[eve] secondary y RAYS DUE TO THE y RAYS OF RADIUM C 9 



counts to some extent for the apparent differences noted at the be- 

 ginning of this paper. 



In a previous paper (Phil. Mag., June, 1908) I have noted that 

 books, carbon, cement or bricks are able to give rise to more penetrat- 

 ing rays than lead, when these substances are exposed to the /îand y 

 rays of radium, provided the electroscope is screened from the secon- 

 dary radiator by a sufficient thickness of aluminium, or by a thick book. 

 It was shown too that these secondary rays came from a depth of at 

 least 4 cms. in the case of carbon, 6 cms. for wood, 3.5 cms. for slate. 

 Moreover, the primary rays had first to penetrate those thicknesses. It 

 is these deep seated penetrating rays that are under consideration. 

 The secondary radiators and screens were all tested, when the radium 

 was not present, and they were found free from any radioactive effects 

 under the conditions of the experiments. 



Fourteen milligrams of pure radium bromide were placed in a test 

 tube within c^dinders of (1) lead, 1 cm. thick, or (2) nickel steel, 2.2 cms. 

 thick. The radiators were 5 cms. or more thick and measured 22.5 x 

 22.5 square centimetres. The necessarily large size and thickness of 

 the radiators made it difficult to procure suitable substances for the 

 experiments, and I have worked mainly with lead, iron, brick and slate. 

 The screens placed in front of the electroscope were either of (1) 

 aluminium or (2) lead. The electroscope, 10 x 10 x 16 cms., was 

 made of zinc and mounted on a platform. One face of the electro- 

 scope was removed and replaced with two very thin aluminium sheets, 

 each .00031 cm. thick. The electroscope and screens were guarded 

 from the radium and the cylinder containing it by blocks of lead 10 

 cms. thick. The effect with no radiator present, except air, was 

 deduced from the effect with the stated radiator present; and this was 

 done for all the screens used. The secondary rays from air are easily 

 absorbed, so that the results given are quite reliable for secondary 

 rays of the y type. 



The electroscope here, as in most cases, is affected not merely by 

 the secondary y rays which traverse it, but also by all the secondary 

 rays caused by them from the sides of the electroscope and from the 

 inner side of the screens. There seems no way of evading this com- 

 plexity, and if all the conditions are identical, except the radiating 

 plates employed, the results obtained do indicate the nature and 

 magnitude of the secondary rays of the y type passing from the radia- 

 tor and entering the electroscope. 



The measurements obtained are given in the following four tables 

 and in the corresponding four diagrams. The radium bromide was in 

 the same position, as nearly as possible, for all four series. The rays 

 under investigation are all " incident," not " emergent." 



