﻿244 
  Prof. 
  H. 
  W. 
  Wood 
  on 
  the 
  Absorption 
  

  

  great 
  as 
  that 
  o£ 
  the 
  atmosphere 
  in 
  the 
  tube. 
  I 
  have 
  long- 
  

   since 
  abandoned 
  the 
  viscosity 
  theory 
  which 
  I 
  first 
  proposed 
  

   to 
  explain 
  the 
  apparent 
  peculiar 
  behaviour 
  of 
  the 
  vapour, 
  

   and 
  now 
  believe 
  that 
  even 
  in 
  the 
  highly 
  exhausted 
  tubes 
  we 
  

   can 
  never 
  have 
  the 
  vapour 
  at 
  a 
  greater 
  pressure 
  than 
  that 
  of 
  

   the 
  residual 
  gas. 
  The 
  error 
  resulted 
  from 
  an 
  over-estimate 
  

   of 
  the 
  density 
  of 
  the 
  metallic 
  vapour, 
  based 
  upon 
  its 
  optical 
  

   properties. 
  

  

  This 
  pressure 
  effect 
  is 
  not 
  to 
  be 
  confused 
  with 
  the 
  

   phenomenon 
  studied 
  by 
  Humphreys 
  and 
  Mohler, 
  for 
  it 
  

   occurs 
  at 
  pressures 
  below 
  one 
  atmosphere, 
  and 
  there 
  is 
  no 
  

   shift, 
  the 
  band 
  merely 
  becoming 
  wider 
  and 
  blacker, 
  precisely 
  

   us 
  if 
  more 
  mercury 
  vapour 
  were 
  present. 
  

  

  Fluorescence. 
  

  

  The 
  fluorescence 
  of 
  the 
  vapour 
  is 
  best 
  shown 
  by 
  enclosing 
  

   a 
  drop 
  of 
  the 
  metal 
  in 
  an 
  exhausted 
  quartz 
  bulb, 
  heating 
  the 
  

   bulb 
  over 
  a 
  Bunsen 
  burner 
  turned 
  down 
  very 
  low, 
  and 
  

   passing 
  a 
  powerful 
  spark 
  between 
  zinc 
  or 
  cadmium 
  electrodes 
  

   placed 
  as 
  close 
  as 
  possible 
  to 
  the 
  bulb. 
  The 
  colour 
  of 
  the 
  

   fluorescent 
  light 
  is 
  a 
  bluish 
  green 
  mixed 
  with 
  a 
  good 
  deal 
  of 
  

   white, 
  i. 
  e. 
  it 
  embraces 
  a 
  region 
  of 
  the 
  spectrum 
  extending 
  

   from 
  the 
  red-yellow 
  well 
  down 
  into 
  the 
  ultra-violet. 
  The 
  

   colour 
  of 
  the 
  fluorescent 
  light 
  varies 
  somewhat 
  with 
  the 
  

   nature 
  of 
  the 
  electrodes 
  used. 
  The 
  spectrum 
  appears 
  con- 
  

   tinuous 
  even 
  with 
  a 
  concave 
  grating 
  of 
  2 
  metres 
  radius. 
  

  

  Photographed 
  with 
  a 
  small 
  quartz 
  spectrograph, 
  the 
  

   spectrum 
  is 
  found 
  to 
  be 
  continuous, 
  extending 
  roughly 
  from 
  

   the 
  yellow 
  down 
  to 
  wave-length 
  3000, 
  with 
  a 
  very 
  pro- 
  

   nounced 
  minimum 
  at 
  wave-length 
  3600. 
  It 
  was 
  at 
  first 
  

   suspected 
  that 
  the 
  visible 
  band 
  and 
  the 
  ultra-violet 
  band 
  

   might 
  be 
  excited 
  by 
  different 
  radiations, 
  and 
  a 
  week 
  or 
  more 
  

   was 
  spent 
  in 
  photographing 
  the 
  spectrum 
  emitted 
  by 
  the 
  

   vapour 
  when 
  stimulated 
  by 
  lines 
  isolated 
  from 
  the 
  spark- 
  

   spectrum 
  by 
  an 
  auxiliary 
  quartz 
  spectrograph. 
  Yery 
  little, 
  

   if 
  any, 
  difference 
  could 
  be 
  detected 
  however. 
  It 
  was 
  found 
  

   that 
  if 
  air 
  was 
  in 
  the 
  bulb 
  it 
  was 
  impossible 
  to 
  excite 
  any 
  

   fluorescence. 
  If, 
  however, 
  we 
  boil 
  the 
  mercury 
  in 
  a 
  flask 
  it 
  

   fluoresces 
  brightly 
  as 
  soon 
  as 
  it 
  is 
  in 
  brisk 
  ebullition, 
  and 
  if 
  

   the 
  absorption 
  spectrum 
  is 
  photographed 
  at 
  the 
  moment 
  at 
  

   which 
  fluorescence 
  appears, 
  we 
  find 
  that 
  the 
  absorption-band 
  

   has 
  contracted 
  on 
  its 
  short 
  wave-length 
  side 
  to 
  its 
  position 
  

   when 
  the 
  vapour 
  is 
  in 
  vacuo. 
  The 
  fluorescence 
  spectrum 
  is 
  

   shown 
  on 
  PI. 
  YII. 
  figs. 
  2 
  & 
  5, 
  excited 
  in 
  this 
  case 
  by 
  the 
  light 
  

   from 
  the 
  cadmium 
  spark. 
  The 
  cadmium 
  lines 
  appear 
  as 
  well 
  

   owing 
  to 
  diffused 
  light. 
  As 
  will 
  be 
  seen, 
  there 
  is 
  in 
  addition 
  

  

  