﻿Spectra 
  oj 
  Iodine 
  under 
  High 
  Dispersion. 
  845 
  

  

  less 
  extent, 
  with 
  other 
  gases 
  such 
  as 
  nitrogen, 
  the 
  total 
  

   intensity 
  of 
  the 
  light 
  being 
  much 
  less, 
  however, 
  for 
  reasons 
  

   given 
  in 
  the 
  papers 
  published 
  by 
  Wood 
  and 
  Franck. 
  To 
  

   reduce 
  the 
  absorption 
  element 
  as 
  much 
  as 
  possible, 
  the 
  rear 
  

   end 
  of 
  the 
  iodine 
  tube 
  was 
  packed 
  in 
  ice, 
  or 
  in 
  a 
  mixture 
  of 
  

   ice 
  and 
  salt, 
  which 
  reduced 
  the 
  density 
  of 
  the 
  iodine 
  vapour 
  

   to 
  a 
  small 
  fraction 
  of 
  its 
  value 
  at 
  room 
  temperature. 
  It 
  

   was 
  found 
  that 
  in 
  this 
  case 
  the 
  band 
  spectrum 
  was 
  quite 
  

   pronounced. 
  With 
  the 
  iodine 
  vapour 
  in 
  a 
  high 
  vacuum, 
  one 
  

   always 
  obtains 
  a 
  faint 
  trace 
  of 
  the 
  band 
  spectrum 
  if 
  a 
  very 
  

   long 
  exposure 
  is 
  given, 
  but 
  it 
  was 
  much 
  stronger 
  in 
  the 
  case 
  

   of 
  the 
  cooled 
  tube. 
  The 
  hypothesis 
  was 
  made 
  that 
  this 
  

   resulted 
  from 
  the 
  circumstance 
  that 
  the 
  band 
  spectrum 
  was 
  

   more 
  strongly 
  absorbed 
  by 
  the 
  iodine 
  vapour 
  than 
  the 
  lines 
  of 
  

   the 
  resonance 
  groups, 
  some 
  of 
  which 
  as 
  we 
  have 
  seen 
  are 
  

   absorbed 
  scarcely 
  at 
  all. 
  This 
  was 
  tested 
  by 
  passing 
  the 
  

   light 
  from 
  the 
  frozen 
  tube 
  through 
  a 
  bulb 
  containing 
  iodine 
  

   vapour 
  at 
  room 
  temperature, 
  before 
  its 
  entrance 
  into 
  the 
  

   spectroscope. 
  The 
  band 
  spectrum 
  at 
  once 
  disappeared, 
  

   showing 
  that 
  the 
  hypothesis 
  was 
  in 
  all 
  probability 
  correct. 
  

   The 
  resonance 
  groups 
  w 
  r 
  ere 
  uninfluenced 
  by 
  the 
  cooling 
  of 
  

   the 
  tube, 
  except 
  that 
  they 
  became 
  fainter. 
  

  

  Examination 
  of 
  the 
  Vapour 
  for 
  Phosphorescence. 
  

  

  An 
  attempt 
  to 
  detect 
  a 
  possible 
  finite 
  duration 
  of 
  the 
  light 
  

   emitted 
  by 
  the 
  vapour 
  after 
  shutting 
  off 
  the 
  exciting 
  beam, 
  

   was 
  made 
  by 
  focussing 
  an 
  image 
  of 
  the 
  sun 
  at 
  the 
  centre 
  of 
  a 
  

   swiftly 
  moving 
  stream 
  of 
  iodine 
  vapour. 
  Two 
  glass 
  bulbs 
  

   were 
  joined 
  by 
  a 
  tube 
  which 
  projected 
  several 
  centimetres 
  

   into 
  one 
  of 
  the 
  bulbs. 
  The 
  iodine 
  crystals 
  were 
  introduced 
  

   into 
  the 
  other 
  bulb 
  and 
  the 
  whole 
  system 
  highly 
  exhausted 
  

   and 
  sealed 
  from 
  the 
  pump. 
  On 
  cooling 
  the 
  first 
  bulb 
  by 
  the 
  

   application 
  of 
  a 
  pad 
  of 
  cotton-wool 
  wet 
  with 
  liquid 
  air, 
  the 
  

   iodine 
  vapour 
  in 
  this 
  bulb 
  immediately 
  condensed, 
  forming 
  

   a 
  very 
  high 
  vacuum 
  into 
  which 
  rushed 
  the 
  vapour, 
  con- 
  

   tinuously 
  formed 
  from 
  the 
  crystals 
  in 
  the 
  other 
  bulb. 
  The 
  

   solar 
  image 
  was 
  formed 
  just 
  at 
  the 
  mouth 
  of 
  the 
  tube, 
  but 
  

   no 
  prolongation 
  of 
  the 
  fluorescent 
  spot 
  could 
  be 
  detected, 
  

   as 
  would 
  be 
  the 
  case 
  if 
  the 
  moving 
  jet 
  of 
  vapour 
  remained 
  

   luminous 
  after 
  passing 
  through 
  the 
  focus. 
  A 
  paper 
  by 
  

   Mr. 
  F. 
  S. 
  Phillips 
  has 
  appeared 
  in 
  the 
  Proc. 
  Roy. 
  Soc. 
  

   (ser. 
  A, 
  vol. 
  lxxxix.) 
  since 
  the 
  completion 
  of 
  my 
  work 
  

   describing 
  similar 
  experiments 
  with 
  mercury 
  vapour, 
  w^hich 
  

   showed 
  very 
  persistent 
  luminosity. 
  It 
  is 
  probable 
  that 
  the 
  

   fluorescence 
  of 
  mercury 
  vapour 
  results, 
  in 
  part 
  at 
  least, 
  from 
  

  

  