Taylor — Retardation of Alpha Rays by Metals. 359 



However, the difference between the ionization in the two 

 cases was a constant value for all positions of the metal sheets 

 above the source of rays. This difference would not be a 

 constant quantity if the scattering of the rays was the occasion 

 of the increase in the ionization produced by moving the metal 

 sheets away from the source of rays. On the contrary, the 

 difference between the ionizations with and without the 

 diaphragm limiting the geometrical beam of rays would be 

 greater when the sheet is far away from the source of rays 

 than when it is near the source of rays if scattering of the rays 

 by the foils was the cause of the increase in the ionization. 

 The fact that the ionization was greater with the diaphragm 

 open than when it just limited the cone of rays signifies that 

 more alpha particles get into the ionization chamber in the 

 former than in the latter case, and therefore confirms the exist- 

 ence of scattering of the rays by metal foils as found by Geiger.* 

 These two methods of investigation, although in the case of 

 the latter showing the existence of the scattering • of the rays, 

 seem to be sufficient to preclude scattering as an explanation 

 for the so-called decrease in the air-equivalents of the metal 

 sheets as they are moved away from the polonium. By 

 measuring the ionizations with and without the diaphragm 

 limiting the cone of rays when there was not a metal sheet 

 over the source of rays, it was found that the ionization was 

 greater in the latter than in the former case, which shows that 

 the rays are scattered by air as well as by metals. These 

 methods, however, are not particularly suitable for measuring 

 the amount of the scattering, and hence no comparison as to 

 how much each metal scatters the rays was attempted. The 

 important fact is that the effect under consideration is not 

 influenced by the scattering of the rays. 



Continuation of Experiments. 



In the first experiments polonium had been used as the 

 source of rays, but in order to extend the study to alpha par- 

 ticles of higher range, radium C has been used in the present 

 experiments. This made it possible to use foils of greater 

 thickness than had been previously used. A thin aluminium 

 foil covered with a thin coating of lacquer was put directly 

 over a capsule containing a thin film of pure radium bromide 

 in order to prevent escape of the emanation. The hole in the 

 brass plug over the radium bromide was of such dimensions 

 that the cone of rays emerging from it fell well within the 

 limits of the ionization chamber. The radium bromide was 

 set at such a distance from the ionization chamber that a part 



*Proceedings of the Royal Society, Series A, vol. lxxxi, No. 546, page 

 J 74. 



