728 Mr. & Mrs. Soddy and Mr. A. S. Russell on the 

 to a very high degree of accuracy under proper conditions. 

 The whole laboratory in which the measurements have been 

 carried out has been preserved scrupulously from contamin- 

 ation, and no radium, except in sealed tubes, has ever been 

 brought in. With regard to the departures at the initial part 

 of the range, it is shown that these are in opposite senses for 

 different materials and vary greatly with the disposition 

 employed. Although these departures have not yet been 

 fully cleared up it is probable that they are due to equilibrium 

 between the primary and secondary radiations, probably as 

 regards distribution in space, not having yet been attained 

 for the initial thickness of absorbing material. 



Confining attention to the 7-rays of radium and lead for 

 which most evidence is available it is difficult to escape from 

 the conclusion that the rays are homogeneous and are expo- 

 nentially absorbed, without scattering of the primary beam. 

 In metals other than lead, softening and scattering of the 

 primary 7-rays may, it is true, occur. But this does not affect 

 the important conclusion that initially the primary 7-rays are 

 probably homogeneous. Now there is considerable evidence 

 that the /3-rays of radium are not homogei eons. Even if the 

 very soft /3-rays due to radium itself and to radium B (0. Hahn 

 and L. Meitner, Phys. Zeit, 1909, x, p. 741) are left out of 

 consideration as possibly too feebly penetrating to produce a 

 detectable 7-radiation, there is evidence that the /3-radiation 

 of radium itself is also complex (compare ibid. p. 697). 

 The view that the 7-rays of radium are homogeneous thus 

 carries with it very strong support of our earlier view that 

 the /3- and 7^-rays are not interdependent, It is not advo- 

 cated that this point of view is as yet established but 

 merely that a considerable body of evidence exists in its 

 favour. We are now at work to see if Prof. Bragg's theory 

 that some metals like zinc soften and scatter the 7-rays before 

 converting them into /3-rays while others like lead trans- 

 form them into /3-rays at one step, will prove helpful in 

 unravelling some of the intricate phenomena observed. It 

 appears, however, if this is so, that zinc will be found to 

 behave to the Y^rays of uranium like lead to the 7-rays of 

 radium, 



Part I.^-Initial part of Absorption Curves of the <y~Rays 

 of Uranium X. 



The disposition employed was identical with that used in 

 Series I. of the earlier measurements (loc. cit. p. 632), all the 

 absorbing plates being clamped up to form the base of the 



