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  SIR 
  DAVID 
  BREWSTER 
  ON 
  THE 
  OPTICAL 
  PHENOMENA 
  OF 
  

  

  titanium 
  solution 
  has 
  deposited 
  all 
  its 
  crystals, 
  the 
  completed 
  crystal 
  of 
  amethyst 
  

   will 
  have 
  its 
  outer 
  surfaces 
  covered 
  with 
  spicular 
  crystals 
  of 
  titanium, 
  or 
  the 
  pyra- 
  

   mid 
  of 
  titanium 
  groups 
  will 
  be 
  on 
  the 
  very 
  outside 
  of 
  the 
  pyramid 
  of 
  amethyst. 
  

   I 
  had 
  the 
  good 
  fortune 
  to 
  find 
  such 
  a 
  crystal, 
  in 
  which 
  the 
  coat 
  containing 
  the 
  

   titanium 
  is 
  laid 
  like 
  varnish 
  on 
  all 
  the 
  faces 
  of 
  the 
  pyramid, 
  but 
  only 
  on 
  the 
  

   upper 
  end 
  of 
  three 
  of 
  them 
  ; 
  the 
  lower 
  end 
  of 
  these 
  three 
  faces 
  having 
  lain 
  on 
  the 
  

   solution 
  protected 
  from 
  the 
  deposition 
  of 
  the 
  titanium. 
  This 
  crystal 
  is, 
  I 
  believe, 
  

   unique, 
  and 
  possesses 
  the 
  great 
  interest 
  of 
  exhibiting 
  the 
  very 
  process 
  by 
  which 
  

   it 
  was 
  formed. 
  

  

  The 
  two 
  phenomena 
  which 
  I 
  have 
  just 
  described 
  are 
  shewn 
  in 
  Figs. 
  4 
  and 
  5. 
  

  

  5. 
  Distribution 
  of 
  Titanium 
  in 
  Brazil 
  Topaz. 
  

  

  In 
  examining 
  a 
  great 
  number 
  of 
  very 
  imperfect 
  crystals 
  of 
  Brazil 
  topaz, 
  I 
  

   found 
  many 
  which 
  contained 
  crystals 
  of 
  titanium 
  of 
  a 
  brilliant 
  scarlet 
  colour, 
  

   with 
  a 
  tinge 
  of 
  yellow. 
  These 
  crystals 
  were 
  perfectly 
  transparent, 
  and 
  occurred 
  

   in 
  seven 
  different 
  forms. 
  

  

  1. 
  In 
  flat 
  amorphous 
  plates, 
  which 
  were 
  highly 
  transparent. 
  

  

  2. 
  In 
  hexagonal 
  plates, 
  lying 
  in 
  different 
  planes. 
  

  

  3. 
  In 
  transparent 
  lines 
  running 
  in 
  different 
  directions, 
  and, 
  though 
  continuous, 
  

   lying 
  in 
  different 
  planes. 
  

  

  4. 
  In 
  lines 
  running 
  inwards 
  from 
  the 
  margin 
  of 
  the 
  specimen, 
  and 
  terminating 
  

   in 
  small 
  flat 
  plates. 
  See 
  Fig. 
  6. 
  

  

  5. 
  In 
  the 
  most 
  remarkable 
  symmetrical 
  forms 
  like 
  sceptres 
  or 
  maces, 
  resem- 
  

   bling 
  some 
  of 
  those 
  symmetrical 
  cavities 
  which 
  I 
  had 
  previously 
  found 
  in 
  the 
  

   white 
  topazes 
  of 
  New 
  Holland 
  * 
  See 
  Fig. 
  7. 
  

  

  6. 
  In 
  some 
  specimens 
  the 
  plates 
  of 
  titanium 
  are 
  actually 
  bent, 
  as 
  in 
  Fig. 
  8. 
  

  

  7. 
  In 
  little 
  groups 
  of 
  transparent 
  circular 
  plates 
  of 
  a 
  scarlet 
  colour, 
  and 
  hav- 
  

   ing 
  concentric 
  rings. 
  

  

  When 
  light 
  is 
  reflected 
  from 
  the 
  separating 
  faces 
  of 
  the 
  titanium 
  and 
  topaz, 
  

   it 
  is 
  almost 
  completely 
  polarised 
  ; 
  and 
  at 
  greater 
  angles 
  than 
  that 
  of 
  maximum 
  

   polarisation, 
  colours 
  of 
  singular 
  brilliancy 
  cross 
  the 
  reflected 
  images. 
  These 
  

   colours 
  are 
  doubtless 
  connected 
  with 
  the 
  fact, 
  that 
  at 
  some 
  of 
  these 
  faces 
  there 
  

   are 
  three 
  images 
  of 
  a 
  luminous 
  object 
  seen 
  by 
  reflexion, 
  one 
  of 
  the 
  two 
  outer 
  ones 
  

   being 
  polarised 
  oppositely 
  to 
  one 
  of 
  the 
  double 
  middle 
  images, 
  as 
  in 
  the 
  case 
  of 
  

   the 
  multiplication 
  of 
  images 
  in 
  composite 
  crystals 
  of 
  calcareous 
  spar.f 
  

  

  6. 
  On 
  the 
  Crystals 
  and 
  Cavities 
  in 
  Garnet. 
  

  

  In 
  the 
  greater 
  number 
  of 
  the 
  crystals 
  of 
  garnet 
  which 
  I 
  have 
  had 
  occasion 
  to 
  exa- 
  

   mine, 
  I 
  have 
  found 
  many 
  crystals 
  and 
  cavities, 
  and 
  much 
  amorphous 
  matter. 
  In 
  

  

  * 
  See 
  Edinburgh 
  Transactions, 
  1826, 
  vol. 
  x., 
  Plate 
  XX. 
  

   f 
  See 
  Phil. 
  Trans., 
  1815, 
  Plate 
  XV., 
  Fig. 
  2. 
  

  

  