﻿Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  381 
  

  

  The 
  behavior 
  of 
  chalcedony 
  in 
  the 
  thermal 
  microscope 
  has 
  

   also 
  been 
  studied. 
  A 
  thin 
  section 
  was 
  ground, 
  approximately 
  

   parallel 
  with 
  the 
  libers, 
  and 
  carefully 
  polished 
  on 
  both 
  sides 
  

   so 
  that 
  there 
  was 
  almost 
  no 
  surface 
  diffusion. 
  This 
  was 
  

   placed 
  in 
  the 
  thermal 
  microscope 
  and 
  the 
  temperature 
  raised 
  

   rather 
  slowly. 
  The 
  birefringence 
  was 
  observed 
  carefully, 
  

   especially 
  through 
  the 
  tridymite, 
  cristobalite, 
  and 
  quartz 
  

   inversion 
  temperatures. 
  No 
  sudden 
  change 
  in 
  birefringence 
  was 
  

   perceptible 
  up 
  to 
  725°. 
  There 
  the 
  section 
  began 
  to 
  break 
  up 
  

   and 
  at 
  820° 
  was 
  badly 
  cracked, 
  though 
  certain 
  areas 
  were 
  

   still 
  quite 
  clear. 
  It 
  was 
  then 
  cooled 
  to 
  400° 
  and 
  no 
  change 
  

   of 
  significance 
  was 
  observed. 
  

  

  All 
  the 
  work 
  done 
  upon 
  chalcedony 
  failed 
  to 
  show 
  any 
  

   relationship 
  with 
  quartz, 
  and 
  its 
  conversion 
  into 
  tridymite 
  at 
  

   800° 
  to 
  850° 
  points 
  strongly 
  to 
  its 
  being 
  a 
  different 
  mineral. 
  

  

  Natural 
  occurrences 
  of 
  chalcedony 
  indicate 
  its 
  formation 
  

   at 
  rather 
  low 
  temperatures, 
  but 
  the 
  evidence 
  is 
  not 
  yet 
  sufficient 
  

   to 
  decide 
  whether 
  it 
  is 
  stable 
  under 
  the 
  conditions 
  of 
  formation 
  

   or 
  whether 
  it 
  is 
  an 
  unstable 
  form 
  whose 
  precipitation 
  is 
  due 
  to 
  

   some 
  peculiarity 
  of 
  conditions. 
  

  

  Fusion 
  of 
  Cristobalite 
  and 
  Quartz. 
  

  

  In 
  the 
  former 
  work 
  of 
  Day 
  and 
  Shepherd* 
  the 
  melting- 
  

   point 
  of 
  silica 
  was 
  placed 
  at 
  approximately 
  1600°. 
  The 
  deter- 
  

   mination 
  was 
  made 
  by 
  exposing 
  powdered 
  quartz 
  in 
  an 
  iridium 
  

   furnace 
  to 
  various 
  temperatures 
  determined 
  by 
  an 
  optical 
  pyrom- 
  

   eter, 
  and 
  observing 
  the 
  lowest 
  point 
  at 
  which 
  evidences 
  of 
  

   melting 
  could 
  be 
  established. 
  Under 
  the 
  conditions 
  of 
  experi- 
  

   ment 
  there 
  may 
  be 
  a 
  question 
  as 
  to 
  whether 
  the 
  melting 
  point 
  

   of 
  quartz 
  or 
  of 
  cristobalite 
  was 
  realized. 
  Lately 
  Endell 
  and 
  

   Riekef 
  have 
  made 
  a 
  new 
  determination 
  by 
  heating 
  cristobalite 
  

   in 
  an 
  iridium 
  resistance-furnace, 
  temperatures 
  being 
  measured 
  

   by 
  means 
  of 
  a 
  carefully 
  calibrated 
  Ir— 
  Ir 
  Ku 
  thermoelement. 
  

   They 
  place 
  the 
  melting 
  temperature 
  at 
  1685° 
  d= 
  10°. 
  Although 
  

   their 
  work 
  shows 
  evidence 
  of 
  great 
  care 
  the 
  results 
  are 
  not 
  

   free 
  from 
  suspicion 
  because 
  of 
  the 
  high 
  volatility 
  of 
  iridium 
  at 
  

   these 
  temperatures 
  and 
  the 
  possibility 
  of 
  contamination 
  of 
  the 
  

   thermoelements, 
  for 
  which 
  proper 
  allowance 
  cannot 
  be 
  made. 
  

  

  In 
  making 
  a 
  new 
  determination 
  I 
  employed 
  a 
  carbon-resist- 
  

   ance 
  furnace 
  through 
  which 
  a 
  current 
  of 
  carbon-monoxide 
  gas 
  

   was 
  continually 
  passing. 
  Through 
  the 
  middle 
  of 
  the 
  furnace 
  

   an 
  inner 
  tube 
  of 
  magnesia 
  was 
  placed 
  and 
  a 
  rapid 
  current 
  of 
  

   dry 
  air 
  passed 
  through 
  it. 
  By 
  this 
  means 
  the 
  atmosphere 
  in 
  

   contact 
  with 
  the 
  carbon 
  walls 
  was 
  kept 
  of 
  a 
  reducing 
  character, 
  

   while 
  the 
  inner 
  tube 
  in 
  which 
  the 
  thermoelement 
  and 
  charge 
  

  

  *A. 
  L. 
  Day 
  and 
  E. 
  S. 
  Shepherd, 
  this 
  Journal 
  (4), 
  xxii, 
  265, 
  1906. 
  

   •J-K. 
  Endell 
  and 
  E. 
  Rieke, 
  Zs. 
  anorg. 
  Chem., 
  lxxix, 
  239-259, 
  1912. 
  

  

  