Absorption  of  a  Rays.  137 
Absorption  of  a  Rays  by  Aluminium. 
The  absorption  produced  by  thin  aluminium -foil  was  also 
investigated.  The  foil  used  was  about  0 '00031  cm.  in 
thickness.  The  absorption  of  different  thicknesses  of  alu- 
minium was  determined  by  measuring,  in  a  manner  similar 
to  that  for  air,  the  ionization  produced  by  the  a  rays  after 
Rff.  3. 
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passing  through  a  definite  number  of  layers  of  the  thin  foil. 
Separate  curves  corresponding  to  the  curve  for  air  were 
obtained  when  various  thicknesses  of  aluminium  were  placed 
in  the  path  of  the  rays.     These  curves  arc  shown  in  figs.  3,  I- 
