﻿350 
  Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals, 
  

  

  product 
  which 
  enters 
  into 
  solution 
  with 
  one 
  or 
  another 
  form 
  of 
  

   silica. 
  If 
  such 
  solution 
  occurred, 
  the 
  inversion-points 
  would 
  

   be 
  displaced 
  and 
  the 
  determinations 
  made 
  would 
  have 
  no 
  

   special 
  significance. 
  It 
  is 
  necessary, 
  therefore, 
  to 
  show 
  that 
  

   the 
  artificial 
  products 
  do 
  not 
  represent 
  solid 
  solutions. 
  For 
  this 
  

   purpose 
  chemical 
  analysis 
  has 
  little 
  value. 
  A 
  number 
  of 
  

   analyses 
  were 
  made 
  by 
  volatilizing 
  the 
  silica 
  with 
  hydrofluoric 
  

   acid 
  and 
  weighing 
  the 
  residue. 
  This 
  residue 
  was 
  always 
  rather 
  

   small 
  (0'19 
  — 
  0*60 
  per 
  cent.), 
  but 
  its 
  effect 
  depended 
  wholly 
  upon 
  

   the 
  question 
  as 
  to 
  whether 
  it 
  was 
  mixed 
  with 
  the 
  crystalline 
  

   silica 
  as 
  a 
  mechanical 
  impurity 
  or 
  whether 
  it 
  had 
  entered 
  into 
  

   solution 
  with 
  it. 
  To 
  settle 
  this, 
  the 
  determination 
  of 
  physical 
  

   properties 
  fortunately 
  provides 
  effective 
  criteria. 
  Certain 
  of 
  

   these 
  properties 
  are 
  of 
  such 
  a 
  nature 
  that 
  they 
  would 
  be 
  even 
  

   more 
  affected 
  by 
  a 
  slight 
  amount 
  of 
  foreign 
  material 
  in 
  solid 
  

   solution 
  than 
  would 
  the 
  transition 
  points 
  mentioned. 
  There- 
  

   fore, 
  by 
  a 
  comparison 
  of 
  the 
  properties 
  of 
  the 
  artificial 
  minerals 
  

   with 
  those 
  of 
  their 
  analogues 
  in 
  nature, 
  the 
  probability 
  of 
  

   identity 
  can 
  be 
  established. 
  Outside 
  of 
  this, 
  the 
  physical 
  con- 
  

   stants 
  are 
  inherently 
  of 
  value 
  and 
  their 
  determinations 
  should 
  

   be 
  recorded. 
  

  

  Properties 
  of 
  Quartz. 
  — 
  The 
  quartz 
  obtained 
  in 
  sodic 
  tung- 
  

   state 
  melts 
  seldom 
  exceeds 
  CI 
  1 
  ™ 
  in 
  length. 
  The 
  crystals 
  

   appear 
  to 
  be 
  simple 
  combinations 
  of 
  prism 
  and 
  pyramids. 
  

   Frequently 
  the 
  forms 
  are 
  rounded 
  or 
  distorted, 
  an 
  effect 
  which, 
  

   with 
  some 
  crystals, 
  can 
  be 
  seen 
  to 
  be 
  due 
  to 
  oscillatory 
  develop- 
  

   ment 
  of 
  faces. 
  Ordinarily, 
  each 
  crystal 
  is 
  a 
  separate 
  individual, 
  

   with 
  double 
  terminations, 
  and 
  a 
  general 
  habit 
  similar 
  to 
  quartz 
  

   phenocrysts 
  in 
  porphyries. 
  Determination 
  of 
  refractive 
  indices 
  

   was 
  made 
  in 
  sodium 
  light 
  by 
  matching 
  the 
  index 
  of 
  the 
  crystals 
  

   with 
  that 
  of 
  various 
  oils 
  by 
  the 
  Becke 
  line 
  method, 
  the 
  index 
  

   of 
  the 
  oil 
  mixture 
  which' 
  matched 
  being 
  immediately 
  deter- 
  

   mined 
  on 
  a 
  total 
  refractometer. 
  The 
  agreement 
  with 
  natural 
  

   quartz 
  was 
  very 
  close. 
  For 
  artificial 
  © 
  = 
  1-544: 
  e 
  = 
  1-551 
  

   (temperature 
  23°), 
  for 
  natural 
  co 
  = 
  1-544 
  e= 
  1*553. 
  

  

  Strong 
  confirmation 
  of 
  its 
  identity 
  with 
  natural 
  quartz 
  was 
  

   furnished 
  by 
  comparison 
  with 
  the 
  a-/3 
  inversion 
  point 
  of 
  the 
  

   natural 
  mineral 
  (to 
  be 
  described 
  later). 
  The 
  quantity 
  of 
  heat 
  

   involved 
  in 
  this 
  transformation 
  is 
  so 
  insignificant 
  (3-4 
  calories* 
  

   per 
  gramme, 
  according 
  to 
  unpublished 
  determinations 
  by 
  W. 
  

   P. 
  White 
  of 
  this 
  Laboratory) 
  that 
  a 
  small 
  amount 
  of 
  material 
  

   in 
  solution 
  would 
  tend 
  to 
  produce 
  a 
  decided 
  shift. 
  The 
  aver- 
  

   age 
  of 
  three 
  determinations 
  gave 
  the 
  point 
  as 
  577*2° 
  on 
  heating 
  

   and 
  568*5° 
  on 
  cooling. 
  Within 
  the 
  limits 
  of 
  error 
  of 
  the 
  

   method, 
  these 
  points 
  coincide 
  with 
  those 
  of 
  natural 
  quartz. 
  

  

  * 
  The 
  exact 
  amount 
  of 
  heat 
  change 
  to 
  be 
  considered 
  is 
  variable 
  because 
  of 
  

   an 
  increase 
  of 
  specific 
  heat 
  just 
  prior 
  to 
  the 
  inversion, 
  which 
  must 
  be 
  taken 
  

   into 
  account 
  when 
  the 
  displacement 
  is 
  considerable. 
  

  

  