Different Types of Beta Rays. 151 



inquires why this effect did not appear in the experiments 

 when tin and lead were used as coverings for the openings 

 into the chamber, and also when a thickness o£ 0'0784 mm. of 

 tinfoil was placed below the aluminium cover. The probable 

 explanation is that the transmitted secondary rays from tin 

 and lead are not so effective ionizing agents or so good exciters 

 of tertiary rays as the secondary rays from aluminium. The 

 effect even in the case of aluminium is small, although quite 

 noticeable ; and it is probable, therefore, with the weaker 

 secondary rays from the tin and lead, that the effect would be 

 very much less and consequently masked by the other in- 

 fluences present. 



The experiments which have just been described are also 

 interesting for the light which they throw on the nature of 

 the transmitted secondary radiation excited in the metals 

 aluminium, tin, and lead by y rays. According to the 

 argument which has been presented, it follows from Bragg's 

 conclusions, since the secondary rays from aluminium are 

 better ionizers than those from tin and lead, that the particles 

 constituting these secondary rays must be endowed with 

 smaller velocities than those constituting the secondary 

 radiation from the other two metals. The transmitted y 

 excited secondary rays from aluminium should therefore, 

 from this point of view, be more easily absorbed than those 

 emitted by tin and lead. 



This conclusion regarding the character of the transmitted 

 secondary radiation excited in aluminium by y rays is in 

 accord with the conclusions of McClelland *, Starke f, and 

 others, who have found an exceptionally high coefficient of 

 absorption for the reflected secondary rays excited by /3 and 

 7 rays in this metal. 



VII. A Comparison of the Secondary Radiations 



EXCITED IN DIFFERENT METALS BY /3 EAYS. 



Some conclusions of interest can also be drawn from 

 the results of the experiments of the present investigation 

 regarding the secondary rays excited in different metals 

 by /3 rays. For the purpose of making a comparison, the 

 thicknesses of the limiting absorbing layers of the three 

 metals studied with both reflected and transmitted rays are 



* McClelland, Trans. Eov. Dublin Soc. viii. p. 1G9 (1905). 

 t Starke, Le Radium, Feb. 1908. 



