﻿370 
  Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  

  

  exposed. 
  In 
  inversion 
  the 
  effects 
  due 
  to 
  the 
  different 
  compo- 
  

   nents 
  of 
  such 
  a 
  mixture 
  would 
  be 
  superposed 
  upon 
  each 
  other 
  

   and 
  could 
  not 
  be 
  distinguished. 
  

  

  Mention 
  has 
  been 
  made 
  of 
  the 
  fact 
  that 
  the 
  temperatures 
  of 
  

   inversion-points 
  can 
  be 
  changed 
  by 
  heating 
  the 
  sample 
  a 
  second 
  

   time 
  at 
  a 
  temperature 
  different 
  from 
  that 
  at 
  which 
  it 
  was 
  first 
  

   prepared. 
  This 
  is 
  shown 
  by 
  the 
  following 
  figures: 
  — 
  No. 
  142 
  

   was 
  prepared 
  from 
  amorphous 
  precipitated 
  silica 
  heated 
  70 
  

   hours 
  at 
  1100°±. 
  Inversion-points 
  230-8° 
  and 
  207*2°. 
  After 
  

   reheating 
  at 
  a 
  high 
  temperature 
  (probably 
  1500°-1600°) 
  in 
  a 
  

   Fletcher 
  gas 
  furnace 
  the 
  in 
  version 
  -points 
  were 
  267*5° 
  and 
  

   235*5°. 
  So. 
  175 
  was 
  from 
  amorphous 
  silica 
  at 
  1030°± 
  for 
  69 
  

   hours. 
  Inversion-points 
  225*1° 
  and 
  205*1°. 
  After 
  reheating 
  

   to 
  1570° 
  the 
  inversion-points 
  were 
  258*3° 
  and 
  217°6. 
  No. 
  166a 
  

   was 
  made 
  by 
  devitrifying 
  silica 
  glass 
  at 
  approximately 
  1025° 
  to 
  

   1075°. 
  Inversion-points 
  219*7° 
  and 
  198*1°; 
  after 
  heating 
  to 
  

   1580°, 
  258*2° 
  and 
  225*8°. 
  

  

  When 
  the 
  temperature 
  of 
  the 
  first 
  heating 
  was 
  high 
  the 
  effect 
  

   of 
  a 
  second 
  heating 
  at 
  a 
  lower 
  temperature 
  was 
  not 
  so 
  striking, 
  

   though 
  still 
  noticeable. 
  No. 
  172 
  was 
  prepared 
  at 
  1580° 
  and 
  

   gave 
  inversion-points 
  268*8° 
  and 
  226*9°. 
  After 
  reheating 
  46 
  

   hours 
  at 
  1200°± 
  the 
  points 
  were 
  263*8° 
  and 
  227*6°. 
  

  

  From 
  the 
  data 
  which 
  have 
  been 
  assembled 
  there 
  is 
  hardly 
  

   room 
  for 
  doubt 
  that 
  the 
  position 
  of 
  the 
  inversion-points 
  is 
  

   dependent 
  upon 
  the 
  previous 
  heat 
  treatment, 
  and 
  the 
  behavior 
  

   of 
  the 
  material 
  seems 
  to 
  be 
  perfectly 
  consistent 
  with 
  the 
  theory 
  

   that 
  the 
  crystals 
  are 
  composed 
  of 
  two 
  different 
  molecular 
  spe- 
  

   cies. 
  It 
  remains 
  to 
  be 
  shown 
  that 
  other 
  explanations 
  which 
  

   have 
  been 
  conceived 
  as 
  possible 
  are 
  not 
  competent 
  to 
  explain 
  

   the 
  facts 
  when 
  put 
  to 
  the 
  test. 
  There 
  are 
  a 
  number 
  of 
  these 
  

   possibilities 
  with 
  a 
  greater 
  or 
  less 
  degree 
  of 
  apparent 
  probability. 
  

  

  1°. 
  The 
  variation 
  may 
  be 
  due 
  to 
  foreign 
  material 
  in 
  solid 
  

   solution. 
  This 
  has 
  already 
  been 
  discussed 
  and 
  its 
  impossibility 
  

   pointed 
  out. 
  

  

  2°. 
  The 
  effect 
  may 
  be 
  due 
  to 
  differences 
  in 
  lag 
  in 
  the 
  

   a- 
  ft 
  inversion. 
  This 
  was 
  considered 
  as 
  a 
  possible 
  explanation 
  

   before 
  the 
  amount 
  of 
  variation 
  was 
  known. 
  When 
  it 
  was 
  dis- 
  

   covered 
  that 
  the 
  inversion 
  on 
  cooling 
  with 
  some 
  preparations 
  

   was 
  above 
  that 
  on 
  heating 
  with 
  others, 
  this 
  possibility 
  was 
  elim- 
  

   inated. 
  Before 
  this, 
  however, 
  evidence 
  of 
  another 
  sort 
  had 
  

   been 
  obtained. 
  It 
  was 
  found 
  that 
  large 
  variations 
  in 
  the 
  rate 
  

   of 
  heating 
  or 
  cooling 
  made 
  no 
  observable 
  change 
  in 
  the 
  inver- 
  

   sion-points. 
  Furthermore, 
  a 
  certain 
  preparation 
  which 
  gave 
  a 
  

   break 
  at 
  272*1° 
  on 
  heating 
  and 
  238-5° 
  on 
  cooling 
  was 
  heated 
  to 
  

   320°, 
  then 
  cooled 
  slowly, 
  holding 
  for 
  1J 
  hrs. 
  at 
  247*5°-244-4° 
  

   (9° 
  to 
  5*9° 
  above 
  the 
  lower 
  inversion-point). 
  The 
  temperature 
  

   was 
  then 
  allowed 
  to 
  drop 
  and 
  the 
  break 
  found 
  at 
  238*0°, 
  prac- 
  

   tically 
  the 
  same 
  as 
  before. 
  Heating 
  from 
  room-temperature 
  

  

  