﻿416 
  Dr. 
  J. 
  Erskine-Murray 
  on 
  

  

  paper 
  saturated 
  with 
  copper 
  sulphate 
  gave 
  about 
  +'10 
  volt 
  

   with 
  the 
  same 
  standard 
  plate. 
  As 
  no 
  special 
  precautions 
  

   were 
  taken 
  in 
  these 
  experiments 
  to 
  obtain 
  a 
  perfectly 
  pure 
  

   liquid 
  surface, 
  one 
  cannot, 
  especially 
  in 
  view 
  of 
  the 
  changes 
  

   described 
  in 
  § 
  46, 
  be 
  quite 
  certain 
  that 
  they 
  represent 
  the 
  true 
  

   potential 
  of 
  copper 
  coated 
  with 
  a 
  solution 
  of 
  its 
  sulphate. 
  

   They 
  agree, 
  however, 
  with 
  Professors 
  Ayrton 
  and 
  Perry's 
  

   values 
  for 
  saturated 
  and 
  non-saturated 
  solutions. 
  

  

  § 
  51. 
  One 
  of 
  the 
  standard 
  gold 
  plates 
  which 
  had 
  been 
  

   polished 
  with 
  HolhVs 
  plate-powder 
  used 
  dry 
  gave, 
  with 
  a 
  

   similar 
  plate 
  which 
  had 
  a 
  layer 
  of 
  alcohol 
  on 
  it, 
  

  

  . 
  --13 
  v. 
  

  

  W 
  hen 
  the 
  alcohol 
  had 
  dried 
  up 
  the 
  value 
  was 
  

  

  -•05 
  v., 
  

  

  the 
  polished 
  plate 
  being 
  negative 
  as 
  before. 
  

  

  § 
  52. 
  The 
  conclusion 
  which 
  I 
  draw 
  from 
  the 
  experiments 
  

   described 
  in 
  this 
  and 
  the 
  preceding 
  chapter 
  and 
  in 
  the 
  tables 
  is, 
  

   that 
  a 
  layer 
  of 
  liquid 
  on 
  a 
  metallic 
  surface 
  does 
  not 
  give, 
  with 
  

   a 
  metal 
  separated 
  from 
  it 
  by 
  air, 
  a 
  definite 
  potential-difference 
  

   of 
  its 
  own, 
  as 
  in 
  similar 
  circumstances 
  a 
  solid 
  conducting 
  film 
  

   would 
  do, 
  but 
  merely 
  adds 
  a 
  certain 
  amount 
  to 
  that 
  of 
  the 
  

   plate 
  on 
  which 
  it 
  lies. 
  Thus 
  two 
  different 
  metals 
  coated 
  with 
  

   layers 
  of 
  the 
  same 
  liquid 
  do 
  not, 
  as 
  a 
  rule 
  in 
  air, 
  give 
  zero 
  

   potential-difference, 
  but 
  usually 
  give 
  nearly 
  the 
  same 
  potential- 
  

   difference 
  as 
  the 
  dry 
  metals. 
  For 
  instance, 
  the 
  potential 
  of 
  

   lead 
  with 
  an 
  alcohol 
  layer 
  *1 
  centim. 
  deep 
  on 
  it 
  is 
  about 
  

   •13 
  volt 
  higher 
  than 
  that 
  of 
  dry 
  lead, 
  while 
  that 
  of 
  wet 
  copper 
  

   is 
  about 
  the 
  same 
  amount 
  higher 
  than 
  the 
  average 
  value 
  for 
  

   dry 
  copper. 
  There 
  is 
  no 
  tendency 
  shown 
  for 
  a 
  liquid 
  film 
  to 
  

   take 
  up 
  a 
  definite 
  potential 
  independent 
  of 
  that 
  of 
  the 
  metal 
  

   on 
  which 
  it 
  lies, 
  with 
  any 
  metal 
  separated 
  from 
  it 
  by 
  a 
  

   dielectric, 
  as 
  a 
  solid 
  conducting 
  film 
  would 
  do. 
  This 
  seems 
  to 
  

   be 
  the 
  most 
  important 
  distinction 
  between 
  solid 
  and 
  liquid 
  

   conductors, 
  and 
  it 
  is 
  in 
  accord 
  with 
  what 
  is 
  known 
  of 
  voltaic 
  

   cells; 
  for 
  if 
  the 
  potential- 
  differences 
  in 
  the 
  chain 
  copper- 
  

   water-zinc 
  were 
  equal 
  and 
  opposite 
  to 
  that 
  of 
  zinc-copper, 
  as 
  

   copper-iron-zinc 
  is 
  to 
  zinc-copper, 
  we 
  should 
  have 
  no 
  electro- 
  

   motive 
  force 
  in 
  the 
  circuit, 
  when 
  the 
  materials 
  are 
  all 
  at 
  one 
  

   temperature. 
  In 
  most 
  of 
  my 
  experiments 
  the 
  type 
  is 
  copper- 
  

   water-nonconductor 
  (air) 
  -water-zinc 
  and 
  not 
  copper- 
  water- 
  

   zinc; 
  the 
  members 
  of 
  the 
  first 
  and 
  last 
  pairs 
  are 
  in 
  contact, 
  

   while 
  a 
  non-conductor 
  intervenes 
  between 
  the 
  two 
  free 
  water 
  

   surfaces, 
  these 
  being 
  able 
  to 
  take 
  up 
  their 
  natural 
  contact- 
  

   potential-differences 
  with 
  the 
  metals 
  they 
  touch 
  ; 
  but 
  the 
  sum 
  

  

  