from Gases subject to X-Rays. 685 



Prof. Rutherford's experiments* show that the a radiation 

 of thorium and radium differ somewhat from that of uranium, 

 being more penetrating. In all probability the radiations 

 from the tinroil, and perhaps from the zinc also, are o£ the 

 same general nature as the a radiations from uranium, 

 thorium, and radium, though differing somewhat in quality. 



LXXY. Secondary Radiation from Gases subject to X-Rays. 

 By Charles Gr. Barkla, 'M.Sc. (Vict. )/B. A. (Cantab.), 

 King's College, Cambridge; Oliver Lodge Fellow, University 

 College, Liverjioolf. 



RADIATION from air through which X-radiation was 

 passing was first noticed by Rontgen J, through the 

 effect produced on a photographic plate which was screened 

 from the direct radiation. 



Sagnac§, who studied secondary radiation from metals 

 subject to X-rays, also obtained a much smaller effect from 

 air, and concluded it was more easily absorbed than the 

 primary radiation producing it. The following is an account 

 of a more complete investigation of the subject of secondary 

 radiation from gases subject to Rontgen rays. 



An X-ray bulb and exciting induction-coil were placed 

 inside a large wooden box which was completely covered 

 with thick sheet lead. A small rectangular aperture in 

 the side of the box near the bulb permitted the radiation to 

 pass in a definitely bounded beam through air or any other 

 gas outside the box. 



In the first experiments no attempt was made to inclose 

 the air through which the radiation was directed, so that 



* Phil. Mag. July 1902, p. 11. Every one must recognize the merit 

 of Prof. Rutherford's most valuable experiments on the absorption of 

 Becquerel rays by air. But I must confess I cannot follow the reasoning 

 by which he seeks to deduce from them the coefficient of absorption. His 

 calculation starts on the assumption that if I be the intensity of radia- 

 tion close to an infinite radioactive surface, the intensity at a distance x 

 is Ie— te, where X is the coefficient of absorption. No donbt if the radia- 

 tion were emitted wholly in a direction normal to the surface this wonld 

 be so. But, as a matter of fact, an element of area of the surface emits 

 much of its radiation in an oblique direction. This obliquely emitted 

 radiation has to pass through a greater thickness of air before it has 

 reached the distance x from the infinite radioactive plane. Thus the re- 

 lation between intensity and distance becomes greatly complicated. It 

 is impossible to calculate it without knowing the relation between the 

 intensity of radiation from an element of .area, and the angle between that 

 element and the direction of propagation. 



t Communicated by Prof. L. R. Wilberforce. 



X Annul. Phi/s. Chem. lxiv. 1, pp. 18-37 (1898). 



§ Comptes Rendus, exxvi. pp. 5*21-523 (1898). 



