﻿Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  337 
  

  

  In 
  determining 
  the 
  inversion-point 
  between 
  tridymite 
  and 
  

   cristobalite 
  the 
  same 
  general 
  method 
  of 
  procedure 
  was 
  followed, 
  

   but 
  the 
  details 
  were 
  slightly 
  different. 
  As 
  before, 
  tempera- 
  

   tures 
  were 
  first 
  found 
  above 
  which 
  tridymite 
  changed 
  to 
  cristo- 
  

   balite 
  and 
  below 
  which 
  cristobalite 
  changed 
  to 
  tridymite, 
  and 
  

   the 
  range 
  was 
  gradually 
  brought 
  within 
  narrow 
  limits. 
  For 
  

   final 
  determination, 
  however, 
  it 
  was 
  not 
  considered 
  advisable 
  

   to 
  depend 
  upon 
  the 
  constancy 
  of 
  the 
  storage 
  batteries 
  over 
  

   night, 
  because 
  of 
  the 
  draught 
  which 
  would 
  be 
  imposed 
  upon 
  

   them 
  by 
  the 
  heavy 
  heating-current. 
  Therefore 
  the 
  charge 
  

   under 
  treatment 
  was 
  withdrawn 
  at 
  night 
  and 
  quickly 
  cooled, 
  

   and 
  replaced 
  in 
  the 
  morning 
  at 
  the 
  same 
  temperature 
  and 
  heat- 
  

   ing 
  continued. 
  No 
  difference 
  in 
  principle 
  was 
  involved 
  in 
  thus 
  

   breaking 
  up 
  the 
  time 
  of 
  heating 
  into 
  several 
  periods. 
  

  

  The 
  final 
  experiments 
  were 
  as 
  follows 
  : 
  

  

  No. 
  114. 
  Mixture 
  of 
  cristobalite 
  and 
  sodic 
  tungstate 
  ; 
  

   length 
  of 
  heating, 
  4f 
  hours 
  at 
  1460° 
  ± 
  2° 
  ; 
  the 
  product 
  is 
  still 
  

   mostly 
  cristobalite, 
  but 
  there 
  is 
  a 
  very 
  appreciable 
  quantity 
  of 
  

   tridymite. 
  

  

  N 
  o. 
  117. 
  Mixture 
  of 
  tridymite 
  and 
  sodic 
  tungstate 
  ; 
  length 
  

   of 
  heating, 
  10 
  hours 
  25 
  minutes 
  at 
  1475° 
  ± 
  2° 
  ; 
  product 
  is 
  

   mostly 
  tridymite, 
  but 
  with 
  considerable 
  cristobalite. 
  

  

  No. 
  122. 
  Tridymite 
  and 
  sodic 
  -tungstate 
  ; 
  21J 
  hours 
  at 
  

   1470° 
  ± 
  2° 
  ; 
  no 
  cristobalite 
  discoverable. 
  

  

  No. 
  120. 
  Cristobalite 
  and 
  sodic 
  tungstate 
  ; 
  16 
  hours 
  at 
  

   1470° 
  ± 
  2° 
  ; 
  no 
  tridymite 
  discoverable. 
  

  

  From 
  these 
  experiments, 
  in 
  connection 
  with 
  many 
  others 
  at 
  

   higher 
  and 
  lower 
  temperatures, 
  which 
  gave 
  consistent 
  results, 
  

   we 
  appear 
  to 
  be 
  perfectly 
  justified 
  in 
  placing 
  the 
  tridymite- 
  

   cristobalite 
  inversion 
  temperature 
  at 
  1470° 
  db 
  10°. 
  

  

  The 
  enantiotropic 
  relations 
  were 
  confirmed 
  by 
  numerous 
  

   experiments, 
  modified 
  in 
  various 
  ways. 
  Starting 
  with 
  quartz 
  

   either 
  tridymite 
  or 
  cristobalite 
  may 
  be 
  obtained, 
  according 
  to 
  

   the 
  temperature 
  used. 
  Likewise, 
  tridymite 
  may 
  be 
  converted 
  

   into 
  quartz 
  or 
  into 
  cristobalite, 
  and 
  cristobalite 
  into 
  quartz 
  or 
  

   tridymite. 
  Silica 
  glass 
  and 
  amorphous 
  precipitated 
  silica 
  have 
  

   likewise 
  been 
  converted 
  at 
  will 
  into 
  any 
  one 
  of 
  the 
  three 
  crystal- 
  

   line 
  modifications. 
  In 
  all 
  its 
  relations 
  to 
  other 
  forms 
  precipi- 
  

   tated 
  silica 
  behaves 
  in 
  the 
  same 
  way 
  as 
  silica 
  glass, 
  and 
  may 
  

   probably 
  be 
  considered 
  as 
  the 
  same 
  chemical 
  substance, 
  differ- 
  

   ing 
  only 
  in 
  its 
  state 
  of 
  physical 
  division. 
  

  

  There 
  can 
  be 
  no 
  doubt 
  that 
  quartz, 
  tridymite, 
  and 
  cristobalite 
  

   are 
  enantiotropic 
  forms, 
  each 
  with 
  a 
  certain 
  range 
  of 
  stability. 
  

   Their 
  general 
  equilibrium 
  relations 
  are 
  shown 
  in 
  fig. 
  1, 
  in 
  

   which 
  the 
  coordinates 
  are 
  temperature 
  and 
  vapor-pressure. 
  

   The 
  absolute 
  values 
  of 
  vapor-pressure 
  are, 
  of 
  course, 
  unknown, 
  

   but 
  we 
  may 
  make 
  use 
  of 
  the 
  principle 
  that 
  the 
  vapor-pressure 
  

   rises 
  with 
  temperature, 
  and 
  that 
  the 
  vapor-pressure 
  of 
  a 
  stable 
  

  

  