﻿336 
  Prof. 
  W. 
  G. 
  Duffield 
  on 
  the 
  

  

  more 
  abundant 
  when 
  the 
  pole 
  was 
  given 
  a 
  negative 
  potential 
  

   of 
  the 
  same 
  amount 
  (PL 
  IX. 
  fig. 
  1). 
  Using 
  lower 
  potentials, 
  

   the 
  difference 
  between 
  positive 
  and 
  negative 
  became 
  still 
  

   more 
  marked, 
  — 
  below 
  about 
  70 
  volts 
  growths 
  were 
  not 
  formed 
  

   upon 
  the 
  positive 
  pole. 
  

  

  Further 
  trials 
  were 
  made 
  with 
  a 
  third 
  electrode 
  of 
  different 
  

   shapes 
  varying 
  from 
  spherical 
  to 
  pointed 
  which 
  was 
  held 
  at 
  

   distances 
  varying 
  from 
  2 
  mm. 
  to 
  about 
  2 
  cm. 
  above 
  the 
  arc, 
  

   but 
  well 
  within 
  the 
  stream 
  of 
  vapour 
  that 
  rose 
  from 
  it. 
  

   Using 
  the 
  spherical 
  terminal 
  no 
  growth 
  formed 
  upon 
  it 
  

   when 
  its 
  potential 
  was 
  either 
  +78 
  or 
  —78 
  volts, 
  though 
  

   some 
  brown 
  powder 
  collected; 
  but 
  a 
  vigorous 
  growth 
  was 
  

   produced 
  when 
  the 
  electrode 
  was 
  pointed 
  and 
  the 
  same 
  

   potential 
  used, 
  there 
  was, 
  however, 
  always 
  a 
  marked 
  pre- 
  

   ference 
  of 
  the 
  growth 
  for 
  the 
  negative 
  sign. 
  This 
  growth 
  

   often 
  extended 
  for 
  2 
  or 
  3 
  cm. 
  up 
  the 
  needle-point, 
  but 
  was 
  

   more 
  prominent 
  at 
  the 
  tip. 
  

  

  The 
  Iron 
  Arc. 
  

  

  Observations 
  of 
  an 
  iron 
  arc 
  burning 
  in 
  air 
  were 
  also 
  made 
  

   microscopically 
  and 
  a 
  growth 
  corresponding 
  to 
  that 
  observed 
  

   in 
  the 
  copper 
  was 
  seen. 
  Its 
  laterals 
  were 
  more 
  fully 
  deve- 
  

   loped 
  giving 
  it 
  a 
  feathery 
  appearance, 
  and 
  it 
  was 
  rather 
  

   more 
  delicate 
  and 
  flexible 
  and 
  more 
  susceptible 
  to 
  draughts 
  

   of 
  air, 
  but 
  when 
  cold 
  it 
  was 
  not 
  so 
  brittle. 
  Unlike 
  the 
  

   copper 
  deposit 
  some 
  of 
  this 
  growth 
  occurred 
  upon 
  the 
  anode, 
  

   but 
  always 
  to 
  a 
  very 
  much 
  less 
  extent 
  than 
  upon 
  the 
  cathode. 
  

   Its 
  rate 
  of 
  growth 
  was 
  not 
  appreciably 
  accelerated 
  in 
  the 
  

   proximity 
  of 
  the 
  anode, 
  but 
  remained 
  fairly 
  constant 
  across 
  

   the 
  arc, 
  but 
  when 
  it 
  touched 
  the 
  anode 
  it 
  adhered 
  to 
  it, 
  and 
  

   separation 
  of 
  the 
  poles 
  spun 
  it 
  out 
  to 
  a 
  greater 
  degree 
  of 
  

   fineness. 
  

  

  The 
  difficulty 
  of 
  photographing 
  these 
  growths 
  is 
  very 
  

   great, 
  both 
  on 
  account 
  of 
  their 
  fragility 
  and 
  because 
  they 
  

   are 
  of 
  such 
  gossamer 
  fineness 
  that 
  they 
  will 
  not 
  readily 
  

   remain 
  still 
  long 
  enough. 
  Besides, 
  unless 
  they 
  can 
  be 
  pho- 
  

   tographed 
  during 
  the 
  running 
  of 
  the 
  arc 
  their 
  appearance 
  

   changes 
  — 
  even 
  though 
  the 
  cathode 
  be 
  not 
  disconnected 
  from 
  

   the 
  supply 
  the 
  laterals 
  collapse 
  to 
  some 
  extent. 
  PI. 
  X. 
  fig. 
  4, 
  

   shows 
  some 
  of 
  the 
  feathery 
  growths 
  which 
  cohered 
  upon 
  the 
  

   stoppage 
  of 
  the 
  arc, 
  they 
  original^ 
  bridged 
  the 
  gap 
  from 
  

   the 
  cathode 
  to 
  the 
  anode 
  (below), 
  but 
  the 
  poles 
  have 
  been 
  

   separated 
  and 
  some 
  of 
  the 
  fine 
  filaments 
  have 
  been 
  broken. 
  

   These 
  filaments 
  are 
  less 
  than 
  1/100 
  mm. 
  in 
  diameter, 
  and 
  

  

  