﻿and 
  Fluorescence 
  of 
  Mercury 
  Vapour. 
  24ii 
  

  

  Avbicli 
  broadens 
  the 
  2536 
  line 
  and 
  shifts 
  it 
  towards 
  the 
  

   shorter 
  wave-length 
  region. 
  The 
  shifted 
  and 
  broadened 
  

   band 
  and 
  the 
  narrower 
  2536 
  band 
  appeared 
  to 
  be 
  dis- 
  

   connected, 
  and 
  it 
  occurred 
  to 
  me 
  that 
  it 
  might 
  be 
  possible 
  

   to 
  obtain 
  the 
  broadened 
  band 
  alone. 
  It 
  seemed 
  as 
  if 
  we 
  

   might 
  be 
  dealing 
  with 
  a 
  molecular 
  aggregate 
  mixed 
  with 
  

   free 
  mercury 
  molecules, 
  in 
  which 
  case 
  it 
  might 
  be 
  possible 
  

   to 
  get 
  the 
  effect 
  of 
  the 
  aggregate 
  alone 
  by 
  employing 
  a 
  very 
  

   long 
  column 
  of 
  air 
  containing 
  but 
  very 
  little 
  mercury 
  vapour. 
  

   I 
  accordingly 
  fitted 
  up 
  a 
  steel 
  tube 
  3 
  metres 
  long, 
  containing 
  

   20 
  small 
  porcelain 
  boats 
  each 
  containing 
  a 
  few 
  drops 
  of 
  

   mercury, 
  and 
  closed 
  at 
  the 
  ends 
  with 
  quartz 
  windows. 
  The 
  

   appearance 
  of 
  the 
  two 
  bands 
  was 
  the 
  same 
  as 
  before 
  how- 
  

   ever. 
  The 
  tube 
  was 
  surrounded 
  by 
  a 
  second 
  tube 
  heated 
  by 
  

   a 
  line 
  of 
  very 
  small 
  flames 
  obtained 
  by 
  drilling 
  pin-holes 
  at 
  

   intervals 
  along 
  a 
  long 
  piece 
  of 
  gas-pipe. 
  The 
  very 
  in- 
  

   teresting 
  observation 
  was 
  made 
  that 
  the 
  2536 
  line 
  was 
  quite 
  

   distinct 
  in 
  the 
  photograph 
  of 
  the 
  absorption 
  spectrum, 
  even 
  

   when 
  the 
  tube 
  was 
  at 
  room 
  temperature. 
  This 
  gives 
  us 
  a 
  

   means 
  of 
  studying 
  mercury 
  vapour 
  at 
  low 
  temperatures, 
  and 
  

   determining 
  whether 
  it 
  can 
  be 
  prevented 
  from 
  entering 
  an 
  

   exhausted 
  receiver 
  by 
  any 
  of 
  the 
  absorbing 
  devices 
  {^e.g. 
  

   gold-leaf) 
  frequently 
  suggested. 
  It 
  will 
  also 
  be 
  possible 
  to 
  

   determine 
  its 
  vapour 
  density 
  at 
  lower 
  temperatures 
  than 
  has 
  

   hitherto 
  been 
  possible. 
  It 
  was 
  found 
  that 
  if 
  the 
  tube 
  at 
  

   room 
  temperature 
  was 
  exhausted, 
  the 
  line 
  became 
  much 
  

   fainter, 
  almost 
  disappearing. 
  On 
  readmitting 
  the 
  air 
  it 
  

   immediately 
  resumed 
  its 
  full 
  intensity. 
  No 
  trace 
  of 
  the 
  

   broadened 
  and 
  displaced 
  band 
  appeared 
  at 
  room 
  temperature,, 
  

   owing 
  to 
  the 
  insufficient 
  density 
  of 
  the 
  vapour. 
  The 
  effect 
  

   of 
  admitting 
  the 
  air 
  is 
  precisely 
  analogous 
  to 
  the 
  observations, 
  

   made 
  by 
  Angstrom 
  on 
  the 
  CO 
  band 
  in 
  the 
  infra-red. 
  It 
  is 
  

   a 
  pressure 
  effect, 
  the 
  effect 
  of 
  admitting 
  the 
  air 
  being 
  

   equivalent 
  to 
  compressing 
  the 
  actual 
  amount 
  of 
  mercury 
  

   vapour 
  present 
  until 
  it 
  is 
  at 
  atmospheric 
  pressure. 
  Ang- 
  

   strom's 
  paper 
  should 
  be 
  referred 
  to 
  in 
  this 
  connexion. 
  I 
  

   have 
  observed 
  the 
  same 
  thing 
  with 
  sodium 
  vapour 
  for 
  many 
  

   years, 
  but 
  never 
  felt 
  quite 
  sure 
  as 
  to 
  the 
  interpretation 
  of 
  

   the 
  phenomenon. 
  The 
  D 
  lines 
  widen 
  and 
  fuse 
  together 
  the 
  

   moment 
  nitrogen 
  or 
  hydrogen 
  is 
  admitted 
  to 
  one 
  of 
  the 
  steel 
  

   tubes. 
  In 
  the 
  case 
  of 
  sodium 
  vapour, 
  however, 
  I 
  could 
  not 
  

   feel 
  sure 
  that 
  the 
  actual 
  number 
  of 
  sodium 
  molecules 
  per 
  

   cubic 
  cm. 
  had 
  not 
  been 
  increased 
  by 
  the 
  admission 
  of 
  the 
  gas, 
  

   for 
  the 
  gas 
  would 
  hinder 
  diffusion 
  to 
  the 
  cooler 
  parts 
  of 
  the 
  

   tube, 
  and 
  it 
  might 
  easily 
  be 
  possible 
  for 
  us 
  to 
  have 
  the 
  

   metallic 
  vapour 
  at 
  the 
  centre 
  of 
  the 
  tube 
  at 
  a 
  pressure 
  as. 
  

  

  