﻿Volta 
  Electricity 
  of 
  Metals, 
  431 
  

  

  This 
  gives 
  a 
  variation 
  of 
  about 
  

  

  '0032 
  v. 
  per 
  degree 
  centigrade, 
  

  

  which 
  is 
  nearly 
  the 
  same 
  as 
  the 
  temperature-variation 
  of 
  bare 
  

   aluminium 
  in 
  air. 
  

  

  § 
  82. 
  A 
  plate 
  of 
  silvered 
  glass 
  used 
  with 
  the 
  glass 
  side 
  

   facing 
  a 
  standard 
  plate 
  gave 
  almost 
  the 
  same 
  potential 
  as 
  

   clean 
  silver 
  in 
  air. 
  This 
  plate, 
  which 
  was 
  practically 
  silver 
  

   coated 
  with 
  glass, 
  gave 
  a 
  temperature-variation 
  larger 
  than 
  

   that 
  of 
  silver 
  in 
  air. 
  In 
  this 
  case 
  we 
  have 
  glass 
  in 
  contact 
  with 
  

   the 
  silver 
  surface 
  instead 
  of 
  air, 
  but 
  the 
  change 
  does 
  not 
  alter 
  

   the 
  potential. 
  It 
  may 
  be 
  mentioned 
  that 
  the 
  back 
  of 
  the 
  

   silver 
  film 
  was 
  painted 
  black, 
  and 
  not 
  coated 
  with 
  glass 
  ; 
  but 
  

   this 
  is 
  of 
  small 
  consequence, 
  since 
  it 
  has 
  been 
  proved 
  by 
  

   experiment 
  that 
  the 
  condition 
  of 
  the 
  back 
  of 
  a 
  plate 
  does 
  not 
  

   sensibly 
  affect 
  the 
  volta 
  contact-potential 
  ; 
  or, 
  more 
  generally, 
  

   that 
  if 
  parts 
  of 
  a 
  plate 
  be 
  in 
  different 
  conditions, 
  the 
  potential 
  

   observed 
  will 
  be 
  the 
  mean 
  of 
  the 
  potentials 
  of 
  the 
  different 
  

   parts, 
  the 
  importance 
  of 
  each 
  part 
  being 
  proportional 
  to 
  its 
  

   capacity. 
  

  

  § 
  83. 
  On 
  account 
  of 
  the 
  great 
  attraction 
  of 
  sodium 
  for 
  

   oxygen, 
  it 
  seemed 
  of 
  interest 
  to 
  measure 
  its 
  potential 
  in 
  cir- 
  

   cumstances 
  which 
  excluded 
  that 
  gas 
  from 
  the 
  surface 
  of 
  the 
  

   metal. 
  In 
  order 
  to 
  effect 
  this 
  two 
  pieces 
  of 
  thin 
  sheet-glass, 
  

   each 
  about 
  6 
  cm. 
  square, 
  were 
  put 
  into 
  a 
  dish 
  of 
  melted 
  

   paraffin-wax 
  together 
  with 
  some 
  clean 
  sodium, 
  and 
  a 
  large 
  

   drop 
  of 
  the 
  sodium 
  was 
  put 
  between 
  the 
  plates 
  of 
  glass 
  and 
  

   squeezed 
  out 
  into 
  a 
  small 
  plate 
  of 
  3 
  or 
  4 
  square 
  cm. 
  area. 
  

   The 
  glass 
  plates, 
  with 
  sodium 
  between 
  them, 
  were 
  taken 
  out 
  

   of 
  the 
  melted 
  wax 
  and 
  allowed 
  to 
  cool. 
  Since 
  the 
  glass 
  plates 
  

   were 
  of 
  much 
  larger 
  diameter 
  than 
  the 
  sodium, 
  the 
  edges 
  of 
  

   the 
  latter 
  were 
  protected 
  by 
  the 
  wax 
  which 
  filled 
  up 
  the 
  space 
  

   between 
  the 
  plates 
  not 
  occupied 
  by 
  sodium. 
  The 
  flat 
  faces 
  

   of 
  the 
  sodium 
  were 
  apparently 
  in 
  contact 
  with 
  the 
  glass. 
  

   The 
  sodium 
  was 
  connected 
  to 
  the 
  electrometer 
  by 
  a 
  fine 
  

   copper 
  wire. 
  The 
  greater 
  part 
  of 
  the 
  sodium 
  surface 
  was 
  

   bright 
  or 
  only 
  slightly 
  tarnished, 
  and 
  it 
  remained 
  in 
  almost 
  

   the 
  same 
  condition 
  for 
  many 
  days, 
  being 
  protected 
  by 
  the 
  

   glass 
  plates 
  and 
  by 
  the 
  wax 
  which 
  filled 
  the 
  space 
  between 
  

   them 
  unoccupied 
  by 
  sodium. 
  The 
  first 
  measurements 
  gave 
  

   sodium 
  

  

  2-86 
  v. 
  

   positive 
  to 
  a 
  tarnished 
  zinc 
  plate, 
  i. 
  e. 
  about 
  

  

  3-56 
  v. 
  

   positive 
  to 
  the 
  standard 
  gold 
  plate. 
  This 
  potential 
  gradually 
  

   decreased. 
  

  

  2 
  G2 
  

  

  