﻿401 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Electrical 
  Resonance 
  of 
  

  

  films 
  so 
  sensitive, 
  that 
  merely 
  moistening 
  the 
  outside 
  of 
  the 
  

   bulb 
  and 
  blowing 
  upon 
  it 
  was 
  sufficient 
  to 
  produce 
  the 
  change. 
  

   On 
  wiping 
  it, 
  or 
  on 
  stroking 
  the 
  bulb 
  with 
  the 
  finger, 
  the 
  

   original 
  colour 
  would 
  return. 
  This 
  was 
  strongly 
  suggestive 
  

   o£ 
  a 
  molecular 
  change, 
  and 
  for 
  a 
  while 
  I 
  was 
  inclined 
  to 
  

   regard 
  it 
  as 
  analogous 
  to 
  the 
  well-known 
  case 
  of 
  iodide 
  of 
  

   mercury. 
  The 
  final 
  solution 
  occurred 
  to 
  me 
  in 
  the 
  following- 
  

   way. 
  I 
  had 
  been 
  for 
  some 
  time 
  endeavouring 
  to 
  introduce 
  

   into 
  the 
  bulb 
  some 
  fluid 
  of 
  high 
  dielectric 
  constant, 
  to 
  deter- 
  

   mine 
  its 
  effect 
  on 
  the 
  colour. 
  All 
  fluids 
  tried 
  apparently 
  

   contained 
  sufficient 
  oxygen 
  to 
  instantly 
  bleach 
  the 
  films. 
  The 
  

   introduction 
  of 
  such 
  a 
  fluid 
  should, 
  it 
  seemed 
  to 
  me, 
  increase 
  

   the 
  capacity, 
  slow 
  down 
  the 
  period, 
  and 
  shift 
  the 
  absorption- 
  

   band 
  towards 
  the 
  red, 
  or 
  perhaps 
  out 
  of 
  the 
  visible 
  spectrum 
  

   entirely. 
  Films 
  hud 
  often 
  been 
  obtained 
  which 
  showed 
  

   absolutely 
  no 
  trace 
  of 
  colour-change 
  on 
  cooling, 
  and 
  it 
  occurred 
  

   to 
  me 
  that 
  possibly 
  traces 
  of 
  hydrocarbon 
  vapour, 
  from 
  the 
  

   ligroin 
  in 
  which 
  the 
  metal 
  had 
  been 
  kept, 
  might 
  be 
  condensed 
  

   on 
  the 
  wall 
  by 
  the 
  application 
  of 
  a 
  cold 
  body. 
  I 
  first 
  prepared 
  

   some 
  films 
  from 
  sodium 
  and 
  potassium 
  which 
  had 
  been 
  heated 
  

   for 
  some 
  time 
  in 
  vacuo, 
  in 
  order 
  to 
  remove 
  all 
  traces 
  of 
  the 
  

   hydrocarbon. 
  These 
  films 
  were 
  invariably 
  unresponsive, 
  to 
  

   either 
  ice 
  or 
  solid 
  carbonic 
  acid. 
  I 
  next 
  made 
  a 
  double 
  bulb, 
  

   and 
  formed 
  a 
  film 
  in 
  the 
  usual 
  way 
  in 
  one 
  of 
  the 
  bulbs. 
  

   This 
  film 
  proved 
  to 
  be 
  exceedingly 
  sensitive 
  to 
  the 
  application 
  

   of 
  ice, 
  changing 
  from 
  apple-green 
  to 
  the 
  colour 
  of 
  dense 
  

   cobalt-glass. 
  I 
  now 
  packed 
  the 
  other 
  bulb 
  in 
  carbon 
  dioxide 
  

   and 
  ether, 
  in 
  order 
  to 
  condense 
  in 
  it 
  all 
  of 
  the 
  suspected 
  

   hydrocarbon 
  vapour. 
  It 
  was 
  then 
  found 
  that 
  the 
  film 
  was 
  

   no 
  longer 
  sensitive 
  to 
  the 
  application 
  of 
  cold. 
  To 
  make 
  

   absolutely 
  sure 
  that 
  the 
  true 
  cause 
  had 
  been 
  found, 
  I 
  cooled 
  a 
  

   small 
  spot 
  on 
  a 
  pink 
  film 
  to 
  a 
  low 
  temperature 
  by 
  means 
  of 
  a 
  

   small 
  piece 
  of 
  carbon 
  dioxide, 
  which 
  was 
  left 
  on 
  the 
  bulb 
  for 
  

   some 
  time. 
  The 
  spot 
  became 
  almost 
  colourless, 
  the 
  absorption 
  

   having 
  passed 
  entirely 
  out 
  of 
  the 
  spectrum. 
  On 
  examining 
  the 
  

   spot 
  quickly 
  under 
  the 
  microscope, 
  I 
  had 
  no 
  trouble 
  in 
  seeing 
  

   minute 
  globules 
  of 
  a 
  transparent 
  liquid, 
  sufficient 
  quantity 
  

   having 
  condensed 
  to 
  form 
  droplets. 
  These 
  grew 
  rapidly 
  

   smaller 
  as 
  the 
  glass 
  warmed, 
  and 
  the 
  instant 
  they 
  disappeared, 
  

   or 
  perhaps 
  a 
  fraction 
  of 
  a 
  second 
  afterwards, 
  the 
  field 
  of 
  the 
  

   instrument 
  became 
  first 
  bluish, 
  then 
  pink, 
  in 
  consequence 
  of 
  

   the 
  entrance 
  of 
  the 
  absorption-band 
  into 
  the 
  spectrum 
  again. 
  

   There 
  is 
  a 
  temperature 
  effect 
  of 
  a 
  different 
  nature, 
  which 
  

   at 
  first 
  seems 
  wholly 
  inexplicable, 
  but 
  which 
  I 
  now 
  think 
  I 
  

   understand. 
  On 
  one 
  or 
  two 
  occasions 
  I 
  have 
  had 
  potassium 
  

   films, 
  originally 
  deep 
  violet, 
  become 
  almost 
  transparent 
  on 
  

  

  