﻿550 
  SIR 
  DAVID 
  BREWSTER 
  ON 
  THE 
  OPTICAL 
  PHENOMENA 
  OF 
  

  

  sides 
  of 
  the 
  hexagon, 
  and 
  six 
  beautiful 
  radiations, 
  like 
  those 
  of 
  the 
  Asterial 
  

   Sapphire, 
  perpendicular 
  to 
  the 
  sides 
  of 
  the 
  hexagon. 
  

  

  The 
  existence 
  of 
  these 
  rectilineal 
  fissures 
  is 
  an 
  important 
  fact 
  in 
  crystallogra- 
  

   phy. 
  It 
  proves 
  that 
  the 
  crystals 
  were 
  in 
  a 
  soft 
  state 
  after 
  they 
  had 
  attained 
  their 
  

   present 
  form 
  ; 
  and 
  that, 
  in 
  the 
  process 
  of 
  induration, 
  the 
  fissures 
  were 
  produced 
  

   by 
  the 
  shrinking 
  of 
  the 
  tourmaline, 
  in 
  the 
  same 
  manner 
  as 
  similar 
  fissures 
  are 
  

   produced 
  during 
  the 
  induration 
  of 
  clay. 
  In 
  the 
  mica 
  which 
  surrounds 
  some 
  of 
  

   the 
  crystals, 
  there 
  is 
  the 
  appearance 
  of 
  considerable 
  disturbance 
  ; 
  but 
  I 
  can 
  find 
  

   no 
  trace 
  of 
  any 
  cavity 
  from 
  which 
  the 
  tourmaline 
  may 
  have 
  been 
  ejected 
  in 
  a 
  

   fluid 
  state. 
  The 
  faces 
  of 
  these 
  crystals 
  are 
  not 
  everywhere 
  in 
  optical 
  contact 
  

   with 
  the 
  mica, 
  and 
  it 
  is 
  very 
  probable 
  that 
  they 
  could 
  be 
  removed 
  without 
  any 
  

   adhering 
  mica, 
  as 
  I 
  have 
  occasionally 
  found 
  crystals 
  of 
  tourmaline 
  that 
  were 
  

   moveable 
  between 
  the 
  laminae. 
  

  

  In 
  the 
  same 
  specimen 
  which 
  contains 
  these 
  tourmalines, 
  and 
  in 
  others, 
  I 
  have 
  

   found, 
  what 
  I 
  believe 
  has 
  never 
  before 
  been 
  observed, 
  the 
  woolly 
  filaments 
  of 
  the 
  

   Penicillum 
  glaucum 
  of 
  Link, 
  with 
  its 
  sporules 
  scattered 
  about 
  between 
  the 
  laminae, 
  

   and 
  sometimes 
  beautifully 
  moniliform, 
  as 
  in 
  the 
  Penicillum 
  glaucum 
  obtained 
  

   from 
  milk 
  by 
  M. 
  Turpin.* 
  

  

  2. 
  On 
  the 
  Distribution 
  of 
  Titanium 
  in 
  Mica. 
  

  

  In 
  examining 
  a 
  remarkable 
  specimen 
  of 
  mica 
  from 
  Irkutsk, 
  in 
  Siberia, 
  I 
  

   found 
  titanium 
  between 
  the 
  laminae 
  in 
  various 
  forms, 
  sometimes 
  in 
  amorphous 
  

   plates, 
  sometimes 
  in 
  a 
  powdery 
  state 
  adhering 
  to 
  the 
  mica, 
  and 
  most 
  frequently 
  

   in 
  beautiful 
  dendritic 
  forms, 
  of 
  various 
  degrees 
  of 
  thickness. 
  At 
  a 
  thickness 
  of 
  

   about 
  the 
  hundredth 
  of 
  an 
  inch, 
  the 
  titanium, 
  in 
  all 
  these 
  forms 
  is 
  \ 
  opaque 
  ; 
  but 
  

   at 
  less 
  thicknesses, 
  it 
  has 
  a 
  brownish 
  transparency, 
  becoming 
  almost 
  perfectly 
  

   transparent 
  at 
  thicknesses 
  which 
  do 
  not 
  seem 
  to 
  exceed 
  the 
  2000th 
  part 
  of 
  an 
  

   inch. 
  In 
  Fig. 
  3 
  I 
  have 
  given 
  a 
  drawing 
  of 
  an 
  opaque 
  group 
  executed 
  for 
  me 
  

   with 
  minute 
  accuracy 
  by 
  my 
  celebrated 
  friend 
  Mr 
  Haidinger 
  of 
  Vienna, 
  during 
  

   his 
  residence 
  in 
  Edinburgh. 
  The 
  transparent 
  groups 
  are 
  much 
  more 
  beautiful 
  

   than 
  the 
  opaque 
  ones, 
  the 
  crystalline 
  ramifications 
  having 
  the 
  most 
  diversified 
  

   forms, 
  resembling 
  often 
  regular 
  organisations. 
  

  

  When 
  the 
  mica 
  is 
  removed 
  from 
  above 
  the 
  titanium, 
  so 
  that 
  only 
  an 
  exceed- 
  

   ingly 
  thin 
  film 
  of 
  it 
  is 
  left, 
  the 
  reflected 
  light 
  is 
  extremely 
  brilliant, 
  and 
  consists 
  

   of 
  the 
  most 
  splendid 
  colours. 
  These 
  colours, 
  which 
  have 
  always 
  the 
  form 
  of 
  the 
  

   titanium, 
  are 
  those 
  which 
  are 
  produced 
  by 
  the 
  thin 
  film 
  of 
  mica 
  which 
  covers 
  

   the 
  titanium, 
  and 
  are 
  not 
  produced, 
  as 
  has 
  been 
  supposed, 
  by 
  a 
  vacuity 
  in 
  the 
  

   mica. 
  

  

  In 
  some 
  specimens 
  of 
  mica 
  from 
  Bengal, 
  the 
  imbedded 
  titanium 
  is 
  spread 
  out 
  

  

  * 
  See 
  Comptes 
  Rendus, 
  torn, 
  v., 
  p. 
  822, 
  1837 
  ; 
  Dec. 
  11. 
  

  

  