﻿620 
  SIR 
  DAVID 
  BREWSTER 
  ON 
  CIRCULAR 
  CRYSTALS. 
  

  

  bells 
  already 
  described, 
  in 
  which 
  oblique 
  refraction 
  and 
  the 
  thinness 
  of 
  the 
  film 
  

   combine 
  to 
  produce 
  beautiful 
  coloured 
  rings, 
  with 
  a 
  black 
  cross. 
  

  

  In 
  other 
  crystals, 
  such 
  as 
  Muriate 
  and 
  Citrate 
  of 
  Quinine, 
  Codeine, 
  and 
  Nitrate 
  

   of 
  Codeine, 
  I 
  have 
  observed 
  the 
  luminous 
  sectors, 
  and 
  the 
  black 
  cross 
  round 
  the 
  

   air-bubbles, 
  which 
  are 
  formed 
  after 
  fusion, 
  a 
  phenomenon 
  exactly 
  the 
  same 
  as 
  

   that 
  which 
  takes 
  places 
  round 
  cavities 
  in 
  diamonds, 
  amber, 
  and 
  other 
  substances. 
  

  

  Having 
  thus 
  described 
  the 
  principal 
  phenomena 
  of 
  circular 
  crystals, 
  I 
  shall 
  

   now 
  proceed 
  to 
  make 
  a 
  few 
  observations 
  on 
  their 
  formation 
  and 
  decomposition. 
  

   Circular 
  crystals 
  are 
  abnormal 
  aggregations, 
  which 
  owe 
  their 
  existence 
  to 
  some 
  

   disturbing 
  cause. 
  The 
  natural 
  tendency 
  of 
  the 
  elementary 
  molecules 
  of 
  the 
  most 
  

   perfect 
  of 
  them, 
  is 
  to 
  combine 
  with 
  their 
  homologous 
  axes 
  parallel 
  to 
  one 
  another, 
  

   and 
  to 
  form 
  regular 
  crystals 
  ; 
  and 
  it 
  is 
  only 
  when 
  this 
  tendency 
  is 
  counteracted 
  

   by 
  the 
  quick 
  application 
  of 
  heat 
  or 
  cold, 
  by 
  pressure, 
  or 
  by 
  the 
  nature 
  of 
  the 
  sol- 
  

   vent 
  or 
  of 
  the 
  combined 
  ingredients, 
  as 
  in 
  the 
  case 
  of 
  borax 
  and 
  'phosphoric 
  acid, 
  

   that 
  the 
  molecules 
  are 
  constrained 
  to 
  arrange 
  themselves 
  round 
  a 
  centre, 
  not 
  

   merely 
  in 
  radiating 
  prisms, 
  as 
  in 
  Wavellite 
  and 
  some 
  other 
  minerals, 
  but 
  accord- 
  

   ing 
  to 
  laws 
  which 
  could 
  not 
  have 
  been 
  anticipated 
  from 
  any 
  known 
  principles 
  of 
  

   crystallisation. 
  If, 
  owing 
  to 
  any 
  disturbing 
  cause, 
  two 
  molecules 
  should 
  be 
  de- 
  

   posited 
  with 
  their 
  axes 
  at 
  right 
  angles 
  to 
  each 
  other, 
  or 
  four 
  with 
  their 
  similar 
  

   poles 
  directed 
  to 
  the 
  same 
  point, 
  this 
  will 
  lead 
  to 
  the 
  formation 
  of 
  a 
  circular 
  disc, 
  

   which 
  will 
  be 
  of 
  limited 
  thickness, 
  if 
  the 
  crystallisation 
  takes 
  place 
  between 
  two 
  

   plates 
  of 
  glass 
  pressed 
  together, 
  or 
  to 
  the 
  formation 
  of 
  a 
  spherical 
  crystal, 
  as 
  in 
  

   the 
  Lithoxanthate 
  of 
  Ammonia, 
  when 
  there 
  is 
  room 
  for 
  its 
  growth 
  in 
  all 
  directions. 
  

   The 
  disc, 
  or 
  the 
  sphere, 
  might 
  thus 
  increase 
  to 
  a 
  considerable 
  size, 
  if 
  there 
  was 
  

   only 
  one 
  centre 
  of 
  crystallisation, 
  but 
  as 
  the 
  same 
  causes 
  have 
  been 
  operating 
  all 
  

   around, 
  the 
  size 
  of 
  the 
  circular 
  crystal 
  is 
  limited 
  by 
  the 
  number 
  of 
  molecules 
  

   within 
  its 
  sphere, 
  or 
  by 
  its 
  junction 
  with 
  the 
  other 
  discs 
  around 
  it. 
  In 
  this 
  last 
  

   case, 
  they 
  form 
  a 
  sort 
  of 
  mosaic, 
  in 
  which 
  their 
  shape 
  is 
  not 
  circular, 
  but 
  hexa- 
  

   gonal, 
  as 
  in 
  manna, 
  oil 
  of 
  mace, 
  and 
  many 
  other 
  substances. 
  

  

  In 
  the 
  greater 
  number 
  of 
  circular 
  crystallisations, 
  the 
  tints 
  are 
  a 
  minimum 
  at 
  

   the 
  centre 
  of 
  the 
  disc, 
  and 
  increase 
  outwards, 
  — 
  that 
  is, 
  the 
  molecules 
  form 
  a 
  

   thinner 
  film 
  at 
  the 
  centre, 
  which 
  increases 
  in 
  thickness 
  towards 
  the 
  circumference 
  ; 
  

   but 
  in 
  other 
  cases 
  the 
  reverse 
  of 
  this 
  takes 
  place, 
  and 
  in 
  the 
  disc 
  represented 
  in 
  

   Fig. 
  3, 
  where 
  the 
  tints 
  are 
  those 
  of 
  Newton's 
  rings, 
  some 
  cause, 
  which 
  we 
  cannot 
  

   even 
  conjecture, 
  must 
  have 
  determined 
  the 
  atoms 
  to 
  unite 
  according 
  to 
  the 
  com- 
  

   plex 
  law 
  which 
  connects 
  these 
  tints 
  with 
  the 
  thicknesses 
  at 
  which 
  they 
  are 
  pro- 
  

   duced. 
  A 
  cause 
  of 
  an 
  opposite 
  kind 
  must 
  have 
  given 
  birth 
  to 
  the 
  disc 
  shewn 
  in 
  

   Fig. 
  4, 
  where 
  the 
  molecules 
  form 
  a 
  thick 
  film 
  at 
  the 
  centre, 
  which 
  diminishes 
  in 
  

   thickness 
  from 
  nine 
  to 
  three 
  as 
  the 
  tint 
  passes 
  from 
  the 
  central 
  blue 
  to 
  the 
  white 
  

   at 
  the 
  circumference. 
  

  

  