185 



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 hard- 

 ened 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 A/ A obtained in 

 the present investigation for the hard y 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 radi- 

 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 y 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 tTiat ob- 

 tained 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 ob- 

 served are, firstly, the lack of symmetry in the amounts, and, 

 secondly, a lack of symmetry in some cases in the quality of 

 the secondary y radiation. 



The asymmetry shown by the secondary emergence and 

 incidence j3 rays, which are produced from the primary y 

 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 ether-pulse theory, recently ad- 

 vanced by Professor J. J. Thomson, may possibly furnish a 

 partial explanation of the facts : but there are many diffi- 

 culties in the way of a pulse theory, even modified to this 

 extent : some of these have been pointed out in a recent paper 

 by Professor Bragg and myself CTrans. Roy. Soc, S. Aus., 

 V. xxxii., 1908). 



When one attempts to further explain by the pulse theory 

 the lack of symmetry which exists in the case of the secor.d- 

 ary y rays, the difficulties become much greater. 



If we are to suppose that the incidence and emergence 

 y rays are true secondary effects produced by vibrations of 

 electrons in the absorbing material, the effect resembles very 



