﻿prod) 
  i 
  red 
  in 
  Gases 
  by 
  Ultra- 
  Violet 
  Light. 
  573 
  

  

  are 
  the 
  same 
  as 
  those 
  produced 
  in 
  a 
  gas 
  by 
  Rontgen 
  rays, 
  

   the 
  values 
  of 
  a 
  should 
  be 
  the 
  same 
  under 
  similar 
  conditions 
  

   of 
  force 
  and 
  pressure. 
  A 
  comparison 
  of 
  the 
  results 
  shows 
  

   that 
  this 
  is 
  the 
  case. 
  There 
  are, 
  however, 
  differences 
  between 
  

   the 
  curves 
  which 
  are 
  given 
  in 
  this 
  paper 
  and 
  those 
  which 
  

   have 
  been 
  already 
  published 
  in 
  the 
  papers 
  to 
  which 
  I 
  have 
  

   referred. 
  The 
  values 
  of 
  a 
  as 
  determined 
  by 
  the 
  experiments 
  

   with 
  Rontgen 
  rays 
  are 
  in 
  some 
  cases 
  much 
  too 
  large. 
  The 
  

   greatest 
  differences 
  between 
  the 
  results 
  arise 
  with 
  the 
  larger 
  

  

  values 
  of 
  — 
  , 
  and 
  are 
  principally 
  due 
  to 
  the 
  fact 
  that 
  when 
  

  

  experimenting 
  with 
  Rontgen 
  rays 
  the 
  forces 
  at 
  the 
  lower 
  

   pressures 
  were 
  so 
  large 
  that 
  other 
  sources 
  of 
  ionization 
  came 
  

   into 
  play. 
  As 
  I 
  have 
  pointed 
  out, 
  it 
  is 
  easy 
  to 
  guard 
  against 
  

   this 
  error 
  when 
  experimenting 
  with 
  ultra-violet 
  light 
  ; 
  but 
  

   at 
  the 
  time 
  the 
  earlier 
  experiments 
  were 
  being 
  made 
  I 
  did 
  

   not 
  realize 
  that 
  the 
  potential 
  must 
  be 
  kept 
  more 
  than 
  100 
  

   volts 
  below 
  the 
  sparking-point. 
  Thus, 
  in 
  the 
  experiments 
  

   with 
  hydrogen 
  (Phil. 
  Mag. 
  June 
  1901) 
  at 
  1*77 
  mm. 
  pressure, 
  

   the 
  plates 
  being 
  5*5 
  millimetres 
  apart, 
  potential-differences 
  up 
  to 
  

   220 
  volts 
  were 
  used 
  ; 
  and 
  at 
  *84 
  mm. 
  the 
  potential 
  was 
  taken 
  

   up 
  to 
  250 
  volts. 
  Referring 
  to 
  Tables 
  X. 
  and 
  XI. 
  of 
  this 
  paper 
  

   it 
  will 
  be 
  seen 
  that, 
  with 
  these 
  pressures, 
  potential-differences 
  

   greater 
  than 
  175 
  volts 
  give 
  rise 
  to 
  increases 
  in 
  conductivity 
  

   which 
  cannot 
  be 
  attributed 
  to 
  the 
  action 
  of 
  negative 
  ions. 
  The 
  

   values 
  of 
  a 
  deduced 
  from 
  those 
  experiments 
  are 
  therefore 
  

   too 
  high, 
  and 
  we 
  cannot 
  expect 
  to 
  find 
  a 
  good 
  agreement 
  

   between 
  the 
  present 
  results 
  and 
  the 
  determinations 
  of 
  a 
  made 
  

  

  -V" 
  

  

  with 
  Rontgen 
  rays 
  for 
  the 
  larger 
  values 
  of 
  — 
  . 
  

  

  V 
  X 
  

  

  The 
  curve 
  corresponding 
  to 
  the 
  smaller 
  values 
  of 
  — 
  deter- 
  

   mined 
  by 
  Rontgen 
  rays 
  for 
  hydrogen 
  lies 
  above 
  the 
  curve 
  

   determined 
  by 
  ultra-violet 
  light, 
  in 
  the 
  position 
  of 
  the 
  dotted 
  

   line 
  in 
  the 
  diagram. 
  

  

  The 
  corresponding 
  curve 
  for 
  air 
  would 
  lie 
  between 
  the 
  air 
  

   and 
  carbonic- 
  acid 
  curves 
  given 
  in 
  the 
  diagram. 
  In 
  order 
  not 
  

   to 
  complicate 
  the 
  figure 
  this 
  line 
  has 
  not 
  been 
  inserted. 
  The 
  

   potentials 
  used 
  with 
  air 
  were 
  also 
  too 
  high 
  in 
  some 
  of 
  the 
  

   experiments, 
  so 
  that 
  the 
  values 
  of 
  a 
  are 
  from 
  10 
  to 
  20 
  per 
  

   cent, 
  larger 
  than 
  those 
  found 
  with 
  ultra-violet 
  light 
  for 
  values 
  

  

  v 
  

  

  of 
  — 
  greater 
  than 
  300. 
  

   p 
  

  

  With 
  carbonic 
  acid 
  larger 
  forces 
  may 
  be 
  used 
  than 
  with 
  

  

  air 
  or 
  hydrogen 
  without 
  producing 
  conductivities 
  which 
  do 
  

  

  Phil. 
  Mag. 
  S. 
  6. 
  Vol. 
  3. 
  No. 
  18. 
  June 
  1002. 
  2 
  Q 
  

  

  