﻿Dissociation 
  Pressures 
  of 
  Sulphides. 
  179 
  

  

  then 
  further 
  constricted* 
  to 
  a 
  thick- 
  walled 
  capillary. 
  The 
  

   open 
  end 
  of 
  the 
  tube, 
  after 
  being 
  drawn 
  down 
  to 
  proper 
  size, 
  

   cut 
  off 
  and 
  smoothed 
  in 
  the 
  flame, 
  is 
  attached 
  to 
  the 
  vacuum 
  

   pump 
  by 
  a 
  piece 
  of 
  heavy-walled 
  rubber 
  tubing, 
  and 
  the 
  pump 
  

   is 
  started. 
  A 
  little 
  caution 
  is 
  required 
  at 
  first 
  to 
  prevent 
  any 
  

   sulphide 
  powder 
  from 
  being 
  sucked 
  into 
  the 
  capillary. 
  When 
  

   the 
  pressure 
  is 
  sufficiently 
  reduced 
  (to 
  l 
  mm 
  or 
  less), 
  the 
  sulphur 
  

   is 
  again 
  melted 
  by 
  the 
  free 
  flame 
  and 
  the 
  apparatus 
  is 
  carefully 
  

   heated 
  throughout 
  its 
  whole 
  length 
  to 
  remove 
  water 
  vapor 
  as 
  

   completely 
  as 
  possible. 
  Wkile 
  the 
  pump 
  is 
  still 
  in 
  action, 
  the 
  

   tube 
  is 
  slowly 
  sealed 
  off 
  at 
  D. 
  

  

  The 
  furnaces. 
  — 
  The 
  above 
  apparatus 
  is 
  heated 
  by 
  a 
  pair 
  of 
  

   coaxial 
  cylindrical 
  furnaces 
  which 
  are 
  attached 
  to 
  carriages 
  in 
  

   a 
  horizontal 
  position 
  in 
  such 
  a 
  way 
  as 
  to 
  slide 
  easily 
  on 
  a 
  track 
  

   (figs. 
  2 
  and 
  3). 
  By 
  this 
  means 
  the 
  two 
  furnaces 
  may 
  be 
  easily 
  

   slipped 
  over 
  the 
  glass 
  tube 
  and 
  brought 
  tightly 
  end 
  to 
  end 
  

   when 
  an 
  experiment 
  is 
  to 
  be 
  made 
  ; 
  or 
  quickly 
  drawn 
  apart 
  

   when 
  the 
  system 
  requires 
  to 
  be 
  chilled 
  after 
  sufficient 
  reaction 
  

   has 
  taken 
  place. 
  The 
  most 
  important 
  consideration 
  in 
  the 
  con- 
  

   struction 
  of 
  the 
  furnaces 
  is 
  the 
  maintenance 
  in 
  both 
  of 
  them 
  of 
  

   constant 
  temperature 
  ranges 
  of 
  sufficient 
  length 
  so 
  that 
  each 
  

   bulb 
  of 
  the 
  glass 
  tube 
  (fig. 
  1) 
  shall 
  be 
  kept 
  at 
  a 
  uniform 
  tem- 
  

   perature 
  throughout, 
  and 
  so 
  that 
  a 
  slight 
  displacement 
  of 
  the 
  

   tube 
  from 
  its 
  proper 
  position 
  in 
  the 
  furnace 
  can 
  not 
  disturb 
  

   this 
  uniformity. 
  To 
  fulfil 
  this 
  important 
  condition 
  it 
  was 
  

   found 
  necessary 
  to 
  make 
  the 
  furnaces 
  of 
  considerable 
  length 
  and 
  

   to 
  use 
  three 
  separately 
  controlled 
  heating 
  coils 
  for 
  each 
  furnace. 
  

   In 
  our 
  most 
  satisfactory 
  installation 
  the 
  furnace 
  which 
  serves 
  

   to 
  heat 
  the 
  sulphide 
  is 
  46 
  cm 
  long, 
  while 
  the 
  other, 
  used 
  for 
  

   heating 
  the 
  sulphur, 
  is 
  68 
  em 
  long. 
  

  

  The 
  two 
  main 
  heating 
  coils 
  are 
  made 
  of 
  nichrome 
  wire 
  

   l'3 
  mm 
  in 
  diameter 
  and 
  having; 
  a 
  resistance 
  of 
  0"75 
  ohms 
  to 
  the 
  

   meter. 
  The 
  coils 
  are 
  wound 
  on 
  alundum 
  tubes 
  about 
  5 
  cm 
  

   external 
  (4 
  cm 
  internal) 
  diameter, 
  8 
  turns 
  to 
  the 
  inch. 
  The 
  

   longer 
  one 
  has 
  a 
  total 
  resistance 
  cold 
  of 
  about 
  23 
  ohms, 
  the 
  

   shorter 
  about 
  11*5 
  ohms. 
  The 
  auxiliary 
  coils 
  at 
  each 
  end 
  of 
  

   the 
  furnaces 
  are 
  made 
  either 
  of 
  nichrome 
  tape, 
  wound 
  spirally 
  

   into 
  a 
  flat 
  disk 
  with 
  heavy 
  asbestos 
  paper 
  as 
  the 
  insulator, 
  or, 
  

   for 
  higher 
  temperatures, 
  they 
  are 
  constructed 
  of 
  flat 
  circular 
  

   disks 
  of 
  alundum 
  with 
  a 
  spiral 
  groove 
  in 
  one 
  side 
  ; 
  the 
  wire 
  is 
  

   laid 
  in 
  the 
  groove 
  and 
  cemented 
  in 
  by 
  a 
  paste 
  of 
  ground 
  alun- 
  

   dum 
  and 
  water, 
  which 
  is 
  then 
  dried 
  and 
  baked. 
  This 
  device 
  

   was 
  contrived 
  by 
  Mr. 
  J. 
  B. 
  Ferguson 
  of 
  this 
  laboratory. 
  For 
  

   these 
  auxiliary 
  coils, 
  wires 
  or 
  tapes 
  about 
  7 
  meters 
  in 
  length 
  

   and 
  having 
  a 
  total 
  resistance 
  of 
  5 
  ohms 
  were 
  found 
  adequate. 
  

  

  *If 
  the 
  tube 
  is 
  not 
  partially 
  constricted 
  before 
  the 
  sulphide 
  is 
  dropped 
  in, 
  

   oxidation 
  is 
  to 
  be 
  feared 
  because 
  of 
  the 
  hotter 
  flame 
  and 
  longer 
  time 
  required 
  

   to 
  draw 
  down 
  the 
  wider 
  tube. 
  

  

  