﻿Polarized 
  Light 
  by 
  Fluorescent 
  Gases. 
  187 
  

  

  The 
  foregoing 
  experiments 
  prove 
  that 
  we 
  must 
  seek 
  else- 
  

   where 
  for 
  the 
  cause 
  of 
  the 
  partial 
  polarization. 
  The 
  decrease 
  

   in 
  the 
  amount 
  of 
  polarization 
  with 
  increasing 
  temperature 
  

   made 
  me 
  think 
  that 
  the 
  increased 
  density 
  of 
  the 
  vapour 
  might 
  

   have 
  something 
  to 
  do 
  with 
  the 
  phenomenon, 
  in 
  other 
  words, 
  

   that 
  the 
  depolarizing 
  factor 
  might 
  be 
  the 
  molecular 
  collisions. 
  

  

  To 
  test 
  this 
  hypothesis 
  a 
  small 
  amount 
  of 
  nitrogen 
  was 
  

   introduced 
  into 
  the 
  tube, 
  sufficient 
  to 
  raise 
  the 
  pressure 
  to 
  

   12 
  mms. 
  The 
  number 
  of 
  collisions 
  must 
  now 
  be 
  vastly 
  in 
  

   excess 
  of 
  what 
  obtains 
  with 
  sodium 
  vapour 
  in 
  vacuo, 
  for 
  in 
  

   this 
  case 
  the 
  actual 
  pressure 
  of 
  the 
  metallic 
  vapour 
  is 
  probably 
  

   never 
  in 
  excess 
  of 
  a 
  millimetre 
  or 
  two, 
  if 
  it 
  is 
  as 
  great 
  as 
  that. 
  

   In 
  fact 
  I 
  now 
  feel 
  sure 
  that 
  the 
  partial 
  pressure 
  of 
  the 
  sodium 
  

   vapour 
  can 
  never 
  be 
  greater 
  than 
  the 
  pressure 
  of 
  the 
  residual 
  

   gas 
  in 
  the 
  tube. 
  In 
  my 
  earlier 
  work 
  I 
  overestimated 
  the 
  

   density 
  of 
  the 
  vapour, 
  and 
  imagined 
  that 
  it 
  must 
  possess 
  a 
  

   viscosity, 
  but 
  I 
  am 
  now 
  convinced 
  that 
  this 
  is 
  not 
  so. 
  

  

  The 
  polarization 
  did 
  not 
  appear 
  to 
  be 
  decreased 
  by 
  the 
  

   presence 
  of 
  the 
  nitrogen, 
  which 
  made 
  it 
  seem 
  probable 
  that 
  

   molecular 
  collision 
  was 
  not 
  .the 
  depolarizing 
  agency. 
  Tem- 
  

   perature 
  appears 
  to 
  be 
  the 
  only 
  remaining 
  factor, 
  and 
  I 
  am 
  

   inclined 
  to 
  attribute 
  the 
  depolarization 
  to 
  the 
  rotation 
  of 
  the 
  

   molecule 
  while 
  it 
  is 
  traversing 
  its 
  mean 
  free 
  path. 
  The 
  speed 
  

   of 
  rotation 
  will 
  increase 
  with 
  the 
  temperature, 
  for 
  the 
  energy 
  

   of 
  a 
  gas 
  is 
  partly 
  energy 
  of 
  translation 
  and 
  partly 
  energy 
  of 
  

   rotation, 
  the 
  ratio 
  being 
  constant. 
  (Sodium 
  is, 
  however, 
  

   regarded 
  as 
  monatomic, 
  and 
  the 
  relation 
  may 
  not 
  hold.) 
  

  

  That 
  there 
  is 
  a 
  depolarizing 
  agent 
  is 
  probable 
  from 
  the 
  most 
  

   elementary 
  theory. 
  If 
  we 
  assume 
  the 
  electrons 
  to 
  be 
  free 
  to 
  

   move 
  in 
  any 
  direction, 
  under 
  the 
  influence 
  of 
  the 
  electric 
  

   forces 
  in 
  the 
  light-waves, 
  it 
  seems 
  probable 
  that 
  with 
  a 
  

   polarized 
  beam 
  of 
  exciting 
  light, 
  we 
  should 
  have 
  plane- 
  

   polarized 
  light 
  emitted 
  by 
  the 
  vapour 
  in 
  directions 
  making 
  

   an 
  angle 
  of 
  90° 
  with 
  the 
  exciting 
  beam. 
  If 
  the 
  stimulating 
  

   light 
  were 
  plane-polarized 
  to 
  start 
  with, 
  there 
  would 
  be 
  no 
  

   fluorescent 
  emission 
  at 
  90° 
  in 
  directions 
  parallel 
  to 
  the 
  electric 
  

   vector 
  in 
  the 
  exciting 
  light. 
  In 
  other 
  words, 
  the 
  vapour 
  

   would 
  behave 
  in 
  much 
  the 
  same 
  way 
  as 
  a 
  fog 
  composed 
  of 
  

   very 
  fine 
  particles, 
  though 
  the 
  physical 
  processes 
  involved 
  

   would 
  be 
  quite 
  different 
  in 
  the 
  two 
  cases. 
  

  

  Let 
  us 
  now 
  assume 
  that 
  the 
  electron 
  is 
  able 
  to 
  move 
  along 
  

   a 
  straight 
  line, 
  the 
  position 
  of 
  which 
  is 
  fixed 
  within 
  the 
  

   molecule. 
  While 
  it 
  is 
  quite 
  improbable 
  that 
  this 
  condition 
  

   actually 
  holds, 
  the 
  conception 
  of 
  it 
  will 
  enable 
  us 
  to 
  see 
  how 
  

   the 
  introduction 
  of 
  constrained 
  motion 
  will 
  decrease 
  the 
  

   amount 
  of 
  polarized 
  light 
  emitted 
  by 
  the 
  vapour. 
  We 
  can 
  

  

  