﻿Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  353 
  

  

  prism. 
  In 
  the 
  transformation 
  from 
  the 
  high 
  temperature 
  to 
  

   the 
  low 
  temperature 
  form 
  there 
  appears 
  to 
  be 
  but 
  little 
  shifting 
  

   of 
  the 
  elements 
  of 
  the 
  space-lattice. 
  

  

  The 
  low 
  temperature 
  inversion 
  of 
  tridymite 
  has 
  long 
  been 
  

   known. 
  Further 
  investigation 
  has 
  made 
  it 
  appear 
  that 
  there 
  

   are 
  in 
  reality 
  two 
  inversions 
  lying 
  less 
  than 
  50° 
  apart, 
  of 
  which 
  

   the 
  lower 
  is 
  the 
  one 
  ordinarily 
  observed. 
  The 
  method 
  of 
  deter- 
  

   mining 
  the 
  temperature 
  of 
  these 
  inversions 
  and 
  their 
  meaning 
  

   will 
  be 
  discussed 
  later. 
  Only 
  a 
  small 
  energy 
  change 
  is 
  involved 
  

   in 
  either, 
  and 
  therefore 
  a 
  small 
  amount 
  of 
  material 
  in 
  solid 
  

   solution 
  would 
  probably 
  cause 
  a 
  noticeable 
  shift 
  in 
  their 
  

   positions. 
  This 
  fact 
  gives 
  to 
  the 
  lower 
  inversion 
  a 
  value 
  as 
  a 
  

   criterion 
  for 
  judging 
  the 
  identity 
  of 
  the 
  natural 
  and 
  artificial 
  

   minerals. 
  Mallard* 
  placed 
  the 
  in 
  version-point 
  of 
  natural 
  tridy- 
  

   mite 
  at 
  130° 
  ± 
  5°. 
  This 
  seems 
  to 
  be 
  the 
  only 
  determination 
  

   recorded. 
  My 
  own 
  observation 
  on 
  natural 
  tridymite 
  from 
  

   Cerro 
  San 
  Cristobal 
  in 
  a 
  thermal 
  microscope 
  indicated 
  a 
  some- 
  

   what 
  lower 
  value, 
  about 
  112°. 
  With 
  artificial 
  tridymite 
  the 
  

   average 
  of 
  a 
  number 
  of 
  closely 
  concordant 
  results 
  obtained 
  by 
  

   methods 
  in 
  which 
  I 
  place 
  greater 
  confidence, 
  gave 
  117*4°. 
  

  

  On 
  the 
  whole, 
  the 
  physical 
  properties 
  of 
  artificial 
  tridymite 
  

   show 
  close 
  agreement 
  with 
  those 
  of 
  the 
  natural 
  mineral, 
  and 
  

   there 
  is 
  little 
  reason 
  to 
  doubt 
  that 
  they 
  are 
  the 
  same 
  substance. 
  

  

  Properties 
  of 
  Cristobalite. 
  — 
  It 
  is 
  a 
  little 
  more 
  difficult 
  to 
  

   prove 
  that 
  the 
  cristobalite 
  obtained 
  from 
  tungstate 
  melts 
  car- 
  

   ries 
  no 
  foreign 
  material 
  in 
  solid 
  solution. 
  A 
  peculiar 
  situation 
  

   arises 
  from 
  the 
  fact 
  that 
  the 
  inversion 
  of 
  a 
  into 
  /3 
  cristobalite 
  

   does 
  not 
  take 
  place 
  at 
  a 
  definite 
  temperature 
  like 
  the 
  corre- 
  

   sponding 
  inversions 
  of 
  quartz 
  and 
  tridymite, 
  but 
  the 
  tempera- 
  

   ture 
  for 
  any 
  given 
  preparation 
  depends 
  upon 
  the 
  conditions 
  

   under 
  which 
  it 
  was 
  formed, 
  and 
  that 
  entirely 
  apart 
  from 
  any 
  

   question 
  of 
  impurity. 
  This 
  variability 
  eliminates 
  it 
  as 
  a 
  crite- 
  

   rion. 
  Moreover, 
  the 
  properties 
  of 
  natural 
  cristobalite 
  are 
  rather 
  

   imperfectly 
  known. 
  Efforts 
  were 
  made 
  by 
  this 
  Laboratory 
  to 
  

   obtain 
  specimens 
  of 
  the 
  mineral 
  from 
  dealers 
  for 
  purposes 
  of 
  

   comparison, 
  but 
  the 
  material 
  submitted 
  was 
  practically 
  useless. 
  

   Nevertheless 
  it 
  is 
  possible 
  to 
  establish 
  a 
  strong 
  presumption 
  of 
  

   the 
  identity 
  of 
  the 
  natural 
  and 
  artificial 
  minerals. 
  

  

  The 
  values 
  of 
  the 
  index 
  of 
  refraction 
  and 
  birefringence 
  

   usually 
  cited 
  in 
  standard 
  works 
  are 
  those 
  of 
  Mallard. 
  t 
  Mallard 
  

   gives 
  the 
  value 
  of 
  the 
  index 
  as 
  1432, 
  which 
  is 
  evidently 
  a 
  mis- 
  

   print 
  for 
  1*182, 
  for 
  he 
  immediately 
  states 
  " 
  c'est-a-dire 
  sensi- 
  

   blement 
  egal, 
  ou 
  peut-etre 
  un 
  peu 
  superieur 
  a 
  celui 
  de 
  la 
  tridy- 
  

   mite." 
  P. 
  Gaubert;}; 
  has 
  called 
  attention 
  to 
  the 
  error, 
  and 
  

   has 
  made 
  a 
  redetermination, 
  which, 
  however, 
  he 
  did 
  not 
  con- 
  

  

  *E. 
  Mallard, 
  Bull. 
  Soc. 
  Min., 
  xiii, 
  171, 
  1890. 
  

  

  f 
  Ibid., 
  175, 
  1890. 
  

  

  JP. 
  Gaubert, 
  ibid., 
  xxvii, 
  42, 
  1904. 
  

  

  