﻿424 
  Dr. 
  J. 
  Erskine-Murray 
  otl 
  

  

  time 
  of 
  exposure 
  to 
  oxygen. 
  Thus, 
  in 
  § 
  63, 
  15 
  minutes 
  in 
  

   oxygen 
  caused 
  a 
  rise 
  of 
  *06 
  v. 
  ; 
  while, 
  in 
  § 
  64, 
  24 
  minutes 
  

   in 
  oxygen 
  caused 
  a 
  rise 
  of 
  *11 
  v., 
  but 
  there 
  is 
  no 
  doubt 
  a 
  limit 
  

   to 
  the 
  change. 
  

  

  § 
  66. 
  In 
  searching 
  for 
  an 
  explanation 
  of 
  the 
  temperature- 
  

   variations 
  described 
  in 
  Chapter 
  IX., 
  I 
  compared 
  them 
  with 
  

   those 
  given 
  above. 
  In 
  the 
  case 
  of 
  copper, 
  the 
  oxygen- 
  film 
  

   variation 
  is 
  in 
  the 
  same 
  direction 
  as 
  the 
  temperature-variation 
  

   of 
  copper 
  in 
  air, 
  which 
  suggests 
  the 
  possibility 
  of 
  the 
  latter 
  

   being 
  caused 
  by 
  an 
  increase 
  of 
  the 
  proportion 
  of 
  oxygen 
  in 
  

   the 
  film 
  at 
  higher 
  temperatures 
  on 
  account 
  of 
  a 
  greater 
  

   attraction 
  between 
  the 
  elements. 
  The 
  same 
  reasoning 
  holds 
  

   as 
  regards 
  zinc 
  and 
  tin, 
  but 
  the 
  results 
  for 
  silver 
  are 
  in 
  direct 
  

   opposition 
  ; 
  while 
  the 
  further 
  experiments 
  described 
  in 
  

   Chapter 
  X., 
  which 
  show 
  that 
  the 
  temperature- 
  variations 
  

   exist 
  in 
  cases 
  where 
  air 
  is 
  entirely 
  excluded 
  from 
  the 
  metallic 
  

   surface, 
  render 
  such 
  an 
  explanation 
  very 
  doubtful. 
  Probably, 
  

   therefore, 
  the 
  temperature-variation 
  is 
  the 
  more 
  general 
  of 
  

   the 
  two, 
  it 
  being 
  a 
  change 
  in 
  contact-potential 
  of 
  the 
  metals, 
  

   which, 
  if 
  they 
  are 
  exposed 
  to 
  a 
  gas, 
  may 
  be 
  complicated 
  by 
  

   alteration 
  of 
  the 
  surface-film. 
  

  

  IX. 
  Temperature 
  Variations. 
  

  

  § 
  67. 
  A 
  large 
  number 
  of 
  determinations 
  were 
  made 
  of 
  the 
  

   variation 
  of 
  contact-electricity 
  with 
  the 
  temperature 
  of 
  the 
  

   conductor. 
  This 
  was 
  done 
  by 
  heating 
  one 
  plate 
  while 
  the 
  

   other 
  was 
  kept 
  cool, 
  and 
  their 
  potential-difference 
  was 
  measured 
  

   from 
  time 
  to 
  time 
  as 
  the 
  warm 
  plate 
  was 
  cooling, 
  their 
  

   temperatures 
  being 
  observed 
  at 
  the 
  same 
  time. 
  In 
  the 
  

   diagrams, 
  the 
  abscissae 
  represent 
  temperature 
  and 
  the 
  

   ordinates 
  potential; 
  each 
  curve, 
  therefore, 
  shows 
  the 
  tem- 
  

   perature-variation 
  of 
  the 
  contact-potential 
  of 
  a 
  particular 
  

   metal. 
  For 
  instance, 
  the 
  potential 
  represented 
  by 
  the 
  point 
  

   which 
  corresponds 
  to 
  16° 
  C. 
  on 
  the 
  gold 
  line 
  is 
  called 
  zero 
  

   in 
  this 
  and 
  other 
  chapters 
  of 
  the 
  present 
  communication. 
  

   Thus 
  a 
  standard 
  gold 
  plate 
  (see 
  § 
  8) 
  at 
  40° 
  C. 
  is 
  *04 
  v. 
  negative 
  

   to 
  one 
  at 
  16° 
  C, 
  and 
  an 
  aluminium 
  plate 
  at 
  40° 
  C. 
  is 
  *10 
  v., 
  

   positive 
  to 
  aluminium 
  at 
  16° 
  C, 
  or 
  1*20 
  v. 
  positive 
  to 
  a 
  gold 
  

   plate 
  at 
  16° 
  C. 
  

  

  It 
  must 
  be 
  remembered 
  that 
  unless 
  stated 
  otherwise 
  these 
  

   variations 
  are 
  for 
  metals 
  in 
  air. 
  In 
  the 
  experiments 
  described 
  

   in 
  Chapter 
  X., 
  however, 
  the 
  metallic 
  surfaces 
  were 
  protected 
  

   by 
  solid 
  non-conducting 
  films, 
  and 
  were 
  not 
  in 
  contact 
  w 
  T 
  ith 
  

   the 
  atmosphere 
  ; 
  nevertheless, 
  temperature-variations 
  were 
  

   found, 
  which 
  in 
  the 
  case 
  of 
  silver 
  were 
  actually 
  larger 
  than 
  

   those 
  which 
  took 
  place 
  in 
  air. 
  

  

  