﻿Deposit 
  upon 
  the 
  Poles 
  of 
  Metallic 
  Airs. 
  337 
  

  

  still 
  finer 
  ones 
  have 
  been 
  observed. 
  This 
  type 
  of 
  iron 
  growth 
  

   is 
  better 
  shown 
  in 
  PI. 
  IX. 
  fig. 
  2, 
  where 
  they 
  are 
  seen 
  much 
  

   magnified 
  growing 
  from 
  a 
  needle-point 
  charged 
  to 
  a 
  negative 
  

   potential 
  of 
  100 
  volts 
  about 
  2 
  mm. 
  above 
  a 
  horizontal 
  arc 
  

   between 
  iron 
  poles. 
  They 
  are 
  easily 
  distinguishable 
  from 
  the 
  

   corresponding 
  copper 
  growths 
  in 
  PI. 
  IX. 
  fig. 
  1. 
  

  

  To 
  produce 
  these 
  growths 
  a 
  small 
  hissing 
  arc 
  proved 
  most 
  

   favourable 
  and 
  they 
  were 
  best 
  developed 
  by 
  a 
  current 
  of 
  

   .about 
  4 
  amperes. 
  

  

  In 
  the 
  iron 
  arc 
  the 
  growths 
  seem 
  rather 
  more 
  influenced 
  

   by 
  convection 
  currents 
  than 
  by 
  the 
  electric 
  forces, 
  and 
  much 
  

   of 
  the 
  deposit 
  forms 
  upon 
  the 
  edges 
  of 
  the 
  upper 
  pole 
  ; 
  here, 
  

   especially 
  if 
  the 
  pole 
  be 
  negative, 
  it 
  accumulates, 
  and 
  may 
  

   become 
  massive. 
  Though 
  these 
  deposits 
  are 
  of 
  a 
  much 
  

   coarser 
  nature 
  than 
  those 
  within 
  the 
  arc, 
  they 
  nevertheless 
  

   assume 
  forms 
  reminiscent 
  of 
  organic 
  structures, 
  and 
  are 
  the 
  

   result 
  of 
  further 
  deposition 
  upon 
  the 
  type 
  of 
  growth 
  first 
  

   described. 
  

  

  A 
  general 
  view 
  of 
  this 
  growth 
  is 
  seen 
  in 
  PI. 
  X. 
  fig. 
  1, 
  which 
  

   shows 
  the 
  negative 
  pole 
  at 
  the 
  top 
  after 
  the 
  poles 
  have 
  been 
  

   separated, 
  the 
  characteristic 
  positive 
  bead 
  being 
  seen 
  on 
  the 
  

   lower 
  pole. 
  The 
  photograph 
  is 
  an 
  enlargement, 
  the 
  actual 
  

   diameter 
  of 
  the 
  poles 
  being 
  |-inch. 
  The 
  deposit, 
  which 
  is 
  of 
  

   rather 
  an 
  indeterminate 
  colour 
  ranging 
  from 
  brown 
  to 
  orange- 
  

   yellow, 
  is 
  seen 
  to 
  be 
  most 
  copious 
  round 
  the 
  rim 
  of 
  the 
  

   crater, 
  but 
  it 
  also 
  extends 
  in 
  ridges 
  for 
  about 
  half 
  an 
  inch 
  

   up 
  the 
  side 
  of 
  the 
  top 
  pole, 
  thinning 
  off 
  to 
  an 
  even 
  layer 
  of 
  a 
  

   powdery 
  nature 
  higher 
  up. 
  

  

  A 
  growth 
  on 
  the 
  right-hand 
  side 
  of 
  the 
  pole 
  is 
  seen 
  turned 
  

   upwards 
  under 
  the 
  influence 
  of 
  the 
  hot 
  vapour 
  rising 
  from 
  

   the 
  arc. 
  In 
  fig. 
  2, 
  a 
  coral-like 
  growth 
  at 
  the 
  corner 
  of 
  the 
  

   top 
  pole 
  is 
  shown 
  more 
  highly 
  magnified. 
  The 
  current 
  used 
  

   to 
  produce 
  the 
  growths 
  in 
  figs. 
  1 
  and 
  2 
  was 
  heavy, 
  varying 
  

   from 
  10 
  to 
  14 
  amperes. 
  Fig. 
  3 
  illustrates 
  a 
  modified 
  form 
  

   of 
  the 
  feathery 
  type, 
  some 
  condensation 
  having 
  occurred 
  

   upon 
  it. 
  The 
  filaments 
  are 
  no 
  longer 
  to 
  be 
  seen, 
  and 
  under 
  

   a 
  high 
  power 
  the 
  whole 
  has 
  the 
  appearance 
  of 
  a 
  nodular 
  

   aggregate. 
  

  

  The 
  deposit 
  collected 
  from 
  the 
  poles 
  was 
  found 
  to 
  be 
  

   magnetic 
  ; 
  that 
  it 
  is 
  not 
  pure 
  magnetic 
  oxide 
  is, 
  however, 
  

   shown 
  by 
  its 
  reddish 
  colour. 
  It 
  may 
  be 
  ferric 
  oxide 
  mixed 
  

   with 
  the 
  magnetic 
  oxide 
  or 
  else 
  with 
  particles 
  of 
  iron 
  — 
  

   though 
  probably 
  the 
  former. 
  It 
  has 
  not 
  been 
  determined 
  

   whether 
  oxidation 
  precedes 
  or 
  succeeds 
  deposition. 
  

  

  