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  Intelligence 
  and 
  Miscellaneous 
  Articles. 
  

  

  powerful 
  beam 
  of 
  light. 
  At 
  the 
  time 
  of 
  writing 
  this 
  paper 
  I 
  was 
  

   unaware 
  that 
  the 
  phenomenon 
  had 
  already 
  been 
  observed 
  by 
  

   Wiedemann 
  and 
  Schmidt, 
  whose 
  paper 
  I 
  have 
  since 
  discovered. 
  

  

  A 
  paragraph 
  in 
  Drude's 
  ' 
  Optics,' 
  which 
  I 
  cited 
  in 
  the 
  original 
  

   paper, 
  led 
  me 
  to 
  infer 
  that 
  such 
  a 
  behaviour 
  of 
  the 
  vapour 
  had 
  

   never 
  been 
  observed, 
  though 
  it 
  had 
  been 
  repeatedly 
  sought 
  for 
  ; 
  

   and 
  I 
  therefore 
  took 
  no 
  trouble 
  to 
  go 
  over 
  the 
  literature 
  on 
  the 
  

   subject. 
  

  

  Wiedemann 
  and 
  Schmidt 
  worked 
  with 
  exhausted 
  glass 
  bulbs, 
  

   which 
  I 
  find, 
  on 
  repeating 
  their 
  experiments, 
  soon 
  become 
  opaque 
  

   to 
  light, 
  owing 
  to 
  the 
  liberation 
  of 
  silicon 
  by 
  the 
  action 
  of 
  the 
  

   sodium 
  vapour 
  on 
  the 
  glass. 
  One 
  of 
  my 
  students 
  is 
  at 
  the 
  present 
  

   time 
  engaged 
  in 
  an 
  investigation 
  of 
  the 
  phenomenon 
  with 
  an 
  im- 
  

   proved 
  apparatus, 
  which 
  permits 
  of 
  a 
  much 
  more 
  careful 
  study 
  of 
  

   the 
  spectrum 
  than 
  can 
  be 
  made 
  with 
  glass 
  bulbs. 
  We 
  are 
  also 
  

   making 
  an 
  exhaustive 
  study 
  of 
  the 
  absorption 
  of 
  the 
  vapour 
  over 
  a 
  

   wide 
  range 
  of 
  density 
  and 
  pressure. 
  I 
  have 
  found, 
  as 
  I 
  said 
  in 
  my 
  

   previous 
  paper, 
  that 
  when 
  the 
  vapour 
  is 
  exceedingly 
  dense 
  the 
  

   transmitted 
  light 
  is 
  of 
  a 
  deep 
  violet 
  colour, 
  the 
  spectrum 
  consisting 
  

   ot 
  a 
  rather 
  narrow 
  bright 
  band 
  in 
  the 
  yellow-green 
  and 
  a 
  broad 
  

   band 
  in 
  the 
  violet, 
  the 
  rest 
  of 
  the 
  spectrum 
  being 
  absorbed, 
  the 
  

   transition 
  from 
  the 
  channelled 
  spectrum 
  first 
  observed 
  by 
  Eoscoe 
  

   and 
  Schuster 
  being 
  a 
  gradual 
  increase 
  in 
  the 
  extent 
  and 
  intensity 
  

   of 
  the 
  flutings, 
  photographs 
  of 
  which 
  are 
  shown 
  in 
  my 
  last 
  paper. 
  

   As 
  the 
  vapour 
  becomes 
  still 
  more 
  dense, 
  I 
  find 
  that 
  a 
  narrow 
  and 
  

   very 
  black 
  line 
  appears 
  almost 
  exactly 
  in 
  the 
  centre 
  of 
  the 
  yellowish- 
  

   green 
  band. 
  My 
  first 
  impression 
  was 
  that 
  this 
  corresponded 
  in 
  

   position 
  to 
  one 
  of 
  the 
  well-known 
  emission-lines 
  of 
  the 
  element 
  ; 
  

   but 
  I 
  now 
  find 
  that 
  it 
  is 
  apparently 
  a 
  new 
  absorption-line, 
  its 
  wave- 
  

   length 
  being 
  very 
  nearly 
  552. 
  

  

  As 
  I 
  have 
  already 
  shown, 
  the 
  channelled 
  absorption 
  extends 
  

   throughout 
  the 
  entire 
  range 
  of 
  visible 
  spectrum. 
  The 
  behaviour 
  

   in 
  the 
  ultra-violet 
  and 
  infra-red 
  we 
  are 
  now 
  examining. 
  I 
  am 
  in 
  

   hopes 
  of 
  detecting 
  photographically 
  anomalous 
  dispersion 
  at 
  the, 
  

   infra-red 
  lines 
  which 
  appear 
  in 
  the 
  emission-spectrum. 
  This 
  is 
  a 
  

   matter 
  of 
  considerable 
  importance, 
  for 
  they 
  are 
  near 
  enough 
  to 
  the 
  

   visible 
  spectrum 
  to 
  modify 
  the 
  dispersion-curve 
  which 
  I 
  have 
  

   already 
  given. 
  

  

  It 
  is 
  also 
  important 
  to 
  determine 
  whether 
  the 
  complicated 
  

   channelled 
  spectrum 
  depends 
  on 
  the 
  density 
  of 
  the 
  vapour 
  or 
  its 
  

   amount 
  ; 
  in 
  other 
  words, 
  whether 
  the 
  thousands 
  of 
  free 
  periods 
  

   are 
  present 
  in 
  the 
  sodium 
  atom 
  or 
  mole 
  c 
  ale 
  as 
  it 
  ordinarily 
  occurs 
  

   in 
  flames 
  or 
  vapour 
  of 
  small 
  density, 
  or 
  whether 
  molecular 
  aggre- 
  

   gates 
  form 
  under 
  the 
  conditions 
  linger 
  which 
  it 
  is 
  observed. 
  I 
  

   propose 
  to 
  investigate 
  this 
  question 
  by 
  means 
  of 
  very 
  long 
  

   absorption-tubes 
  heated 
  to 
  a 
  rather 
  low 
  temperature. 
  

  

  Baltimore, 
  January 
  28, 
  1902. 
  

  

  