﻿4:22 
  Dr. 
  J. 
  Erskine-Murray 
  on 
  

  

  It 
  was 
  again 
  warmed 
  slightly, 
  and 
  when 
  at 
  about 
  30° 
  C. 
  gave 
  

  

  Time. 
  Contact-potential. 
  

  

  h. 
  m. 
  v. 
  

  

  42 
  --010. 
  

  

  When 
  16° 
  C, 
  

  

  4 
  48 
  --040; 
  

  

  its 
  potential 
  thus 
  coming 
  back 
  to 
  very 
  nearly 
  the 
  original 
  

   value. 
  

  

  This 
  experiment 
  shows: 
  (1) 
  that 
  the 
  change 
  requires 
  

   considerable 
  time 
  ; 
  for 
  even 
  a 
  fairly 
  strong 
  jet 
  of 
  oxygen 
  

   playing 
  on 
  the 
  plate 
  for 
  two 
  or 
  three 
  minutes 
  produces 
  no 
  

   appreciable 
  effect, 
  while 
  twenty-five 
  minutes 
  in 
  still 
  oxygen 
  

   causes 
  a 
  rise 
  of 
  *20 
  v. 
  : 
  (2) 
  that 
  gentle 
  heating 
  does 
  not 
  

   produce 
  a 
  rise, 
  as 
  it 
  would 
  do 
  with 
  clean 
  unoxygenized 
  copper 
  

   (see 
  Chap. 
  IX.), 
  but 
  it 
  must 
  be 
  remembered 
  that 
  the 
  copper 
  

   was, 
  when 
  heated, 
  already 
  above 
  the 
  potential 
  to 
  which 
  heat 
  

   alone 
  would 
  have 
  raised 
  it 
  ; 
  hence 
  this 
  experiment 
  does 
  not 
  

   show 
  any 
  connexion 
  between 
  temperature-variation 
  and 
  

   density 
  of 
  the 
  oxygen 
  film 
  as 
  might 
  at 
  first 
  sight 
  be 
  supposed. 
  

   § 
  60. 
  A 
  zinc 
  plate 
  which 
  had 
  been 
  polished 
  on 
  glass-paper 
  

   gave 
  with 
  a 
  standard 
  copper 
  plate 
  

  

  *81 
  v., 
  zinc 
  positive. 
  

   It 
  was 
  then 
  put 
  into 
  oxygen 
  and 
  left 
  for 
  fifteen 
  minutes. 
  

   After 
  being 
  taken 
  out, 
  its 
  potential 
  was 
  again 
  measured 
  with 
  

   the 
  same 
  standard 
  plate, 
  and 
  was 
  as 
  follows 
  . 
  — 
  

  

  Time. 
  Contact-potential, 
  

  

  h. 
  m. 
  v. 
  

  

  2 
  -89 
  

  

  10 
  -87 
  

  

  4 
  18 
  '86 
  

  

  4 
  35 
  -85 
  

  

  This 
  shows 
  that 
  zinc 
  also 
  is 
  more 
  positive 
  after 
  immersion 
  

   in 
  oxygen. 
  

  

  § 
  61. 
  The 
  same 
  zinc 
  plate 
  was 
  again 
  polished 
  on 
  glass- 
  

   paper 
  and 
  gave 
  

  

  + 
  •73 
  v. 
  

   with 
  the 
  standard 
  copper. 
  A 
  short 
  time 
  later 
  it 
  gave 
  

  

  + 
  •70 
  v. 
  

   It 
  was 
  now 
  put 
  into 
  oxygen 
  for 
  ten 
  minutes, 
  and 
  after 
  being 
  

  

  taken 
  out 
  gave 
  

  

  

  

  

  Time. 
  

  

  Contact-potential. 
  

  

  

  h. 
  m. 
  

  

  v. 
  

  

  

  3 
  

  

  + 
  •80 
  

  

  

  13 
  

  

  + 
  •78 
  

  

  

  17 
  1 
  

  

  + 
  •74 
  

  

  