﻿Resonance 
  Spectra 
  of 
  Iodine. 
  237 
  

  

  system 
  of 
  fluted 
  bands, 
  few 
  in 
  number 
  and 
  regularly 
  spaced 
  

   if 
  the 
  iodine 
  is 
  in 
  vacuo, 
  increasing 
  in 
  number 
  and 
  com- 
  

   plexity 
  if 
  a 
  gas 
  of 
  the 
  helium 
  group 
  is 
  mixed 
  with 
  the 
  iodine, 
  

   or 
  if 
  more 
  than 
  a 
  single 
  iodine 
  absorption 
  line 
  is 
  excited 
  by 
  

   the 
  mercury 
  lamp. 
  It 
  is 
  probable 
  that 
  the 
  lines 
  forming 
  the 
  

   doublets 
  are 
  themselves 
  constituents 
  of 
  the 
  fluted 
  bands, 
  and 
  

   the 
  transfer 
  of 
  energy 
  from 
  one 
  part 
  of 
  the 
  vibrating 
  system 
  

   to 
  another, 
  as 
  a 
  result 
  of 
  collisions 
  between 
  iodine 
  and 
  

   helium 
  molecules, 
  enables 
  us 
  to 
  build 
  up, 
  so 
  to 
  speak, 
  the 
  

   complicated 
  system 
  of 
  fluted 
  bands 
  shown 
  in 
  the 
  absorption 
  

   spectrum, 
  out 
  of 
  a 
  number 
  of 
  simpler 
  systems 
  which 
  can 
  be 
  

   excited 
  separately. 
  This 
  constitutes 
  a 
  very 
  great 
  advance 
  in 
  

   the 
  analysis 
  of 
  band 
  spectra, 
  and 
  brings 
  us 
  a 
  step 
  nearer 
  to 
  the 
  

   point 
  at 
  which 
  we 
  can 
  picture 
  some 
  idea 
  of 
  the 
  vibrating 
  

   mechanism. 
  

  

  In 
  the 
  more 
  recent 
  work, 
  a 
  method 
  of 
  illumination 
  has 
  

   been 
  employed 
  which 
  is 
  distinctly 
  superior 
  to 
  any 
  previously 
  

   used, 
  and 
  as 
  it 
  is 
  w 
  T 
  ell 
  adapted 
  to 
  purposes 
  of 
  demonstration 
  

   I 
  shall 
  describe 
  it 
  in 
  some 
  detail. 
  The 
  iodine 
  tubes 
  which 
  I 
  

   now 
  employ 
  are 
  of 
  soft 
  glass, 
  about 
  40 
  cm. 
  long 
  and 
  3 
  cm. 
  

   in 
  diameter. 
  One 
  end 
  is 
  blown 
  out 
  into 
  a 
  thin 
  bulb, 
  taking 
  

   care 
  to 
  avoid 
  having 
  the 
  thick 
  drop 
  near 
  the 
  centre 
  of 
  the 
  

   bulb. 
  This 
  is 
  best 
  accomplished 
  by 
  drawing 
  off 
  the 
  tube 
  in 
  

   an 
  oblique 
  direction, 
  which 
  brings 
  the 
  drop 
  — 
  formed 
  by 
  the 
  

   melting 
  down 
  of 
  the 
  pointed 
  end 
  — 
  well 
  to 
  one 
  side. 
  If 
  this 
  

   is 
  not 
  done 
  the 
  drop 
  is 
  apt 
  to 
  form 
  a 
  small 
  lens 
  on 
  the 
  surface 
  

   of 
  the 
  bulb 
  exactly 
  on 
  the 
  axis 
  of 
  the 
  tube. 
  

  

  The 
  other 
  end 
  is 
  drawn 
  down, 
  and 
  a 
  few 
  flakes 
  of 
  iodine 
  

   introduced 
  into 
  the 
  tube. 
  It 
  is 
  a 
  good 
  plan 
  to 
  provide 
  the 
  

   tube 
  with 
  a 
  lateral 
  branch, 
  by 
  which 
  the 
  density 
  of 
  the 
  vapour 
  

   can 
  be 
  controlled, 
  though 
  this 
  is 
  not 
  necessary 
  for 
  demonstra- 
  

   tion 
  purposes. 
  The 
  iodine 
  flakes 
  are 
  now 
  brought 
  into 
  the 
  

   bulb, 
  or 
  to 
  the 
  bottom 
  of 
  the 
  lateral 
  tube, 
  and 
  the 
  tube 
  joined 
  

   to 
  a 
  Gaede 
  pump, 
  interposing 
  a 
  U-tube 
  immersed 
  in 
  liquid 
  

   air 
  or 
  solid 
  C0 
  2; 
  or 
  a 
  tube 
  filled 
  with 
  caustic 
  potash, 
  to 
  keep 
  

   the 
  iodine 
  out 
  of 
  the 
  pump. 
  During 
  the 
  exhaustion 
  it 
  is 
  a 
  

   good 
  plan 
  to 
  heat 
  the 
  walls 
  with 
  a 
  bunsen 
  flame, 
  except 
  

   where 
  the 
  iodine 
  is 
  located. 
  Then 
  allow 
  the 
  tube 
  to 
  cool 
  

   down 
  to 
  the 
  temperature 
  of 
  the 
  room, 
  and 
  heat 
  the 
  portion 
  

   where 
  the 
  iodine 
  is 
  located. 
  The 
  flakes 
  will 
  sublime 
  rapidly 
  

   and 
  crystallize 
  on 
  the 
  cooler 
  portions 
  of 
  the 
  wall. 
  The 
  tube 
  

   is 
  now 
  sealed 
  off 
  from 
  the 
  pump 
  and 
  the 
  drawn-down 
  end 
  

   painted 
  black 
  for 
  a 
  distance 
  of 
  a 
  few 
  centimetres. 
  For 
  the 
  

   illumination 
  I 
  used 
  a 
  very 
  simple 
  modification 
  of 
  the 
  " 
  light 
  

   furnace" 
  described 
  in 
  the 
  earlier 
  paper. 
  

  

  The 
  iodine 
  tube 
  is 
  fastened 
  alongside 
  of 
  and 
  in 
  contact 
  

   Phil. 
  Mag. 
  S. 
  6. 
  Vol. 
  35. 
  No. 
  207. 
  March 
  1918. 
  S 
  

  

  