﻿414 
  Dr. 
  J. 
  Erskine-Murray 
  on 
  

  

  VII. 
  Thick 
  Liquid 
  Films 
  on 
  Metals. 
  

  

  § 
  45. 
  In 
  experimenting 
  with 
  films 
  of 
  sensible 
  thickness 
  

   the 
  liquid 
  was 
  usually 
  placed 
  on 
  a 
  flat 
  metal 
  plate, 
  its 
  surface- 
  

   tension 
  being 
  sufficient 
  to 
  retain 
  it 
  if 
  its 
  depth 
  were 
  not 
  more 
  

   than 
  *3 
  or 
  *4 
  centim. 
  In 
  other 
  cases 
  a 
  shallow 
  metal 
  dish 
  

   was 
  used. 
  The 
  behaviour 
  of 
  water 
  puzzled 
  me 
  for 
  some 
  

   time, 
  and 
  made 
  me 
  realize 
  the 
  extreme 
  difficulty 
  of 
  obtaining 
  

   a 
  clean 
  liquid 
  surface. 
  After 
  some 
  preliminary 
  experiments 
  

   I 
  found 
  that 
  distilled 
  water 
  which 
  has 
  not 
  been 
  exposed 
  

   to 
  the 
  atmosphere 
  gives 
  fairly 
  constant 
  results 
  when 
  lying 
  to 
  

   a 
  depth 
  of 
  *2 
  centim. 
  on 
  one 
  of 
  the 
  standard 
  gold 
  plates. 
  

   When 
  water 
  on 
  gold 
  formed 
  the 
  one 
  plate 
  and 
  a 
  dry 
  standard 
  

   gold 
  plate 
  (see 
  § 
  8) 
  the 
  other, 
  the 
  value 
  found 
  was 
  '10 
  volt, 
  

   the 
  water 
  being 
  negative. 
  Thus 
  clean 
  water 
  in 
  contact 
  with 
  

   gold 
  is 
  about 
  *10 
  volt 
  negative 
  to 
  it. 
  The 
  value 
  found 
  is 
  

   quite 
  different 
  if 
  the 
  water 
  has 
  been 
  exposed 
  to 
  the 
  air 
  for 
  

   some 
  time, 
  and 
  is 
  usually 
  positive 
  instead 
  of 
  negative 
  to 
  the 
  

   standard 
  plate. 
  

  

  § 
  46. 
  Air 
  expelled 
  from 
  the 
  lungs 
  produces 
  a 
  very 
  marked 
  

   change 
  in 
  the 
  contact-potential 
  of 
  water. 
  By 
  blowing 
  through 
  

   a 
  fine 
  glass 
  tube 
  which 
  dipped 
  under 
  the 
  surface 
  of 
  a 
  layer 
  

   of 
  distilled 
  water 
  on 
  a 
  gold 
  plate, 
  the 
  potential 
  was 
  changed 
  

   from 
  

  

  -•10 
  v. 
  

  

  w 
  7 
  ith 
  the 
  standard 
  plate 
  to 
  

  

  + 
  •16 
  v., 
  

  

  and 
  remained 
  at 
  this 
  latter 
  value 
  for 
  more 
  than 
  half 
  an 
  hour, 
  

   showing 
  that 
  the 
  change 
  was 
  not 
  due 
  to 
  a 
  rise 
  in 
  temperature. 
  

   The 
  curious 
  point 
  is 
  that 
  the 
  change 
  seems 
  to 
  be 
  only 
  in 
  the 
  

   surface 
  of 
  the 
  liquid; 
  for 
  when 
  most 
  of 
  it 
  was 
  shaken 
  off 
  the 
  

   potential 
  fell 
  to 
  

  

  -•05 
  v., 
  

  

  although 
  the 
  plate 
  was 
  still 
  wet 
  all 
  over. 
  This 
  appears 
  as 
  if 
  

   the 
  alteration 
  were 
  due 
  to 
  a 
  surface 
  film 
  of 
  oily 
  or 
  dusty 
  

   matter 
  on 
  the 
  water. 
  It 
  was 
  found 
  that 
  distilled 
  water 
  which 
  

   had 
  been 
  exposed 
  to 
  the 
  air 
  of 
  the 
  room 
  for 
  some 
  time 
  gave 
  

   the 
  same 
  value 
  as 
  water 
  which 
  had 
  been 
  breathed 
  through. 
  

  

  § 
  47. 
  A 
  lead 
  plate 
  and 
  a 
  flat 
  circular 
  leaden 
  dish 
  were 
  

   scraped 
  clean 
  with 
  a 
  knife, 
  and 
  next 
  morning, 
  when 
  com- 
  

   pared 
  with 
  each 
  other, 
  they 
  gave 
  

  

  + 
  '10 
  v., 
  

  

  the 
  plate 
  being 
  positive 
  to 
  the 
  dish. 
  Water 
  was 
  now 
  poured 
  

   into 
  the 
  dish 
  until 
  it 
  was 
  nearly 
  full, 
  *4 
  centim. 
  deep, 
  the 
  

  

  