﻿through 
  the 
  Gases 
  HCl, 
  HBr, 
  and 
  HI. 
  469 
  

  

  Results 
  of 
  Measurements. 
  

  

  § 
  4. 
  Measurements 
  were 
  made 
  for 
  a 
  series 
  of 
  pressures 
  

   varying 
  from 
  about 
  0'25 
  to 
  2*4 
  mm. 
  Hg, 
  and 
  for 
  each 
  

   pressure 
  the 
  distribution 
  of 
  potential 
  was 
  measured 
  for 
  a 
  

   series 
  of 
  currents. 
  In 
  general 
  the 
  same 
  voltage 
  from 
  the 
  

   cells 
  was 
  used 
  for 
  measurements 
  at 
  the 
  same 
  pressure; 
  the 
  

   current 
  was 
  changed 
  by 
  changing 
  the 
  resistance. 
  

  

  The 
  discharges 
  to 
  be 
  compared 
  for 
  different 
  gases 
  must 
  

   have 
  the 
  same 
  structure, 
  and 
  for 
  comparison 
  of 
  the 
  po- 
  

   tential 
  gradients 
  of 
  the 
  positive 
  column 
  the 
  latter 
  must 
  be 
  

   uniform. 
  

  

  It 
  is 
  found 
  that 
  for 
  the 
  greatest 
  part 
  of 
  the 
  current 
  interval 
  

   that 
  gives 
  a 
  uniform 
  discharge 
  the 
  electric 
  force 
  is 
  very 
  

   nearly 
  constant 
  all 
  along 
  the 
  positive 
  column. 
  For 
  very 
  

   small 
  currents 
  it 
  is 
  found 
  (especially 
  for 
  HCl, 
  HBr, 
  and 
  O9) 
  

   that 
  the 
  P.D. 
  curve 
  is 
  slightly 
  curved, 
  and 
  in 
  such 
  a 
  way 
  that 
  

   the 
  electric 
  force 
  increases 
  towards 
  the 
  anode. 
  This 
  is 
  in 
  

   agreement 
  with 
  results 
  found 
  by 
  Matthies 
  for 
  discharge 
  

   through 
  vapours 
  of 
  HgCl, 
  HgBr, 
  and 
  Hgl. 
  

  

  Fig. 
  4 
  (PL 
  XIV.) 
  gives 
  some 
  P.D. 
  curves 
  for 
  HBr 
  at 
  pressure 
  

   of 
  0"55 
  mm., 
  showing 
  the 
  general 
  character 
  of 
  these 
  curves 
  for 
  

   varying 
  currents. 
  

  

  In 
  the 
  case 
  of 
  this 
  variation 
  in 
  the 
  force 
  we 
  shall 
  for 
  the 
  

   comparison 
  use 
  the 
  average 
  force, 
  which 
  in 
  our 
  case 
  can 
  be 
  

   put 
  equal 
  to 
  the 
  potential-difference 
  between 
  the 
  explorers 
  

   (1) 
  and 
  (4) 
  divided 
  by 
  their 
  distances. 
  

  

  Discharge 
  through 
  HCl. 
  

  

  § 
  5. 
  The 
  general 
  character 
  of 
  the 
  discharge 
  through 
  

   HCl 
  is 
  the 
  same 
  as 
  for 
  an 
  ordinary 
  gas. 
  Three 
  forms 
  

   of 
  discharge 
  have 
  been 
  observed, 
  the 
  uniform, 
  the 
  striated, 
  

   and 
  one 
  form 
  which 
  is 
  got 
  when 
  the 
  positive 
  column 
  is 
  

   driven 
  into 
  the 
  anode 
  and 
  which 
  we 
  shall 
  call 
  the 
  dark 
  

   discharge. 
  

  

  Some 
  results 
  of 
  the 
  measurements 
  of 
  potential 
  distribution 
  

   in 
  HCl 
  made 
  with 
  the 
  tube 
  described 
  are 
  given 
  in 
  the 
  

   following 
  table 
  : 
  — 
  

  

  Gm 
  is 
  the 
  mean 
  P.G. 
  (potential 
  gradient). 
  

  

  U 
  means 
  uniform 
  discharge. 
  

  

  S, 
  striated 
  discharge. 
  

  

  D, 
  dark 
  discharge. 
  

  

  