﻿Volta 
  Electricity 
  of 
  Metals. 
  409 
  

  

  slightly 
  purple 
  towards 
  the 
  centre 
  (the 
  brown 
  parts 
  turn 
  

   purple 
  it' 
  moistened). 
  When 
  cold, 
  compared 
  with 
  the 
  clean 
  

   plate 
  it 
  now 
  gave 
  

  

  -•79 
  v., 
  

  

  a 
  fall 
  of 
  *75 
  v. 
  Thus 
  this 
  oxidized 
  zinc 
  plate 
  has 
  practically 
  

   the 
  same 
  potential 
  as 
  copper. 
  This 
  great 
  fall 
  of 
  potential 
  

   does 
  not 
  take 
  place 
  by 
  exposure 
  to 
  the 
  atmosphere 
  at 
  ordinary 
  

   temperatures, 
  unless 
  possibly 
  after 
  the 
  lapse 
  of 
  many 
  years, 
  

   for 
  the 
  potential 
  of 
  zinc 
  plates 
  which 
  have 
  been 
  left 
  unpolished 
  

   for 
  many 
  months 
  is 
  only 
  two 
  or 
  three 
  tenths 
  of 
  a 
  volt 
  below 
  

   that 
  of 
  clean 
  zinc 
  (see 
  § 
  34) 
  . 
  

  

  § 
  25. 
  Similar 
  experiments 
  were 
  made 
  with 
  copper. 
  The 
  

   film 
  of 
  oxide 
  was 
  gradually 
  increased 
  in 
  thickness 
  by 
  

   repeatedly 
  heating 
  the 
  plate, 
  and 
  the 
  potential 
  measured 
  in 
  

   each 
  stage 
  of 
  oxidation, 
  the 
  plate 
  being 
  cold. 
  There 
  was 
  a 
  

   change 
  of 
  about 
  *03 
  v, 
  in 
  the 
  negative 
  direction 
  before 
  the 
  

   oxide-film 
  became 
  visible, 
  and 
  further 
  increase 
  in 
  the 
  thickness 
  

   of 
  the 
  film 
  was 
  judged 
  by 
  the 
  gradual 
  change 
  of 
  colour, 
  until 
  

   the 
  well-known 
  dull 
  purplish 
  black 
  of 
  massive 
  copper 
  oxide 
  

   was 
  attained. 
  The 
  potential, 
  measured 
  each 
  time 
  when 
  the 
  

   plate 
  was 
  cold, 
  gradually 
  fell 
  until 
  it 
  reached 
  a 
  limit 
  of 
  about 
  

   *30 
  volt 
  negative 
  to 
  a 
  standard 
  gold 
  plate, 
  which 
  value 
  may 
  

   therefore 
  be 
  taken 
  as 
  the 
  potential 
  of 
  a 
  mass 
  of 
  copper 
  oxide 
  

   at 
  15° 
  C. 
  It 
  may 
  be 
  remarked 
  that 
  there 
  are 
  good 
  reasons, 
  

   which 
  will 
  be 
  given 
  in 
  Chap. 
  IX., 
  for 
  supposing 
  that 
  oxida- 
  

   tion 
  does 
  not 
  commence 
  in 
  air 
  unless 
  the 
  temperature 
  of 
  the 
  

   plate 
  be 
  raised 
  above 
  80° 
  (J. 
  

  

  § 
  26. 
  A 
  plate 
  of 
  copper 
  which 
  had 
  been 
  polished 
  on 
  glass- 
  

   paper 
  and 
  then 
  on 
  wash-leather 
  gave 
  

  

  + 
  •20 
  v. 
  

  

  with 
  a 
  standard 
  gold 
  plate, 
  and 
  was 
  next 
  held 
  in 
  iodine 
  vapour 
  

   for 
  a 
  short 
  time. 
  It 
  looked 
  moist 
  at 
  first, 
  but 
  quickly 
  became 
  

   drv. 
  In 
  this 
  state 
  it 
  gave 
  

  

  -•34 
  v. 
  

   with 
  the 
  same 
  standard, 
  a 
  change 
  of 
  "54 
  v. 
  in 
  the 
  negative 
  

   direction. 
  The 
  surface 
  had 
  a 
  dull 
  colour 
  with 
  a 
  whitish 
  

   " 
  bloom 
  " 
  on 
  it, 
  the 
  tint 
  resembling 
  that 
  of 
  clean 
  copper, 
  

   though 
  rather 
  darker. 
  Exposure 
  to 
  iodine 
  vapour 
  thus 
  makes 
  

   the 
  surface 
  of 
  copper 
  nearly 
  as 
  negative 
  as 
  continued 
  ex- 
  

   posure 
  to 
  the 
  atmosphere 
  at 
  a 
  high 
  temperature. 
  

  

  § 
  27. 
  The 
  potential 
  of 
  a 
  clean 
  zinc 
  plate 
  fell 
  about 
  *12 
  v. 
  

   when 
  it 
  had 
  been 
  exposed 
  to 
  iodine 
  vapour. 
  

  

  § 
  28. 
  The 
  iodine 
  compounds 
  formed 
  on 
  the 
  metallic 
  surfaces 
  

   were 
  not 
  stable, 
  as 
  the 
  appearance 
  and 
  potential 
  of 
  the 
  plates 
  

   altered 
  considerably 
  during 
  twenty 
  hours' 
  exposure 
  to 
  the 
  

   atmosphere. 
  

  

  