﻿Fluorescence 
  of 
  Mercury 
  Vapour. 
  247 
  

  

  boiling 
  metal. 
  The 
  appearance 
  is 
  quite 
  striking, 
  a 
  layer 
  of 
  

   glowing 
  vapour 
  clinging 
  close 
  to 
  the 
  metal, 
  reminding 
  one 
  

   o£ 
  the 
  discharge 
  at 
  the 
  cathode 
  in 
  a 
  vacuum-tube. 
  The 
  

   temperature 
  of 
  the 
  vapour 
  when 
  it 
  leaves 
  the 
  metal 
  is 
  about 
  

   360, 
  consequently 
  it 
  fluoresces 
  brightly 
  : 
  as 
  it 
  rises 
  its 
  

   temperature 
  is 
  speedily 
  raised 
  to 
  the 
  point 
  at 
  which 
  fluores- 
  

   cence 
  disappears. 
  

  

  A 
  still 
  better 
  way 
  of 
  showing 
  the 
  eflPect 
  o£ 
  temperature 
  

   upon 
  the 
  fluorescence 
  is 
  illustrated 
  in 
  fig. 
  1, 
  PL 
  YIII. 
  The 
  

   exhausted 
  bulb 
  previously 
  described 
  is 
  used. 
  It 
  is 
  heated 
  

   from 
  below 
  by 
  a 
  small 
  flame 
  from 
  a 
  Bunsen 
  burner 
  until 
  the 
  

   fluorescence 
  excited 
  by 
  a 
  cadmium 
  spark 
  placed 
  as 
  close 
  a& 
  

   possible 
  is 
  at 
  its 
  brightest. 
  The 
  small 
  pointed 
  flame 
  from 
  a 
  

   blowpipe 
  is 
  then 
  directed 
  against 
  the 
  side 
  for 
  a 
  few 
  seconds. 
  

   The 
  fluorescence 
  promptly 
  disappears 
  at 
  the 
  super-heated 
  

   spot, 
  reappearing 
  again 
  as 
  soon 
  as 
  the 
  local 
  heating 
  i& 
  

   stopped. 
  

  

  If 
  the 
  ultra-violet 
  light 
  of 
  the 
  spark 
  is 
  focussed 
  at 
  the 
  

   centre 
  of 
  the 
  bulb, 
  a 
  narrow 
  cone 
  of 
  brilliant 
  green 
  light 
  

   extends 
  from 
  the 
  point 
  at 
  which 
  the 
  light 
  enters 
  the 
  bulb 
  

   nearly 
  to 
  the 
  opposite 
  wall, 
  as 
  shown 
  on 
  PL 
  YIII. 
  fig. 
  2. 
  The 
  

   flame 
  of 
  a 
  blast-lamp 
  directed 
  against 
  the 
  wall 
  of 
  the 
  bulb 
  

   at 
  the 
  point 
  where 
  the 
  fluorescence 
  begins 
  causes 
  the 
  

   luminous 
  cone 
  to 
  retreat 
  from 
  the 
  wall 
  to 
  a 
  distance 
  of 
  

   several 
  millimetreSj 
  much 
  as 
  the 
  positive 
  column 
  separates 
  

   from 
  the 
  cathode 
  in 
  a 
  vacuum-tube 
  discharge. 
  A 
  photograph 
  

   of 
  the 
  efl'ect 
  is 
  shown 
  on 
  PL 
  VIII. 
  (right-hand 
  figure). 
  

  

  The 
  failure 
  of 
  the 
  2536 
  line 
  to 
  appear 
  in 
  the 
  fluorescence 
  

   spectrum 
  except 
  when 
  the 
  vapour 
  is 
  at 
  comparatively 
  small 
  

   density 
  is 
  very 
  remarkable. 
  It 
  occurred 
  to 
  me 
  that 
  it 
  might 
  

   be 
  due 
  to 
  disappearance 
  of 
  diffuse 
  radiation 
  by 
  the 
  combina- 
  

   tion 
  of 
  the 
  secondary 
  waves 
  from 
  the 
  molecular 
  resonators 
  

   into 
  a 
  regularly 
  reflected 
  wave. 
  I 
  have 
  shown 
  in 
  a 
  previous 
  

   paper 
  (" 
  Selective 
  Reflexion 
  of 
  Monochromatic 
  Light 
  by 
  

   Mercury 
  Vapour 
  " 
  in 
  this 
  Journal, 
  supra, 
  p. 
  187) 
  that 
  this 
  

   takes 
  place 
  when 
  the 
  vapour 
  is 
  illuminated 
  by 
  the 
  light 
  of 
  

   the 
  2536 
  line 
  from 
  the 
  mercury 
  arc, 
  and 
  its 
  density 
  increased 
  

   to 
  ten 
  or 
  more 
  atmospheres. 
  It 
  is 
  possible 
  that 
  something 
  

   of 
  the 
  same 
  nature 
  might 
  explain 
  the 
  failure 
  of 
  the 
  line 
  to 
  

   appear 
  when 
  the 
  vapour 
  was 
  excited 
  by 
  the 
  very 
  shortest 
  

   ultra-violet 
  waves. 
  It 
  must 
  be 
  remembered 
  that 
  in 
  this 
  case 
  

   there 
  is 
  a 
  change 
  of 
  wave-length 
  involved, 
  and 
  it 
  is 
  hard 
  to 
  

   see 
  how 
  the 
  necessary 
  phase 
  relations 
  for 
  selective 
  reflexion 
  

   could 
  hold. 
  Experiment 
  showed 
  that 
  the 
  hypothesis 
  was 
  

  

  S2 
  

  

  