﻿Dispersion 
  of 
  Sodium 
  Vapour. 
  143 
  

  

  light 
  in 
  combination 
  with 
  the 
  method 
  above 
  alluded 
  to 
  of 
  

   reflecting 
  the 
  light 
  from 
  the 
  mirror 
  of 
  molten 
  sodium, 
  I 
  find 
  

   that 
  the 
  spectrum 
  consists 
  of 
  two 
  exceedingly 
  brilliant 
  narrow 
  

   green 
  bands 
  with 
  a 
  narrow 
  dark 
  line 
  between 
  them, 
  and 
  a 
  

   band 
  in 
  the 
  violet 
  which 
  is 
  partially 
  filled 
  with 
  flutings. 
  

   This 
  spectrum 
  I 
  have 
  observed 
  but 
  once, 
  and 
  have 
  given 
  it 
  

   no 
  careful 
  study 
  as 
  yet. 
  

  

  The 
  flutings 
  in 
  the 
  green-blue 
  portion 
  of 
  the 
  spectrum 
  are 
  

   similar 
  in 
  appearance 
  to 
  those 
  in 
  the 
  red, 
  but 
  they 
  make 
  

   their 
  appearance 
  when 
  the 
  vapour 
  is 
  less 
  dense, 
  this 
  part 
  of 
  

   the 
  spectrum 
  being 
  blotted 
  out 
  entirely 
  when 
  the 
  red 
  bands 
  

   are 
  at 
  their 
  best. 
  

  

  I 
  have 
  not 
  yet 
  investigated 
  the 
  question 
  of 
  whether 
  an 
  

   increase 
  in 
  the 
  thickness 
  of 
  the 
  layer 
  of 
  vapour 
  is 
  the 
  equivalent 
  

   of 
  increasing 
  its 
  density, 
  but 
  I 
  feel 
  quite 
  sure 
  that 
  such 
  is 
  not 
  

   the 
  case. 
  There 
  remains 
  too 
  the 
  question 
  of 
  pressure. 
  All 
  

   of 
  the 
  experiments 
  that 
  I 
  have 
  made 
  thus 
  far 
  have 
  been 
  at 
  

   atmospheric 
  pressure, 
  the 
  variations 
  in 
  density 
  being 
  in 
  all 
  

   probability 
  merely 
  dilution 
  of 
  the 
  metallic 
  vapour 
  with 
  

   hydrogen. 
  

  

  Sodium 
  vapour 
  possesses 
  another 
  property 
  which 
  requires 
  

   further 
  investigation, 
  namely, 
  the 
  power 
  of 
  emitting 
  light 
  in 
  

   virtue 
  of 
  temperature 
  alone. 
  This 
  was 
  first 
  observed 
  by 
  

   Evershed 
  "*. 
  Iodine 
  and 
  a 
  number 
  of 
  other 
  substances 
  behave 
  

   in 
  a 
  similar 
  manner, 
  the 
  temperature 
  necessary 
  to 
  produce 
  

   the 
  luminous 
  radiations 
  being 
  but 
  little 
  above 
  a 
  red 
  heat. 
  

   On 
  repeating 
  Evershed's 
  experiment 
  with 
  iodine, 
  my 
  first 
  

   impression 
  was 
  that 
  anomalous 
  dispersion 
  might 
  be 
  at 
  the 
  

   bottom 
  of 
  it. 
  The 
  vapour 
  was 
  heated 
  in 
  a 
  tube, 
  and 
  the 
  purple 
  

   luminosity 
  or 
  glow 
  hung 
  close 
  to 
  the 
  inner 
  wall, 
  which 
  was 
  

   red 
  hot. 
  It 
  seemed 
  quite 
  possible 
  that 
  the 
  phenomenon 
  

   might 
  be 
  a 
  mirage 
  effect, 
  the 
  rays 
  of 
  suitable 
  wave-length 
  

   coming 
  from 
  the 
  red-hot 
  wall 
  moving 
  in 
  curved 
  paths 
  through 
  

   the 
  non-homogeneous 
  vapour. 
  To 
  test 
  this 
  I 
  devised 
  a 
  method 
  

   by 
  which 
  this 
  possibility 
  was 
  entirely 
  eliminated, 
  and 
  found 
  

   that 
  the 
  glow 
  was 
  more 
  brilliant 
  than 
  ever. 
  A 
  spiral 
  of 
  

   platinum 
  wire 
  mounted 
  in 
  a 
  small 
  flask 
  in 
  which 
  iodine 
  was 
  

   vaporized 
  was 
  raised 
  to 
  a 
  bright 
  red 
  heat 
  by 
  an 
  electric 
  

   current. 
  The 
  convection 
  current 
  of 
  iodine 
  which 
  rose 
  through 
  

   the 
  spiral 
  was 
  luminous 
  to 
  a 
  height 
  of 
  about 
  2 
  cm. 
  above 
  the 
  

   spiral, 
  waving 
  from 
  side 
  to 
  side 
  in 
  the 
  form 
  of 
  a 
  reddish-purple 
  

   flame. 
  Focussing 
  the 
  image 
  of 
  an 
  electric 
  arc 
  on 
  the 
  spiral 
  

   increased 
  its 
  luminosity 
  ten-fold, 
  without, 
  however, 
  altering 
  the 
  

   intensity 
  of 
  the 
  glowing 
  vapour. 
  Probably 
  the 
  luminescence 
  

  

  * 
  Phil. 
  Mag-. 
  [5] 
  xxxix. 
  p. 
  460 
  (May 
  1895). 
  

  

  