﻿334 
  Prof. 
  W. 
  G. 
  Duffield 
  on 
  the 
  

  

  other 
  forms 
  of 
  iron 
  deposit 
  are 
  derived 
  this 
  copper 
  growth 
  

   is 
  described 
  first. 
  

  

  The 
  Copper 
  Arc. 
  

  

  The 
  ^copper 
  rods 
  used 
  were 
  1 
  cm. 
  in 
  diameter 
  and 
  were 
  

   usually 
  held 
  horizontally 
  in 
  a 
  hand-fed 
  arc, 
  the 
  arc 
  itself 
  

   springing 
  across 
  the 
  base 
  of 
  the 
  gap 
  and 
  its 
  vapour 
  curling 
  

   upwards 
  fairly 
  evenly 
  between 
  the 
  two 
  poles. 
  The 
  current 
  

   was 
  either 
  derived 
  from 
  town 
  mains 
  supplying 
  continuous 
  

   current 
  at 
  200 
  volts 
  or 
  from 
  a 
  storage 
  battery 
  of 
  50 
  cells. 
  

   The 
  current 
  was 
  reduced 
  by 
  rheostats 
  to 
  about 
  4 
  amperes,, 
  

   the 
  potential 
  difference 
  across 
  the 
  terminals 
  being 
  then 
  

   between 
  25 
  and 
  40 
  volts. 
  These 
  were 
  not 
  the 
  only 
  con- 
  

   ditions 
  under 
  which 
  the 
  growths 
  were 
  formed, 
  but 
  they 
  

   proved 
  very 
  favourable. 
  When 
  the 
  arc-length 
  was 
  more 
  

   than 
  3 
  mm. 
  they 
  were 
  not 
  produced 
  with 
  a 
  current 
  greater 
  

   than 
  6*5 
  amperes, 
  nor 
  were 
  they 
  formed 
  with 
  a 
  current 
  

   of 
  9*5 
  amperes 
  when 
  the 
  gap 
  was 
  2 
  mm., 
  indicating 
  that 
  

   both 
  heavy 
  currents 
  and 
  long 
  arcs 
  militated 
  against 
  their 
  

   formation. 
  

  

  Under 
  favourable 
  conditions 
  of 
  current 
  strength 
  and 
  arc- 
  

   length 
  delicate 
  tree-like 
  structures 
  rapidly 
  grew 
  from 
  the 
  

   negative 
  pole 
  in 
  the 
  direction 
  of 
  the 
  anode, 
  eventually 
  

   bridging 
  the 
  gap 
  and 
  possibly 
  assisting 
  in 
  the 
  passage 
  of 
  the 
  

   current. 
  These 
  growths 
  invariably 
  made 
  their 
  appearance 
  

   upon 
  the 
  cathode 
  and 
  never 
  upon 
  the 
  anode, 
  whether 
  the 
  

   arc 
  were 
  vertical 
  or 
  horizontal 
  and 
  whatever 
  the 
  direction 
  

   of 
  the 
  current. 
  The 
  growths 
  were 
  nearly 
  black 
  and 
  consisted 
  

   it 
  would 
  seem 
  of 
  cupric 
  oxide. 
  They 
  were 
  extremely 
  light 
  

   and 
  fragile, 
  and 
  though 
  urged 
  upwards 
  by 
  the 
  hot 
  gases 
  

   rising 
  from 
  the 
  horizontal 
  arc, 
  only 
  those 
  round 
  the 
  rims 
  were 
  

   permanently 
  deflected, 
  and 
  then 
  but 
  slightly 
  ; 
  the 
  others 
  were 
  

   chiefly 
  influenced 
  by 
  the 
  direction 
  of 
  the 
  electric 
  field. 
  

  

  The 
  arc 
  was 
  examined 
  under 
  a 
  microscope 
  with 
  the 
  aid 
  of 
  

   coloured 
  glass. 
  Starting 
  with 
  copper 
  poles 
  which 
  had 
  been 
  

   filed 
  flat 
  before 
  the 
  arc 
  was 
  struck, 
  the 
  negative 
  pole 
  quickly 
  

   became 
  dotted 
  with 
  numerous 
  tiny 
  black 
  spikes, 
  which 
  

   rapidly 
  developed 
  normally 
  to 
  the 
  surface 
  and 
  remained 
  as 
  

   a 
  rule 
  straight 
  until 
  a 
  point 
  was 
  reached 
  at 
  which 
  branching- 
  

   began, 
  when 
  two 
  or 
  usually 
  more 
  laterals 
  started 
  out 
  from 
  the 
  

   main 
  trunk. 
  These 
  sent 
  out 
  sub-laterals 
  which 
  in 
  their 
  turn 
  

   developed 
  tiny 
  off-shoots, 
  but 
  as 
  the 
  growth 
  of 
  the 
  laterals 
  

   was 
  not 
  as 
  rapid 
  as 
  that 
  of 
  the 
  main 
  stem, 
  the 
  whole 
  growth 
  

   remained 
  fairly 
  open 
  and 
  resembled 
  a 
  tree 
  of 
  the 
  monopodial 
  

   type 
  in 
  which 
  the 
  main 
  axis 
  predominates. 
  The 
  phenomenon 
  

   was 
  very 
  beautiful 
  to 
  watch, 
  the 
  miniature 
  trees 
  being 
  of 
  

  

  