﻿Deposit 
  upon 
  the 
  Poles 
  of 
  Metallic 
  Arcs. 
  343 
  

  

  obvious 
  that 
  the 
  laterals 
  which 
  are 
  at 
  right 
  angles 
  to 
  the 
  

   main 
  stem 
  or 
  axis 
  of 
  the 
  growth 
  experience 
  little 
  or 
  no 
  

   electric 
  force 
  which 
  might 
  occasion 
  the 
  further 
  deposition 
  

   upon 
  them 
  necessary 
  for 
  their 
  extension, 
  but 
  if 
  they 
  are 
  suffi- 
  

   ciently 
  flexible 
  to 
  bend 
  round 
  towards 
  the 
  anode 
  they 
  are 
  as 
  

   favourably 
  oriented 
  as 
  the 
  main 
  axis 
  itself. 
  The 
  iron 
  growth 
  

   appears 
  to 
  be 
  capable 
  of 
  doing 
  this 
  and 
  hence 
  its 
  foliage 
  is 
  

   profuse 
  and 
  dense, 
  but 
  the 
  copper 
  and 
  silver 
  filaments 
  are 
  

   more 
  brittle 
  and 
  less 
  flexible 
  and 
  their 
  laterals 
  remain 
  un- 
  

   developed, 
  the 
  main 
  axis 
  being 
  consequently 
  more 
  pronounced; 
  

   this 
  may 
  account 
  for 
  the 
  organic 
  appearance 
  of 
  the 
  deposit: 
  

   in 
  this 
  connexion 
  it 
  is 
  interesting 
  to 
  note 
  that 
  experiments 
  

   upon 
  the 
  growth 
  of 
  various 
  crops 
  indicate 
  that 
  they 
  too 
  thrive 
  

   best 
  when 
  they 
  spring 
  from 
  a 
  soil 
  discharging 
  electricity: 
  — 
  

   it 
  is 
  pertinent 
  to 
  inquire 
  if 
  their 
  growth 
  is 
  also 
  regu- 
  

   lated 
  by 
  the 
  relative 
  mobilities 
  of 
  positive 
  and 
  negative 
  

   ions. 
  

  

  The 
  rapidity 
  of 
  the 
  growth 
  in 
  the 
  neighbourhood- 
  of 
  the 
  

   anode 
  is 
  evidently 
  occasioned 
  by 
  the 
  increase 
  in 
  the 
  steep- 
  

   ness 
  of 
  the 
  potential 
  gradient 
  close 
  to 
  it 
  as 
  the 
  growth 
  (which 
  

   is 
  in 
  connexion 
  with 
  the 
  cathode 
  and 
  consequently 
  at 
  the 
  

   same 
  potential) 
  extends 
  towards 
  it, 
  while 
  the 
  efficacy 
  of 
  a 
  

   pointed 
  electrode 
  is 
  simply 
  explained 
  by 
  the 
  greater 
  electric 
  

   force 
  in 
  the 
  proximity 
  of 
  a 
  charge 
  of 
  large 
  surface 
  density, 
  

   assisted 
  it 
  may 
  be 
  by 
  the 
  incidence 
  upon 
  it 
  of 
  ultra-violet 
  

   light 
  in 
  the 
  manner 
  already 
  suggested 
  to 
  account 
  for 
  the 
  

   extension 
  of 
  a 
  growth 
  that 
  has 
  already 
  started 
  between 
  the 
  

   poles. 
  Moreover, 
  near 
  it 
  the 
  electric 
  field 
  is 
  very 
  strong 
  

   and 
  not 
  uniform, 
  and 
  any 
  conducting 
  particles 
  would 
  be 
  

   urged 
  towards 
  it. 
  

  

  The 
  origin 
  of 
  the 
  material 
  of 
  the 
  growths. 
  — 
  The 
  experiments 
  

   upon 
  arcs 
  between 
  poles 
  of 
  dissimilar 
  metals 
  point 
  to 
  the 
  

   origin 
  of 
  the 
  material 
  of 
  the 
  growth 
  being 
  the 
  cathode 
  — 
  

   though 
  this 
  is 
  not 
  invariably 
  the 
  case, 
  since 
  a 
  meagre 
  deposit 
  

   of 
  iron 
  may 
  be 
  found 
  upon 
  a 
  cathode 
  of 
  copper. 
  This 
  con- 
  

   clusion 
  is 
  not 
  in 
  conformity 
  with 
  the 
  view 
  that 
  the 
  expulsion 
  

   of 
  positive 
  ions 
  from 
  the 
  anode 
  is 
  the 
  sole 
  source 
  of 
  positive 
  

   ions, 
  such 
  ions 
  seem 
  to 
  be 
  more 
  readily 
  derived 
  from 
  the 
  

   material 
  of 
  the 
  cathode, 
  probably 
  through 
  the 
  impact 
  of 
  

   negative 
  electrons 
  ejected 
  from 
  the 
  cathode 
  with 
  the 
  vapour 
  

   that 
  has 
  diffused 
  from 
  that 
  pole. 
  

  

  In 
  the 
  following 
  Table 
  (p. 
  344) 
  details 
  are 
  given 
  of 
  a 
  few 
  

   typical 
  experiments. 
  

  

  