﻿340 
  Prof. 
  W. 
  G. 
  Duffield 
  on 
  the 
  

  

  In 
  interpreting 
  these 
  experiments 
  it 
  is 
  to 
  be 
  borne 
  in 
  mind 
  

   that 
  the 
  arc 
  itself 
  only 
  occupies 
  a 
  small 
  portion 
  of 
  the 
  gap 
  

   between 
  the 
  poles, 
  and 
  that 
  it 
  usually 
  burned 
  at 
  the 
  bottom 
  

   of 
  a 
  pole-space 
  2 
  mm. 
  wide, 
  the 
  diameter 
  of 
  the 
  poles 
  being 
  

   about 
  1 
  cm. 
  The 
  growths 
  usually 
  appeared 
  above 
  the 
  arc 
  

   proper. 
  

  

  Three 
  lines 
  of 
  argument 
  are 
  now 
  developed 
  to 
  account 
  for 
  

   these 
  phenomena. 
  (1) 
  It 
  is 
  known 
  that 
  the 
  gases 
  rising 
  from 
  

   a 
  metallic 
  arc 
  are 
  ionized, 
  and 
  we 
  expect 
  the 
  space 
  between 
  

   the 
  poles 
  above 
  the 
  arc 
  to 
  be 
  filled 
  with 
  a 
  mixture 
  of 
  ions 
  

   derived 
  from 
  the 
  metals 
  and 
  the 
  air, 
  some 
  of 
  the 
  particles 
  

   being 
  charged 
  positively, 
  some 
  negatively, 
  others 
  being- 
  

   neutral. 
  At 
  first 
  sight 
  it 
  might 
  be 
  expected 
  that 
  of 
  these 
  

   the 
  positives 
  would 
  be 
  drawn 
  to 
  the 
  cathode, 
  the 
  negatives 
  to 
  

   the 
  anode, 
  and 
  that 
  the 
  neutrals 
  would 
  escape 
  with 
  the 
  con- 
  

   vection 
  currents; 
  but 
  different 
  observers 
  have 
  noted 
  that 
  the 
  

   ions 
  are 
  only 
  removed 
  from 
  the 
  arc 
  with 
  difficulty, 
  and 
  it 
  

   seems 
  that 
  the 
  neutral 
  atoms 
  are 
  not 
  the 
  sole 
  constituent 
  of 
  

   the 
  escaping 
  vapours 
  in 
  spite 
  of 
  the 
  electric 
  field 
  between 
  

   the 
  poles, 
  which 
  with 
  a 
  2 
  mm. 
  gap 
  amounts 
  to 
  about 
  200 
  

   volts 
  per 
  cm. 
  

  

  This 
  potential 
  gradient 
  is 
  sufficient 
  in 
  the 
  case 
  of 
  copper 
  

   and 
  iron 
  to 
  sweep 
  some 
  at 
  least 
  of 
  the 
  metallic 
  vapour 
  onto 
  

   the 
  cathode, 
  where 
  it 
  assumes 
  the 
  organic 
  form 
  under 
  dis- 
  

   cussion, 
  but 
  a 
  larger 
  difference 
  of 
  potential 
  is 
  required 
  to 
  

   accomplish 
  this 
  for 
  the 
  ionized 
  silver 
  vapour. 
  In 
  all 
  cases 
  a 
  

   greater 
  potential 
  difference 
  was 
  necessary 
  to 
  sweep 
  the 
  

   material 
  onto 
  the 
  anode. 
  The 
  explanation 
  of 
  this 
  seems 
  to 
  

   consist, 
  at 
  least 
  in 
  part, 
  in 
  the 
  greater 
  mobility 
  of 
  the 
  positive 
  

   ions. 
  It 
  is 
  true 
  that 
  in 
  air 
  alone 
  the 
  negative 
  ions 
  are 
  more 
  

   mobile, 
  but 
  their 
  mobility 
  has 
  been 
  shown 
  to 
  be 
  greatly 
  

   reduced 
  when 
  the 
  air 
  is 
  moist* 
  owing 
  to 
  the 
  deposition 
  of 
  

   moisture 
  upon 
  them, 
  the 
  mobility 
  of 
  the 
  positive 
  ions 
  being 
  

   but 
  slightly 
  affected, 
  as 
  they 
  do 
  not 
  so 
  readily 
  act 
  as 
  centres 
  

   of 
  condensation. 
  G. 
  T. 
  R. 
  Wilson 
  t 
  found 
  later 
  that 
  under 
  

   conditions 
  favourable 
  for 
  the 
  further 
  deposition 
  of 
  moisture, 
  

   the 
  negative 
  ions 
  tended 
  to 
  monopolize 
  the 
  whole 
  of 
  the 
  

   aqueous 
  vapour 
  unless 
  special 
  precautions 
  were 
  taken. 
  It 
  is 
  

   suspected 
  that 
  in 
  the 
  experiments 
  with 
  the 
  arc 
  the 
  negative 
  

   ions 
  act 
  in 
  the 
  same 
  way, 
  and 
  provide 
  the 
  chief 
  source 
  of 
  

   nuclei 
  for 
  the 
  condensation 
  of 
  the 
  metallic 
  vapour, 
  and 
  that 
  

   they 
  give 
  rise 
  to 
  a 
  much 
  more 
  massive 
  aggregate 
  than 
  do 
  

   the 
  positive 
  ions. 
  

  

  * 
  Zelenv, 
  Phil. 
  Trans. 
  A. 
  cxcv. 
  p. 
  193 
  (1900). 
  

   t 
  C 
  T. 
  K. 
  Wilson, 
  Phil. 
  Trans, 
  cxciii. 
  p. 
  289. 
  

  

  