﻿by 
  Collision 
  of 
  Positive 
  and 
  Negative 
  Ions 
  in 
  a 
  Gas. 
  2&D 
  

  

  with 
  the 
  rest 
  of 
  the 
  apparatus, 
  so 
  that 
  the 
  experiments 
  on 
  the 
  

   conductivity 
  could 
  be 
  performed 
  immediately 
  after 
  the 
  gas 
  

   was 
  purified. 
  All 
  the 
  experiments 
  made 
  under 
  these 
  con- 
  

   ditions 
  showed 
  that 
  the 
  sparking 
  potentials 
  were 
  lowered 
  by 
  

   purifying 
  the 
  gas. 
  A 
  complete 
  set 
  of 
  experiments 
  were 
  not 
  

   made, 
  but 
  determinations 
  were 
  made 
  at 
  definite 
  pressures 
  and 
  

   forces 
  to 
  compare 
  the 
  results 
  with 
  those 
  corresponding 
  to 
  the 
  

   slightly 
  impure 
  gas. 
  All 
  our 
  experiments 
  lead 
  to 
  the 
  conclusion 
  

   that 
  purification 
  lowers 
  the 
  minimum 
  sparking 
  potential 
  in 
  

   agreement 
  with 
  the 
  observations 
  of 
  Strutt*. 
  There 
  is, 
  how- 
  

   ever, 
  no 
  numerical 
  agreement 
  between 
  the 
  two 
  observations. 
  

   Even 
  with 
  impure 
  helium 
  the 
  minimum 
  sparking 
  potential 
  

   was 
  found 
  to 
  be 
  231 
  volts, 
  as 
  against 
  Strutt' 
  s 
  lowest 
  value 
  

   of 
  261 
  volts. 
  After 
  purification, 
  as 
  will 
  be 
  seen 
  later, 
  the 
  

   most 
  probable 
  value 
  to 
  be 
  expected 
  for 
  this 
  quantity 
  is 
  201 
  

   volts, 
  an 
  error 
  of 
  more 
  than 
  10 
  volts 
  either 
  way 
  being 
  

   improbable. 
  

  

  It 
  was 
  found 
  that 
  after 
  the 
  passage 
  of 
  a 
  spark 
  not 
  only 
  

   the 
  currents 
  but 
  also 
  the 
  sparking 
  potential 
  was 
  affected, 
  in 
  

   such 
  a 
  way 
  that 
  an 
  addition 
  of 
  twenty 
  volts 
  was 
  sometimes 
  

   necessary 
  to 
  spark 
  again 
  just 
  after 
  the 
  first 
  spark 
  had 
  passed. 
  

   The 
  passage 
  of 
  a 
  spark 
  in 
  the 
  reverse 
  direction 
  facilitated 
  

   recovery, 
  thus 
  pointing 
  to 
  some 
  polarization 
  or 
  other 
  effect 
  

   of 
  the 
  spark 
  discharge 
  on 
  the 
  zinc 
  electrodes 
  employed. 
  

  

  For 
  low 
  values 
  of 
  X/jt? 
  and 
  large 
  values 
  of 
  p 
  x 
  a 
  the 
  

   purification 
  did 
  not 
  have 
  very 
  great 
  effect. 
  In 
  the 
  following 
  

   table 
  the 
  most 
  probable 
  values 
  of 
  a/p 
  for 
  the 
  pure 
  gas 
  are 
  

   shown 
  for 
  small 
  values 
  of 
  X/p, 
  the 
  corresponding 
  a/p's 
  taken 
  

   from 
  the 
  mean 
  of 
  observations 
  on 
  the 
  impure 
  gas 
  being 
  shown 
  

   side 
  by 
  side. 
  

  

  Table 
  XV. 
  

  

  X 
  

   p' 
  

  

  — 
  for 
  pure 
  helium. 
  

  

  ■f 
  

  

  impure 
  helium. 
  

  

  10 
  

  

  •12 
  

  

  

  •12 
  

  

  20 
  

  

  •40 
  

  

  

  •39 
  

  

  30 
  

  

  •77 
  

  

  

  •67 
  

  

  40 
  

  

  10 
  

  

  

  •89 
  

  

  50 
  

  

  1-20 
  

  

  

  106 
  

  

  60 
  

  

  ! 
  

  

  1-35 
  

  

  

  1-21 
  

  

  The 
  values 
  of 
  /3/p 
  for 
  such 
  small 
  values 
  of 
  X/p 
  are 
  difficult 
  

   to 
  determine 
  accurately, 
  but 
  it 
  was 
  clear 
  that 
  the 
  effect 
  of 
  

   the 
  positive 
  ions 
  is 
  very 
  much 
  increased 
  by 
  purification. 
  

   Thus 
  for 
  X/jo 
  = 
  40, 
  /3 
  is 
  more 
  than 
  doubled 
  by 
  purification. 
  

   * 
  Hon. 
  R. 
  J. 
  Strutt, 
  Phil. 
  Trans. 
  A, 
  cxciii. 
  p. 
  377 
  (1900). 
  

  

  