﻿372 
  Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  

  

  small, 
  forces 
  of 
  surface 
  energy 
  would 
  come 
  into 
  play 
  and 
  might 
  

   produce 
  analogous 
  effects 
  to 
  those 
  considered 
  under 
  3°. 
  But 
  

   it 
  was 
  found, 
  as 
  shown 
  above, 
  that 
  surface 
  energy 
  is 
  not 
  an 
  

   important 
  factor 
  in 
  displacing 
  the 
  inversion-point. 
  However, 
  

   it 
  was 
  thought 
  desirable 
  to 
  see 
  if 
  any 
  perceptible 
  change 
  could 
  

   be 
  effected 
  by 
  great 
  variations 
  in 
  the 
  rate 
  of 
  cooling 
  through 
  

   the 
  inversion-point. 
  The 
  preparation' 
  selected 
  gave 
  inversions 
  

   at 
  266*2° 
  and 
  236*2°. 
  A 
  portion 
  was 
  heated 
  16 
  minutes 
  at 
  

   4l6°-377° 
  and 
  then 
  plunged 
  directly 
  into 
  water. 
  It 
  gave 
  

   265*6° 
  and 
  235*4°. 
  The 
  same 
  charge 
  was 
  heated 
  to 
  308° 
  and 
  

   then 
  held 
  overnight 
  at 
  about 
  270°. 
  It 
  was 
  then 
  cooled 
  and 
  

   inversion-points 
  determined. 
  Results 
  were 
  262*4° 
  and 
  237*4°. 
  

   The 
  charge 
  was 
  then 
  heated 
  to 
  286° 
  and 
  cooled 
  very 
  slowly, 
  

   requiring 
  three 
  hours 
  to 
  pass 
  from 
  280° 
  to 
  229°. 
  The 
  inversion- 
  

   points 
  were 
  then 
  266 
  '2° 
  and 
  236*7°. 
  Apparently, 
  therefore, 
  

   no 
  treatment 
  of 
  this 
  kind 
  had 
  any 
  effect 
  in 
  changing 
  the 
  tem- 
  

   perature 
  of 
  inversion. 
  

  

  5°. 
  As 
  an 
  efficient 
  cause 
  an 
  hypothesis 
  might 
  be 
  considered 
  

   which 
  seems 
  rather 
  far-fetched 
  but 
  which 
  should 
  be 
  taken 
  into 
  

   account 
  ; 
  that 
  is, 
  that 
  the 
  high-temperature 
  form 
  which 
  appears 
  

   to 
  be 
  isometric 
  is 
  in 
  reality 
  the 
  same 
  as 
  the 
  low-temperature 
  

   form, 
  the 
  apparent 
  high 
  degree 
  of 
  symmetry 
  being 
  due 
  to 
  sub- 
  

   microscopic 
  twinning. 
  Under 
  this 
  conception 
  the 
  so-called 
  

   inversion-point 
  would 
  be 
  merely 
  a 
  point 
  at 
  which 
  the 
  submi- 
  

   croscopic 
  areas 
  suddenly 
  grew 
  to 
  perceptible 
  size. 
  This 
  possi- 
  

   bility, 
  however, 
  seems 
  inherently 
  improbable 
  from 
  our 
  general 
  

   knowledge 
  of 
  crystal 
  structure 
  and 
  is 
  further 
  controverted 
  by 
  

   the 
  crystal 
  characteristics 
  of 
  the 
  a 
  and 
  /3 
  forms 
  as 
  described 
  on 
  

   p. 
  355. 
  It 
  was 
  shown 
  there 
  that 
  not 
  only 
  is 
  the 
  exterior 
  form 
  

   of 
  the 
  high-temperature 
  cristobalite 
  consistent 
  with 
  isometric 
  

   symmetry, 
  but 
  that 
  the 
  manner 
  in 
  which 
  the 
  axes 
  became 
  

   twinned 
  during 
  growth 
  showed 
  twinning 
  after 
  the 
  spinel 
  law, 
  

   and 
  gave 
  angles 
  Of 
  45° 
  and 
  60° 
  between 
  adjacent 
  portions 
  of 
  

   the 
  axes. 
  In 
  the 
  low-temperature 
  form, 
  on 
  the 
  other 
  hand, 
  

   the 
  twinning 
  was 
  around 
  a 
  45° 
  pyramid 
  and 
  caused 
  the 
  axes 
  to 
  

   assume 
  positions 
  at 
  90° 
  with 
  each 
  other. 
  

  

  As 
  a 
  result 
  of 
  the 
  investigations 
  on 
  the 
  a 
  and 
  j3 
  inversions 
  

   of 
  cristobalite, 
  we 
  appear 
  to 
  be 
  brought 
  to 
  the 
  conclusion 
  that 
  

   the 
  variations 
  are 
  due 
  to 
  the 
  presence 
  of 
  two 
  or 
  more 
  molecular 
  

   species 
  in 
  the 
  mineral, 
  whose 
  proportions 
  are 
  conditioned 
  by 
  

   the 
  previous 
  heat 
  treatment. 
  Certain 
  minor 
  variations 
  arise 
  

   from 
  other 
  causes, 
  but 
  their 
  magnitude 
  is 
  so 
  small 
  that 
  the 
  

   main 
  effect 
  is 
  not 
  seriously 
  obscured 
  thereby. 
  

  

  The 
  Low-Temperature 
  Tridymite 
  Inversions. 
  

  

  The 
  method 
  used 
  for 
  the 
  determination 
  of 
  the 
  temperature 
  

   of 
  the 
  tridymite 
  inversions 
  was 
  the 
  same 
  as 
  has 
  been 
  described 
  

  

  