﻿426 
  Dr. 
  J. 
  Erskine-Murray 
  on 
  

  

  plate, 
  was 
  removed 
  after 
  each 
  observation 
  and 
  was 
  replaced 
  

   only 
  the 
  moment 
  before 
  the 
  next. 
  In 
  other 
  experiments 
  the 
  

   upper 
  plate 
  was 
  merely 
  drawn 
  up 
  as 
  far 
  as 
  possible 
  (about 
  

   10 
  cm.) 
  to 
  prevent 
  its 
  warming 
  the 
  lower 
  one. 
  The 
  tempera- 
  

   ture 
  of 
  the 
  lower 
  plate 
  was 
  also 
  observed, 
  but 
  as 
  a 
  rule 
  it 
  

   varied 
  only 
  a 
  very 
  few 
  degrees. 
  

  

  §71. 
  At 
  first 
  1 
  used 
  to 
  apply 
  a 
  hot 
  soldering-bolt 
  to 
  the 
  

   back 
  of 
  the 
  plate 
  in 
  order 
  to 
  heat 
  it, 
  but 
  latterly 
  I 
  heated 
  two 
  

   or 
  three 
  small 
  blocks 
  of 
  tinned 
  copper 
  and 
  placed 
  them 
  on 
  

   the 
  back 
  of 
  the 
  upper 
  plate. 
  By 
  this 
  second 
  method 
  it 
  was 
  

   possible 
  to 
  observe 
  the 
  variation 
  of 
  the 
  potential 
  during 
  the 
  

   rise 
  as 
  well 
  as 
  the 
  fall 
  of 
  temperature. 
  

  

  § 
  72. 
  In 
  attempting 
  to 
  determine 
  the 
  temperature-coefficient 
  

   of 
  copper 
  we 
  were 
  long 
  baffled 
  by 
  curious 
  anomalies. 
  Some- 
  

   times 
  the 
  plate 
  was 
  positive 
  when 
  hot, 
  other 
  times 
  negative, 
  

   and 
  occasionally 
  it 
  did 
  not 
  vary 
  at 
  all. 
  The 
  clue 
  to 
  this 
  was 
  

   found 
  in 
  observing 
  that 
  during 
  one 
  experiment 
  while 
  the 
  

   copper 
  was 
  cooling 
  it 
  was 
  at 
  first 
  positive, 
  then 
  negative, 
  and 
  

   then 
  it 
  gradually 
  became 
  positive 
  again, 
  though 
  never 
  quite 
  

   reaching 
  its 
  original 
  value. 
  

  

  Now 
  it 
  had 
  been 
  found 
  that 
  copper 
  oxide 
  is 
  negative 
  to 
  

   copper, 
  and 
  that 
  it 
  became 
  temporarily 
  more 
  negative 
  when 
  

   hot 
  ; 
  hence 
  it 
  was 
  guessed 
  that 
  the 
  successively 
  positive 
  and 
  

   negative 
  variation 
  must 
  be 
  due 
  to 
  hot 
  clean 
  copper 
  being 
  posi- 
  

   tive 
  to 
  cold 
  copper, 
  but 
  that 
  it 
  had 
  finally 
  become 
  oxidized 
  

   and 
  therefore 
  negative 
  whether 
  hot 
  or 
  cold, 
  the 
  small 
  perma- 
  

   nent 
  change 
  being 
  due 
  to 
  the 
  thin 
  coating 
  of 
  oxide 
  formed. 
  

  

  § 
  73. 
  The 
  copper 
  was 
  heated 
  much 
  more 
  gently 
  next 
  time, 
  

   and 
  gave 
  the 
  expected 
  result 
  that 
  clean 
  copper 
  becomes 
  

   rapidly 
  more 
  positive 
  as 
  its 
  temperature 
  rises, 
  and 
  that, 
  on 
  

   cooling, 
  its 
  potential 
  returns 
  to 
  its 
  original 
  value 
  unless 
  the 
  

   temperature 
  has 
  exceeded 
  a 
  certain 
  limit. 
  If 
  this 
  limit 
  has 
  

   been 
  exceeded 
  its 
  potential 
  rapidly 
  becomes 
  negative 
  and 
  

   does 
  not 
  return 
  to 
  its 
  original 
  value. 
  

  

  § 
  74. 
  At 
  ordinary 
  atmospheric 
  temperatures 
  the 
  surface 
  of 
  

   clean 
  copper 
  remains 
  for 
  a 
  long 
  time 
  almost 
  unaltered 
  either 
  

   visibly 
  or 
  electrically 
  (see 
  § 
  31), 
  and 
  the 
  film 
  which 
  ultimately 
  

   forms 
  on 
  the 
  surface 
  cannot 
  be 
  pure 
  copper 
  oxide 
  because 
  

   the 
  potential 
  of 
  tarnished 
  copper 
  is 
  higher 
  than 
  that 
  of 
  

   copper 
  oxide 
  obtained 
  by 
  heating 
  in 
  air. 
  If, 
  however, 
  the 
  

   temperature 
  of 
  the 
  copper 
  is 
  raised 
  to 
  about 
  80° 
  C. 
  it 
  imme- 
  

   diately 
  begins 
  to 
  oxidize, 
  though 
  heating 
  to 
  a 
  temperature 
  

   below 
  this 
  limit 
  does 
  not 
  rapidly 
  produce 
  any 
  permanent 
  

   change. 
  Thus 
  there 
  is, 
  as 
  it 
  were, 
  an 
  ignition 
  point 
  for 
  

   copper 
  and 
  oxygen 
  in 
  air 
  ; 
  below 
  it, 
  little 
  action 
  takes 
  place 
  ; 
  

   above 
  it, 
  combination 
  proceeds 
  vigorously. 
  

  

  