﻿Gaseous 
  Ionization 
  and 
  Pressure. 
  881 
  

  

  manometer-tube, 
  and 
  its 
  bore 
  (1*8 
  mms 
  ) 
  was 
  carefully 
  

   calibrated 
  by 
  the 
  mercury-thread 
  method. 
  The 
  thoroughly 
  

   cleaned 
  tube 
  was 
  tilled 
  with 
  air 
  which 
  had 
  been 
  dried 
  

   over 
  P 
  2 
  5 
  ; 
  it 
  was 
  then 
  sealed 
  at 
  one 
  end. 
  A 
  thread 
  of 
  

   mercury 
  several 
  centimetres 
  long 
  enclosed 
  a 
  75 
  cm. 
  length 
  

   of 
  this 
  air 
  at 
  atmospheric 
  pressure. 
  There 
  was 
  very 
  little 
  

   sticktion, 
  and 
  the 
  mercury 
  thread 
  was 
  found 
  always 
  to 
  return 
  

   to 
  its 
  zero, 
  showing 
  that 
  there 
  was 
  no 
  leakage 
  past 
  the 
  

   mercury. 
  A 
  short 
  length 
  of 
  mercury 
  also 
  occupied 
  the 
  closed 
  

   end 
  of 
  the 
  manometer 
  ; 
  by 
  this 
  means 
  the 
  determination 
  of 
  

   the 
  volume 
  end 
  correction 
  was 
  avoided. 
  The 
  readings 
  of 
  the 
  

   manometer 
  were 
  made 
  to 
  0*1 
  mm. 
  by 
  means 
  of 
  a 
  travelling- 
  

   microscope. 
  

  

  Assuming 
  a 
  spherical 
  bounding 
  surface 
  for 
  the 
  mercury 
  

   and 
  an 
  angle 
  of 
  contact 
  with 
  glass 
  of 
  53°, 
  the 
  height 
  of 
  the 
  

   meniscus 
  works 
  out 
  to 
  be 
  almost 
  exactly 
  1 
  /3r, 
  where 
  r 
  is 
  the 
  

   radius 
  of 
  the 
  tube. 
  A 
  reading 
  taken 
  at 
  the 
  tip 
  of 
  the 
  meniscus 
  

   should 
  be 
  corrected 
  by 
  an 
  amount 
  1/6 
  r 
  to 
  give 
  the 
  length 
  of 
  

   the 
  air-thread. 
  

  

  In 
  calculating 
  the 
  pressure 
  (up 
  to 
  17 
  atmospheres) 
  from 
  

   the 
  manometer 
  readings, 
  it 
  was 
  assumed 
  that 
  Boyle's 
  law 
  

   held 
  exactly, 
  for 
  Amagat 
  has 
  shown 
  that 
  for 
  pressures 
  up 
  

   to 
  20 
  atmospheres 
  the 
  above 
  law 
  is 
  true 
  to 
  half 
  a 
  per 
  cent. 
  

   for 
  dry 
  air. 
  The 
  reagents 
  in 
  the 
  drying-tube, 
  through 
  which 
  

   the 
  whole 
  of 
  the 
  gas 
  entering 
  the 
  apparatus 
  had 
  to 
  pass, 
  were 
  

   CaCl 
  2 
  and 
  P 
  2 
  5 
  separated 
  by 
  glass-wool. 
  As 
  a 
  safeguard 
  

   against 
  explosion 
  the 
  glass 
  drying-tube 
  was 
  provided 
  with 
  

   a 
  brass 
  sheath. 
  A 
  metal 
  T-piece 
  was 
  joined 
  by 
  sealing-wax 
  

   to 
  the 
  remote 
  end 
  of 
  the 
  drying-tube. 
  One 
  limb 
  of 
  the 
  T 
  

   was 
  soldered 
  to 
  a 
  length 
  of 
  flexible 
  metal 
  tubing 
  connected 
  

   to 
  a 
  cylinder 
  of 
  compressed 
  gas 
  ; 
  the 
  other 
  limb 
  was 
  joined 
  

   to 
  a 
  motor-tyre 
  valve, 
  which 
  could 
  be 
  used 
  to 
  release 
  the 
  

   pressure 
  in 
  the 
  apparatus. 
  Hydrogen 
  and 
  carbon 
  dioxide 
  

   were 
  used 
  direct 
  from 
  the 
  gas 
  cylinder, 
  which 
  gave 
  of 
  course 
  

   the 
  desired 
  pressure 
  without 
  difficulty. 
  The 
  carbon 
  dioxide 
  

   obtained 
  in 
  this 
  way 
  from 
  the 
  liquid 
  is 
  known 
  to 
  be 
  very 
  

   pure 
  ; 
  the 
  hydrogen 
  contained 
  from 
  2 
  to 
  3 
  per 
  cent, 
  of 
  light 
  

   gaseous 
  impurity. 
  With 
  air 
  the 
  pressure 
  was 
  obtained 
  by 
  a 
  

   motor-tyre 
  pump 
  joined 
  to 
  the 
  valve 
  for 
  the 
  time 
  being. 
  

  

  When 
  a 
  new 
  gas 
  was 
  being 
  admitted 
  into 
  the 
  apparatus, 
  

   the 
  pressure 
  was 
  several 
  times 
  slowly 
  raised 
  to 
  about 
  

   15 
  atmospheres, 
  and 
  then 
  lowered 
  to 
  atmospheric 
  pressure 
  by 
  

   opening 
  the 
  valve. 
  

  

  The 
  final 
  form 
  of 
  ionization-chamber 
  adopted 
  is 
  shown 
  in 
  

   section 
  in 
  fig 
  are 
  2, 
  which 
  is 
  drawn 
  to 
  scale. 
  It 
  allowed 
  a 
  

   uniform 
  field 
  and 
  a 
  pressure 
  of 
  20 
  atmospheres 
  to 
  be 
  obtained. 
  

  

  Phil. 
  May. 
  S. 
  6. 
  Vol. 
  16. 
  No. 
  96. 
  Dec. 
  1908. 
  3 
  M 
  

  

  