﻿162 
  Lubricating 
  Properties 
  of 
  Thin 
  Oily 
  Films. 
  

  

  and 
  as 
  regards 
  olive 
  oil, 
  Miss 
  Poekels 
  showed 
  that 
  the 
  

   behaviour 
  of 
  purified 
  oil 
  is 
  quite 
  different 
  from 
  that 
  of 
  

   the 
  common 
  oil. 
  She 
  quotes 
  Richter 
  (Nature, 
  vol. 
  xlix. 
  

   p. 
  488) 
  as 
  expressing 
  the 
  opinion 
  that 
  the 
  tendency 
  of 
  oil 
  

   to 
  spread 
  itself 
  on 
  water 
  is 
  only 
  due 
  to 
  the 
  free 
  oleic 
  acid 
  

   contained 
  in 
  it, 
  and 
  that 
  if 
  it 
  were 
  possible 
  to 
  completely 
  

   purify 
  the 
  oil 
  from 
  oleic 
  acid, 
  it 
  would 
  not 
  spread 
  at 
  all*. 
  

   Some 
  confusion 
  arises 
  from 
  the 
  different 
  meanings 
  attached 
  

   to 
  the 
  word 
  " 
  spreading." 
  I 
  suppose 
  no 
  one 
  disputes 
  the 
  

   rapid 
  spreading 
  upon 
  a 
  clean 
  surface 
  which 
  results 
  in 
  the 
  

   formation 
  of 
  the 
  invisible 
  mono-molecular 
  layer. 
  Miss 
  

   Poekels 
  calls 
  this 
  a 
  solution 
  current 
  — 
  a 
  rather 
  misleading 
  

   term, 
  which 
  has 
  tended 
  to 
  obscure 
  the 
  meaning 
  of 
  her 
  

   really 
  valuable 
  work. 
  It 
  is 
  the 
  second 
  kind 
  of 
  spreading 
  in 
  

   a 
  thicker 
  layer, 
  resulting 
  in 
  more 
  or 
  less 
  rapid 
  subsequent 
  

   transformations, 
  which 
  is 
  attributed 
  to 
  the 
  presence 
  of 
  oleic 
  

   acid. 
  Miss 
  Poekels 
  says 
  : 
  — 
  " 
  The 
  Provence 
  oil 
  used 
  in 
  my 
  

   experiment 
  was 
  shaken 
  up 
  twice 
  with 
  pure 
  alcohol, 
  and 
  the 
  

   rest 
  (residue) 
  of 
  the 
  latter 
  being 
  carefully 
  removed, 
  a 
  drop 
  

   of 
  the 
  oil 
  was 
  placed 
  upon 
  the 
  freshly 
  formed 
  water-surface 
  

   in 
  a 
  small 
  dish 
  by 
  means 
  of 
  a 
  brass 
  wire 
  previously 
  cleaned 
  

   by 
  ignition. 
  The 
  oil 
  did 
  not 
  really 
  spread, 
  but 
  after 
  a 
  

   momentary 
  centrifugal 
  movement, 
  during 
  which 
  several 
  

   small 
  drops 
  were 
  separated 
  from 
  it, 
  it 
  contracted 
  itself 
  

   in 
  the 
  middle 
  of 
  the 
  surface, 
  and 
  a 
  second 
  drop 
  deposited 
  

   on 
  the 
  same 
  vessel 
  remained 
  absolutely 
  motionless." 
  I 
  have 
  

   repeated 
  this 
  experiment, 
  using 
  oil 
  which 
  is 
  believed 
  to 
  

   have 
  come 
  direct 
  from 
  Italy. 
  A 
  drop 
  of 
  this 
  placed 
  upon 
  

   a 
  clean 
  water-surface 
  at 
  once 
  drives 
  dust 
  to 
  the 
  boundary 
  in 
  

   forming 
  the 
  mono-molecular 
  layer, 
  and 
  in 
  addition 
  flattens 
  

   itself 
  out 
  into 
  a 
  disk 
  of 
  considerable 
  size, 
  which 
  rapidly 
  

   undergoes 
  the 
  transformations 
  well 
  described 
  and 
  figured 
  by 
  

   Devaux. 
  The 
  same 
  oil, 
  purified 
  by 
  means 
  of 
  alcohol 
  on 
  

   Miss 
  Poekels' 
  plan, 
  behaves 
  quite 
  differently. 
  The 
  first 
  

   spreading, 
  driving 
  dust 
  to 
  the 
  boundary, 
  takes 
  place 
  

   entirely 
  as 
  before. 
  But 
  the 
  drop 
  remains 
  upon 
  the 
  water 
  as 
  

   a 
  lens, 
  and 
  flattens 
  itself 
  out, 
  if 
  at 
  all, 
  only 
  very 
  slowly. 
  Small 
  

   admixtures 
  of 
  the 
  original 
  oil 
  with 
  the 
  purified 
  oil 
  behave 
  in 
  

   an 
  intermediate 
  manner, 
  flattening 
  out 
  slowly 
  and 
  allowing 
  

   the 
  beautiful 
  transformations 
  which 
  follow 
  to 
  be 
  observed 
  at 
  

   leisure. 
  

  

  Another 
  point 
  of 
  importance 
  does 
  not 
  appear 
  to 
  have 
  been 
  

  

  noticed. 
  Water-surfaces 
  on 
  which 
  purified 
  olive 
  oil 
  stands 
  

  

  in 
  drops 
  still 
  allow 
  the 
  camphor 
  movements. 
  Very 
  small 
  

  

  fragments 
  spin 
  merrily, 
  while 
  larger 
  ones 
  by 
  their 
  slower 
  

  

  * 
  'Nature,' 
  vol. 
  1. 
  p. 
  223 
  (1894). 
  

  

  