﻿456 
  Prof. 
  0, 
  W. 
  Richardson 
  on 
  the 
  

  

  Some 
  experiments 
  with 
  cadmium 
  iodide 
  by 
  the 
  slit 
  method, 
  

   although 
  not 
  altogether 
  satisfactory 
  in 
  some 
  respects, 
  confirm 
  

   the 
  above 
  results 
  so 
  far 
  as 
  they 
  go. 
  In 
  order 
  to 
  have 
  enough 
  

   of 
  the 
  salt 
  to 
  give 
  an 
  emission 
  lasting 
  for 
  a 
  sufficiently 
  long 
  

   time, 
  a 
  considerable 
  amount 
  of 
  it 
  was 
  melted 
  on 
  to 
  the 
  strip. 
  

   The 
  lowest 
  temperature 
  at 
  which 
  measurements 
  could 
  be 
  

   made 
  was 
  507° 
  Ft. 
  Under 
  these 
  conditions 
  values 
  of 
  w/H 
  

   as 
  high 
  as 
  440 
  were 
  obtained, 
  but 
  the 
  curves 
  were 
  very 
  flat 
  

   and 
  of 
  the 
  form 
  obtained 
  when 
  there 
  is 
  a 
  considerable 
  pres- 
  

   sure 
  of 
  gas 
  in 
  the 
  apparatus. 
  This 
  probably 
  means 
  that 
  the 
  

   measurements 
  were 
  spoiled 
  by 
  the 
  presence 
  of 
  the 
  iodine 
  

   vapour 
  which 
  is 
  given 
  off 
  by 
  the 
  heated 
  salt. 
  If 
  there 
  is 
  any 
  

   considerable 
  quantity 
  of 
  gas 
  or 
  vapour 
  present 
  the 
  method, 
  

   as 
  Davisson 
  has 
  shown, 
  leads 
  to 
  spurious 
  hij:h 
  values 
  of 
  wj/H. 
  

   When 
  the 
  strip 
  had 
  been 
  heated 
  for 
  several 
  hours 
  two 
  sets 
  

   of 
  observations 
  led 
  to 
  the 
  following 
  respective 
  values 
  : 
  — 
  

  

  1. 
  e\m 
  = 
  146-5, 
  m/R 
  = 
  65'6. 
  

  

  2. 
  ejm 
  = 
  167, 
  m/R 
  = 
  57*9. 
  

  

  The 
  mean 
  of 
  these 
  values 
  is 
  61*7, 
  which 
  agrees 
  with 
  Cd 
  ++ 
  

   within 
  the 
  limits 
  of 
  experimental 
  error. 
  The 
  temperature 
  

   in 
  the 
  final 
  experiments 
  was 
  930° 
  Pt. 
  

  

  The 
  above 
  experiments 
  show 
  that 
  when 
  a 
  strip 
  of 
  platinum 
  

   on 
  which 
  cadmium 
  iodide 
  has 
  been 
  placed 
  is 
  heated, 
  ions 
  are 
  

   given 
  off 
  which 
  have 
  an 
  electric 
  atomic 
  w 
  eight 
  corresponding 
  

   to 
  Cd 
  ++ 
  . 
  The 
  high 
  temperatures 
  at 
  which 
  this 
  effect 
  is 
  

   observed 
  are 
  against 
  the 
  view 
  that 
  the 
  ions 
  are 
  produced 
  

   directly 
  by 
  the 
  action 
  of 
  heat 
  on 
  the 
  iodide 
  itself 
  ; 
  since 
  at 
  

   these 
  low 
  pressures 
  it 
  would 
  presumably 
  have 
  evaporated 
  

   before 
  the 
  temperatures 
  recorded 
  were 
  attained. 
  I 
  have 
  not 
  

   been 
  able 
  to 
  obtain 
  sufficient 
  ionization 
  to 
  mnke 
  measurements 
  

   with, 
  at 
  the 
  relatively 
  low 
  temperatures 
  at 
  which 
  the 
  ioniz- 
  

   ation 
  from 
  Cdl 
  2 
  has 
  been 
  observed 
  * 
  under 
  other 
  conditions. 
  

   The 
  ions 
  here 
  investigated 
  probably 
  arise 
  either 
  from 
  the 
  

   action 
  on 
  the 
  hot 
  platinum 
  of 
  cadmium 
  iodide 
  vapour 
  which 
  

   comes 
  from 
  cooler 
  parts 
  of 
  the 
  strip, 
  or 
  else 
  from 
  some 
  less 
  

   volatile 
  substance 
  with 
  which 
  the 
  iodide 
  is 
  contaminated 
  (cf. 
  

   also 
  the 
  section 
  below 
  which 
  deals 
  with 
  negative 
  ions.) 
  It 
  is 
  

   possible 
  that 
  somewhat 
  similar 
  considerations 
  apply 
  to 
  the 
  

   ions 
  emitted 
  by 
  zinc 
  iodide. 
  

  

  Strontium 
  Iodide. 
  Srl 
  2 
  . 
  

  

  Although 
  this 
  substance 
  gave 
  a 
  large 
  emission 
  of 
  negative 
  

   ions 
  at 
  a 
  low 
  temperature, 
  the 
  positive 
  emission 
  was 
  not 
  

  

  * 
  Cf. 
  Cx. 
  C. 
  Schmidt, 
  Ann 
  der 
  Phys. 
  vol. 
  xxxv. 
  p. 
  404 
  (1911), 
  and 
  

   C. 
  Sheaid, 
  Phil. 
  Mag. 
  vol. 
  xxv. 
  p. 
  370 
  (1913). 
  

  

  