﻿618 
  SIR 
  DAVID 
  BREWSTER 
  ON 
  CIRCULAR 
  CRYSTALS. 
  

  

  Other 
  three 
  rings 
  follow 
  in 
  succession, 
  the 
  white 
  tint 
  rising 
  to 
  the 
  yellow, 
  and 
  again 
  

   falling 
  to 
  its 
  original 
  colour. 
  Each 
  of 
  these 
  five 
  rings 
  have 
  precisely 
  the 
  same 
  

   tints 
  through 
  ^ut 
  their 
  circumference, 
  and 
  when 
  a 
  number 
  of 
  such 
  crystals 
  appear 
  

   in 
  the 
  dark 
  field, 
  they 
  form 
  objects 
  of 
  singular 
  beauty. 
  

  

  In 
  some 
  specimens, 
  the 
  discs 
  have 
  the 
  appearance 
  of 
  cones, 
  as 
  in 
  Mannite. 
  

   They 
  have, 
  in 
  the 
  centre 
  of 
  the 
  black 
  cross, 
  another 
  cross 
  whose 
  arms 
  bisect 
  the 
  

   sectors, 
  having 
  sometimes 
  a 
  white, 
  or 
  yellow, 
  ox 
  green 
  tint. 
  This 
  cross 
  is 
  surrounded 
  

   with 
  a 
  faint 
  ring, 
  which 
  separates 
  it 
  from 
  large 
  sectors 
  of 
  a 
  bright 
  pink 
  colour. 
  

  

  The 
  circular 
  discs 
  are 
  often 
  composed 
  of 
  radial 
  lines 
  of 
  different 
  thicknesses, 
  

   and 
  in 
  imperfect 
  optical 
  contact. 
  Their 
  tints 
  consequently 
  vary 
  throughout 
  the 
  

   disc, 
  and 
  have 
  a 
  remarkable 
  appearance. 
  When 
  the 
  crystals 
  are 
  very 
  small, 
  they 
  

   produce 
  the 
  polarised 
  halos 
  given 
  by 
  oil 
  of 
  mace. 
  

  

  19. 
  Hippuric 
  Acid. 
  — 
  This 
  salt 
  gives 
  imperfect 
  discs 
  when 
  melted. 
  With 
  water, 
  

   it 
  gives 
  good 
  circular 
  crystals, 
  but 
  very 
  fine 
  ones 
  with 
  alcohol. 
  They 
  have 
  a 
  great 
  

   variety 
  of 
  forms 
  and 
  tints, 
  depending 
  on 
  the 
  strength 
  of 
  the 
  solution 
  ; 
  but 
  they 
  

   differ 
  from 
  other 
  circular 
  crystals 
  in 
  two 
  points. 
  The 
  radial 
  lines 
  are 
  often 
  sepa- 
  

   rated 
  from 
  one 
  another 
  by 
  black 
  spaces 
  of 
  the 
  same 
  breadth 
  as 
  the 
  luminous 
  

   radial 
  lines, 
  and 
  the 
  whole 
  disc 
  is 
  covered 
  with 
  almost 
  invisible 
  concentric 
  black 
  

   circles, 
  at 
  equal 
  distances 
  from 
  one 
  another. 
  They 
  are 
  seen 
  most 
  distinctly 
  in 
  the 
  

   white 
  field. 
  The 
  four 
  central 
  sectors 
  are 
  often 
  surrounded 
  with 
  a 
  ring 
  separated 
  

   from 
  them 
  by 
  a 
  black 
  space 
  entirely 
  free 
  from 
  matter. 
  In 
  some 
  specimens, 
  the 
  

   discs 
  consist 
  of 
  eight 
  or 
  ten 
  sectors 
  of 
  uniform 
  thickness 
  and 
  tint, 
  which 
  become 
  

   black 
  when 
  in 
  the 
  plane 
  of 
  primitive 
  polarisation. 
  In 
  other 
  specimens, 
  the 
  crys- 
  

   tallisations 
  are 
  large, 
  irregular, 
  and 
  highly 
  coloured. 
  

  

  Having 
  thus 
  described 
  the 
  phenomena 
  exhibited 
  by 
  some 
  of 
  the 
  more 
  impor- 
  

   tant 
  circular 
  crystals, 
  I 
  shall 
  give 
  a 
  tabular 
  list 
  of 
  the 
  other 
  substances 
  in 
  which 
  I 
  

   have 
  found 
  the 
  property 
  of 
  giving 
  circular 
  crystallisations, 
  arranging 
  them 
  under 
  

   the 
  heads 
  of 
  Positive 
  and 
  Negative, 
  as 
  formerly 
  explained. 
  

  

  Positive 
  Circular 
  Crystals. 
  

  

  Sulphate 
  of 
  ammonia 
  and 
  magnesia. 
  

  

  red 
  oxide 
  of 
  manganese. 
  

   Hydrate 
  of 
  potash. 
  

   Citrate 
  of 
  potash. 
  

   Muriate 
  of 
  morphia. 
  

  

  magnesia. 
  

   Almond 
  soap. 
  

   Starch. 
  

   Substance 
  in 
  garnet. 
  

  

  mica. 
  

   Chloride 
  of 
  strontian. 
  

   Sulphate 
  of 
  cobalt 
  and 
  ammonia. 
  

  

  Sulphate 
  of 
  iron 
  and 
  ammonia, 
  

   potash. 
  

  

  manganese 
  and 
  ammonia. 
  

  

  magnesia 
  and 
  ammonia. 
  

  

  zinc 
  and 
  potash. 
  

   Disulphate 
  of 
  mercury. 
  

   Mannite. 
  

  

  Citrate 
  of 
  ammonia. 
  

   Myristic 
  acid. 
  

  

  Cuprose 
  sulphate 
  of 
  potash. 
  

   Kreatinine. 
  

  

  