﻿Alpha 
  Rays 
  by 
  Metals 
  and 
  Gases, 
  619 
  

  

  In 
  conclusion, 
  I 
  wish 
  to 
  express 
  my 
  gratitude 
  to 
  Professor 
  

   Bumstead, 
  at 
  whose 
  suggestion 
  these 
  experiments 
  were 
  under- 
  

   taken, 
  for 
  his 
  valuable 
  suggestions 
  and 
  interest 
  in 
  the 
  work 
  ; 
  

   also 
  to 
  Professor 
  Bolt 
  wood, 
  who 
  kindly 
  prepared 
  the 
  polonium 
  

   and 
  secured 
  the 
  radium 
  bromide 
  for 
  me, 
  and 
  gave 
  me 
  many 
  

   valuable 
  suo-o-estions. 
  

  

  Summary 
  of 
  Results, 
  

  

  1. 
  The 
  air-equivalents 
  of 
  metal 
  foils 
  decrease 
  with 
  the 
  

   speed 
  of 
  the 
  alpha-particles 
  entering 
  them. 
  The 
  decrease 
  is 
  

   very 
  small 
  for 
  thin 
  foils 
  of 
  the 
  lighter 
  metals 
  when 
  the 
  speed 
  

   of 
  the 
  alpha-particle 
  is 
  high 
  ; 
  but 
  when 
  the 
  speed 
  is 
  low 
  for 
  

   thin 
  sheets, 
  or 
  when 
  the 
  sheets 
  are 
  thicker, 
  the 
  change 
  

   becomes 
  more 
  marked. 
  For 
  different 
  sheets 
  of 
  the 
  same 
  

   metal 
  the 
  rates 
  of 
  change 
  are 
  proportional 
  to 
  the 
  air- 
  

   equivalents 
  of 
  the 
  sheets. 
  For 
  sheets 
  of 
  different 
  metals 
  

   of 
  equal 
  air-equivalents 
  the 
  rates 
  of 
  change 
  are 
  approximately 
  

   proportional 
  to 
  the 
  square 
  roots 
  of 
  the 
  respective 
  atomic 
  

   weights. 
  

  

  2. 
  The 
  air-equivalents 
  of 
  hydrogen 
  cells 
  or 
  sheets 
  increase 
  

   as 
  the 
  speed 
  of 
  the 
  entering 
  particle 
  decreases, 
  while 
  the 
  air- 
  

   equivalents 
  of 
  sheets 
  of 
  paper 
  and 
  celloidin 
  remain 
  constant. 
  

  

  3. 
  The 
  hydrogen-equivalents 
  of 
  sheets 
  of 
  paper, 
  films 
  of 
  

   celloidin, 
  and 
  air 
  do 
  not 
  remain 
  constant, 
  but 
  decrease 
  as 
  the 
  

   speed 
  of 
  the 
  alpha-particle 
  decreases. 
  The 
  rate 
  at 
  which 
  

   the 
  hydrogen-equivalent 
  of 
  a 
  celloidin 
  film 
  decreases 
  with 
  the 
  

   speed 
  of 
  the 
  entering 
  alpha-particle 
  is 
  numerically 
  equal 
  to 
  

   the 
  rate 
  at 
  which 
  the 
  air-equivalent 
  of 
  a 
  hydrogen 
  sheet 
  of 
  

   corresponding 
  thickness 
  increases. 
  

  

  4. 
  The 
  result 
  obtained 
  by 
  Bragg, 
  that 
  the 
  total 
  ionization 
  

   produced 
  by 
  the 
  alpha-particle 
  in 
  air 
  is 
  the 
  same 
  as 
  that 
  in 
  

   hydrogen, 
  is 
  confirmed 
  by 
  a 
  more 
  direct 
  method. 
  

  

  5. 
  It 
  is 
  very 
  probable 
  that 
  for 
  the 
  high 
  ranges 
  the 
  alpha- 
  

   particle 
  loses 
  its 
  energy, 
  in 
  passing 
  through 
  substances, 
  

   more 
  rapidly 
  the 
  higher 
  the 
  atomic 
  weight 
  of 
  the 
  substance 
  ; 
  

   but 
  that 
  this 
  difference 
  decreases 
  slowly, 
  until 
  in 
  the 
  low 
  

   ranges 
  the 
  loss 
  ot" 
  energy 
  is 
  the 
  more 
  rapid 
  the 
  lower 
  the 
  

   atomic 
  weight 
  of 
  the 
  substance. 
  

  

  6. 
  A 
  comparison 
  of 
  the 
  Bragg 
  curves 
  for 
  air 
  and 
  hydrogen 
  

   indicates 
  that 
  the 
  large 
  ionization 
  at 
  low 
  ranges 
  (knee 
  of 
  the 
  

   curve) 
  is 
  due 
  at 
  least 
  in 
  part 
  to 
  the 
  fact 
  that 
  the 
  particle 
  loses 
  

   its 
  energy 
  more 
  rapidly 
  in 
  this 
  part 
  of 
  the 
  range, 
  and 
  not 
  

   wholly 
  to 
  the 
  higher 
  ionizing 
  efficiency 
  of 
  particles 
  of 
  low 
  

   speed. 
  

  

  Sloane 
  Laboratory, 
  

  

  Yale 
  University, 
  

  

  New 
  Haven, 
  Conn., 
  U.S.A. 
  

   2T2 
  

  

  