﻿and 
  Fluorescence 
  of 
  Mercury 
  Vapour. 
  241 
  

  

  other 
  observations. 
  The 
  study 
  of 
  the 
  vapour 
  has 
  been 
  

   attended 
  with 
  great 
  difficulty, 
  since 
  all 
  of 
  the 
  interesting- 
  

   things 
  are 
  in 
  the 
  ultra-violet, 
  and 
  can 
  be 
  picked 
  up 
  only 
  by 
  

   the 
  aid 
  of 
  photography. 
  

  

  The 
  work 
  has 
  been 
  suspended 
  from 
  time 
  to 
  time, 
  and 
  then 
  

   taken 
  up 
  again 
  as 
  new 
  methods 
  of 
  attack 
  have 
  suggested 
  

   themselves. 
  

  

  I 
  have 
  at 
  last 
  succeeded 
  in 
  devising 
  a 
  satisfactory 
  piece 
  of 
  

   apparatus 
  for 
  the 
  study 
  of 
  the 
  anomalous 
  dispersion 
  of 
  the 
  

   vapour. 
  This 
  is 
  much 
  to 
  be 
  desired^ 
  for 
  the 
  vapour, 
  unlike 
  

   that 
  of 
  sodium, 
  does 
  not 
  attack 
  the 
  quartz 
  vessel 
  in 
  which 
  it 
  

   is 
  contained. 
  It 
  is 
  thus 
  possible 
  to 
  work 
  with 
  prisms 
  of 
  

   known 
  angle, 
  and 
  vapour 
  of 
  known 
  and 
  uniform 
  density. 
  

   If 
  we 
  work 
  with 
  prisms 
  we 
  shall 
  need 
  a 
  cement 
  capable 
  of 
  

   resisting 
  the 
  vapour 
  and 
  of 
  standing 
  a 
  temperature 
  of 
  say 
  

   300°. 
  I 
  have 
  some 
  hopes 
  of 
  a 
  mixture 
  of 
  the 
  fused 
  haloids 
  of 
  

   silver 
  and 
  am 
  now 
  making 
  experiments 
  in 
  this 
  direction. 
  A 
  

   quantitative 
  determination 
  of 
  the 
  dispersion 
  would 
  be 
  of 
  very 
  

   great 
  interest, 
  on 
  account 
  of 
  the 
  asymmetry 
  of 
  the 
  absorption- 
  

   band. 
  

  

  The 
  absorption 
  spectrum 
  of 
  the 
  vapour 
  is 
  shown 
  on 
  PI. 
  YII. 
  

   fig. 
  1. 
  A 
  small 
  globule 
  of 
  mercury 
  was 
  placed 
  in 
  a 
  quartz 
  

   bulb 
  3 
  cms. 
  in 
  diameter, 
  which 
  was 
  thoroughly 
  exhausted 
  

   and 
  sealed. 
  The 
  spectrograms 
  were 
  taken 
  in 
  succession, 
  the 
  

   temperature 
  of 
  the 
  bulb 
  being 
  gradually 
  raised. 
  There 
  are 
  

   three 
  distinct 
  absorption-bands, 
  all 
  in 
  the 
  ultra-violet 
  region. 
  

   The 
  one 
  at 
  wave-length 
  2536 
  appears 
  first 
  as 
  a 
  pair 
  of 
  fine 
  

   lines, 
  not 
  unlike 
  the 
  D 
  lines 
  in 
  appearance, 
  one 
  of 
  them 
  

   (\ 
  = 
  2539"4) 
  relatively 
  much 
  weaker 
  than 
  the 
  other 
  (\ 
  = 
  

   2.536*7). 
  As 
  the 
  density 
  of 
  the 
  vapour 
  increases 
  they 
  fuse 
  

   toofether, 
  formino- 
  a 
  sino-le 
  band 
  which 
  then 
  widens 
  in 
  a 
  

   remarkable 
  manner 
  towards 
  the 
  reo-ion 
  of 
  lono-er 
  wave- 
  

  

  o 
  o 
  

  

  length, 
  its 
  boundarv 
  on 
  the 
  other 
  remainino- 
  almost 
  fixed 
  in 
  

   position. 
  

  

  In 
  addition 
  to 
  the 
  absorption-band 
  at 
  wave-length 
  2536, 
  

   there 
  is 
  a 
  group 
  of 
  narrow 
  bands 
  just 
  above 
  the 
  three 
  bright 
  

   cadmium 
  lines 
  further 
  down 
  in 
  the 
  ultra-violet. 
  These 
  

   bands 
  do 
  not 
  appear 
  until 
  the 
  vapour 
  has 
  acquired 
  a 
  con- 
  

   siderable 
  density 
  : 
  they 
  form 
  a 
  series, 
  the 
  head 
  of 
  which 
  is 
  

   turned 
  towards 
  the 
  visible 
  spectrum. 
  The 
  individual 
  bands 
  

   which 
  make 
  up 
  the 
  group 
  decrease 
  in 
  intensity 
  with 
  

   decreasing 
  wave-length 
  and 
  lie 
  closer 
  together, 
  resembling 
  

   a 
  Balmer 
  series 
  on 
  a 
  very 
  small 
  scale. 
  The 
  wave-lengths 
  of 
  

   the 
  four 
  bands 
  which 
  make 
  up 
  the 
  group 
  are 
  as 
  follows 
  : 
  — 
  

   2346-1, 
  2339-4, 
  2334*4, 
  2331*2. 
  They 
  can 
  be 
  seen 
  in 
  figs. 
  1 
  

   and 
  7 
  of 
  PI. 
  YII. 
  They 
  are 
  so 
  nearly 
  fused 
  together 
  that 
  it 
  is 
  

  

  