﻿TOURMALINE, 
  &c, 
  WITHIN 
  MICA 
  AND 
  OTHER 
  MINERALS. 
  553 
  

  

  one 
  specimen, 
  in 
  particular, 
  the 
  included 
  crystals 
  form 
  a 
  larger 
  mass 
  than 
  the 
  

   garnet, 
  which 
  is 
  merely 
  a 
  cement 
  for 
  holding 
  them 
  together. 
  These 
  crystals 
  have 
  

   various 
  crystalline 
  forms, 
  while 
  some 
  are 
  amorphous, 
  though 
  regularly 
  crystallised 
  

   in 
  their 
  interior. 
  All 
  these 
  crystals 
  are 
  doubly 
  refracting, 
  and 
  give 
  the 
  colours 
  of 
  

   polarised 
  light 
  from 
  their 
  small 
  size. 
  

  

  In 
  another 
  specimen, 
  many 
  of 
  the 
  crystals, 
  in 
  the 
  form 
  of 
  hexagons 
  and 
  

   rhombic 
  plates, 
  are 
  opaque, 
  and 
  exhibit 
  by 
  polarised 
  light 
  the 
  remarkable 
  pheno- 
  

   menon, 
  which 
  I 
  had 
  never 
  before 
  seen, 
  of 
  having 
  luminous 
  edges, 
  so 
  that 
  when 
  

   the 
  rest 
  of 
  the 
  crystal 
  and 
  all 
  the 
  field 
  of 
  view 
  is 
  dark, 
  we 
  observe 
  hexagons 
  and 
  

   rhombs, 
  and 
  other 
  geometrical 
  figures, 
  depicted 
  in 
  lines 
  of 
  red 
  light. 
  It 
  is 
  not 
  

   easy 
  to 
  ascertain 
  the 
  cause 
  of 
  this 
  singular 
  appearance, 
  because 
  we 
  cannot 
  see 
  the 
  

   form 
  of 
  the 
  crystals 
  where 
  the 
  light 
  exists; 
  but 
  I 
  have 
  no 
  doubt 
  that 
  the 
  lumi- 
  

   nous 
  lines 
  consist 
  of 
  light 
  depolarised 
  by 
  reflexion 
  from 
  the 
  sides 
  of 
  the 
  hexagonal 
  

   and 
  rhombic 
  plates, 
  because 
  the 
  illuminating 
  pencil 
  is 
  much 
  larger 
  than 
  the 
  crys- 
  

   tals, 
  and 
  the 
  crystals 
  much 
  smaller 
  than 
  the 
  pupil 
  of 
  the 
  eye, 
  so 
  that 
  light 
  

   must 
  be 
  reflected 
  from 
  the 
  prismatic 
  faces 
  of 
  the 
  hexagons 
  and 
  rhombic 
  plates, 
  

   if 
  they 
  have 
  sufficiently 
  broad 
  faces, 
  and 
  that 
  light 
  so 
  reflected 
  must 
  enter 
  the 
  

   pupil 
  of 
  the 
  eye. 
  

  

  In 
  this 
  specimen 
  and 
  in 
  others 
  there 
  are 
  many 
  spherical 
  cavities, 
  surrounded 
  

   with 
  sectors 
  of 
  polarised 
  light, 
  and 
  also 
  several 
  amorphous 
  masses 
  of 
  matter, 
  

   round 
  which 
  there 
  is 
  also 
  polarised 
  light, 
  indicating, 
  as 
  all 
  the 
  phenomena 
  of 
  the 
  

   crystals 
  do, 
  that 
  the 
  matter 
  of 
  the 
  garnet 
  must 
  have 
  been 
  in 
  a 
  soft 
  state, 
  and 
  com- 
  

   pressed 
  by 
  some 
  force 
  emanating 
  from 
  these 
  cavities. 
  

  

  In 
  another 
  specimen 
  of 
  garnet, 
  a 
  large 
  fissure 
  in 
  its 
  interior 
  is 
  occupied 
  with 
  

   granular 
  matter, 
  which 
  must 
  have 
  issued 
  either 
  from 
  a 
  burst 
  cavity 
  containing 
  a 
  

   fluid 
  or 
  a 
  gas, 
  or 
  both 
  ; 
  but 
  what 
  is 
  very 
  interesting, 
  and 
  what 
  I 
  have 
  never 
  ob- 
  

   served 
  in 
  any 
  other 
  mineral, 
  the 
  matter 
  has, 
  in 
  several 
  places, 
  formed 
  circular 
  

   crystals 
  of 
  singular 
  beauty, 
  some 
  being 
  very 
  simple, 
  and 
  others 
  very 
  composite. 
  

  

  St 
  Leonard's 
  College, 
  St 
  Andrews, 
  

   December 
  11, 
  1852. 
  

  

  