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  XXXVII. 
  — 
  On 
  the 
  Optical 
  Phenomena 
  and 
  Crystallisation 
  of 
  Tourmaline, 
  Titanium, 
  

   and 
  Quartz, 
  within 
  Mica, 
  Amethyst, 
  and 
  Topaz. 
  By 
  Sir 
  David 
  Brewster, 
  

   K.H., 
  D.C.L., 
  F.R.S., 
  and 
  V.P.R.S. 
  Edin. 
  (With 
  a 
  Plate.) 
  

  

  (Read 
  4th 
  January 
  1853.) 
  

  

  The 
  existence 
  of 
  certain 
  minerals 
  imbedded 
  in 
  others, 
  — 
  the 
  optical 
  phenomena 
  

   which 
  they 
  exhibit, 
  — 
  their 
  form 
  and 
  mode 
  of 
  distribution, 
  and 
  the 
  mechanical 
  in- 
  

   fluence 
  which 
  has 
  been 
  exerted 
  during 
  their 
  formation 
  on 
  the 
  mineral 
  that 
  con- 
  

   tains 
  them, 
  are 
  among 
  the 
  most 
  curious 
  and 
  instructive 
  facts 
  in 
  physical 
  science. 
  

  

  The 
  dissemination 
  of 
  perfectly-formed 
  crystals 
  of 
  titanium, 
  both 
  in 
  the 
  form 
  

   of 
  titanite 
  and 
  anatase, 
  in 
  Brazilian 
  crystals 
  of 
  quartz, 
  is 
  a 
  fact 
  so 
  well 
  known 
  

   that 
  I 
  shall 
  take 
  no 
  farther 
  notice 
  of 
  it, 
  but 
  shall 
  proceed 
  to 
  give 
  an 
  account 
  of 
  a 
  

   series 
  of 
  facts 
  of 
  a 
  much 
  more 
  general 
  and 
  interesting 
  character, 
  which 
  I 
  have 
  

   had 
  occasion 
  to 
  observe, 
  during 
  an 
  extensive 
  examination 
  of 
  minerals, 
  undertaken 
  

   with 
  a 
  different 
  object. 
  

  

  1. 
  On 
  the 
  Distribution 
  of 
  Tourmaline 
  in 
  Mica. 
  

  

  When 
  fluids 
  and 
  condensed 
  gases 
  are 
  imprisoned 
  in 
  the 
  cavities 
  of 
  topaz 
  and 
  

   other 
  hard 
  minerals, 
  they 
  retain 
  their 
  place 
  till 
  some 
  powerful 
  agent 
  releases 
  

   them 
  from 
  confinement, 
  or 
  till 
  heat 
  gives, 
  them 
  such 
  an 
  expansive 
  force 
  as 
  to 
  

   burst 
  the 
  mineral. 
  In 
  mica, 
  however, 
  where 
  the 
  laminae 
  of 
  which 
  it 
  is 
  com- 
  

   posed 
  are 
  held 
  together 
  by 
  a 
  very 
  feeble 
  cohesive 
  force, 
  the 
  fluids 
  in 
  their 
  cavi- 
  

   ties, 
  and 
  the 
  extraneous 
  materials 
  which 
  were 
  present 
  at 
  their 
  formation, 
  have 
  

   experienced 
  no 
  difficulty 
  in 
  quitting 
  their 
  place, 
  and 
  spreading 
  themselves 
  be- 
  

   tween 
  the 
  plates 
  of 
  the 
  mineral. 
  

  

  Tourmaline 
  and 
  quartz, 
  though 
  thus 
  distributed 
  between 
  the 
  laminse 
  of 
  mica 
  

   subsequent 
  to 
  its 
  crystallisation, 
  have 
  yet 
  found 
  a 
  place 
  in 
  it 
  contemporaneously 
  

   with 
  the 
  crystallisation 
  of 
  the 
  mica 
  itself. 
  In 
  this 
  case 
  they 
  are 
  large 
  crystals, 
  

   equivalent 
  in 
  thickness 
  to 
  many 
  laminse, 
  and 
  may 
  be 
  taken 
  out 
  and 
  subjected 
  to 
  

   examination. 
  Some 
  of 
  the 
  crystals 
  of 
  tourmaline 
  are 
  so 
  large, 
  indeed, 
  that 
  I 
  

   have 
  used 
  them 
  with 
  their 
  own 
  natural 
  faces 
  as 
  analysing 
  prisms 
  ; 
  and 
  the 
  quartz 
  

   crystals, 
  which 
  are 
  amorphous, 
  and 
  very 
  irregularly 
  formed, 
  occupy 
  a 
  still 
  greater 
  

   space. 
  In 
  both 
  cases, 
  however, 
  the 
  tourmaline 
  and 
  the 
  quartz, 
  when 
  taken 
  out, 
  

   leave 
  large 
  openings 
  in 
  the 
  laminse, 
  and 
  have 
  greatly 
  disturbed 
  the 
  structure 
  of 
  

   the 
  mica 
  around 
  them. 
  

  

  The 
  crystals 
  of 
  tourmaline 
  thus 
  formed 
  in 
  the 
  mica, 
  have 
  almost 
  always 
  the 
  

   faces 
  of 
  the 
  flattened 
  hexagonal 
  prism 
  parallel 
  to 
  the 
  laminse 
  of 
  the 
  mica. 
  I 
  have 
  

   found, 
  however, 
  a 
  few 
  cases 
  in 
  which 
  the 
  flat 
  summit 
  of 
  the 
  hexagonal 
  prism 
  is 
  

   parallel 
  to 
  the 
  laminse. 
  The 
  crystallisations 
  of 
  quartz 
  have 
  also 
  the 
  axis 
  of 
  the 
  

   prism, 
  or 
  its 
  hexagonal 
  faces 
  parallel 
  to 
  the 
  laminse. 
  

  

  VOL. 
  XX. 
  PART 
  IV. 
  7 
  I 
  

  

  