﻿Dissociation 
  Pressures 
  of 
  Sulphides. 
  193 
  

  

  Heat 
  of 
  dissociation 
  of 
  covellite 
  and 
  py 
  rite. 
  — 
  The 
  two 
  cases 
  

   of 
  dissociation 
  which 
  we 
  have 
  studied 
  are 
  unusually 
  compli- 
  

   cated. 
  Not 
  only 
  does 
  the 
  sulphur 
  vapor 
  change 
  in 
  density 
  

   with 
  change 
  of 
  temperature, 
  but 
  the 
  solid 
  products, 
  pyrrhotite 
  

   and 
  covellite, 
  vary 
  in 
  composition. 
  The 
  validity 
  of 
  the 
  Yan't 
  

   Hoff 
  equation 
  : 
  

  

  log 
  

  

  ft-ip 
  g 
  g»%q/ 
  T 
  .- 
  T 
  . 
  

  

  T. 
  T 
  R 
  \ 
  T, 
  T 
  

  

  may 
  be 
  doubted 
  here, 
  but 
  assuming 
  that 
  it 
  applies, 
  the 
  experi- 
  

   mental 
  data 
  are 
  not 
  sufficiently 
  accurate 
  for 
  the 
  calculation 
  of 
  

   satisfactory 
  values 
  of 
  Q. 
  An 
  error 
  of 
  1° 
  in 
  the 
  temperature 
  

   of 
  the 
  sulphur 
  causes 
  an 
  error 
  in 
  p 
  of 
  8-10 
  mm 
  in 
  certain 
  parts 
  

   of 
  the 
  curve 
  (450-500 
  mm 
  ) 
  which 
  occasion 
  a 
  very 
  large 
  error 
  in 
  

   the 
  calculated 
  value 
  of 
  Q. 
  Small 
  errors 
  in 
  the 
  temperature 
  

   interval 
  T 
  2 
  — 
  T 
  1 
  also 
  cause 
  large 
  errors 
  in 
  the 
  value 
  of 
  Q. 
  

  

  IV. 
  Advantages 
  and 
  Disadvantages 
  of 
  the 
  Method. 
  

  

  1. 
  The 
  above 
  method 
  for 
  the 
  determination 
  of 
  dissociation 
  

   pressures 
  has 
  the 
  important 
  advantage 
  of 
  approaching 
  the 
  

   equilibrium 
  from 
  both 
  directions 
  so 
  that, 
  if 
  the 
  temperature 
  

   measurements 
  are 
  accurately 
  made, 
  the 
  course 
  of 
  the 
  curve 
  

   within 
  the 
  limits 
  set 
  should 
  be 
  certain. 
  It 
  will 
  be 
  noted 
  that 
  

   the 
  chilling 
  of 
  the 
  glass 
  tube 
  as 
  a 
  consequence 
  of 
  its 
  light 
  

   w 
  T 
  eight 
  and 
  the 
  method 
  of 
  manipulation, 
  is 
  very 
  quickly 
  accom- 
  

   plished. 
  It 
  is 
  possible 
  that 
  at 
  high 
  pressures 
  there 
  might 
  be 
  

   some 
  reversal 
  of 
  the 
  reaction 
  in 
  this 
  operation, 
  but 
  nothing 
  of 
  

   the 
  kind 
  has 
  been 
  observed 
  in 
  our 
  experiments. 
  

  

  2. 
  Either 
  the 
  sulphide 
  or 
  its 
  dissociation 
  product 
  may 
  be 
  

   used 
  and 
  the 
  presence 
  of 
  free 
  sulphur 
  would 
  have 
  no 
  effect, 
  

   advantages 
  not 
  possessed 
  by 
  the 
  spiral 
  gauge 
  method. 
  The 
  

   influence 
  of 
  moisture 
  or 
  other 
  foreign 
  gases 
  would 
  probably 
  be 
  

   less 
  serious 
  than 
  in 
  other 
  methods. 
  

  

  3. 
  It 
  is 
  unnecessary 
  to 
  wait 
  till 
  the 
  sulphur 
  vapor 
  has 
  come 
  

   to 
  equilibrium 
  with 
  the 
  sulphide 
  ; 
  all 
  that 
  is 
  needed 
  is 
  to 
  know 
  

   with 
  certainty 
  the 
  direction 
  in 
  which 
  the 
  reaction 
  is 
  proceeding 
  

   under 
  given 
  conditions. 
  With 
  the 
  spiral 
  gauge 
  method 
  it 
  

   would 
  of 
  course 
  be 
  necessary 
  to 
  a 
  correct 
  measurement 
  that 
  the 
  

   sulphide 
  should 
  be 
  held 
  at 
  constant 
  temperature 
  until 
  equili- 
  

   brium 
  was 
  reached. 
  This 
  necessity 
  would 
  cause 
  much 
  trouble 
  

   if 
  the 
  reaction 
  moved 
  very 
  slowly, 
  because 
  the 
  temperatures 
  

   require 
  constant 
  watching. 
  The 
  dynamic 
  method 
  could 
  not 
  

   be 
  used 
  at 
  all 
  in 
  such 
  a 
  case, 
  for 
  it 
  depends 
  on 
  the 
  assumption 
  

   that 
  the 
  partial 
  pressure 
  of 
  the 
  sulphur 
  in 
  the 
  stream 
  of 
  indif- 
  

   ferent 
  gas 
  is 
  always 
  equal 
  to 
  the 
  dissociation 
  pressure.* 
  This 
  

  

  * 
  Wasjuchnowa 
  proved 
  that 
  equilibrium 
  under 
  her 
  working 
  conditions 
  was 
  

   quickly 
  established 
  between 
  covellite, 
  chalcocite 
  and 
  sulphur 
  vapor. 
  

  

  