﻿Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  373 
  

  

  for 
  cristobalite 
  ; 
  that 
  is, 
  a 
  differential 
  method 
  by 
  which 
  the 
  

   differences 
  in 
  temperature 
  between 
  the 
  tridymite 
  charge 
  and 
  a 
  

   neutral 
  substance 
  were 
  read. 
  Previous 
  work 
  with 
  natural 
  

   tridymite 
  had 
  shown 
  that 
  there 
  was 
  a 
  change 
  in 
  optical 
  

   properties 
  at 
  a 
  temperature 
  which 
  had 
  been 
  placed 
  at 
  about 
  

   130° 
  ; 
  and 
  the 
  intention 
  was 
  to 
  determine 
  this 
  point 
  rather 
  

   more 
  accurately. 
  It 
  was 
  found, 
  however, 
  that 
  when 
  the 
  curve 
  

   of 
  temperature-differences 
  against 
  temperatures 
  was 
  plotted, 
  

   two 
  well-defined 
  breaks 
  were 
  revealed, 
  as 
  shown 
  in 
  fig. 
  8. 
  It 
  

   was 
  then 
  considered 
  desirable 
  to 
  investigate 
  this 
  phenomenon 
  

   more 
  closely 
  and 
  to 
  determine 
  the 
  properties 
  of 
  tridymite 
  

   formed 
  under 
  varying 
  conditions 
  and 
  ascertain 
  whether 
  the 
  

   two 
  points 
  were 
  always 
  shown 
  and 
  whether 
  their 
  temperatures 
  

   were 
  constant. 
  The 
  results 
  are 
  given 
  in 
  Table 
  II. 
  

  

  Fig. 
  8. 
  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  A 
  

  

  \ 
  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  _ 
  _J 
  

  

  

  \ 
  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  \ 
  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  [ 
  

  

  

  

  

  

  

  

  

  

  

  

  

  / 
  

  

  \ 
  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  *- 
  1 
  

  

  > 
  < 
  

  

  o 
  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  Fig. 
  8. 
  Heating 
  curve 
  of 
  the 
  a 
  — 
  /3 
  X 
  — 
  /? 
  2 
  tridymite 
  inversions. 
  Tempera- 
  

   ture-differences 
  plotted 
  against 
  temperatures. 
  Horizontal 
  scale 
  : 
  1 
  square 
  = 
  

   500 
  microvolts 
  (approximately 
  10°) 
  ; 
  vertical 
  scale 
  : 
  1 
  square=10 
  microvolts 
  

   (approximately 
  - 
  2°). 
  

  

  From 
  this 
  table 
  it 
  will 
  be 
  seen 
  that 
  variations 
  in 
  conditions 
  

   of 
  formation 
  appear 
  to 
  have 
  no 
  effect 
  upon 
  the 
  inversion. 
  The 
  

   two 
  breaks 
  always 
  appear 
  and 
  the 
  temperatures 
  are 
  practically 
  

   constant. 
  

  

  A 
  number 
  of 
  attempts 
  were 
  made 
  to 
  determine 
  by 
  the 
  same 
  

   method 
  the 
  corresponding 
  breaks 
  on 
  the 
  cooling 
  curves, 
  but 
  

   nothing 
  which 
  seemed 
  significant 
  could 
  be 
  detected. 
  In 
  order 
  

   to 
  ascertain 
  the 
  reason 
  for 
  this, 
  optical 
  methods 
  were 
  resorted 
  

   to. 
  Several 
  tridymite 
  crystals 
  were 
  taken 
  from 
  a 
  specimen 
  of 
  

   rock 
  from 
  Cerro 
  San 
  Cristobal, 
  which 
  the 
  IT. 
  S. 
  National 
  

   Museum 
  kindly 
  furnished 
  us. 
  These 
  were 
  placed 
  on 
  a 
  glass 
  

   slide, 
  immersed 
  in 
  a 
  heavy 
  oil, 
  and 
  a 
  cover 
  slip 
  placed 
  above. 
  

   The 
  slide 
  was 
  laid 
  above 
  a 
  hole 
  in 
  a 
  heavy 
  copper 
  bar 
  on 
  the 
  

  

  