﻿972 
  

  

  Mr. 
  R. 
  W, 
  Lawson 
  on 
  the 
  Spectra 
  of 
  

  

  b, 
  whilst 
  at 
  lower 
  pressures 
  it 
  expands 
  laterally 
  to 
  a 
  small 
  

   degree, 
  but 
  contracts 
  towards 
  the 
  capillary 
  (see 
  d). 
  Con- 
  

   comitant 
  with 
  this 
  effect 
  the 
  negative 
  glow 
  (dotted) 
  discharge, 
  

  

  Fig. 
  4. 
  

  

  II 
  w 
  II 
  I' 
  

  

  PRE«VR.£ 
  

  

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  whilst 
  not 
  visible 
  in 
  6, 
  extends 
  still 
  further 
  down 
  the 
  tube 
  in 
  

   d 
  than 
  in 
  c. 
  These 
  effects 
  would 
  account 
  for 
  results 
  obtained 
  

   with 
  hydrogen, 
  nitrogen, 
  and 
  oxygen, 
  where 
  it 
  was 
  found 
  

   that 
  increase 
  of 
  current 
  in 
  the 
  bulb 
  at 
  about 
  1 
  mm. 
  pressure, 
  

   intensified 
  the 
  negative-glow 
  spectrum 
  in 
  the 
  bulb, 
  and 
  made 
  

   it 
  extend 
  nearer 
  towards 
  the 
  capillary. 
  In 
  addition, 
  it 
  was 
  

   observed 
  that 
  the 
  effect 
  of 
  reduction 
  of 
  pressure 
  in 
  the 
  dis- 
  

   charge 
  tube 
  was 
  to 
  produce 
  a 
  gradual 
  transition 
  of 
  the 
  

   spectrum 
  of 
  the 
  bulb 
  from 
  pure 
  positive 
  column 
  to 
  almost 
  

   pure 
  negative 
  glow. 
  In 
  this 
  last 
  case 
  a 
  slight 
  admixture 
  of 
  

   the 
  less 
  refrangible 
  parts 
  of 
  the 
  spectrum 
  of 
  the 
  positive 
  

   column 
  caused 
  the 
  lack 
  of 
  purity. 
  When 
  the 
  pressure 
  of 
  

   the 
  gas 
  in 
  the 
  discharge 
  tube 
  reached 
  a 
  certain 
  critical 
  value 
  

   the 
  form 
  and 
  colour 
  of 
  the 
  bulb 
  discharge 
  changed, 
  and 
  the 
  

   bulb 
  was 
  filled 
  with 
  a 
  glow 
  of 
  uniform 
  intensity. 
  The 
  

   change 
  appeared 
  to 
  be 
  quite 
  abrupt, 
  and 
  corresponded 
  to 
  the 
  

   stage 
  when 
  the 
  negative 
  glow 
  just 
  filled 
  the 
  bulb. 
  The 
  effect 
  

   was 
  very 
  striking 
  in 
  the 
  case 
  of 
  nitrogen, 
  for 
  just 
  previous 
  

   to 
  the 
  attainment 
  of 
  this 
  critical 
  pressure 
  the 
  bulb 
  was 
  filled 
  

   with 
  a 
  blue-violet 
  glow 
  characteristic 
  of 
  the 
  negative 
  glow 
  

   in 
  nitrogen, 
  then 
  it 
  suddenly 
  changed 
  colour 
  from 
  the 
  above 
  

   to 
  a 
  bright 
  rosy 
  purple. 
  For 
  nitrogen 
  this 
  critical 
  pressure 
  

   was 
  about 
  0*4 
  mm. 
  In 
  the 
  case 
  of 
  hydrogen, 
  the 
  effect 
  was 
  

   by 
  no 
  means 
  so 
  marked, 
  but, 
  nevertheless, 
  it 
  was 
  observed. 
  

   The 
  occurrence 
  of 
  both 
  the 
  primary 
  and 
  secondary 
  spectra 
  

   of 
  hydrogen 
  in 
  the 
  bulb 
  and 
  capillary 
  rendered 
  the 
  colour- 
  

   changes 
  less 
  distinct 
  with 
  hydrogen, 
  and 
  in 
  this 
  case 
  the 
  

   critical 
  pressure 
  appeared 
  to 
  be 
  in 
  the 
  neighbourhood 
  of 
  3 
  mm. 
  

   That 
  this 
  value 
  should 
  be 
  so 
  different 
  from 
  that 
  for 
  nitrogen 
  

   was 
  unexpected, 
  but 
  repetition 
  led 
  to 
  the 
  same 
  results 
  in 
  

  

  