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  . 
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  the 
  electrodes 
  was 
  140 
  volts, 
  the 
  dark 
  space 
  was 
  about 
  1 
  mm. 
  

   thick 
  at 
  the 
  bottom 
  of 
  the 
  electrode 
  and 
  somewhat 
  more 
  than 
  

   this 
  at 
  the 
  top. 
  No 
  doubt 
  the 
  reason 
  that 
  this 
  dark 
  space 
  

   did 
  not 
  extend 
  more 
  than 
  this 
  was 
  that 
  the 
  drop 
  in 
  potential 
  

   was 
  all 
  in 
  the 
  immediate 
  neighbourhood 
  of 
  the 
  negative 
  

   electrode, 
  as 
  in 
  one 
  of 
  the 
  previous 
  experiments. 
  The 
  dark 
  

   space 
  extends 
  somewhat 
  further 
  at 
  the 
  top 
  of 
  the 
  electrode, 
  

   since 
  there 
  are 
  fewer 
  ions 
  there, 
  and 
  their 
  electrostatic 
  effect 
  

   is 
  not 
  so 
  marked. 
  

  

  A 
  magnetic 
  field 
  was 
  then 
  placed 
  perpendicular 
  to 
  the 
  line 
  

   joining 
  the 
  electrodes 
  and 
  also 
  to 
  the 
  direction 
  in 
  which 
  the 
  

   vapour 
  was 
  moving. 
  When 
  the 
  strength 
  of 
  this 
  field 
  was 
  

   5560 
  lines 
  per 
  sq. 
  cm., 
  there 
  was 
  a 
  great 
  diminution 
  in 
  the 
  

   amount 
  of 
  light 
  beyond 
  the 
  electrodes. 
  The 
  ions 
  were 
  

   apparently 
  forced 
  out 
  of 
  the 
  region 
  by 
  the 
  magnetic 
  field, 
  

   leaving 
  the 
  vapour 
  non-luminous. 
  With 
  the 
  magnetic 
  field 
  

   the 
  lessening 
  of 
  the 
  brightness 
  did 
  not 
  seem 
  to 
  occur 
  at 
  one 
  

   electrode 
  more 
  than 
  at 
  the 
  other. 
  

  

  Investigation 
  was 
  also 
  made 
  on 
  the 
  conductivity 
  of 
  the 
  

   vapour 
  above 
  F, 
  and 
  while 
  it 
  confirmed 
  what 
  has 
  been 
  said 
  

   about 
  the 
  electrical 
  properties 
  of 
  the 
  vapour, 
  it 
  did 
  not 
  

   appear 
  to 
  give 
  any 
  further 
  knowledge 
  concerning 
  the 
  cause 
  

   of 
  the 
  light 
  and 
  may 
  be 
  omitted 
  from 
  this 
  discussion. 
  

  

  It 
  was 
  also 
  observed 
  that 
  with 
  the 
  vapour 
  freely 
  rising 
  from 
  

   the 
  arc, 
  and 
  with 
  a 
  potential 
  difference 
  of 
  more 
  than 
  15 
  volts 
  

   between 
  F 
  and 
  the 
  arc, 
  the 
  luminosity 
  was 
  increased 
  and 
  was 
  

   changed 
  from 
  a 
  yellow 
  to 
  a 
  green 
  tint. 
  This 
  was 
  found 
  to 
  be 
  

   due 
  to 
  a 
  strengthening 
  of 
  the 
  green 
  line 
  in 
  the 
  mercury 
  

   spectrum. 
  I 
  hope 
  to 
  be 
  able 
  to 
  report 
  on 
  this 
  more 
  fully 
  in 
  

   a 
  later 
  article. 
  

  

  Summary. 
  

  

  The 
  vapour 
  passing 
  from 
  the 
  mercury 
  arc 
  in 
  a 
  vacuum 
  to 
  

   a 
  condensing 
  chamber 
  is 
  luminous 
  and 
  has 
  a 
  line 
  spectrum 
  

   similar 
  to 
  the 
  spectrum 
  of 
  the 
  arc. 
  This 
  luminosity 
  can 
  be 
  

   destroyed 
  by 
  an 
  electric 
  field. 
  It 
  cannot 
  be 
  concentrated 
  at 
  

   any 
  point 
  and 
  is 
  not 
  due 
  to 
  radiation 
  from 
  the 
  arc. 
  These 
  

   facts 
  cannot 
  be 
  explained 
  by 
  assuming 
  that 
  the 
  light 
  is 
  due 
  

   to 
  chemical 
  action 
  nor 
  to 
  ionization 
  of 
  the 
  vapour 
  either 
  in 
  

   the 
  arc 
  or 
  in 
  the 
  region 
  above 
  it. 
  They 
  can, 
  however, 
  

   be 
  satisfactorily 
  explained 
  by 
  assuming 
  that 
  the 
  light 
  is 
  

   caused 
  by 
  the 
  recombination 
  of 
  the 
  ions 
  to 
  form 
  uncharged 
  

   molecules. 
  

  

  Colgate 
  University, 
  U.S.A. 
  

   May 
  1913. 
  

  

  