﻿Dispersion 
  of 
  Sodium 
  Vapour. 
  139 
  

  

  The 
  coloured 
  drawing 
  of 
  the 
  spectrum, 
  Plate 
  II., 
  to 
  which 
  I 
  

   have 
  already 
  alluded, 
  was 
  made 
  when 
  the 
  spectrum 
  was 
  

   obtained 
  in 
  this 
  manner. 
  A 
  single 
  prism 
  gives 
  a 
  very 
  pretty 
  

   anomalous 
  spectrum, 
  but 
  the 
  magnificent 
  effect 
  produced 
  by 
  

   the 
  battery 
  makes 
  the 
  slight 
  amount 
  of 
  extra 
  trouble 
  well 
  

   worth 
  while. 
  If 
  the 
  electric 
  arc 
  is 
  employed 
  as 
  the 
  source 
  of 
  

   light, 
  extreme 
  violet 
  will 
  be 
  found 
  to 
  occupy 
  the 
  position 
  of 
  

   the 
  undeviated 
  image 
  of 
  the 
  slit. 
  Then 
  comes 
  the 
  blue, 
  

   sometimes 
  in 
  contact 
  with 
  the 
  violet 
  and 
  sometimes 
  slightly 
  

   separated 
  by 
  a 
  fine 
  dark 
  line, 
  owing 
  to 
  the 
  fact 
  that 
  the 
  violet 
  

   light 
  comes 
  from 
  the 
  fluted 
  carbon 
  band 
  of 
  the 
  arc, 
  which 
  is 
  

   separated 
  from 
  the 
  blue 
  by 
  a 
  comparatively 
  dark 
  region. 
  

   Then 
  comes 
  a 
  wide 
  gap 
  corresponding 
  to 
  light 
  absorbed 
  by 
  

   the 
  sodium 
  vapour 
  in 
  the 
  blue-green 
  region 
  (the 
  channelled 
  

   spectrum), 
  and 
  above 
  this 
  a 
  beautiful 
  flare 
  of 
  colour 
  ranging 
  

   from 
  blue-green 
  through 
  grass-green 
  to 
  yellow. 
  The 
  red 
  and 
  

   orange 
  portion 
  of 
  the 
  spectrum 
  is 
  on 
  the 
  other 
  side 
  of 
  or 
  below 
  

   the 
  undeviated 
  image, 
  forming 
  another 
  brilliant 
  flare 
  of 
  colour. 
  

   It 
  is 
  separated 
  from 
  the 
  violet 
  by 
  a 
  wide 
  dark 
  band, 
  due 
  to 
  

   the 
  absorption 
  in 
  the 
  vicinity 
  of 
  the 
  D 
  lines., 
  If 
  the 
  density 
  

   of 
  the 
  vapour 
  is 
  increased 
  by 
  heating 
  the 
  tube 
  to 
  a 
  higher 
  

   temperature, 
  the 
  red 
  flare 
  extends 
  lower 
  down, 
  grows 
  fainter, 
  

   and 
  finally 
  fades 
  away 
  owing 
  to 
  the 
  presence 
  of 
  the 
  fluted 
  

   absorption-bands 
  in 
  the 
  red. 
  The 
  green 
  and 
  blue 
  persist, 
  

   however, 
  becoming 
  more 
  widely 
  separated, 
  but 
  finally 
  the 
  

   green 
  disappears 
  almost 
  entirely. 
  It 
  is 
  best 
  to 
  arrange 
  the 
  

   gas-cock 
  so 
  that 
  the 
  height 
  of 
  the 
  flames 
  can 
  be 
  controlled 
  

   without 
  leaving 
  the 
  eyepiece, 
  for 
  it 
  is 
  surprising 
  how 
  slight 
  

   a 
  change 
  is 
  necessary 
  to 
  completely 
  alter 
  the 
  general 
  

   appearance 
  of 
  the 
  spectrum. 
  

  

  The 
  glass 
  tube 
  should 
  not 
  be 
  allowed 
  to 
  cool 
  until 
  the 
  

   experiment 
  is 
  at 
  an 
  end, 
  otherwise 
  it 
  will 
  immediately 
  fly 
  to 
  

   pieces 
  as 
  soon 
  as 
  the 
  flame 
  is 
  again 
  applied 
  to 
  it. 
  

  

  While 
  glass 
  answers 
  very 
  well 
  if 
  the 
  experiment 
  is 
  to 
  be 
  

   of 
  short 
  duration, 
  sheet 
  iron 
  is 
  much 
  more 
  satisfactory. 
  

   Suitable 
  tubes 
  can 
  be 
  made 
  by 
  any 
  tinsmith. 
  They 
  should 
  

   be 
  made 
  of 
  thin 
  sheet-iron, 
  and 
  the 
  turned-over 
  seam 
  ham- 
  

   mered 
  until 
  a 
  tight 
  joint 
  is 
  formed. 
  These 
  tubes 
  can 
  be 
  

   heated 
  and 
  cooled 
  any 
  number 
  of 
  times, 
  and 
  can 
  be 
  kept 
  in 
  

   operation 
  for 
  an 
  hour 
  or 
  two, 
  at 
  the 
  end 
  of 
  which 
  time 
  the 
  

   sodium 
  is 
  generally 
  used 
  up, 
  a 
  moss-like 
  deposit 
  of 
  oxide 
  

   gradually 
  filling 
  up 
  the 
  tube. 
  These 
  tubes 
  can 
  be 
  used 
  over 
  

   and 
  over 
  again 
  without 
  deterioration, 
  and 
  are 
  most 
  satisfactory 
  

   in 
  every 
  respect. 
  Their 
  only 
  fault 
  lies 
  in 
  their 
  conductivity, 
  

   the 
  sealing- 
  W 
  ax 
  softening 
  and 
  the 
  glass 
  plates 
  falling 
  off, 
  but 
  

   this 
  can 
  be 
  prevented 
  by 
  wrapping 
  a 
  strip 
  of 
  cloth 
  around 
  

  

  