﻿410 
  Dr. 
  J. 
  Erski 
  ne-Murray 
  on 
  

  

  § 
  29. 
  The 
  effects 
  of 
  sulphur-fumes, 
  hydrogen 
  sulphide, 
  

   &c, 
  were 
  tried, 
  but, 
  though 
  in 
  most 
  cases 
  considerable 
  changes 
  

   were 
  observed, 
  the 
  results 
  obtained 
  were 
  not 
  satisfactory. 
  

  

  Y. 
  Atmospheric 
  or 
  Time 
  Effects. 
  

  

  § 
  80. 
  A 
  small 
  variation 
  of 
  the 
  potential 
  is 
  usually 
  found 
  to 
  

   take 
  place 
  during 
  a 
  short 
  time 
  after 
  the 
  plate 
  has 
  been 
  polished, 
  

   and 
  before 
  it 
  has 
  settled 
  down 
  to 
  a 
  more 
  or 
  less 
  permanent 
  

   value. 
  This 
  is 
  partly 
  due 
  to 
  the 
  fact 
  that 
  the 
  plate 
  has 
  been 
  

   slightly 
  warmed 
  during 
  polishing, 
  and 
  takes 
  some 
  time 
  to 
  

   come 
  to 
  the 
  atmospheric 
  temperature 
  (see 
  Chap. 
  IX.). 
  As 
  

   regards 
  changes 
  which 
  are 
  not 
  due 
  to 
  variation 
  of 
  temperature, 
  

   I 
  shall 
  give 
  some 
  typical 
  experiments 
  in 
  the 
  following 
  sections 
  

   of 
  this 
  chapter. 
  

  

  § 
  31. 
  Two 
  plates 
  of 
  " 
  high 
  conductivity" 
  copper 
  were 
  

   polished 
  on 
  fine 
  glass-paper, 
  and 
  several 
  experiments 
  by 
  

   burnishing 
  them 
  by 
  mutual 
  friction 
  were 
  then 
  made 
  (see 
  

   § 
  16). 
  Forty-seven 
  minutes 
  after 
  the 
  original 
  polishing 
  their 
  

   potentials 
  with 
  the 
  standard 
  plate 
  were 
  

  

  (a) 
  -1- 
  -035 
  v. 
  

   {b) 
  + 
  '045 
  v. 
  

  

  The 
  plate 
  (a) 
  was 
  left 
  in 
  the 
  apparatus 
  all 
  night, 
  that 
  is 
  to 
  

   say, 
  in 
  a 
  position 
  sheltered 
  from 
  dust, 
  and 
  in 
  air 
  partially 
  

   dried 
  by 
  the 
  sulphuric 
  acid 
  in 
  the 
  lower 
  part 
  of 
  the 
  case 
  ; 
  the 
  

   potential 
  next 
  morning, 
  20 
  11 
  3 
  m 
  after 
  the 
  measurements 
  given 
  

   above, 
  was 
  

  

  + 
  -045 
  v. 
  

  

  with 
  the 
  same 
  standard 
  plate. 
  It 
  was 
  thus 
  nearly 
  constant. 
  

   The 
  plate 
  (b) 
  was 
  left 
  in 
  a 
  rack 
  in 
  the 
  laboratory 
  without 
  

   special 
  protection, 
  with 
  the 
  result 
  that 
  next 
  morning 
  the 
  

   potential, 
  owing 
  to 
  exposure 
  to 
  the 
  air 
  of 
  the 
  room 
  for 
  about 
  

   20 
  h 
  ; 
  had 
  fallen 
  to 
  

  

  -•056 
  v., 
  

  

  a 
  change 
  of 
  — 
  *101 
  v. 
  The 
  plate 
  (b) 
  was 
  now 
  placed 
  in 
  the 
  

   apparatus, 
  and 
  remained 
  nearly 
  constant 
  for 
  two 
  hours 
  at 
  

   least. 
  Plate 
  (a) 
  was 
  left 
  in 
  the 
  rack, 
  and 
  its 
  potential 
  fell 
  

   *065 
  v. 
  in 
  about 
  2 
  hours. 
  It 
  is 
  thus 
  evident 
  that 
  at 
  ordinary 
  

   temperatures 
  in 
  a 
  rather 
  dry 
  atmosphere, 
  and 
  in 
  a 
  place 
  pro- 
  

   tected 
  from 
  dust 
  and 
  light, 
  the 
  rate 
  of 
  variation 
  of 
  the 
  con- 
  

   tact-potential 
  of 
  copper 
  is 
  very 
  small 
  ; 
  in 
  an 
  exposed 
  place, 
  

   however, 
  it 
  is 
  by 
  no 
  means 
  negligible. 
  

  

  § 
  32. 
  A 
  plate 
  of 
  block 
  tin 
  was 
  polished 
  on 
  glass-paper. 
  

   Its 
  potential 
  varied 
  with 
  time 
  as 
  follows, 
  the 
  time 
  being 
  

   counted 
  from 
  the 
  completion 
  of 
  the 
  polishing 
  : 
  — 
  

  

  