﻿30 
  W. 
  P. 
  White— 
  Silicate 
  Specific 
  Heats. 
  

  

  Below 
  1470 
  cristobalite 
  is 
  metastable, 
  as 
  is 
  quartz 
  

   above 
  870°, 
  and 
  between 
  870 
  and 
  1470 
  tridymite, 
  not 
  

   investigated 
  in 
  the 
  present 
  work, 
  is 
  the 
  stable 
  form 
  of 
  

   silica. 
  The 
  transformations 
  of 
  these 
  forms 
  into 
  each 
  

   other 
  are 
  very 
  sluggish 
  ; 
  for 
  this 
  reason, 
  and 
  on 
  account 
  

   of 
  the 
  relative 
  smoothness 
  of 
  the 
  specific 
  heat 
  curves 
  of 
  

   quartz 
  and 
  cristobalite 
  above 
  600°, 
  their 
  transformations 
  

   are 
  to 
  be 
  classed 
  rather 
  with 
  that 
  of 
  calcium 
  metasilicate 
  

   (wollastonite-pseudo-wollastonite) 
  than 
  with 
  the 
  prompt 
  

   change 
  between 
  the 
  different 
  forms 
  of 
  quartz, 
  a 
  and 
  /?. 
  

  

  We 
  have, 
  then, 
  represented 
  in 
  the 
  present 
  substances, 
  

   besides 
  the 
  magnesium 
  silicate 
  inversion, 
  which 
  is 
  mono- 
  

   tropic, 
  two 
  different 
  kinds 
  of 
  enantiotropic 
  inversion: 
  

  

  (1) 
  Between 
  /?-quartz 
  and 
  /3-tridymite, 
  ^-tridymite 
  and 
  

   ^-cristobalite, 
  and 
  wollastonite 
  and 
  pseudo-wollastonite, 
  

   sluggish 
  inversions 
  marked 
  by 
  no 
  perceptible 
  change 
  of 
  

   specific 
  heat 
  near 
  the 
  inversion 
  points, 
  though 
  with 
  dif- 
  

   ferences 
  of 
  1 
  or 
  2 
  per 
  cent 
  between 
  the 
  two 
  forms. 
  

  

  (2) 
  In 
  quartz 
  and 
  probably 
  in 
  cristobalite, 
  a 
  prompt 
  

   inversion 
  characterized 
  by 
  change 
  in 
  specific 
  heat 
  and 
  in 
  

   many 
  other 
  properties 
  below 
  the 
  inversion. 
  

  

  In 
  trying 
  to 
  relate 
  these 
  facts 
  to 
  others, 
  we 
  naturally 
  

   start 
  with 
  the 
  picture 
  of 
  crystal 
  structure 
  which 
  is 
  given 
  

   by 
  the 
  recent 
  X-ray 
  investigations. 
  This 
  is 
  a 
  picture 
  of 
  

   atoms 
  joined 
  in 
  a 
  way 
  that 
  seems 
  quite 
  regardless 
  of 
  the 
  

   chemical 
  molecule 
  as 
  formerly 
  known, 
  but 
  doubtless 
  con- 
  

   nected 
  by 
  linkages 
  which 
  are 
  somewhat 
  similar 
  to 
  the 
  

   valence 
  bands 
  of 
  ordinary 
  chemical 
  union, 
  and 
  which 
  are 
  

   not 
  all 
  of 
  the 
  same 
  kind 
  except 
  in 
  crystals 
  of 
  the 
  simplest 
  

   composition. 
  38 
  A 
  change 
  in 
  crystalline 
  form 
  must 
  be 
  the 
  

   result 
  of 
  a 
  change 
  in 
  these 
  linkages, 
  and 
  by 
  analogy 
  with 
  

   the 
  behavior 
  of 
  chemical 
  valences 
  we 
  may 
  feel 
  fairly 
  

   certain 
  that 
  the 
  change 
  consists 
  in 
  a 
  virtual 
  weakening 
  

   of 
  them, 
  perhaps 
  a 
  suppression 
  of 
  some, 
  with 
  rising 
  tem- 
  

   perature. 
  Observation 
  shows 
  that 
  the 
  change 
  may 
  have 
  

   one 
  of 
  at 
  least 
  two 
  different 
  characters. 
  The 
  crystal 
  

   may 
  change 
  en 
  masse, 
  or 
  as 
  nearly 
  so 
  as 
  is 
  consistent 
  

   with 
  the 
  fact 
  that 
  heat 
  must 
  be 
  gained 
  or 
  dissipated 
  as 
  

   the 
  change 
  takes 
  place. 
  This, 
  as 
  might 
  be 
  supposed, 
  

   appears 
  to 
  occur 
  when 
  the 
  change 
  of 
  form 
  is 
  slight, 
  pre- 
  

   sumably 
  involving 
  rather 
  a 
  stretching 
  than 
  a 
  rearrange- 
  

   ment 
  of 
  the 
  atomic 
  pattern. 
  This 
  is 
  the 
  case 
  with 
  the 
  

  

  38 
  Irving' 
  Langmuir, 
  The 
  Constitution 
  and 
  Fundamental 
  Properties 
  of 
  

   Solids 
  and 
  Liquids, 
  J. 
  Am. 
  Chem. 
  Soc., 
  38, 
  2233, 
  1916. 
  

  

  