﻿400 
  Dr. 
  J. 
  Erskine-Murray 
  on 
  

  

  difference 
  as 
  the 
  dry 
  metals 
  on 
  which 
  they 
  lie 
  (see 
  Chaps. 
  VI. 
  

  

  and 
  VII.). 
  

  

  (e) 
  A 
  very 
  thin 
  film 
  of 
  oxide 
  on 
  a 
  metal 
  produces 
  only 
  a 
  

   very 
  small 
  change 
  in 
  the 
  potential, 
  and 
  every 
  increase 
  in 
  

   thickness 
  of 
  the 
  film 
  is 
  attended 
  by 
  a 
  further 
  change 
  in 
  

   potential 
  until 
  a 
  limiting 
  value 
  is 
  reached, 
  which 
  is 
  that 
  of 
  a 
  

   mass 
  of 
  the 
  oxide 
  (see 
  Chap. 
  IV.). 
  

  

  (/) 
  Exposure 
  to 
  the 
  atmosphere 
  at 
  ordinary 
  temperatures 
  

   does 
  not 
  as 
  a 
  rule 
  produce 
  any 
  rapid 
  change 
  in 
  volta-potential, 
  

   especially 
  if 
  the 
  air 
  be 
  comparatively 
  dry 
  and 
  free 
  from 
  dust. 
  

   The 
  ultimate 
  change 
  is 
  usually 
  in 
  the 
  negative 
  direction 
  

   (see 
  Chap. 
  V.) 
  . 
  

  

  (</) 
  I 
  have 
  extended 
  Lord 
  Kelvin's 
  experiments 
  on 
  the 
  

   effect 
  of 
  temporary 
  immersion 
  of 
  a 
  metal 
  in 
  a 
  gas 
  to 
  the 
  

   cases 
  of 
  copper, 
  zinc, 
  tin, 
  and 
  silver 
  in 
  oxygen, 
  and 
  find 
  that 
  

   copper, 
  zinc, 
  and 
  silver 
  become 
  temporarily 
  positive, 
  while 
  

   tin 
  becomes 
  negative 
  in 
  consequence 
  of 
  this 
  treatment 
  (see 
  

   Chap. 
  VIII.). 
  

  

  The 
  research, 
  suggested 
  by 
  Lord 
  Kelvin, 
  was 
  carried 
  out 
  

   in 
  the 
  Natural 
  Philosophy 
  Laboratory 
  of 
  Glasgow 
  University 
  

   during 
  the 
  Sessions 
  1893-94-95, 
  and 
  during 
  1895-96* 
  in 
  the 
  

   Cavendish 
  Laboratory 
  of 
  Cambridge 
  University. 
  My 
  thanks 
  

   are 
  due 
  to 
  Lord 
  Kelvin 
  for 
  many 
  suggestions 
  and 
  much 
  

   valuable 
  advice, 
  both 
  in 
  regard 
  to 
  experiments 
  and 
  to 
  the 
  

   discussion 
  of 
  results 
  ; 
  and 
  to 
  Professor 
  J. 
  J. 
  Thomson 
  for 
  

   similar 
  kindnesses 
  during 
  my 
  work 
  in 
  the 
  Cavendish 
  

   Laboratory. 
  A 
  small 
  portion 
  of 
  this 
  investigation, 
  on 
  the 
  

   effect 
  of 
  Rontgen 
  #-rays 
  on 
  the 
  contact 
  electricity 
  of 
  metals, 
  

   was 
  published 
  in 
  the 
  Proceedings 
  of 
  the 
  Royal 
  Society 
  for 
  

   March 
  1896. 
  

  

  I 
  have 
  also 
  to 
  thank 
  Professor 
  James 
  Holm, 
  M.A."*, 
  and 
  

   Mr. 
  George 
  E. 
  Allan, 
  B.Sc, 
  for 
  the 
  part 
  they 
  took 
  in 
  the 
  

   earlier 
  portion 
  of 
  the 
  work. 
  

  

  II. 
  Method 
  of 
  Experiment. 
  

  

  § 
  3. 
  The 
  measurement 
  of 
  the 
  natural 
  potential-difference 
  

   between 
  any 
  pair 
  of 
  conductors 
  was 
  usually 
  made, 
  in 
  air, 
  by 
  

   the 
  null 
  method 
  described 
  very 
  briefly 
  by 
  Lord 
  Kelvin 
  in 
  the 
  

   Report 
  of 
  the 
  British 
  Association 
  for 
  1880, 
  p. 
  494. 
  To 
  make 
  

   clear 
  the 
  exact 
  circumstances 
  in 
  which 
  the 
  potentials 
  were 
  

   measured 
  it 
  will 
  be 
  advantageous 
  to 
  describe 
  the 
  apparatus 
  and 
  

   general 
  method 
  of 
  experiment 
  in 
  detail. 
  

  

  § 
  4. 
  A 
  circular 
  disk 
  of 
  one 
  of 
  the 
  metals, 
  usually 
  about 
  

  

  * 
  When 
  this 
  was 
  written 
  last 
  August 
  my 
  friend 
  and 
  former 
  fellow-^ 
  

   worker 
  was 
  Professor 
  of 
  Applied 
  Mathematics 
  in 
  the 
  South-African 
  

   College, 
  Cape 
  Town. 
  He 
  died 
  in 
  October. 
  

  

  