Mr Glasson, Secondary Rontgen Rays from Metallic Salts. 437 



Secondary Rontgen Rays from Metallic Salts. By J. L. 

 Glasson, 1851 Exhibition Scholar of Adelaide University, Gon- 

 ville and Caius College. (Communicated by Prof. Sir J. J. 

 Thomson.) 



[Read 14 March 1910.] 



It has been shown by Barkla that when an element of atomic 

 weight greater than that of Calcium is struck by a primary 

 X-ray beam of suitable hardness, then the secondary X-rays 

 emitted by it form a homogeneous beam characteristic of the 

 radiating element. 



The object of the present experiments was to determine 

 whether the nature of this characteristic secondary X-radiation 

 is affected by the state of chemical combination of the element. 



Barkla made some experiments to test this point (Phil. Mag. 

 June 1906), but that was before the conception of "character- 

 istic homogeneous radiation" was introduced. The enormous 

 simplification brought about by its introduction, makes further 

 experiments interesting and their interpretation simple. 



In particular it was desired to investigate the effect of a 

 change in the valency of the metal upon the characteristic 

 radiation from it. 



II. 



The arrangement of the apparatus is shown in figure 1. The 

 metal or salt to be examined was placed at A, the salt being 

 contained in a thin paper tray about 1 cm. deep. B and G are 

 two thick lead screens, 20 cm. apart with apertures 2 cm. square 

 cut in them. The rays enter the ionisation chamber D by a 

 window of aluminium leaf W, and the ionisation produced is 

 measured by a Wilson electroscope and a stop watch. No balance 

 chamber was used as it was found that the coil, fitted with a 

 hammer break, gave a very constant discharge; the variations 

 in a series of consecutive readings amounted to about two or 

 three per cent. 



The screens for absorbing the rays were placed at B. They 

 were of aluminium when an absolute determination of the ab- 

 sorption coefficient was required; but in some cases filter papers 

 were used when only comparative measurements were required. 



The absorption curve was determined in the same way as that 

 used by Barkla in his work on homogeneous X-rays. The per- 

 centage absorption caused by a given screen (II) was measured, 

 after different proportions of the beam had laeen previously 



