﻿TOURMALINE 
  &c, 
  WITHIN 
  MICA 
  AND 
  OTHER 
  MINERALS. 
  549 
  

  

  tinctly 
  seen 
  in 
  the 
  mica, 
  and 
  cracks 
  or 
  luminous 
  streaks 
  often 
  occur 
  in 
  the 
  

   tourmaline 
  plates 
  themselves. 
  I 
  have 
  observed, 
  too, 
  in 
  portions 
  of 
  the 
  mica 
  

   where 
  I 
  cannot 
  find 
  any 
  cavities 
  or 
  crystals, 
  distinct 
  luminous 
  sectors 
  of 
  polar- 
  

   ised 
  light, 
  which 
  could 
  only 
  be 
  produced 
  by 
  a 
  force 
  emanating 
  from 
  their 
  centre. 
  

   This 
  force 
  may 
  have 
  been 
  that 
  of 
  gas 
  discharged 
  from 
  some 
  neighbouring 
  cavity, 
  

   and 
  driven 
  by 
  change 
  of 
  temperature 
  to 
  some 
  other 
  part 
  of 
  the 
  mica 
  plate 
  ; 
  and 
  

   in 
  the 
  following 
  remarkable 
  phenomenon 
  we 
  may 
  perhaps 
  find 
  some 
  evidence 
  in 
  

   favour 
  of 
  this 
  opinion. 
  

  

  Plates 
  of 
  mica 
  contain 
  many 
  beautiful 
  systems 
  of 
  Newton's 
  rings, 
  occupying 
  a 
  

   circular 
  space 
  where 
  the 
  laminae 
  have 
  been 
  separated 
  by 
  some 
  cause 
  or 
  other, 
  

   and 
  where, 
  of 
  course, 
  there 
  must 
  be 
  either 
  air 
  or 
  some 
  gaseous 
  body. 
  The 
  colours 
  

   of 
  the 
  first 
  order 
  are 
  at 
  the 
  circumference 
  of 
  the 
  circular 
  space 
  where 
  the 
  laminae 
  

   are 
  in 
  optical 
  contact, 
  and 
  the 
  higher 
  orders 
  of 
  colour 
  extend 
  towards, 
  and 
  often 
  

   to 
  the 
  centre 
  of 
  the 
  space. 
  Now 
  it 
  is 
  a 
  curious 
  fact, 
  that 
  wherever 
  there 
  is 
  a 
  

   cavity 
  which 
  has 
  projected 
  its 
  fluid 
  and 
  probably 
  gaseous 
  contents, 
  it 
  is 
  situated 
  

   in 
  the 
  circumference 
  of 
  one 
  of 
  these 
  circular 
  spaces. 
  When 
  two 
  cavities 
  have 
  been 
  

   near 
  each 
  other, 
  the 
  circular 
  spaces 
  unite 
  and 
  lose 
  their 
  form, 
  and 
  when 
  the 
  cavi- 
  

   ties 
  have 
  been 
  more 
  numerous, 
  the 
  circular 
  spaces 
  unite 
  into 
  very 
  irregular 
  

   shapes. 
  That 
  these 
  circular 
  hollows 
  or 
  spaces 
  between 
  the 
  laminae 
  have 
  been 
  

   produced 
  by 
  something 
  which 
  has 
  issued 
  from 
  the 
  cavity 
  to 
  which 
  they 
  are 
  so 
  

   constantly 
  related, 
  cannot 
  admit 
  of 
  a 
  doubt. 
  That 
  it 
  has 
  not 
  been 
  a 
  fluid 
  is 
  evi- 
  

   dent, 
  and 
  therefore 
  it 
  must 
  have 
  been 
  a 
  gas, 
  which 
  is 
  either 
  there 
  still, 
  or 
  has 
  

   escaped 
  through 
  some 
  minute 
  openings 
  between 
  the 
  laminae, 
  where 
  optical 
  contact 
  

   has 
  been 
  restored.* 
  

  

  There 
  are 
  some 
  specimens 
  of 
  mica 
  in 
  which 
  the 
  crystals 
  of 
  tourmaline 
  are 
  

   large 
  and 
  opaque, 
  and 
  exhibit 
  phenomena 
  which 
  I 
  believe 
  have 
  not 
  been 
  recog- 
  

   nised 
  in 
  any 
  other 
  mineral. 
  The 
  most 
  interesting 
  specimen 
  of 
  this 
  kind 
  I 
  owe 
  to 
  

   Professor 
  Fleming, 
  who 
  pointed 
  out 
  to 
  me 
  one 
  of 
  the 
  peculiarities 
  which 
  it 
  con- 
  

   tains. 
  This 
  specimen 
  is 
  accurately 
  represented, 
  of 
  the 
  natural 
  size, 
  in 
  Fig. 
  2. 
  

   The 
  largest 
  of 
  the 
  five 
  crystals 
  is 
  0'28 
  of 
  an 
  inch 
  broad, 
  and 
  the 
  smallest 
  0*08 
  of 
  

   an 
  inch. 
  Their 
  thickness 
  cannot 
  greatly 
  exceed 
  the 
  thousandth 
  of 
  an 
  inch, 
  and 
  yet 
  

   it 
  is 
  with 
  difficulty 
  that 
  the 
  strongest 
  sun-light 
  can 
  be 
  seen 
  through 
  them. 
  The 
  

   form 
  of 
  the 
  smallest 
  is 
  a 
  perfect 
  hexagon, 
  and 
  in 
  the 
  rest 
  the 
  same 
  form 
  is 
  more 
  

   or 
  less 
  distinct. 
  In 
  the 
  oval 
  crystal 
  there 
  are 
  numerous 
  holes, 
  and 
  in 
  all 
  of 
  them 
  

   there 
  are 
  numbers 
  of 
  rectilineal 
  cracks 
  parallel 
  to 
  the 
  sides 
  of 
  the 
  hexagon, 
  and 
  

   some 
  of 
  them 
  so 
  narrow 
  that 
  light 
  can 
  scarcely 
  pass 
  through 
  them. 
  When 
  we 
  

   look 
  at 
  the 
  sun 
  through 
  one 
  of 
  these 
  crystals, 
  a 
  curious 
  optical 
  phenomenon 
  

   is 
  seen, 
  a 
  luminous 
  hexagonal 
  surface, 
  composed 
  of 
  lines 
  of 
  light, 
  parallel 
  to 
  the 
  

  

  * 
  A 
  fluid 
  even 
  may 
  have 
  thus 
  escaped, 
  and 
  the' 
  circular 
  hollow 
  remained 
  as 
  before. 
  In 
  

   support 
  of 
  this 
  opinion, 
  see 
  Edinburgh 
  Transactions, 
  vol. 
  x., 
  p. 
  11 
  ; 
  hut 
  especially 
  vol. 
  xvi., 
  p. 
  13 
  ; 
  

   or 
  Phil. 
  Mag., 
  vol. 
  xxxi., 
  p. 
  101, 
  August 
  1847. 
  

  

  