Reflexion of the (3-Particles by Matter. 



865 



some reason gives slightly higher value than Pb) than the 

 substances o£ lower atomic weight. 



Experiments were next performed by using various radio- 

 active substances as the sources of /3-rays of different velo- 

 cities but using lead in all cases as the reflector. The results 

 are given in Table VIII. 



Table VIII. 



Ionization due to multiple reflexion of /3-particles of 

 various penetrating powers. 



KaE. 



ActO. 



ThA+D. 



EaB+C. 



EaO. 



ThA=ThD. 



ThD = 90perct. 



2360 



2500 



74-0 



225-0 



2430 



2020 



The numbers in the various columns give the percentages 

 of ionization due to multiple reflexion alone. The multiple 

 reflexion increased in value for the more penetrating rays, 

 but apparently reaches a maximum and then begins to 

 decrease again. This agrees with the previous observations. 



Conclusions. 



(1) Scattering of the /3-particles may produce a rise in the 

 initial portion of the absorption curve under certain con- 

 ditions, and a steep incline under other conditions. The 

 effect of this scattering appears to be a function of the 

 velocity. 



(2) In measurements of absorption the absorbing material 

 should be placed directly against the opening of the electro- 

 scope in order to avoid errors due to scattering. 



(3) The coefficient of absorption of the /3-particles from 

 thin layers varies with the substance underneath the active 

 material, being greater for elements of greater atomic 

 weight. 



(4) To avoid the steep incline in the initial portion of 

 the absorption curve caused by reflexion of the /3-particles 

 by the absorbing layers, it is necessary to place underneath 

 the thin active layer as many absorbing sheets as above. 



(5) The percentage of reflexion, measured by the ionization 

 method, of the /3-particles from thin active layers by any 

 one substance is a function of the velocity, and for rays whose 



