﻿tliroiigli 
  the 
  Gases 
  HCl^ 
  HBr, 
  and 
  HI. 
  475 
  

  

  o£ 
  secondary 
  process 
  which 
  is 
  most 
  marked 
  in 
  the 
  iodine 
  

   vapours. 
  The 
  great 
  diminution 
  in 
  the 
  P.G. 
  with 
  increase 
  o£ 
  

   current 
  which 
  is 
  found 
  by 
  Matthies 
  for 
  the 
  iodine 
  compounds 
  

   would 
  point 
  to 
  a 
  secondary 
  effect, 
  for 
  if 
  it 
  existed 
  it 
  ought 
  to 
  

   be 
  more 
  marked 
  for 
  larger 
  currents. 
  Then 
  to 
  make 
  the 
  

   secondary 
  effect 
  the 
  least 
  possible 
  it 
  would 
  be 
  advisable 
  to 
  

   make 
  the 
  comparison 
  for 
  very 
  small 
  currents, 
  as 
  actually 
  has 
  

   been 
  done 
  for 
  HCl, 
  HBr, 
  and 
  HI. 
  

  

  The 
  Cathode 
  Fall. 
  

   § 
  11. 
  The 
  normal 
  cathode 
  rail 
  corresponding 
  to 
  a 
  partly 
  

   covered 
  cathode 
  is 
  determined 
  in 
  a 
  number 
  of 
  measurements 
  

   for 
  different 
  pressures 
  and 
  currents. 
  As 
  the 
  mean 
  of 
  the 
  

   observed 
  values 
  we 
  get 
  : 
  

  

  For 
  HCl 
  329'6 
  volts 
  platinum 
  electrodes. 
  

  

  „ 
  HBr 
  ... 
  350-3 
  „ 
  

  

  T, 
  Oo 
  364 
  „ 
  „ 
  „ 
  

  

  Fio-. 
  9 
  oives 
  the 
  cathode 
  fall 
  above 
  the 
  state 
  of 
  saturation 
  

   of 
  the 
  cathode 
  for 
  the 
  tube 
  described. 
  We 
  notice 
  the 
  charac- 
  

   teristic 
  difference 
  between 
  the 
  curves 
  for 
  high 
  and 
  low 
  

   pressures. 
  The 
  horizontal 
  line 
  gives 
  the 
  normal 
  cathode 
  fall, 
  

   and 
  where 
  the 
  curves 
  cut 
  this 
  line 
  we 
  have 
  the 
  biggest 
  

   current 
  that 
  gives 
  the 
  normal 
  cathode 
  fall 
  for 
  the 
  tube 
  and 
  

   pressure 
  considered. 
  The 
  relation 
  between 
  saturation 
  current 
  

   and 
  pressure 
  is 
  shown 
  in 
  PI. 
  XIV. 
  fig. 
  10. 
  These 
  curves 
  cut 
  

   the 
  p 
  axis, 
  and 
  for 
  all 
  three 
  gases 
  at 
  the 
  same 
  point. 
  Thus 
  for 
  

   each 
  gas 
  there 
  is 
  a 
  certain 
  pressure, 
  below 
  which 
  no 
  current 
  

   that 
  can 
  be 
  made 
  to 
  pass 
  can 
  give 
  the 
  normal 
  cathode 
  fall, 
  

   and 
  which 
  is 
  the 
  same 
  for 
  the 
  three 
  gases. 
  This 
  result 
  

   suggests 
  that 
  for 
  the 
  same 
  tube 
  the 
  minimum 
  pressure 
  for 
  a 
  

   normal 
  cathode 
  fall 
  is 
  independent 
  of 
  the 
  nature 
  of 
  the 
  gas. 
  

   For 
  the 
  tube 
  used 
  the 
  minimum 
  pressure 
  = 
  0*25 
  mm. 
  

  

  The 
  Anode 
  Fall. 
  

  

  § 
  12. 
  To 
  get 
  a 
  strict 
  definition 
  of 
  the 
  anode 
  fall 
  we 
  shall 
  let 
  

   the 
  P. 
  D. 
  curves 
  (see 
  PI. 
  XIV. 
  fig. 
  4) 
  continue 
  in 
  their 
  direction 
  

   past 
  the 
  explorer 
  (1) 
  and 
  their 
  points 
  of 
  intersection 
  with 
  

   the 
  vertical 
  axis 
  through 
  the 
  anode 
  will 
  give 
  the 
  anode 
  fall. 
  

  

  In 
  order 
  to 
  find 
  the 
  influence 
  of 
  current 
  and 
  pressure 
  on 
  

   the 
  anode 
  fall 
  other 
  conditions 
  must 
  be 
  unaltered. 
  Thus 
  the 
  

   discharge 
  must 
  be 
  reversible, 
  which 
  in 
  our 
  case 
  requires 
  a 
  

   uniform 
  structure. 
  Further, 
  the 
  tube 
  must 
  be 
  run 
  for 
  some 
  

   time 
  in 
  the 
  gas 
  to 
  be 
  tested 
  before 
  any 
  measurements 
  are 
  

   taken. 
  It 
  is 
  found 
  that 
  the 
  anode 
  fall 
  when 
  a 
  new 
  gns 
  is 
  

   introduced 
  will 
  begin 
  with 
  a 
  too 
  large 
  value. 
  This 
  is 
  probably 
  

   due 
  to 
  changes 
  in 
  the 
  surface-conditions 
  of 
  the 
  electrodes, 
  for 
  

  

  2 
  K2 
  

  

  