440 Dr. J. P. V. Madsen on 



has broken the original 7 ray up, immediately sending out a 

 ft ray, or has so shattered the original y ray as to make it 

 easy for Pb to subsequently complete the process. 



In the case of Zn, &c, the effect of the collision has not 

 been so definite ; much scattered y radiation is produced, 

 somewhat softened by the effects of collision, but still able 

 to suffer further modification as a 7 ray before being even- 

 tually broken up and sending out the ft ray. 



It must be noticed that in experiments upon the secondary 

 incidence 7 rays, such as have been carried out by Kleeman 

 and by Eve, the nature of the secondary effect may be 

 modified to a very considerable extent if the Ra be covered 

 directly by a considerable mass of a substance such as Zn. 

 Where a similar screen of lead would appear to have 

 hardened the primary beam which is being experimented 

 with, the screen of Zn may very well appear in some cases 

 to have softened the primary beam, more especially if the 

 secondary effects produced by hard and by soft rays do not 

 conform to the same law of distribution. 



It may be observed that the value of X/A obtained in the 

 present investigation for the hard 7 rays from Ra is *028, 

 whereas the mean value obtained by Wigger was '021. The 

 difference probably arises from the fact that Wigger appears 

 to have taken the natural ionization in the chamber as the 

 amount to be subtracted from all the readings. It seems 

 quite possible that a considerable amount of secondary radia- 

 tion from surrounding bodies may have been able to enter 

 the chamber, in which case the correction should have been 

 greater, and this applied to all the readings would increase 

 the value of A/A which he obtained. 



Again, since the hard 7 rays produce a considerable 

 amount of secondary radiation in Pb, it is not surprising 

 that with a long, narrow chamber, such as Wigger used, 

 the shape of the absorption curve for Pb differs from that 

 obtained in the present experiments. 



We have now sufficient information to enable us to con- 

 struct at least a working theory. The main points to be 

 observed are, firstly, the lack of symmetry in the amounts, 

 •and secondly, a lack of symmetry in some cases in the 

 quality of the secondary 7 radiation. 



The asymmetry shown by the secondary emergence and 

 incidence ft rays, which are produced from the primary 7 

 rays of Ra, has already been put forward as an argument 

 in favour of the "material'" theory propounded by Professor 

 Bragg. 



The modification of the aether-pulse theory, recently 

 advanced by Professor Sir J. J. Thomson, may possibly 



