﻿242 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Absorption 
  

  

  doubtful 
  if 
  they 
  will 
  appear 
  separated 
  in 
  the 
  reproduction. 
  

   There 
  is 
  also 
  a 
  general 
  absorption 
  further 
  down 
  in 
  the 
  ultra- 
  

   violet, 
  which 
  wipes 
  out 
  the 
  last 
  cadmium 
  line 
  first, 
  and 
  then 
  

   the 
  others 
  in 
  succession. 
  The 
  apparent 
  absorption-band 
  at 
  

   the 
  extreme 
  right 
  of 
  fig. 
  1, 
  PI. 
  VII., 
  is 
  merely 
  the 
  region 
  of 
  

   minimum 
  sensibility 
  in 
  the 
  blue-green 
  region 
  of 
  the 
  ortho- 
  

   chromatic 
  plate. 
  

  

  If 
  the 
  bulb 
  contains 
  air 
  or 
  any 
  other 
  chemically 
  inert 
  gas 
  

   at 
  atmospheric 
  pressure, 
  the 
  band 
  (2536) 
  widens 
  sym- 
  

   metrically 
  at 
  first, 
  attaining 
  a 
  width 
  of 
  about 
  8 
  Angstrom 
  

   units. 
  Beyond 
  this 
  point 
  a 
  further 
  increase 
  in 
  the 
  density 
  

   of 
  the 
  mercury 
  vapour 
  causes 
  a 
  widening 
  in 
  one 
  direction 
  

   only, 
  as 
  is 
  the 
  case 
  when 
  the 
  vapour 
  is 
  in 
  vacuo. 
  

  

  If, 
  instead 
  of 
  sealing 
  the 
  mercury 
  up 
  in 
  a 
  bulb 
  filled 
  with 
  

   air, 
  w^e 
  place 
  it 
  in 
  a 
  quartz 
  flask 
  provided 
  wdth 
  a 
  long 
  neck 
  

   and 
  gradually 
  raise 
  the 
  flame 
  below 
  the 
  flask, 
  we 
  get 
  a 
  

   remarkable 
  series 
  of 
  spectrograms 
  (PL 
  VII. 
  fig. 
  4). 
  The 
  band 
  

   widens 
  and 
  then 
  appears 
  to 
  drift 
  towards 
  the 
  longer 
  wave- 
  

   lengths, 
  without 
  further 
  increase 
  in 
  width. 
  This 
  apparent 
  

   drift 
  is 
  due 
  to 
  the 
  expulsion 
  of 
  the 
  air 
  by 
  the 
  boiling 
  

   mercury, 
  the 
  band 
  contracting 
  on 
  one 
  side 
  more 
  rapidly 
  

   than 
  it 
  widens 
  on 
  tbe 
  other. 
  

  

  This 
  action 
  of 
  the 
  air 
  in 
  modifying 
  the 
  appearance, 
  and 
  in 
  

   some 
  cases 
  the 
  apparent 
  position 
  of 
  the 
  absorption-band^ 
  

   cannot 
  be 
  attributed 
  to 
  chemical 
  action, 
  for 
  the 
  same 
  efi'ect 
  

   was 
  found 
  with 
  hydrogen, 
  nitrogen, 
  and 
  helium. 
  As 
  we 
  

   shall 
  see 
  presently, 
  the 
  vapour 
  is 
  deprived 
  of 
  its 
  power 
  of 
  

   fluorescing 
  when 
  it 
  is 
  mixed 
  with 
  another 
  gas. 
  

  

  It 
  was 
  found, 
  by 
  making 
  a 
  more 
  careful 
  study, 
  that 
  the 
  

   effect 
  of 
  air 
  upon 
  the 
  band 
  at 
  2536 
  was 
  a 
  little 
  more 
  com- 
  

   plicated 
  than 
  was 
  at 
  first 
  supposed. 
  In 
  vacuo 
  the 
  broadening 
  

   is 
  almost 
  entirely 
  in 
  the 
  direction 
  of 
  

   longer 
  wave-lengths. 
  If 
  air 
  is 
  present 
  Tig- 
  1- 
  

  

  a 
  hazy 
  band 
  appears 
  on 
  the 
  short 
  

   wave-length 
  side 
  of 
  the 
  line, 
  and 
  if 
  

   the 
  time 
  of 
  exposure 
  and 
  vapour 
  

   density 
  are 
  just 
  right 
  the 
  band 
  and 
  

   line 
  are 
  separated 
  by 
  a 
  narrow 
  strip 
  

   slightly 
  lighter 
  than 
  the 
  band. 
  The 
  form 
  of 
  the 
  absorption 
  

   curve 
  in 
  the 
  two 
  cases 
  is 
  indicated 
  in 
  fig. 
  1. 
  

  

  A 
  photograph 
  is 
  reproduced 
  on 
  PL 
  YII. 
  fig. 
  3, 
  short 
  

   wave-lengths 
  being 
  to 
  the 
  left. 
  It 
  is 
  difficult 
  to 
  obtain 
  an 
  

   enlargement 
  which 
  shows 
  the 
  lighter 
  region 
  between 
  the 
  

   line 
  and 
  the 
  band, 
  though 
  it 
  is 
  not 
  difficult 
  to 
  make 
  it 
  out 
  in 
  

   the 
  negative. 
  The 
  appearance 
  of 
  the 
  band 
  suggests 
  that 
  

   mercury 
  + 
  air 
  gives 
  us 
  some 
  sort 
  of 
  a 
  molecular 
  aggregate 
  

  

  Fig. 
  L 
  

  

  