﻿Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  349 
  

  

  This 
  single 
  observation 
  requires 
  support 
  from 
  other 
  direc- 
  

   tions. 
  If 
  it 
  should 
  be 
  confirmed 
  its 
  interpretation 
  leads 
  to 
  in- 
  

   teresting 
  deductions. 
  The 
  explanation 
  which 
  first 
  suggests 
  

   itself 
  is 
  that 
  under 
  great 
  pressure 
  quartz 
  phenocrysts 
  had 
  

   formed 
  in 
  the 
  magma 
  at 
  a 
  temperature 
  considerably 
  above 
  the 
  

   870° 
  inversion-point, 
  and 
  that 
  with 
  relief 
  of 
  pressure 
  accom- 
  

   panying 
  the 
  movement 
  toward 
  the 
  surface, 
  the 
  position 
  of 
  the 
  

   inversion-point 
  was 
  lowered 
  to 
  such 
  a 
  degree 
  that 
  tridymite 
  

   became 
  the 
  stable 
  phase 
  and 
  transformation 
  followed 
  as 
  a 
  

   natural 
  consequence 
  ; 
  but 
  we 
  are 
  not 
  justified 
  in 
  accepting 
  this 
  

   explanation 
  at 
  once. 
  We 
  cannot 
  focus 
  our 
  attention 
  upon 
  this 
  

   one 
  phenomenon 
  and 
  neglect 
  the 
  results 
  which 
  would 
  arise 
  

   among 
  the 
  accompanying 
  constituents, 
  that 
  is, 
  upon 
  the 
  magma 
  

   as 
  a 
  whole, 
  from 
  relief 
  of 
  pressure. 
  In 
  a 
  mixture 
  of 
  such 
  

   great 
  complexity 
  as 
  a 
  partly 
  solidified 
  magma, 
  consisting 
  of 
  

   solids, 
  liquids, 
  and 
  dissolved 
  gases, 
  a 
  change 
  of 
  pressure 
  will 
  be 
  

   accompanied 
  by 
  transformations 
  and 
  reactions 
  among 
  all 
  the 
  

   components 
  tending 
  toward 
  a 
  new 
  condition 
  of 
  equilibrium. 
  

   The 
  direction 
  of 
  all 
  such 
  reactions 
  will 
  be 
  governed 
  by 
  a 
  single 
  

   principle, 
  that 
  the 
  net 
  result 
  shall 
  be 
  an 
  increase 
  of 
  volume 
  of 
  

   the 
  mass 
  as 
  a 
  whole 
  when 
  the 
  pressure 
  is 
  decreased. 
  Neces- 
  

   sarily 
  such 
  reactions 
  will 
  be 
  attended 
  by 
  an 
  evolution 
  or 
  absorp- 
  

   tion 
  of 
  heat, 
  but 
  this 
  factor 
  does 
  not 
  influence 
  the 
  direction 
  of 
  

   reaction 
  except 
  secondarily, 
  and, 
  moreover, 
  there 
  is 
  no 
  general 
  

   parallelism 
  between 
  the 
  amount 
  of 
  the 
  volume 
  change 
  and 
  the 
  

   quantity 
  of 
  heat 
  evolved 
  or 
  absorbed. 
  It 
  is 
  certain 
  that 
  from 
  

   such 
  internal 
  reactions 
  (neglecting 
  losses 
  of 
  heat 
  by 
  conduction 
  

   or 
  radiation 
  to 
  the 
  surroundings) 
  the 
  temperature 
  of 
  the 
  

   magma 
  will 
  either 
  rise 
  or 
  fall, 
  but 
  observations 
  of 
  volcanic 
  

   phenomena 
  have 
  not 
  yet 
  supplied 
  data 
  from 
  which 
  it 
  is 
  possible 
  

   to 
  affirm 
  which 
  is 
  the 
  general 
  result. 
  There 
  is, 
  therefore, 
  at 
  

   least 
  the 
  possibility 
  that 
  in 
  the 
  rise 
  o£ 
  a 
  magma 
  from 
  the 
  depths 
  

   the 
  temperature 
  may 
  actually 
  become 
  greater, 
  perhaps 
  even 
  to 
  

   a 
  notable 
  degree. 
  If 
  this 
  should 
  be 
  the 
  case, 
  the 
  transforma- 
  

   tion 
  from 
  quartz 
  to 
  tridymite 
  might 
  well 
  "be 
  explained 
  from 
  

   this 
  alone, 
  and 
  the 
  fact 
  that 
  the 
  direction 
  of 
  volume-change 
  in 
  

   this 
  one 
  constituent 
  is 
  that 
  d 
  emanded 
  of 
  the 
  magma 
  as 
  a 
  whole 
  

   would 
  be 
  a 
  mere 
  coincidence. 
  < 
  

  

  Physical 
  Peopeeties 
  of 
  Aetificial 
  Qtjaetz, 
  Teidtmite, 
  and 
  

  

  Ceistobalite. 
  

  

  In 
  determining 
  the 
  transition 
  points 
  between 
  quartz 
  and 
  

   tridymite 
  and 
  between 
  tridymite 
  and 
  cristobalite, 
  it 
  was 
  neces- 
  

   sary, 
  as 
  previously 
  explained, 
  to 
  use 
  a 
  solvent 
  or 
  catalytic 
  agent 
  

   in 
  order 
  to 
  cause 
  the 
  transformation 
  to 
  proceed 
  at 
  an 
  appreci- 
  

   able 
  rate, 
  and 
  sodic 
  tungstate 
  was 
  selected 
  for 
  the 
  purpose. 
  

   The 
  use 
  of 
  this 
  material 
  is 
  permissible 
  if 
  it 
  gives 
  rise 
  to 
  no 
  

  

  