Reflexion of the (3-Particles by Matter. 



85a 



large to more than compensate for the loss suffered by the 

 normal rays due to absorption, reflexion, and slight side 

 scattering. 



When a thick layer of radioactive material was used, e. g. 

 uranium oxide, the effect described was not observed owing 

 to the fact that complete scattering took place within the 

 material itself except for the uppermost layer. The radiation 

 from the latter was relatively too weak to produce an 

 observable effect. 



It is clear, therefore, that scattering may produce a rise 

 in the initial portion of absorption curves as well as a steep 

 incline, or no observable effect, depending entirely on the 

 experimental arrangements. In order to obviate the effect 

 of the scattering of the /3-particles on the initial portion of 

 the absorption curve, the absorbing screens should be placed 

 directly against the opening of the electroscope, in which 

 case none of the scattered /3-particles will be lost. 



Apparatus. 



In the ordinary cylindrical or rectangular shaped ionization 

 vessels, the /3-particles do not have equal paths, and conse- 

 quently do not produce equal numbers of ions within the 

 vessel. For this reason it was decided to use a hemispherical 

 ionization vessel in the following investigation. A copper 

 hemisphere, 30 cms. in diameter, was used for this purpose, 

 and an appropriate electrode and electroscope were constructed 

 as shown in fig. 2. The bottom of the ionization vessel was 



removable. The active material was in all cases deposited as 

 a thin layer on a thin aluminium foil (0*00025 cm.), and 



