﻿Fennei' 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  371 
  

  

  and 
  holding 
  for 
  an 
  hour 
  at 
  7° 
  below 
  the 
  upper 
  inversion-point 
  

   likewise 
  failed 
  to 
  produce 
  inversion. 
  With 
  the 
  same 
  prepara- 
  

   tion 
  a 
  device 
  was 
  arranged 
  by 
  which 
  a 
  platinum 
  wire 
  passing 
  

   through 
  the 
  charge 
  could 
  be 
  brought 
  to 
  a 
  red 
  heat 
  by 
  an 
  auxil- 
  

   iary 
  current 
  at 
  any 
  instant. 
  This 
  would, 
  of 
  course, 
  convert 
  

   the 
  material 
  immediately 
  in 
  contact 
  with 
  it 
  from 
  the 
  a 
  to 
  the 
  j3 
  

   form 
  and 
  might 
  induce 
  transformation 
  in 
  the 
  remainder 
  of 
  the 
  

   charge 
  at 
  a 
  lower 
  temperature 
  than 
  before. 
  The 
  auxiliary 
  

   current 
  was 
  applied 
  at 
  different 
  points 
  during 
  heating, 
  but 
  

   transformation 
  of 
  the 
  charge 
  as 
  a 
  whole 
  did 
  not 
  occur 
  until 
  

   269'1° 
  was 
  reached, 
  which, 
  within 
  the 
  limits 
  of 
  error, 
  may 
  be 
  

   considered 
  the 
  same 
  point 
  as 
  before. 
  

  

  Probably 
  there 
  is 
  some 
  lag-effect, 
  but 
  so 
  small 
  in 
  amount 
  

   as 
  to 
  be 
  almost 
  negligible. 
  

  

  3°. 
  The 
  effect 
  may 
  be 
  due 
  to 
  differences 
  in 
  fineness 
  of 
  mate- 
  

   rial. 
  With 
  very 
  fine 
  material, 
  size 
  of 
  grain 
  is 
  a 
  theoretical 
  fac- 
  

   tor, 
  as 
  the 
  amount 
  of 
  free 
  energy 
  involved 
  in 
  passing 
  from 
  the 
  

   a 
  to 
  the 
  j3 
  form 
  is 
  dependent 
  upon 
  it. 
  Because 
  of 
  the 
  small 
  

   total 
  energy-change 
  involved 
  in 
  the 
  inversion, 
  any 
  means 
  by 
  

   which 
  the 
  amount 
  of 
  free 
  energy 
  of 
  one 
  form 
  over 
  that 
  of 
  the 
  

   other 
  was 
  changed 
  might 
  be 
  conceived 
  to 
  have 
  a 
  large 
  effect 
  in 
  

   shifting 
  the 
  inversion-point. 
  Experiments 
  were 
  made 
  to 
  deter- 
  

   mine 
  the 
  amount 
  of 
  the 
  effect 
  which 
  fine 
  grinding 
  would 
  pro- 
  

   duce, 
  and 
  the 
  results 
  seemed 
  to 
  show 
  that 
  the 
  inversion-point 
  

   might 
  be 
  shifted 
  a 
  few 
  degrees 
  by 
  this 
  means, 
  but 
  that 
  nothing 
  

   approaching 
  the 
  differences 
  of 
  40° 
  or 
  50° 
  which 
  had 
  been 
  

   attained 
  could 
  be 
  produced 
  by 
  this 
  process. 
  A 
  certain 
  prepa- 
  

   ration 
  (No. 
  160a) 
  gave 
  inversion-points 
  at 
  261-3° 
  and 
  230-4°. 
  

   A 
  portion 
  of 
  this, 
  which 
  was 
  so 
  fine 
  as 
  to 
  settle 
  from 
  suspen- 
  

   sion 
  in 
  water 
  only 
  after 
  long 
  standing, 
  gave 
  262*0° 
  and 
  227*3°. 
  

   Another 
  portion 
  was 
  ground 
  for 
  about 
  35 
  hours 
  in 
  a 
  mechani- 
  

   cally 
  driven 
  mortar, 
  being 
  kept 
  moist 
  with 
  kerosene. 
  At 
  the 
  

   end 
  the 
  grains 
  were 
  so 
  small 
  that 
  they 
  were 
  almost 
  beyond 
  the 
  

   resolving 
  powers 
  of 
  a 
  No. 
  9 
  objective. 
  The 
  kerosene 
  was 
  

   driven 
  off 
  bv 
  gentle 
  heat 
  and 
  the 
  resultant 
  powder 
  gave 
  inver- 
  

   sion-points 
  at 
  259-6° 
  and 
  218*5°. 
  

  

  Another 
  preparation 
  was 
  divided 
  into 
  two 
  portions 
  by 
  a 
  200- 
  

   mesh 
  sieve. 
  The 
  coarser 
  portions 
  gave 
  inversions 
  at 
  2664° 
  

   and 
  236*0°. 
  The 
  portion 
  which 
  passed 
  through 
  was 
  further 
  

   ground 
  in 
  a 
  mortar 
  and 
  gave 
  inversions 
  at 
  271*2° 
  and 
  231*3°. 
  

  

  Several 
  experiments 
  gave 
  similar 
  results. 
  There 
  is 
  evidently 
  

   some 
  effect, 
  but 
  its 
  nature 
  may 
  be 
  an 
  increase 
  of 
  the 
  lag 
  in 
  

   inversion 
  rather 
  than 
  a 
  real 
  shift 
  of 
  the 
  inversion-point, 
  and 
  in 
  

   any 
  case 
  the 
  effect 
  is 
  of 
  rather 
  a 
  minor 
  order. 
  

  

  4°. 
  The 
  effect 
  may 
  be 
  due 
  to 
  variations 
  among 
  different 
  

   preparations 
  in 
  the 
  size 
  of 
  the 
  twinned 
  areas 
  in 
  the 
  a 
  form. 
  

   If 
  one 
  preparation 
  in 
  passing 
  from 
  the 
  isotropic 
  to 
  the 
  bire- 
  

   fringent 
  state 
  gave 
  rise 
  to 
  large 
  twinned 
  areas, 
  and 
  another 
  to 
  

  

  