﻿176 
  Allen 
  and 
  Lombard 
  — 
  Determination 
  of 
  

  

  volume 
  of 
  nitrogen, 
  the 
  gas 
  being 
  passed 
  over 
  the 
  sulphide 
  

   while 
  the 
  latter 
  is 
  held 
  at 
  constant 
  temperature 
  ; 
  a 
  static 
  

   method 
  in 
  which 
  the 
  sulphide 
  is 
  maintained 
  at 
  constant 
  

   temperature 
  while 
  sulphur 
  vapor 
  at 
  a 
  known 
  partial 
  pressure 
  

   in 
  a 
  mixture 
  with 
  nitrogen 
  is 
  passed 
  over 
  it, 
  the 
  sulphide 
  losing 
  

   or 
  gaining 
  in 
  weight 
  according 
  as 
  its 
  dissociation 
  pressure 
  is 
  

   greater 
  or 
  less 
  than 
  the 
  sulphur 
  vapor 
  pressure 
  ; 
  and 
  finally 
  a 
  

   static 
  method 
  in 
  which 
  the 
  sulphur 
  vapor 
  pressure 
  is 
  directly 
  

   measured 
  by 
  the 
  very 
  ingenious 
  spiral 
  quartz-glass 
  gauge 
  

   devised 
  by 
  F. 
  M. 
  G-. 
  Johnson.* 
  The 
  first 
  two 
  methods 
  were 
  

   used 
  by 
  Wasjuchnowa,f 
  the 
  last;}; 
  by 
  Preunner 
  and 
  Brock- 
  

   moller. 
  

  

  We 
  had 
  prepared 
  to 
  use 
  the 
  spiral 
  gauge 
  in 
  the 
  investiga- 
  

   tion 
  of 
  the 
  stability 
  relations 
  of 
  the 
  copper-iron 
  sulphides, 
  but 
  

   the 
  outbreak 
  of 
  the 
  European 
  war 
  about 
  that 
  time 
  prevented 
  

   the 
  importation 
  of 
  quartz-glass 
  gauges, 
  and 
  the 
  construction 
  of 
  

   sufficiently 
  sensitive 
  instruments 
  in 
  this 
  country 
  proved 
  im- 
  

   possible. 
  

  

  II. 
  JVew 
  Method 
  for 
  Dissociation 
  Pressures. 
  

  

  In 
  this 
  dilemma 
  the 
  present 
  method 
  was 
  devised. 
  It 
  con- 
  

   sists 
  in 
  balancing 
  the 
  dissociation 
  pressure 
  to 
  be 
  determined 
  by 
  

   a 
  known 
  vapor 
  pressure 
  of 
  liquid 
  sulphur. 
  For 
  this 
  purpose 
  

   an 
  evacuated 
  glass 
  tube 
  is 
  required, 
  having 
  at 
  each 
  end 
  a 
  small 
  

   bulb 
  (fig. 
  1). 
  One 
  of 
  these 
  bulbs 
  contains 
  the 
  sulphide, 
  the 
  

   other 
  the 
  liquid 
  sulphur. 
  Now 
  while 
  the 
  sulphide 
  is 
  held 
  at 
  

   any 
  desired 
  temperature, 
  the 
  experimenter 
  ascertains 
  the 
  

   temperature 
  at 
  which 
  the 
  sulphur 
  must 
  be 
  heated 
  in 
  order 
  that 
  

   its 
  vapor 
  pressure 
  may 
  equal 
  the 
  dissociation 
  pressure 
  of 
  the 
  

   sulphide. 
  It 
  is 
  obvious 
  that 
  the 
  sulphur 
  bulb 
  must 
  be 
  the 
  

   coolest 
  part 
  of 
  the 
  system, 
  and 
  that 
  the 
  method 
  applies 
  only 
  to 
  

   cases 
  where 
  sulphur 
  is 
  the 
  only 
  volatile 
  product. 
  In 
  practice 
  

   it 
  is 
  necessary 
  to 
  find 
  by 
  trial 
  two 
  temperatures 
  for 
  the 
  sulphur, 
  

   thus 
  fixing 
  two 
  pressures, 
  at 
  one 
  of 
  which 
  the 
  sulphide 
  loses 
  

   sulphur, 
  at 
  the 
  other 
  remains 
  unchanged 
  ; 
  or 
  gains, 
  if 
  the 
  dis- 
  

   sociation 
  product 
  was 
  originally 
  chosen 
  for 
  experiment. 
  At 
  

   the 
  first 
  temperature 
  it 
  is 
  evident 
  that 
  the 
  vapor 
  pressure 
  of 
  

   the 
  sulphur 
  is 
  lower, 
  at 
  the 
  second 
  higher 
  than 
  the 
  dissocia- 
  

   tion 
  pressure 
  of 
  the 
  sulphide. 
  Between 
  these 
  two 
  pressures 
  

   lies 
  the 
  dissociation 
  pressure. 
  In 
  an 
  actual 
  determination, 
  the 
  

   interval 
  between 
  the 
  two 
  sulphur 
  temperatures 
  is 
  narrowed 
  

   down 
  as 
  far 
  as 
  practicable, 
  the 
  size 
  of 
  the 
  interval 
  at 
  any 
  point 
  

   depending 
  naturally 
  on 
  the 
  steepness 
  of 
  the 
  dissociation 
  

   pressure 
  curve. 
  

  

  * 
  Z. 
  phys. 
  Chem. 
  lxi, 
  457, 
  1908. 
  

  

  fDas 
  Gleichgewicht 
  Cupro-cuprisulfid. 
  Dissertation, 
  Berlin, 
  1909. 
  

   JZ. 
  phys. 
  Chern., 
  lxxxi, 
  149, 
  1912. 
  Preunner 
  and 
  Brockmoller 
  used 
  the 
  

   spiral 
  gauge 
  as 
  a 
  zero 
  instrument. 
  

  

  