﻿408 
  Dr. 
  J. 
  Erskine-Murray 
  on 
  

  

  the 
  results 
  for 
  harder 
  metals, 
  is 
  really 
  in 
  accordance 
  with 
  

   them, 
  for 
  different 
  polishers 
  do 
  not 
  produce 
  appreciably 
  dif- 
  

   ferent 
  states 
  of 
  polish 
  on 
  a 
  soft 
  metal. 
  This 
  remark 
  applies 
  

   equally 
  to 
  the 
  fact 
  that 
  a 
  smaller 
  variation 
  was 
  observed 
  with 
  

   " 
  high 
  conductivity 
  " 
  copper 
  than 
  with 
  the 
  old 
  copper 
  plate, 
  

   as 
  the 
  old 
  p'ate 
  was 
  found 
  to 
  be 
  considerably 
  harder 
  than 
  the 
  

   purer 
  " 
  high 
  conductivity 
  " 
  copper. 
  

  

  § 
  20. 
  It 
  is 
  probable 
  that 
  the 
  variation 
  described 
  in 
  this 
  

   chapter 
  is 
  not 
  directly 
  due 
  to 
  roughness 
  or 
  smoothness, 
  but 
  

   rather 
  to 
  alteration, 
  either 
  by 
  change 
  in 
  the 
  outer 
  layer 
  of 
  

   the 
  metal, 
  or 
  in 
  the 
  film 
  of 
  condensed 
  air 
  which 
  no 
  doubt 
  

   exists 
  on 
  it, 
  of 
  the 
  interface 
  between 
  metal 
  and 
  air. 
  

  

  IV. 
  Thin 
  Solid 
  Films 
  of 
  Oxides, 
  Iodides, 
  Sfc. 
  

  

  § 
  21. 
  Among 
  the 
  earlier 
  experiments 
  were 
  many 
  in 
  which 
  

   the 
  changes 
  due 
  to 
  films 
  of 
  oxide 
  of 
  different 
  thicknesses 
  

   were 
  studied 
  ; 
  indeed 
  this 
  was 
  the 
  primary 
  object 
  of 
  investi- 
  

   gation 
  suggested 
  by 
  Lord 
  Kelvin. 
  But 
  constant 
  difficulties 
  

   and 
  ambiguities 
  presented 
  themselves, 
  hindering 
  the 
  inter- 
  

   pretation 
  of 
  even 
  the 
  simplest 
  experiments, 
  and 
  it 
  was 
  found 
  

   necessary 
  to 
  enlarge 
  the 
  scope 
  of 
  the 
  research 
  in 
  order 
  to 
  

   remove 
  or 
  explain 
  them. 
  

  

  § 
  22. 
  The 
  experiments 
  detailed 
  below 
  are 
  among 
  the 
  most 
  

   definite 
  of 
  those 
  carried 
  out. 
  They 
  show 
  large 
  changes 
  in 
  

   potential 
  on 
  account 
  of 
  very 
  thin 
  films 
  of 
  oxide, 
  and 
  prove 
  

   that 
  the 
  amount 
  of 
  change 
  is 
  dependent 
  on 
  the 
  thickness 
  of 
  

   the 
  film 
  when 
  below 
  a 
  certain 
  limit. 
  

  

  § 
  23. 
  Two 
  plates 
  of 
  cast 
  zinc, 
  which 
  had 
  been 
  carefully 
  

   polished 
  on 
  medium 
  glass-paper 
  the 
  day 
  before, 
  gave 
  with 
  

   one 
  another 
  

  

  + 
  •02 
  v., 
  

  

  the 
  upper 
  plate 
  being 
  positive. 
  The 
  upper 
  plate 
  was 
  now 
  

   taken 
  out 
  and 
  heated 
  on 
  the 
  back 
  by 
  means 
  of 
  a 
  very 
  

   small 
  blowpipe 
  flame. 
  Its 
  face 
  became 
  slightly 
  blistered, 
  

   but 
  not 
  much 
  discoloured, 
  and 
  when 
  cold 
  it 
  gave 
  with 
  the 
  

   other, 
  which 
  had 
  not 
  been 
  altered 
  in 
  any 
  way, 
  

  

  --44 
  v., 
  

  

  the 
  " 
  minus 
  " 
  indicating 
  that 
  the 
  upper 
  is 
  now 
  negative. 
  

   This 
  shows 
  a 
  permanent 
  fall 
  of 
  '46 
  v. 
  due 
  to 
  heating 
  in 
  air. 
  

   § 
  24. 
  A 
  plate 
  of 
  rolled 
  zinc, 
  which 
  gave 
  

  

  -•04 
  v. 
  

  

  with 
  another 
  plate 
  of 
  the 
  same 
  material, 
  was 
  heated 
  as 
  in 
  the 
  

   last 
  experiment 
  but 
  to 
  a 
  higher 
  temperature, 
  very 
  nearly 
  to 
  

   its 
  melting-point. 
  Its 
  face 
  became 
  a 
  streaky 
  yellowish 
  brown, 
  

  

  