﻿338 
  Prof. 
  W. 
  G. 
  Duffield 
  on 
  the 
  

  

  The 
  Silver 
  Arc. 
  

  

  Arcs 
  struck 
  between 
  poles 
  of 
  silver 
  \ 
  inch 
  in 
  diameter 
  

   and 
  also 
  between 
  poles 
  £ 
  inch 
  in 
  diameter 
  showed 
  no 
  deposit 
  

   o£ 
  the 
  above 
  nature 
  even 
  after 
  half 
  an 
  hour's 
  run. 
  There 
  

   were 
  in 
  the 
  horizontal 
  arc 
  small 
  inconspicuous 
  lumps 
  upon 
  

   the 
  negative 
  pole 
  of 
  what 
  looked 
  under 
  a 
  magnifying 
  glass 
  

   like 
  an 
  aggregate 
  of 
  tiny 
  crystals 
  in 
  no 
  regular 
  form. 
  

  

  In 
  these 
  experiments 
  arc-lengths 
  of 
  1*5 
  to 
  2 
  mm. 
  were 
  

   used 
  with 
  a 
  current 
  of 
  4 
  amperes, 
  and 
  the 
  voltage 
  varied 
  

   from 
  25 
  to 
  32 
  volts 
  across 
  the 
  arc. 
  

  

  The 
  introduction 
  of 
  a 
  third 
  pointed 
  electrode 
  about 
  2 
  mm. 
  

   above 
  the 
  horizontal 
  silver 
  arc 
  caused 
  no 
  deposition 
  upon 
  it 
  

   if 
  it 
  were 
  positively 
  charged 
  to 
  100 
  volts, 
  but 
  if 
  negatively 
  

   charged 
  50 
  volts 
  were 
  sufficient 
  to 
  ensure 
  a 
  vigorous 
  growth 
  

   upon 
  the 
  point 
  and 
  some 
  distance 
  up 
  it. 
  This 
  growth 
  was 
  

   very 
  like 
  that 
  formed 
  in 
  the 
  copper 
  arc, 
  but 
  was 
  hardier 
  and 
  

   seemed 
  a 
  little 
  heavier; 
  it 
  was 
  black 
  in 
  colour, 
  indicating 
  

   that 
  it 
  consisted 
  of 
  an 
  oxide 
  of 
  silver. 
  The 
  positive 
  needle 
  is 
  

   not 
  without 
  some 
  effect 
  as 
  a 
  layer 
  of 
  a 
  powdery 
  nature 
  slowly 
  

   accumulates 
  upon 
  it, 
  but 
  it 
  does 
  not 
  exhibit 
  the 
  structure 
  

   presented 
  by 
  a 
  needle 
  charged 
  negatively. 
  PI. 
  IX. 
  fig. 
  3 
  

   shows 
  the 
  silver 
  growth 
  upon 
  a 
  needle-point 
  at 
  a 
  negative 
  

   potential 
  of 
  100 
  volts 
  after 
  the 
  arc 
  below 
  had 
  been 
  burning 
  

   for 
  3 
  minutes. 
  In 
  fig. 
  4 
  the 
  needle 
  is 
  seen 
  with 
  a 
  powdery 
  

   deposit 
  ; 
  it 
  had 
  been 
  connected 
  to 
  the 
  positive 
  terminal 
  of 
  the 
  

   100 
  volt 
  battery, 
  the 
  other 
  terminal 
  of 
  which 
  was 
  earthed. 
  

   No 
  growths 
  were 
  formed 
  though 
  the 
  arc 
  had 
  been 
  burning 
  

   for 
  9 
  minutes. 
  

  

  Arcs 
  betiveen 
  different 
  Metals. 
  

  

  Experiments 
  were 
  made 
  with 
  horizontal 
  arcs 
  of 
  pairs 
  of 
  

   metals, 
  e. 
  g. 
  copper 
  and 
  iron, 
  silver 
  and 
  iron, 
  silver 
  and 
  

   copper, 
  iron 
  and 
  carbon, 
  to 
  ascertain 
  the 
  effect 
  of 
  the 
  anode 
  

   upon 
  the 
  cathode 
  growth. 
  When 
  iron 
  and 
  copper 
  were 
  used 
  

   and 
  the 
  copper 
  was 
  the 
  cathode, 
  the 
  growth 
  upon 
  it 
  was 
  very 
  

   meagre 
  indeed, 
  consisting 
  partly 
  of 
  iron 
  (it 
  was 
  magnetic) 
  

   which 
  had 
  evidently 
  been 
  derived 
  from 
  the 
  positive 
  pole. 
  

   When 
  the 
  iron 
  was 
  negative 
  the 
  growth 
  was 
  also 
  magnetic 
  

   and 
  very 
  profuse. 
  

  

  Using 
  a 
  silver 
  pole 
  with 
  iron 
  or 
  copper 
  as 
  the 
  other 
  elec- 
  

   trode, 
  no 
  appreciable 
  growth 
  was 
  obtained 
  when 
  the 
  silver 
  

   was 
  negative, 
  but 
  when 
  it 
  was 
  positive 
  the 
  growths 
  charac- 
  

   teristic 
  of 
  iron 
  or 
  copper 
  appeared 
  on 
  the 
  cathode. 
  

  

  