﻿Ions 
  from 
  Hot 
  Salts. 
  461 
  

  

  of 
  barium 
  iodide, 
  a 
  few 
  square 
  millimetres 
  in 
  area, 
  before 
  

   it 
  was 
  visible 
  in 
  an 
  ordinarily 
  lighted 
  room. 
  The 
  emission 
  

   increases 
  very 
  rapidly 
  with 
  the 
  temperature 
  when 
  other 
  

   conditions 
  are 
  unchanged. 
  At 
  low 
  temperatures 
  a 
  relatively 
  

   large 
  proportion 
  of 
  this 
  emission 
  is 
  carried 
  by 
  heavy 
  ions. 
  

   This 
  proportion 
  diminishes 
  steadily 
  as 
  the 
  temperature 
  is 
  

   raised, 
  until 
  at 
  temperatures 
  comparable 
  with 
  700° 
  practically 
  

   all 
  the 
  current 
  is 
  carried 
  by 
  electrons. 
  

  

  In 
  the 
  experiment 
  to 
  which 
  the 
  above 
  numbers 
  refer 
  

   a 
  glow 
  appeared 
  (although 
  the 
  pressure 
  of 
  gas 
  in 
  the 
  

   apparatus 
  was 
  only 
  0*001 
  mm. 
  on 
  the 
  McLeod 
  gauge) 
  when 
  

   the 
  temperature 
  was 
  raised 
  to 
  725°, 
  and 
  was 
  accompanied 
  by 
  

   a 
  large 
  current 
  when 
  the 
  magnetic 
  field 
  was 
  off. 
  The 
  

   numbers 
  taken 
  under 
  these 
  conditions 
  are 
  indicated 
  thus 
  *. 
  

   At 
  766° 
  the 
  potential 
  was 
  reduced 
  from 
  —80 
  to 
  —40 
  volts, 
  

   which 
  caused 
  the 
  glow 
  to 
  disappear. 
  The 
  observations 
  taken 
  

   with 
  40 
  volts 
  instead 
  of 
  80 
  are 
  shown 
  thus 
  f 
  . 
  Under 
  these 
  

   conditions 
  the 
  current 
  was 
  not 
  saturated, 
  and 
  it 
  is 
  probable 
  

   that 
  the 
  magnetic 
  field 
  kept 
  back 
  some 
  of 
  the 
  heavy 
  ions 
  

   as 
  well 
  as 
  the 
  light 
  ones. 
  These 
  numbers 
  have 
  not 
  therefore 
  

   the 
  same 
  significance 
  as 
  the 
  others. 
  

  

  The 
  above 
  numbers 
  indicate 
  that 
  the 
  proportionate 
  rate 
  

   of 
  increase 
  of 
  the 
  emission 
  with 
  rising 
  temperature 
  is 
  very 
  

   much 
  larger 
  at 
  low 
  than 
  at 
  high 
  temperatures. 
  Part 
  at 
  

   least 
  of 
  this 
  effect 
  is 
  spurious, 
  as 
  the 
  emission 
  at 
  constant 
  

   temperature 
  was 
  increasing 
  with 
  time 
  at 
  the 
  low 
  tempera- 
  

   tures 
  and 
  diminishing 
  with 
  time 
  at 
  the 
  high 
  temperatures 
  

   in 
  these 
  experiments 
  (see 
  below). 
  

  

  In 
  view 
  of 
  the 
  large 
  proportion 
  of 
  the 
  current 
  which 
  was 
  

   carried 
  by 
  heavy 
  ions 
  at 
  the 
  lowest 
  temperature 
  in 
  the 
  

   experiments 
  just 
  recorded, 
  new 
  measurements 
  were 
  carried 
  

   out 
  with 
  a 
  fresh 
  specimen 
  of 
  salt 
  at 
  the 
  lowest 
  temperature 
  

   at 
  which 
  the 
  emission 
  could 
  be 
  measured 
  conveniently. 
  In 
  

   comparing 
  the 
  emission 
  with 
  and 
  without 
  a 
  magnetic 
  field 
  

   a 
  difficulty 
  was 
  experienced 
  owing 
  to 
  the 
  change 
  with 
  time. 
  

   To 
  eliminate 
  this, 
  alternate 
  readings 
  with 
  and 
  without 
  the 
  

   field 
  were 
  taken 
  as 
  rapidly 
  as 
  possible 
  after 
  one 
  another. 
  

   Under 
  these 
  circumstances 
  the 
  changes 
  of 
  the 
  effect 
  from 
  

   one 
  measurement 
  under 
  given 
  conditions 
  to 
  the 
  next 
  was 
  not 
  

   very 
  great, 
  and 
  the 
  mean 
  of 
  the 
  two 
  readings 
  was 
  taken 
  

   to 
  represent 
  the 
  value 
  of 
  the 
  effect 
  at 
  the 
  intermediate 
  

   time 
  at 
  which 
  the 
  emission 
  under 
  the 
  alternate 
  condition 
  

   was 
  measured. 
  Measurements 
  carried 
  out 
  in 
  this 
  way 
  at 
  

   325° 
  led 
  to 
  the 
  following 
  successive 
  values 
  for 
  the 
  ratio 
  of 
  the 
  

   emission 
  with 
  the 
  stopping 
  field 
  to 
  the 
  emission 
  without 
  the 
  

   field:— 
  1-033, 
  0*974, 
  0*992, 
  1*034. 
  The 
  mean 
  of 
  these 
  is 
  1*008. 
  

  

  Phil. 
  Mag. 
  S. 
  6. 
  Vol. 
  26. 
  No. 
  153. 
  Sept. 
  1913. 
  2 
  I 
  

  

  