140  Mr.  R.  K.  McClung  on  the 
As  seen  from  the  diagram,  only  a  very  small  portion  of  the 
curve  below  the  maximum  point  can  be  obtained  for  a 
thickness  of  aluminium  greater  than  eight  layers,  since  the 
maximum  point  for  a  thickness  beyond  this  occurs  so  close  to 
Fig.  6. 
60 
50 
a  n 
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UMIN 
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20 
10 
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eo 
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the  gauze.  It  is  not  practicable  to  place  the  source  of  activity 
closer  to  the  gauze  than  about  6  mm.,  as  the  hole  through 
which  the  rays  emerge  must  be  at  least  about  5  mm.  in  depth 
in  order  that  the  angle  of  the  cone  of  rays  may  not  be  too 
large. 
Comparison  of  the  Absorption  by  Aluminium  and  Air. 
From  these  curves  for  air  and  aluminium  the  absorption  of 
the  a  rays  by  these  substances  can  easily  be  compared  by 
