Different Types of Beta Rays. 153 



intersect in the figure, that they do not typify the same 

 absorption law. It will be seen from the curve B that the 

 maximum penetrabilities of the secondary /3 rays, as deter- 

 mined by the reflexion experiments, approach very closely 

 to a linear relation which exhibits in a striking manner the 

 important result that secondary rays excited in plates of 

 different metals when j3 rays are allowed to fall on them are 

 the more penetrative the greater the density of the metal of 

 which the reflector is made. 



VIII. Summary of Results. 



(1) The /3 radiation from radium bromide, which includes 

 the /5 radiations from all the radium products in the equi- 

 librium state, will not produce any ionization on the far side 

 of a plate of aluminium 7 mms. in thickness, of a plate of 

 lin 2*5 mms. in thickness, or of a plate of lead *9 mm. in 

 thickness. 



(2) The maximum secondary radiation emitted from the 

 front side of plates of the metals aluminium, tin, and lead, 

 when bombarded by /3 rays, are given by the following- 

 thicknesses : — 



Aluminium O'l mm. 



Tin 0-24 „ 



Lead O'lG „ 



(3) The transmitted secondary radiations excited by y rays 

 in lead and tin are more penetrating than the transmitted 

 secondary radiation excited in aluminium by the same rays. 



(4) When /3 rays are allowed to fall in turn on reflectors 

 of different metals, it is found that the greater the density of 

 the metal from which the reflector is made the greater is the 

 penetrability of the reflected secondary rays excited by the 

 J3 radiation. 



(5) From the experiments on the transmission of (3 rays 

 through sheets of aluminium-foil, it has been shown that when 

 very thin sheets of the metal are used the ionization at first 

 contributed by the transmitted secondary radiation excited by 

 the /3 rays is greater than that lost through absorption of the 

 primary rays. 



In conclusion, I wish to express my best thanks to Prof. 

 McLennan, at whose suggestion the investigation was under- 

 taken, for his help and advice and unfailing kindness 

 throughout the course of the research. 



