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  II. 
  On 
  the 
  Changes 
  of 
  Pressure 
  which 
  accompany 
  Point 
  

   Discharge 
  through 
  Hydrogen, 
  containing 
  Oxygen 
  and 
  

   Nitrogen. 
  By 
  A. 
  P. 
  Chattock, 
  Professor 
  of 
  Physics, 
  

   and 
  A. 
  M. 
  Tyndall, 
  B.Sc, 
  Lecturer 
  in 
  Physics, 
  University 
  

   College, 
  Bristol 
  *. 
  

  

  TT^HE 
  need 
  for 
  the 
  experiments 
  described 
  in 
  this 
  paper 
  

   JL 
  arose 
  in 
  connexion 
  with 
  an 
  attempt 
  upon 
  which 
  we 
  

   have 
  been 
  intermittently 
  engaged 
  for 
  some 
  years, 
  to 
  measure 
  

   the 
  molecular 
  complexity 
  of 
  the 
  ions 
  in 
  point 
  discharge 
  

   through 
  gases 
  at 
  normal 
  temperature 
  and 
  pressure. 
  

  

  Our 
  original 
  plan 
  was 
  to 
  discharge 
  through 
  pure 
  hydrogen 
  

   from 
  a 
  point 
  against 
  a 
  platinum 
  or 
  palladium 
  plate, 
  in 
  the 
  

   hope 
  that 
  these 
  substances 
  might 
  absorb 
  the 
  ions 
  and 
  hold 
  

   them 
  long 
  enough 
  for 
  a 
  measurement 
  of 
  the 
  resulting 
  

   pressure 
  change 
  to 
  be 
  made. 
  

  

  If 
  the 
  volume 
  and 
  pressure 
  of 
  the 
  hydrogen 
  are 
  Y 
  and 
  P, 
  

   and 
  a 
  decrease 
  of 
  pressure 
  of 
  p 
  is 
  observed 
  after 
  discharging 
  

   c 
  coulombs, 
  the 
  corresponding 
  change 
  of 
  volume, 
  for 
  P 
  

   constant, 
  is 
  Vp/P 
  = 
  i\ 
  

  

  Now 
  the 
  volume 
  of 
  hydrogen 
  given 
  off 
  by 
  one 
  coulomb 
  in 
  

   a 
  water 
  voltameter 
  is 
  0*116 
  c.c. 
  at 
  0° 
  C. 
  and 
  760 
  mm. 
  Hence 
  

   the 
  number 
  of 
  complete 
  molecules 
  which 
  must 
  have 
  been 
  

   removed 
  from 
  the 
  gas, 
  or 
  at 
  least 
  have 
  ceased 
  to 
  exist 
  as 
  

   separate 
  pressure-producing 
  bodies 
  when 
  the 
  charge 
  on 
  one 
  

   monovalent 
  ion 
  is 
  passed 
  through 
  it 
  is 
  

  

  2 
  x 
  0-116 
  x 
  c 
  

  

  where 
  v 
  is 
  the 
  change 
  of 
  volume 
  at 
  0° 
  C. 
  and 
  760 
  mm. 
  

   (The 
  2 
  comes 
  in 
  because 
  a 
  molecule 
  of 
  hydrogen 
  contains 
  

   two 
  monovalent 
  ions.) 
  

  

  In 
  what 
  follows 
  the 
  expression 
  " 
  per 
  ion 
  " 
  always 
  means 
  

   " 
  per 
  hydrogen 
  atom 
  set 
  free 
  in 
  a 
  water 
  voltameter 
  placed 
  in 
  

   series 
  with 
  the 
  discharge 
  vessel/'' 
  

  

  Suppose 
  now 
  that 
  the 
  ions 
  carrying 
  the 
  point 
  discharge 
  

   are 
  in 
  some 
  way 
  removed 
  from 
  the 
  gas 
  when 
  they 
  reach 
  the 
  

   electrodes. 
  If 
  each 
  ion 
  consists 
  of 
  a 
  single 
  atom 
  of 
  hydrogen, 
  

   !N 
  will 
  be 
  0*5 
  ; 
  but 
  if 
  it 
  carries 
  with 
  it 
  a 
  cluster 
  of 
  hydrogen 
  

   atoms, 
  N 
  will 
  be 
  half 
  the 
  number 
  of 
  atoms 
  in 
  the 
  cluster. 
  

  

  We 
  found, 
  as 
  was 
  to 
  be 
  expected, 
  that 
  the 
  purity 
  of 
  the 
  

   hydrogen 
  exercised 
  an 
  enormous 
  influence 
  on 
  the 
  contractions 
  

   observed, 
  and 
  therefore 
  on 
  the 
  calculated 
  values 
  of 
  N 
  ; 
  quite 
  

  

  * 
  Communicated 
  by 
  the 
  Authors. 
  

  

  