﻿Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  351 
  

  

  Fig. 
  2. 
  

  

  Properties 
  of 
  Tridymite. 
  — 
  The 
  most 
  frequent 
  form 
  of 
  tridy- 
  

   mite 
  as 
  obtained 
  by 
  the 
  inversion 
  of 
  quartz 
  in 
  a 
  tungstate 
  

   melt 
  is 
  as 
  aggregates 
  of 
  crystals 
  of 
  random 
  orientation 
  replac- 
  

   ing 
  each 
  quartz 
  grain. 
  In 
  addition, 
  large 
  numbers 
  of 
  perfectly 
  

   formed 
  hexagonal 
  plates 
  are 
  almost 
  always 
  present 
  in 
  the 
  same 
  

   preparation. 
  Ordinarily 
  the 
  crystals 
  are 
  quite 
  minute, 
  but 
  it 
  

   is 
  not 
  difficult 
  to 
  produce 
  them 
  at 
  will 
  of 
  such 
  size 
  that 
  individ- 
  

   ual 
  crystals 
  are 
  plainly 
  visible 
  to 
  the 
  naked 
  eye. 
  The 
  essentials 
  

   seem 
  to 
  be 
  a 
  long 
  period 
  of 
  heating 
  and 
  a 
  moderately 
  high 
  tem- 
  

   perature. 
  After 
  one 
  experiment, 
  conducted 
  at 
  1300° 
  for 
  23 
  

   hours, 
  the 
  crust 
  of 
  the 
  mass 
  in 
  the 
  crucible 
  appeared 
  somewhat 
  

   fissured 
  and 
  the 
  openings 
  were 
  lined 
  with 
  relatively 
  large, 
  

   separate 
  crystals 
  of 
  a 
  tabular 
  form. 
  Under 
  a 
  binocular 
  of 
  

   moderate 
  power 
  their 
  hexagonal 
  form 
  could 
  be 
  distinguished. 
  

   Interpenetration 
  twinning 
  was 
  developed 
  to 
  a 
  high 
  degree, 
  and 
  

   although 
  the 
  crystals 
  were 
  too 
  frail 
  for 
  goniometric 
  work, 
  the 
  

   resemblance 
  to 
  the 
  twins 
  and 
  trillings 
  figured 
  in 
  Dana 
  and 
  

   Hintze 
  was 
  striking. 
  In 
  another 
  experiment 
  the 
  heating 
  was 
  

   continued 
  for 
  140 
  hours 
  at 
  a 
  temperature 
  varying 
  from 
  900° 
  to 
  

   1200° 
  and 
  still 
  better 
  crystals 
  were 
  obtained. 
  

  

  The 
  crystals 
  in 
  random 
  aggregates 
  frequently 
  show 
  elongated 
  

   or 
  lath-like 
  shapes, 
  due 
  to 
  their 
  be- 
  

   ing 
  cross- 
  sections 
  of 
  the 
  thin 
  scales. 
  

   In 
  such 
  cases 
  the 
  extinction 
  is 
  par- 
  

   allel 
  to 
  the 
  elongation, 
  and 
  the 
  

   elongation 
  has 
  the 
  vibration 
  direc- 
  

   tion 
  a. 
  In 
  other 
  cases 
  the 
  wedge- 
  

   like 
  twinning 
  frequently 
  noted 
  in 
  

   descriptions 
  appears. 
  This 
  has 
  the 
  

   appearance 
  shown 
  in 
  fig. 
  2. 
  

  

  The 
  hexagonal 
  scales, 
  when 
  of 
  

   the 
  thinness 
  ordinarily 
  obtained, 
  

   appear 
  perfectly 
  isotropic 
  when 
  

   lying 
  on 
  the 
  base. 
  The 
  larger 
  ones 
  

   secured 
  by 
  special 
  effort 
  are 
  found 
  

   to 
  be 
  divided 
  into 
  slightly 
  birefrin- 
  

   gent 
  fields, 
  as 
  shown 
  in 
  fig. 
  3. 
  The 
  

   acute 
  bisectrix 
  in 
  each 
  distinct 
  area 
  is 
  normal 
  to 
  the 
  plate 
  and 
  

   the 
  optical 
  character 
  is 
  positive. 
  The 
  planes 
  of 
  the 
  optic 
  axes 
  

   are 
  related 
  to 
  the 
  exterior 
  crystal 
  boundaries 
  in 
  such 
  a 
  way 
  as 
  

   to 
  be 
  always 
  normal 
  to 
  an 
  edge. 
  The 
  shape 
  of 
  the 
  fields, 
  

   though 
  quite 
  irregular, 
  is 
  also 
  plainly 
  related 
  to 
  the 
  crystal 
  

   outline. 
  The 
  hyperbolic 
  brushes 
  are 
  broad 
  and 
  rather 
  faint, 
  

   and 
  the 
  value 
  of 
  the 
  axial 
  angle 
  is 
  therefore 
  difficult 
  of 
  accurate 
  

   determination. 
  Three 
  measurements 
  gave 
  the 
  following 
  results 
  

   for 
  2 
  Y: 
  — 
  32-6°, 
  38'0°, 
  35*8°, 
  average 
  35'5°, 
  or 
  2E 
  = 
  58;6°. 
  

   Determination 
  of 
  refractive 
  indices 
  in 
  sodium 
  light 
  by 
  the 
  im- 
  

   mersion 
  method 
  gave 
  for 
  vibrations 
  parallel 
  to 
  plates 
  {a 
  and 
  (3) 
  

  

  