Absorption  of  x  Rays.  135 
thus  showing,  as  Bragg  has  pointed  out,  that  as  the  a. 
particle  moves  farther  from  the  source  within  a  certain  limit 
it  becomes  abetter  ionizer,  although  the  velocity  has  become 
less  after  passing  through  a  certain  thickness  of  air. 
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After  the  maximum  point  is  reached  and  the  distance 
increased,  the  ionization  falls  off  extremely  rapidly  for  a 
small  increase  in  the  distance  until  a  distance  of  about 
6" 8  cms.  is  reached,  when  the  ionization  ceases  altogether. 
The  part  of  the  curve  which  rises  rapidly  from  this  point 
is  due  almost  entirely  to  the  natural  leak  of  the  vessel  and 
the  0  and  7  ray  effects  which  are  present. 
The  range  in  air  over  which  the  a.  ray  is  capable  of  pro- 
ducing ionization  is  therefore  about  6* 8  cms.  This  agrees 
very  well  with  the  result  obtained  by  Bragg  and  Kleeman. 
who  obtained  a  distance  of  6*7  cms. 
