﻿Dr. 
  J. 
  Erskine-Murray 
  on 
  Volta 
  Electricity 
  of 
  Metals. 
  399 
  

  

  the 
  temporary 
  variations 
  of 
  potential 
  due 
  to 
  change 
  of 
  tem- 
  

   perature 
  of 
  a 
  copper, 
  iron, 
  or 
  zinc 
  plate 
  : 
  and 
  (3) 
  his 
  

   experiments 
  which 
  prove 
  the 
  smallness 
  of 
  the 
  changes 
  pro- 
  

   duced 
  in 
  the 
  potential-difference 
  of 
  copper 
  and 
  zinc 
  hy 
  

   varying 
  either 
  the 
  pressure 
  or 
  the 
  nature 
  of 
  the 
  gas 
  surround- 
  

   ing 
  them. 
  

  

  § 
  2 
  a. 
  I 
  shall 
  now 
  give 
  a 
  short 
  summary 
  of 
  the 
  results 
  

   detailed 
  in 
  the 
  present 
  paper 
  : 
  — 
  

  

  (a) 
  Metals 
  covered 
  with 
  nonconducting 
  solid 
  films 
  of 
  wax 
  

   or 
  glass, 
  except 
  at 
  their 
  point 
  of 
  contact, 
  give 
  nearly 
  the 
  same 
  

   potential 
  as 
  the 
  bare 
  metals 
  in 
  air. 
  The 
  substitution 
  of 
  wax 
  

   for 
  air 
  next 
  the 
  metal 
  only 
  causes 
  a 
  small 
  change 
  which 
  may 
  

   be 
  in 
  the 
  same 
  direction 
  and 
  of 
  approximately 
  equal 
  amounts 
  

   for 
  metals 
  whose 
  potentials 
  in 
  air 
  are 
  very 
  different 
  ; 
  e. 
  g. 
  the 
  

   volta-potential-difference 
  between 
  zinc 
  and 
  copper 
  when 
  both 
  

   are 
  coated 
  with 
  solid 
  paraffin-wax 
  is 
  very 
  nearly 
  the 
  same 
  as 
  

   that 
  between 
  bare 
  zinc 
  and 
  copper. 
  In 
  this 
  connexion 
  I 
  may 
  

   mention 
  that 
  I 
  have 
  measured 
  the 
  potential 
  of 
  sodium 
  coated 
  

   with 
  wax 
  and 
  glass, 
  and 
  find 
  it 
  to 
  be 
  about 
  3'56 
  volts 
  positive 
  

   to 
  a 
  standard 
  gold 
  plate 
  (see 
  Chap. 
  X.). 
  

  

  (b) 
  A 
  metal, 
  cleaned 
  by 
  careful 
  polishing 
  and 
  scratching 
  

   with 
  emery-cloth 
  or 
  glass-paper 
  is 
  less 
  positive 
  when 
  its 
  surface 
  

   is 
  in 
  a 
  sharply 
  scratched 
  condition 
  than 
  when 
  smoothed 
  or 
  

   burnished, 
  the 
  difference 
  frequently 
  amounting 
  to 
  *2 
  or 
  *3 
  of 
  

   a 
  volt. 
  This 
  result 
  is 
  not 
  in 
  opposition 
  to 
  that 
  of 
  M. 
  Pellat, 
  

   mentioned 
  above, 
  § 
  2 
  (1), 
  for 
  the 
  conditions 
  were 
  different, 
  

   as 
  his 
  plates 
  were 
  washed 
  with 
  alcohol 
  after 
  polishing 
  while 
  

   mine 
  were 
  not 
  (see 
  Chap. 
  III. 
  and 
  also 
  §§ 
  39 
  el 
  seq.). 
  

  

  (c) 
  The 
  temperature 
  variations, 
  between 
  15° 
  C. 
  and 
  60° 
  C, 
  of 
  

   the 
  volta-potential 
  of 
  many 
  metals 
  have 
  been 
  determined, 
  both 
  

   for 
  clean 
  dry 
  metals 
  in 
  air 
  and 
  for 
  metals 
  coated 
  with 
  liquid 
  

   or 
  nonconducting 
  solid 
  films 
  ; 
  and 
  it 
  has 
  been 
  found 
  that 
  they 
  

   are 
  of 
  considerable 
  magnitude 
  in 
  both 
  cases. 
  The 
  curves 
  

   representing 
  the 
  variation 
  of 
  potential 
  with 
  temperature 
  

   appear 
  as 
  if 
  they 
  should 
  meet 
  at 
  a 
  point 
  below 
  —200° 
  C, 
  at 
  

   a 
  potential 
  about 
  '4 
  volt 
  positive 
  to 
  a 
  standard 
  gold 
  plate 
  at 
  

   16° 
  C; 
  this 
  suggests 
  that 
  voita-potential-differences 
  may 
  

   possibly 
  vanish 
  at 
  a 
  very 
  low 
  temperature 
  (see 
  Chap. 
  IX.). 
  

  

  (d) 
  A 
  liquid 
  film, 
  even 
  if 
  of 
  extreme 
  thinness, 
  may 
  cause 
  

   a 
  considerable 
  change 
  in 
  the 
  potential 
  of 
  a 
  dry 
  polished 
  plate 
  

   which 
  continues 
  permanent 
  for 
  many 
  hours 
  and 
  even 
  days 
  

   afier 
  the 
  disappearance 
  of 
  the 
  film. 
  

  

  Two 
  films 
  of 
  the 
  same 
  liquid 
  opposed 
  to 
  one 
  another 
  on 
  the 
  

   surfaces 
  of 
  two 
  plates 
  of 
  different 
  metals 
  do 
  not 
  usually 
  give 
  

   zero 
  potential-difference, 
  as 
  solid 
  conducting 
  films 
  of 
  one 
  

   material 
  would 
  do, 
  but 
  give 
  nearly 
  the 
  same 
  potential- 
  

  

  2E 
  2 
  

  

  