﻿120 
  JV. 
  L. 
  Bowen 
  — 
  Sodium- 
  Potassium 
  JVephelites. 
  

  

  usually 
  possible 
  to 
  deduce 
  a 
  complete 
  equilibrium 
  diagram. 
  

   Dependence 
  had 
  to 
  be 
  placed 
  on 
  this 
  method 
  to 
  a 
  considerable 
  

   extent 
  in 
  the 
  present 
  case. 
  The 
  temperatures 
  at 
  which 
  begin- 
  

   ning 
  of 
  melting 
  and 
  inversion 
  and 
  completion 
  of 
  melting 
  and 
  

   inversion 
  take 
  place 
  were 
  measured 
  by 
  the 
  quenching 
  method. 
  

   These, 
  when 
  plotted 
  against 
  composition, 
  give 
  a 
  complete 
  

   equilibrium 
  diagram 
  from 
  which 
  the 
  composition 
  of 
  the 
  crys- 
  

   talline 
  phases 
  can 
  be 
  determined 
  even 
  though 
  this 
  cannot 
  be 
  

   done 
  by 
  optical 
  means. 
  Even 
  in 
  these 
  determinations 
  difficul- 
  

   ties 
  were 
  encountered 
  as 
  a 
  result 
  of 
  the 
  near 
  agreement 
  of 
  the 
  

   refractive 
  index 
  of 
  all 
  the 
  glasses 
  with 
  that 
  of 
  carnegieite. 
  

   Thus 
  when 
  one 
  takes 
  crystalline 
  nephelite-like 
  material 
  and 
  

   holds 
  it 
  at 
  successively 
  higher 
  temperatures, 
  a 
  temperature 
  is 
  

   finally 
  reached 
  at 
  which 
  a 
  small 
  trace 
  of 
  interstitial 
  material 
  of 
  

   low 
  refraction 
  is 
  formed 
  but 
  one 
  is 
  at 
  a 
  loss 
  to 
  decide 
  whether 
  

   it 
  is 
  carnegieite 
  or 
  glass. 
  By 
  raising 
  the 
  temperature 
  the 
  size 
  

   of 
  the 
  interstitial 
  patches 
  can 
  be 
  increased 
  until 
  their 
  nature 
  is 
  

   determinable. 
  Unfortunately, 
  however, 
  one 
  cannot 
  be 
  sure 
  

   that 
  small 
  patches 
  which 
  were 
  carnegieite 
  have 
  not, 
  with 
  the 
  

   rise 
  of 
  temperature, 
  been 
  changed 
  to 
  patches 
  of 
  glass, 
  indeed 
  

   there 
  are 
  some 
  mixtures 
  in 
  which 
  this 
  does 
  take 
  place 
  within 
  a 
  

   narrow 
  range 
  of 
  temperature. 
  

  

  Then 
  again 
  there 
  is 
  the 
  difficulty 
  of 
  determining 
  the 
  temper- 
  

   ature 
  at 
  which 
  carnegieite 
  mix-crystals 
  begin 
  to 
  melt, 
  for 
  the 
  first 
  

   trace 
  of 
  glass 
  cannot 
  be 
  found 
  under 
  the 
  microscope 
  on 
  account 
  

   of 
  the 
  correspondence 
  of 
  refraction 
  in 
  glass 
  and 
  carnegieite. 
  

   The 
  microscope 
  will 
  detect 
  the 
  glass 
  only 
  when 
  enough 
  has 
  

   been 
  formed 
  to 
  give 
  definite 
  isotropic 
  patches. 
  It 
  was 
  found, 
  

   however, 
  that 
  the 
  first 
  appearance 
  of 
  glass 
  could 
  best 
  be 
  deter- 
  

   mined 
  macroscopically 
  in 
  this 
  case. 
  Before 
  the 
  formation 
  of 
  

   glass 
  the 
  material 
  is 
  a 
  white, 
  opaque, 
  sintered 
  cake 
  but 
  on 
  the 
  

   formation 
  of 
  glass 
  the 
  interstices 
  become 
  filled 
  with 
  it, 
  and 
  since 
  

   the 
  index 
  matches 
  that 
  of 
  the 
  crystals 
  the 
  charge 
  becomes 
  

   transparent 
  and 
  to 
  all 
  appearances 
  entirely 
  glassy 
  even 
  when 
  

   under 
  the 
  microscope 
  no 
  unmistakable 
  glass 
  can 
  be 
  detected. 
  

   Some 
  difficulty 
  is 
  experienced, 
  too, 
  in 
  determining 
  the 
  tempera- 
  

   ature 
  at 
  which 
  final 
  solution 
  of 
  carnegieite 
  takes 
  place. 
  Rare 
  

   crystals 
  of 
  carnegieite 
  in 
  glass 
  are 
  easily 
  overlooked, 
  for 
  they 
  

   appear 
  merely 
  as 
  indefinite 
  birefringent 
  patches 
  without 
  distin- 
  

   guishable 
  outline 
  against 
  the 
  glass. 
  

  

  These 
  difficulties 
  were 
  finally 
  solved, 
  especially 
  as 
  a 
  result 
  of 
  

   the 
  assembling 
  and 
  correlation 
  of 
  all 
  the 
  results, 
  but 
  during 
  the 
  

   course 
  of 
  the 
  investigation 
  they 
  were 
  often 
  very 
  discouraging. 
  

  

  Results 
  of 
  Quenching 
  Experiments. 
  

  

  The 
  results 
  of 
  the 
  quenching 
  experiments 
  may 
  now 
  be 
  given 
  

   in 
  tabular 
  form. 
  (Table 
  I.) 
  

  

  