﻿Dispersion 
  of 
  Sodium 
  Vapour. 
  131 
  

  

  detail 
  the 
  manner 
  of 
  preparing 
  and 
  using 
  these 
  dispersion- 
  

   tubes, 
  which 
  I 
  have 
  found 
  most 
  advantageous. 
  The 
  ends 
  of 
  

   the 
  tubes 
  are 
  first 
  warmed 
  and 
  thickly 
  coated 
  with 
  sealing- 
  

   wax 
  ; 
  one 
  of 
  the 
  glass 
  straws 
  is 
  then 
  placed 
  in 
  position, 
  and 
  

   a 
  small 
  piece 
  of 
  plate 
  glass, 
  previously 
  warmed, 
  pressed 
  

   against 
  the 
  wax, 
  any 
  crevices 
  around 
  the 
  straw 
  being 
  closed 
  

   with 
  wax. 
  The 
  leading-in 
  tube 
  is 
  next 
  placed 
  in 
  position, 
  

   and 
  a 
  piece 
  of 
  freshly-cut 
  sodium 
  (about 
  5 
  mm. 
  on 
  a 
  side) 
  

   inserted. 
  The 
  other 
  window 
  is 
  then 
  cemented 
  to 
  the 
  tube, 
  

   and 
  the 
  current 
  of 
  hydrogen 
  started 
  as 
  soon 
  as 
  possible. 
  

   Some 
  experience 
  is 
  necessary 
  properly 
  to 
  regulate 
  the 
  hydrogen 
  

   stream 
  during 
  the 
  experiment. 
  When 
  the 
  tube 
  is 
  first 
  

   heated 
  much 
  white 
  smoke 
  forms. 
  If 
  a 
  stream 
  corresponding 
  

   to 
  about 
  one 
  bubble 
  per 
  second 
  is 
  allowed 
  to 
  flow, 
  the 
  smoke 
  

   will 
  usually 
  clear 
  up 
  in 
  a 
  few 
  minutes 
  and 
  give 
  little 
  trouble. 
  

   The 
  tube 
  should 
  be 
  heated 
  by 
  means 
  of 
  a 
  Bunsen 
  burner 
  

   turned 
  down 
  low, 
  the 
  tip 
  of 
  the 
  flame 
  playing 
  against 
  the 
  

   bottom 
  of 
  the 
  tube. 
  If 
  a 
  sodium 
  flame 
  is 
  placed 
  behind 
  the 
  

   tube 
  the 
  formation 
  of 
  the 
  vapour 
  can 
  be 
  watched, 
  for 
  it 
  

   appears 
  almost 
  jet-black 
  against 
  the 
  flame, 
  though 
  quite 
  

   colourless 
  in 
  white 
  light. 
  The 
  behaviour 
  of 
  the 
  vapour 
  is 
  

   somewhat 
  peculiar. 
  It 
  grows 
  out 
  from 
  the 
  sodium 
  globule 
  

   as 
  a 
  dark 
  atmosphere 
  with 
  a 
  sharply 
  defined 
  surface, 
  which 
  

   clings 
  to 
  the 
  globule 
  with 
  great 
  tenacity. 
  It 
  resembles 
  at 
  

   first 
  a 
  thick 
  growth 
  of 
  mould 
  more 
  than 
  anything 
  else 
  that 
  

   I 
  can 
  think 
  of, 
  and 
  a 
  sudden 
  gust 
  of 
  hydrogen 
  scarcely 
  

   moves 
  it 
  at 
  all. 
  A 
  wire 
  pushed 
  up 
  through 
  it 
  drags 
  a 
  certain 
  

   amount 
  above 
  the 
  free 
  surface 
  in 
  much 
  the 
  same 
  manner 
  as 
  

   a 
  stick 
  pushed 
  up 
  through 
  the 
  surface 
  of 
  thick 
  molasses 
  

   would 
  do. 
  If 
  the 
  tube 
  be 
  inverted 
  the 
  black 
  cloud 
  clings 
  to 
  

   the 
  upper 
  surface, 
  behaving 
  on 
  the 
  whole 
  like 
  a 
  very 
  viscous 
  

   mass. 
  It 
  is 
  even 
  possible 
  to 
  dip 
  some 
  of 
  it 
  up 
  on 
  a 
  wire. 
  

  

  These 
  peculiar 
  physical 
  properties 
  of 
  the 
  metallic 
  vapour 
  I 
  

   have 
  as 
  yet 
  only 
  studied 
  in 
  a 
  very 
  superficial 
  manner, 
  and 
  

   I 
  mention 
  them 
  now, 
  only 
  because 
  it 
  appears 
  to 
  me 
  that 
  

   there 
  is 
  some 
  connexion 
  between 
  them 
  and 
  the 
  optical 
  

   behaviour 
  of 
  the 
  medium. 
  

  

  I 
  am 
  of 
  the 
  opinion 
  that 
  the 
  apparent 
  viscosity 
  is 
  an 
  

   illusion, 
  and 
  that 
  the 
  sharply- 
  defined 
  surface 
  is 
  merely 
  the 
  

   boundary 
  at 
  which 
  either 
  condensation 
  or 
  chemical 
  action 
  

   (the 
  hydrogen 
  not 
  being 
  pure) 
  is 
  taking 
  place. 
  The 
  process 
  

   of 
  dipping 
  the 
  vapour 
  up 
  on 
  the 
  wire 
  might 
  be 
  explained 
  by 
  

   condensation 
  on 
  the 
  wire 
  followed 
  by 
  vaporization. 
  A 
  more 
  

   careful 
  study 
  of 
  the 
  physical 
  behaviour 
  of 
  the 
  vapour 
  will 
  be 
  

   made 
  some 
  time 
  in 
  the 
  future. 
  

  

  The 
  apparatus 
  employed 
  in 
  the 
  study 
  of 
  the 
  dispersion 
  

  

  K2 
  

  

  