﻿594 
  Mr. 
  A. 
  B. 
  Wood 
  on 
  the 
  Range 
  of 
  the 
  

  

  difference 
  between 
  these 
  two 
  curves 
  is 
  shown 
  in 
  curve 
  B. 
  

   This 
  curve 
  gives 
  the 
  variation 
  of 
  ionization 
  produced 
  by 
  

   recoil 
  atoms 
  at 
  different 
  pressures. 
  

  

  Assuming 
  that 
  ionization 
  is 
  independent 
  o£ 
  velocity, 
  the 
  

   ionization 
  in 
  the 
  chamber 
  due 
  to 
  each 
  recoil 
  atom 
  which 
  

   passes 
  through 
  it 
  is 
  proportional 
  to 
  the 
  number 
  o£ 
  collisions 
  

   it 
  makes 
  with 
  air 
  molecules 
  in 
  the 
  chamber. 
  Hence 
  the 
  

   ionization 
  per 
  recoil 
  atom 
  is 
  proportional 
  to 
  the 
  pressure 
  

   within 
  the 
  vessel. 
  But 
  we 
  have 
  seen 
  that 
  the 
  number 
  N 
  of 
  

   recoil 
  atoms 
  reaching 
  the 
  chamber 
  decreases 
  with 
  increase 
  

   of 
  pressure 
  owing 
  to 
  absorption 
  in 
  the 
  gas 
  between 
  the 
  source 
  

   and 
  the 
  chamber. 
  Hence 
  

  

  N 
  = 
  %hb>, 
  

  

  where 
  N 
  is 
  the 
  number 
  of 
  recoil 
  atoms 
  leaving 
  the 
  source 
  

   and 
  ty(p) 
  is 
  the 
  fraction 
  absorbed 
  in 
  the 
  gas. 
  

  

  Therefore 
  the 
  ionization 
  I 
  at 
  any 
  pressure 
  p 
  is 
  given 
  by 
  

  

  I 
  <x 
  N 
  p 
  f(p), 
  

   i. 
  e. 
  

  

  a 
  relation 
  giving 
  the 
  form 
  of 
  the 
  absorption 
  curve. 
  Conse- 
  

   quently, 
  if 
  we 
  plot 
  the 
  relation 
  between 
  -£ 
  and 
  p, 
  we 
  shall 
  

  

  obtain 
  a 
  curve 
  giving 
  us 
  the 
  form 
  of 
  the 
  function 
  yfr(p). 
  

  

  Fig. 
  4, 
  curve 
  C, 
  shows 
  the 
  result 
  of 
  plotting 
  the 
  relation 
  

   between 
  " 
  ionization 
  per 
  mm. 
  pressure 
  " 
  and 
  " 
  pressure." 
  

   The 
  general 
  form 
  of 
  the 
  curve 
  was 
  the 
  same 
  in 
  all 
  experi- 
  

   ments. 
  It 
  will 
  be 
  noticed 
  that 
  all 
  the 
  recoil 
  atoms 
  are 
  

   prevented 
  from 
  reaching 
  the 
  ionization 
  chamber 
  at 
  a 
  pressure 
  

   of 
  about 
  5 
  mm. 
  The 
  mean 
  value 
  of 
  this 
  pressure 
  was 
  found 
  

   to 
  be 
  49 
  mm., 
  corresponding 
  to 
  a 
  range 
  of 
  recoil 
  of 
  '175 
  mm. 
  

   in 
  air 
  at 
  atmospheric 
  pressure. 
  The 
  form 
  of 
  the 
  absorption 
  

   curve 
  is 
  rather 
  peculiar. 
  The 
  ionization 
  per 
  mm. 
  pressure 
  

   falls 
  rapidly 
  for 
  the 
  first 
  few 
  millimetres 
  pressure, 
  then 
  more 
  

   slowly 
  and 
  again 
  more 
  rapidly 
  with 
  further 
  increase 
  of 
  

   pressure. 
  The 
  curve 
  is 
  unlike 
  that 
  obtained 
  by 
  the 
  first 
  

   method 
  of 
  determining 
  the 
  range 
  — 
  this 
  point 
  will 
  be 
  referred 
  

   to 
  later. 
  

  

  Recoil 
  from 
  Thorium 
  Active 
  Deposit 
  in 
  Hydrogen. 
  

  

  In 
  these 
  experiments 
  the 
  hydrogen 
  was 
  prepared 
  electro- 
  

   lytically, 
  washed 
  by 
  bubbling 
  through 
  concentrated 
  sulphuric 
  

   acid, 
  and 
  finally 
  dried 
  by 
  passing 
  over 
  bulbs 
  containing 
  

  

  