﻿18 
  Brackett 
  and 
  Williams 
  — 
  Newtonite 
  and 
  Rectorite. 
  

  

  If 
  the 
  water 
  given 
  off 
  at 
  110°-115° 
  C. 
  be 
  regarded 
  mainly 
  

   as 
  water 
  of 
  crystallization 
  it 
  is 
  evident 
  that 
  it 
  corresponds 
  to 
  

   one 
  molecule, 
  and 
  the 
  compound 
  would 
  have 
  the 
  formula, 
  

   AI 
  9 
  3 
  . 
  2Si0 
  2 
  . 
  H 
  2 
  0+aq. 
  

  

  Under 
  the 
  microscope 
  a 
  cleavage 
  plate 
  usually 
  shows 
  a 
  few 
  

   spots 
  where 
  it 
  is 
  evident 
  that 
  only 
  one 
  plate 
  is 
  included 
  in 
  the 
  

   thickness, 
  while 
  the 
  most 
  of 
  the 
  section 
  is 
  made 
  up 
  of 
  two 
  or 
  

   more 
  plates 
  lying 
  one 
  over 
  another. 
  In 
  the 
  single 
  plate 
  there 
  

   is 
  one 
  comparatively 
  distinct 
  system 
  of 
  parallel 
  lines 
  in 
  the 
  

   direction 
  of 
  which 
  a 
  sharp 
  extinction 
  takes 
  place. 
  There 
  is 
  

   usually 
  also 
  a 
  much 
  less 
  distinct 
  system 
  of 
  lines 
  which 
  lie 
  at 
  

   nearly 
  right 
  angles 
  to 
  the 
  first* 
  In 
  the 
  thicker 
  portions 
  of 
  

   the 
  plate 
  two 
  or 
  more 
  such 
  pairs 
  of 
  line 
  systems 
  are 
  often 
  

   found 
  superimposed 
  one 
  upon 
  the 
  other. 
  In 
  such 
  cases 
  the 
  

   extinction 
  parallel 
  to 
  either 
  system 
  is 
  very 
  indistinct. 
  

  

  The 
  index 
  of 
  refraction 
  is 
  low 
  — 
  lower 
  than 
  that 
  of 
  Canada 
  

   balsam 
  — 
  and 
  the 
  peculiar 
  structure 
  of 
  the 
  plates 
  gives 
  to 
  the 
  

   thin 
  section, 
  especially 
  when 
  viewed 
  without 
  the 
  microscope, 
  a 
  

   peculiar 
  undulating 
  and 
  glistening 
  appearance. 
  

  

  In 
  convergent 
  polarized 
  light, 
  the 
  simple 
  plates 
  show 
  a 
  strong 
  

   double 
  refraction, 
  and 
  give 
  very 
  beautiful 
  biaxial 
  interference 
  

   figures. 
  The 
  acute 
  bisectrix 
  appears 
  to 
  stand 
  perpendicular 
  to 
  

   the 
  cleavage 
  plane. 
  f 
  The 
  angle 
  between 
  the 
  hyperbolas 
  varies 
  

   much 
  in 
  size, 
  in 
  some 
  cases 
  being 
  not 
  more 
  than 
  5°, 
  and 
  

   in 
  others 
  approaching 
  nearer 
  to 
  15° 
  or 
  20°. 
  The 
  rings 
  about 
  

   the 
  axes 
  join 
  each 
  other 
  forming 
  ellipses 
  so 
  that 
  the 
  determina- 
  

   tion 
  of 
  the 
  dispersion 
  of 
  the 
  axes 
  and 
  bisectrix 
  is 
  uncertain. 
  

   It 
  appears, 
  however, 
  as 
  if 
  the 
  angle 
  for 
  red 
  were 
  greater 
  than 
  

   that 
  for 
  blue, 
  p^>u. 
  Dispersion 
  of 
  the 
  bisectrix 
  appears 
  to 
  be 
  

   wanting. 
  The 
  fact 
  that 
  in 
  many 
  cases 
  two 
  plates 
  lie 
  one 
  over 
  

   the 
  other 
  gives 
  rise 
  to 
  apparent 
  optical 
  anomalies 
  which 
  are, 
  

   however, 
  only 
  caused 
  by 
  this 
  superimposition. 
  Thus 
  in 
  some 
  

   cases 
  beautiful 
  examples 
  of 
  what 
  is 
  known 
  as 
  the 
  " 
  optical 
  

   spectacles 
  " 
  (Optische 
  Brillen) 
  may 
  be 
  observed. 
  

  

  Among 
  the 
  inclusions 
  of 
  foreign 
  material 
  which 
  appear 
  in 
  

   this 
  substance 
  may 
  be 
  mentioned 
  the 
  following: 
  The 
  hydrous 
  

   oxide 
  of 
  iron, 
  which 
  has 
  already 
  been 
  noted, 
  appears 
  in 
  small 
  

   round 
  masses 
  or 
  globules, 
  which 
  are 
  for 
  the 
  most 
  part 
  deposited 
  

   between 
  the 
  individual 
  jnates 
  of 
  which 
  the 
  mass 
  is 
  made 
  up. 
  

   Some 
  member 
  of 
  the 
  pyroxene 
  or 
  amphibole 
  group 
  has 
  also 
  

   been 
  observed 
  lying 
  in 
  the 
  cleavage 
  planes. 
  

  

  These 
  impurities 
  occur 
  in 
  sufficiently 
  large 
  quantities 
  to 
  

   exert 
  a 
  decided 
  influence 
  over 
  the 
  results 
  of 
  the 
  chemical 
  

  

  * 
  86° 
  and 
  88° 
  have 
  been 
  measured. 
  

  

  f 
  A 
  plate 
  cut 
  at 
  right, 
  angles 
  to 
  the 
  cleavage 
  plane 
  seemed 
  to 
  show 
  extinction 
  

   parallel 
  and 
  perpendicular 
  to 
  that 
  plane, 
  but 
  owing 
  to 
  the 
  wavy 
  form 
  of 
  the 
  plate 
  

   it 
  was 
  impossible 
  to 
  determine 
  it 
  accurately. 
  

  

  