﻿on 
  Columnar 
  Ionization. 
  523 
  

  

  In 
  fig. 
  5 
  is 
  also 
  given 
  a 
  curve 
  obtained 
  by 
  the 
  direct 
  

   measurement 
  of 
  the 
  ionization 
  current 
  in 
  the 
  longitudinal 
  

   field 
  when 
  the 
  gas 
  in 
  the 
  vessel 
  was 
  C0 
  2 
  (carefully 
  dried) 
  at 
  

   a 
  pressure 
  of 
  750 
  mm. 
  of 
  mercury, 
  the 
  polonium 
  being 
  at 
  a 
  

   distance 
  of 
  1'75 
  cm. 
  from 
  the 
  centre 
  of 
  the 
  ionization 
  vessel. 
  

   This 
  curve 
  represents 
  very 
  accurately 
  the 
  limiting 
  curve 
  for 
  

   C0 
  2 
  , 
  corresponding 
  to 
  the 
  ionization 
  by 
  a 
  single 
  alpha 
  par- 
  

   ticle. 
  It 
  seemed 
  unnecessary 
  to 
  obtain 
  the 
  limiting 
  points 
  

   in 
  the 
  manner 
  employed 
  in 
  the 
  case 
  of 
  air, 
  inasmuch 
  as 
  the 
  

   shape 
  of 
  the 
  curve 
  for 
  C0 
  2 
  is 
  determined 
  almost 
  entirely 
  by 
  

   values 
  of 
  the 
  electric 
  field 
  which 
  are 
  easily 
  able 
  to 
  prevent 
  

   volume 
  recombination. 
  This 
  curve 
  was 
  also 
  in 
  good 
  agree- 
  

   ment 
  with 
  that 
  obtained 
  by 
  Moulin. 
  The 
  saturation 
  current 
  

   for 
  C0 
  2 
  was 
  not 
  obtainable 
  experimentally; 
  its 
  value 
  was 
  

   calculated 
  by 
  means 
  of 
  a 
  formula 
  given 
  by 
  Langevin, 
  as 
  is 
  

   indicated 
  in 
  the 
  following 
  section. 
  

  

  Theoretical 
  Considerations. 
  

  

  5. 
  According 
  to 
  the 
  view 
  brought 
  forward 
  by 
  Moulin 
  * 
  

   the 
  saturation 
  current 
  is 
  obtained 
  for 
  alpha-ray 
  ionization 
  

   only 
  when 
  the 
  component 
  of 
  the 
  electric 
  field 
  perpendicular 
  

   to 
  the 
  alpha 
  particle 
  column 
  has 
  a 
  value 
  which 
  is 
  sufficient 
  

   to 
  separate 
  the 
  ions 
  before 
  recombination 
  occurs. 
  After 
  the 
  

   column 
  has 
  been 
  formed 
  two 
  effects 
  are 
  operative 
  which 
  tend 
  

   to 
  separate 
  the 
  ions 
  ; 
  namely 
  diffusion 
  combined 
  with 
  mole- 
  

   cular 
  agitation, 
  and 
  the 
  transverse 
  component 
  of 
  the 
  electric 
  

   field. 
  Moulin 
  distinctly 
  states 
  that 
  a 
  field 
  strictly 
  longi- 
  

   tudinal 
  has 
  no 
  effect 
  in 
  breaking 
  up 
  the 
  columns 
  ; 
  that 
  the 
  

   reason 
  the 
  saturation 
  current 
  is 
  attained 
  in 
  practice 
  for 
  

   longitudinal 
  fields 
  is 
  that 
  these 
  fields 
  really 
  have 
  a 
  com- 
  

   ponent 
  perpendicular 
  to 
  the 
  trajectories. 
  He 
  has 
  also 
  shown 
  

   by 
  calculations 
  based 
  on 
  the 
  geometrical 
  conditions 
  of 
  his 
  

   apparatus, 
  that 
  when 
  the 
  saturation 
  current 
  is 
  attained 
  in 
  

   such 
  a 
  longitudinal 
  field 
  the 
  transverse 
  component 
  of 
  the 
  

   field 
  has 
  the 
  same 
  value 
  as 
  would 
  be 
  necessary 
  to 
  secure 
  

   saturation 
  in 
  a 
  transverse 
  field. 
  With 
  a 
  field 
  which 
  is 
  

   strictly 
  longitudinal 
  on 
  his 
  view 
  we 
  should 
  obtain 
  only 
  that 
  

   fraction 
  of 
  the 
  saturation 
  current 
  which 
  is 
  due 
  to 
  diffusion. 
  

   Curve 
  A, 
  fig. 
  5, 
  shows 
  that 
  this 
  fraction 
  is 
  approximately 
  

   0*74 
  for 
  air 
  at 
  a 
  pressure 
  of 
  one 
  atmosphere, 
  which 
  is 
  in 
  

   good 
  agreement 
  with 
  the 
  value 
  previously 
  found 
  by 
  Moulin. 
  

  

  The 
  excellent 
  agreement 
  which 
  Moulin 
  obtains 
  between 
  

   theory 
  and 
  experiment 
  lends 
  strong 
  support 
  to 
  this 
  view 
  : 
  at 
  

   the 
  same 
  time 
  we 
  would 
  like 
  to 
  indicate 
  briefly 
  a 
  possible 
  

   * 
  L. 
  c. 
  p. 
  80. 
  

  

  