﻿Fennei' 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  335 
  

  

  end 
  of 
  a 
  diameter 
  so 
  as 
  to 
  form 
  a 
  sort 
  of 
  bucket, 
  was 
  then 
  sus- 
  

   pended 
  from 
  corresponding 
  wires 
  hanging 
  from 
  the 
  lower 
  end 
  

   of 
  a 
  Marquardt 
  porcelain 
  tube. 
  A 
  thermoelement, 
  of 
  the 
  stand- 
  

   ard 
  material 
  used 
  in 
  the 
  Geophysical 
  Laboratory 
  (pure 
  plati- 
  

   num 
  against 
  90 
  Pt 
  10 
  Rh), 
  was 
  run 
  down 
  through 
  the 
  Marquardt 
  

   tube 
  and 
  into 
  the 
  charge. 
  The 
  thermojunction 
  within 
  the 
  

   charge 
  was 
  bare, 
  while 
  the 
  upper 
  portions 
  of 
  the 
  wires 
  were 
  

   insulated 
  from 
  each 
  other 
  by 
  inclosing 
  them 
  within 
  capillary 
  

   porcelain 
  tubes. 
  The 
  upper 
  ends 
  of 
  the 
  thermoelement 
  wires 
  

   were 
  attached 
  to 
  corresponding 
  terminals 
  of 
  the 
  ice-bath, 
  from 
  

   which 
  copper 
  wires 
  led 
  to 
  the 
  potentiometer 
  in 
  the 
  usual 
  man- 
  

   ner. 
  Electromotive 
  force 
  was 
  read 
  from 
  a 
  mirror 
  galvanom- 
  

   eter 
  in 
  connection 
  with 
  the 
  potentiometer, 
  in 
  the 
  usual 
  form 
  

   adopted 
  by 
  the 
  Geophysical 
  Laboratory, 
  whose 
  details 
  have 
  

   been 
  fully 
  described 
  in 
  previous 
  papers* 
  and 
  need 
  not 
  be 
  gone 
  

   into. 
  Full 
  precautions 
  against 
  leakage 
  of 
  electric 
  current 
  into 
  

   the 
  galvanometer 
  circuit 
  were 
  employed. 
  A 
  check 
  was 
  main- 
  

   tained 
  on 
  the 
  accuracy 
  of 
  the 
  readings 
  of 
  the 
  thermoelements 
  

   by 
  occasional 
  calibration 
  by 
  comparison 
  with 
  the 
  melting 
  points 
  

   of 
  standard 
  substances 
  (MgCa(Si0 
  3 
  ) 
  a 
  = 
  139L2°, 
  Li 
  2 
  SiO 
  3 
  =1200°, 
  

   gold 
  = 
  10624°, 
  Na 
  2 
  S0 
  4 
  = 
  884°, 
  zinc 
  = 
  419-4°). 
  

  

  By 
  means 
  of 
  the 
  device 
  described, 
  the 
  bucket 
  containing 
  the 
  

   charge 
  could 
  be 
  inserted 
  into 
  a 
  furnace 
  heated 
  by 
  an 
  electric 
  

   current 
  passing 
  through 
  a 
  platinum 
  resistance 
  coil, 
  and 
  sub- 
  

   jected 
  to 
  whatever 
  heat 
  treatment 
  was 
  desirable. 
  

  

  If 
  one 
  heats 
  ground 
  quartz 
  in 
  sodic 
  tungstate 
  at 
  1000° 
  or 
  

   more 
  for 
  several 
  hours, 
  it 
  is 
  found 
  to 
  have 
  been 
  more 
  or 
  less 
  

   completely 
  converted 
  into 
  tridymite. 
  On 
  the 
  other 
  hand, 
  if 
  

   tridymite 
  so 
  prepared 
  is 
  mixed 
  with 
  tungstate 
  and 
  heated 
  at 
  

   800° 
  for 
  a 
  long 
  period, 
  innumerable 
  small 
  quartz 
  crystals 
  can 
  

   be 
  perceived 
  in 
  the 
  resultant 
  product. 
  Somewhere 
  between 
  

   these 
  two 
  temperatures, 
  therefore, 
  there 
  must 
  lie 
  an 
  inversion- 
  

   point. 
  At 
  high 
  temperatures 
  the 
  transformation 
  of 
  quartz 
  into 
  

   tridymite 
  can 
  easily 
  be 
  carried 
  to 
  completion. 
  The 
  reversion 
  

   of 
  tridymite 
  into 
  quartz 
  can 
  likewise 
  be 
  completely 
  carried 
  out 
  

   but 
  is 
  more 
  sluggish, 
  and 
  generally 
  no 
  attempt 
  was 
  made 
  to 
  

   convert 
  the 
  whole 
  charge 
  because 
  of 
  the 
  length 
  of 
  time 
  which 
  

   would 
  be 
  required. 
  The 
  quantity 
  of 
  quartz 
  increases 
  with 
  the 
  

   period 
  of 
  heating, 
  but 
  having 
  once 
  established 
  the 
  reversibility 
  

   of 
  the 
  process, 
  nothing 
  would 
  be 
  gained 
  by 
  continuing 
  it 
  for 
  

   excessive 
  periods. 
  It 
  simply 
  remained 
  to 
  determine 
  a 
  temper- 
  

   ature 
  above 
  which 
  tridymite 
  could 
  be 
  recognized 
  as 
  having 
  

   been 
  obtained 
  from 
  quartz 
  and 
  below 
  which 
  quartz 
  as 
  obtained 
  

   from 
  tridymite. 
  It 
  was 
  found 
  that 
  within 
  a 
  few 
  degrees 
  of 
  the 
  

  

  * 
  A. 
  L. 
  Day, 
  E. 
  T. 
  Allen, 
  and 
  J. 
  P. 
  Iddings, 
  Publication 
  No. 
  31, 
  Carnegie 
  

   Inst, 
  of 
  Washington 
  ; 
  A. 
  L. 
  Day, 
  E. 
  S. 
  Shepherd, 
  and 
  F. 
  E. 
  Wright, 
  in 
  this 
  

   Journal, 
  (4), 
  xxii, 
  265-302, 
  1906 
  ; 
  W. 
  P.White, 
  in 
  Phys. 
  Rev., 
  xxv, 
  334-352, 
  

   1907 
  ; 
  and 
  in 
  this 
  Journal, 
  (4), 
  xxviii, 
  459-489, 
  1909. 
  

  

  