﻿SIR 
  DAVID 
  BREWSTER 
  ON 
  CIRCULAR 
  CRYSTALS. 
  617 
  

  

  gated 
  crystallisations, 
  the 
  elementary 
  particles 
  are 
  in 
  perfect 
  optical 
  contact, 
  the 
  - 
  

   tint 
  which 
  they 
  produce 
  being 
  a 
  bright 
  white 
  of 
  the 
  first 
  order. 
  At 
  this 
  period 
  

   of 
  their 
  formation, 
  a 
  crystallised 
  and 
  semi-opaque 
  crust 
  is 
  formed 
  above 
  many 
  of 
  

   them, 
  the 
  opacity 
  arising 
  from 
  an 
  imperfection 
  in 
  the 
  optical 
  contact. 
  This 
  crust 
  

   sometimes 
  cracks 
  and 
  falls 
  off, 
  leaving 
  the 
  perfect 
  crystal 
  beneath, 
  or 
  when 
  it 
  

   merely 
  cracks, 
  shewing 
  the 
  perfect 
  crystal 
  through 
  the 
  fissure. 
  These 
  incrusta- 
  

   tions 
  sometimes 
  occupy 
  the 
  middle 
  of 
  the 
  spaces 
  between 
  the 
  black 
  arches, 
  m 
  n, 
  

   op, 
  &c, 
  and 
  raise 
  the 
  tint 
  to 
  an 
  orange-brown. 
  In 
  a 
  specimen 
  preserved 
  in 
  

   Canada 
  balsam, 
  the 
  balsam 
  has 
  insinuated 
  itself 
  between 
  the 
  imperfectly-united 
  

   elementary 
  crystals, 
  and 
  made 
  the 
  crust 
  so 
  transparent, 
  that 
  the 
  crystal 
  beneath 
  

   it 
  is 
  most 
  distinctly 
  seen, 
  as 
  if 
  through 
  a 
  piece 
  of 
  glass. 
  

  

  In 
  some 
  specimens, 
  the 
  optical 
  contact 
  is 
  so 
  imperfect, 
  that 
  groups 
  of 
  discs 
  

   have 
  a 
  pale 
  nut-brown 
  semi-transparency, 
  with 
  the 
  concentric 
  black 
  bands 
  finely 
  

   developed. 
  

  

  In 
  other 
  specimens, 
  we 
  have 
  every 
  degree 
  of 
  transparency, 
  up 
  to 
  absolute 
  

   opacity. 
  In 
  some 
  discs, 
  the 
  black 
  cross 
  is 
  scarcely 
  seen, 
  and 
  they 
  seem 
  as 
  if 
  they 
  

   were 
  composed 
  of 
  fine 
  threads 
  of 
  worsted, 
  from 
  the 
  sides 
  of 
  which 
  other 
  finer 
  

   threads 
  diverge. 
  Such 
  crystals 
  are 
  beautifully 
  white 
  by 
  reflected 
  light, 
  and 
  look 
  

   as 
  if 
  they 
  were 
  formed 
  of 
  fibres 
  of 
  white 
  satin. 
  

  

  An 
  interesting 
  peculiarity 
  in 
  the 
  larger 
  discs 
  is 
  shewn 
  in 
  Fig. 
  21, 
  where 
  each 
  

   successive 
  ring 
  is 
  formed 
  by 
  radiations 
  from 
  the 
  margin 
  of 
  the 
  preceding 
  ring. 
  

   These 
  radiations 
  or 
  tufts 
  are 
  occasionally 
  separate, 
  as 
  in 
  the 
  figure, 
  but 
  generally 
  in 
  

   optical 
  contact, 
  so 
  as 
  to 
  form 
  a 
  luminous 
  ring 
  in 
  which 
  the 
  tints 
  are 
  not 
  uniform. 
  

  

  In 
  weak 
  solutions 
  of 
  mannite, 
  the 
  crystallisations 
  are 
  exceedingly 
  delicate, 
  

   and 
  the 
  light 
  which 
  they 
  polarise 
  scarcely 
  visible. 
  

  

  18. 
  Oxalurate 
  of 
  Ammonia 
  {pure.) 
  — 
  This 
  salt, 
  to 
  which 
  my 
  attention 
  was 
  

   called 
  by 
  Professor 
  Gregory, 
  and 
  which, 
  according 
  to 
  that 
  chemist, 
  is 
  probably 
  

   identical 
  with 
  the 
  Lithoxanthate 
  of 
  Ammonia, 
  gives 
  very 
  beautiful 
  negative 
  cir- 
  

   cular 
  crystals. 
  

  

  With 
  weak 
  aqueous 
  solutions 
  the 
  discs 
  are 
  small 
  and 
  beautiful, 
  and 
  very 
  much 
  

   like 
  those 
  from 
  Lithoxanthate 
  of 
  Ammonia, 
  the 
  cross 
  sometimes 
  consisting 
  wholly 
  

   of 
  circular 
  discs, 
  and 
  at 
  other 
  times 
  of 
  a 
  few 
  discs 
  interspersed 
  among 
  dendritic 
  

   crystallisations. 
  

  

  From 
  strong 
  solutions 
  the 
  discs 
  are 
  often 
  nearly 
  opaque, 
  and 
  round 
  them 
  are 
  

   formed 
  concentric 
  rings, 
  consisting 
  of 
  marginal 
  radiations, 
  as 
  in 
  Fig. 
  20, 
  their 
  

   elements 
  being 
  often 
  in 
  optical 
  contact, 
  and 
  yielding 
  different 
  polarised 
  tints. 
  

   Occasionally 
  we 
  find 
  discs, 
  sometimes 
  large 
  ones, 
  in 
  which 
  the 
  central 
  circle 
  con- 
  

   sists 
  of 
  tints 
  of 
  the 
  green 
  of 
  the 
  second 
  order, 
  with 
  a 
  feebly-developed 
  black 
  cross, 
  

   descending 
  to 
  the 
  white 
  of 
  the 
  first 
  order. 
  This 
  is 
  followed 
  by 
  a 
  narrow 
  black 
  

   concentric 
  space, 
  beyond 
  which 
  the 
  white 
  tint 
  reappears, 
  and 
  rises 
  to 
  the 
  yellow 
  of 
  

   the 
  second 
  order, 
  which 
  again 
  descends 
  to 
  white, 
  thus 
  completing 
  the 
  second 
  ring. 
  

  

  