922 Mr. D. C. It. Florancc 



on 



examined closely the radiations from various metals, and from 

 his results concluded that the primary and secondary 7-rays 

 could be divided into several homogeneous groups. Madsen*, 

 by a study of the tw emergent " radiation, i. e. the radiation 

 emitted from a screen in the direction of the primary <y rays, 

 divided the primary radiation into two homogeneous groups. 

 One was "hard" or very penetrating, and the other a soft 

 group or one easily absorbed. He has shown that there 

 existsa marked lack of symmetry in the quantity of secondary 

 radiation emitted from i lie two sides of the plate, and in some 

 cases a considerable difference in the penetrating power of 

 the radiation. He considers that tins secondary radiation is 

 derived Erom the primary by a scattering process. 



The question of the distribution and character of the 

 secondary 7-rays is very complicated, and although a large 

 amount of work has been done, many points still remain to 

 !>»' settled. It is of great importance, for example, to settle 

 whether the secondary radiation of the 7-ray type is merely 

 pari of the primary rays which have been scattered in their 

 pa<-;iev through matter, or is a true secondary radiation 

 excited by the passage of the 7-rays through matter. In the 

 latter case, it i^ to be expected that the secondary 7-ravs 

 would differ in quality from the primary. One of the main 

 difficulties of the subject is the apparent complexity of the 

 primary 7-rays, to which attention lias been drawn by Eve, 

 K Iceman, and Madsen. Soddy has found under special con- 

 ditions that the 7-rays from radium are absorbed according 

 to an exponential law, and has concluded consequently that 

 the radiations are homogeneous. This view is, however, 

 difficult to reconcile with the evidence obtained by a study 

 of the secondary 7-rays f. 



The following experiments were undertaken to see if any 

 conclusive evidence could be obtained to settle between these 

 hypotheses. The results given in this paper extend and 

 somewhat modify those obtained by Madsen. The 7-rays 

 appear to he entirely heterogeneous, and the terms "hard" 

 and "soft" can only be used for convenience and not to 

 denote two distinct groups of homogeneous rays. 



The paper consists of two parts: — 



I. A short investigation of the initial absorption of 7-rays 

 by lead under ordinary experimental conditions. 



* Phil. Mag. March 1909. 



t Note recent letters: Kleeman, Phil. Mag. July 1910; Soddy, PHI. 

 Majr. August 1910. 



