﻿SIR 
  DAVID 
  BREWSTER 
  ON 
  CIRCULAR 
  CRYSTALS. 
  613 
  

  

  of 
  rings 
  seen 
  round 
  the 
  star 
  Capella, 
  with 
  annular 
  apertures, 
  and 
  drawn 
  by 
  Sir 
  

   John 
  Herschel.* 
  

  

  4. 
  Manna. 
  — 
  This 
  substance 
  gives 
  fine 
  circular 
  crystals, 
  which 
  are 
  negative, 
  

   whether 
  obtained 
  from 
  fusion 
  or 
  an 
  aqueous 
  solution. 
  The 
  crystals 
  obtained 
  by 
  

   melting 
  the 
  Manna 
  are 
  the 
  most 
  perfect 
  and 
  beautiful. 
  The 
  intersection 
  of 
  the 
  

   arms 
  of 
  the 
  black 
  cross 
  is 
  so 
  sharp 
  that 
  it 
  sometimes 
  requires 
  a 
  considerable 
  

   power 
  to 
  develope 
  it, 
  and 
  the 
  four 
  minute 
  sectors 
  around 
  it. 
  Beyond 
  this 
  the 
  

   crystals 
  radiate 
  uninterruptedly 
  till 
  they 
  are 
  stopped 
  by 
  meeting 
  with 
  other 
  

   crystals, 
  and 
  the 
  whole 
  of 
  them 
  are 
  joined 
  together 
  in 
  a 
  hexagonal 
  mosaic 
  pave- 
  

   ment. 
  The 
  colours 
  are 
  very 
  bright, 
  varying 
  from 
  the 
  white 
  of 
  the 
  first 
  to 
  the 
  

   blue 
  and 
  green 
  of 
  the 
  second 
  order, 
  and 
  there 
  is 
  a 
  uniformity 
  in 
  the 
  tints, 
  and 
  

   consequently 
  in 
  the 
  shading 
  of 
  the 
  black 
  cross, 
  which 
  indicates 
  great 
  equality 
  in 
  

   the 
  elementary 
  prisms, 
  and 
  in 
  the 
  forces 
  which 
  keep 
  them 
  in 
  optical 
  contact. 
  

   The 
  discs 
  are 
  seldom 
  found 
  separate, 
  and 
  they 
  have 
  no 
  rims, 
  no 
  annuli, 
  and 
  no 
  

   concentric 
  cracks. 
  

  

  5. 
  Disulphate 
  of 
  Mercury. 
  — 
  This 
  salt, 
  dissolved 
  in 
  nitric 
  acid, 
  gives 
  no 
  circular 
  

   crystals 
  by 
  rapid 
  cooling 
  ; 
  but, 
  when 
  the 
  solution 
  is 
  cooled 
  slowly, 
  it 
  yields 
  posi- 
  

   tive 
  circular 
  crystallisations 
  of 
  a 
  square 
  form, 
  as 
  shewn 
  in 
  Fig. 
  14, 
  which 
  under- 
  

   go 
  interesting 
  variations. 
  The 
  rectangular 
  cross 
  is 
  sometimes 
  wanting, 
  and 
  is, 
  

   as 
  it 
  were, 
  replaced 
  by 
  black 
  lines, 
  which 
  meet 
  at 
  the 
  centre. 
  These 
  lines 
  are 
  

   sometimes 
  black 
  in 
  the 
  white 
  field, 
  and 
  are 
  then 
  junction 
  lines 
  where 
  the 
  optical 
  

   contact 
  is 
  imperfect. 
  The 
  greater 
  number 
  of 
  the 
  crystals 
  in 
  which 
  these 
  lines 
  are 
  

   more 
  or 
  less 
  perfectly 
  seen 
  are 
  rounded 
  at 
  the 
  angles. 
  Sometimes 
  they 
  are 
  

   nearly 
  circular, 
  and 
  the 
  tint 
  which 
  they 
  polarise 
  is 
  very 
  little 
  above 
  the 
  beginning 
  

   of 
  black 
  of 
  Newton's 
  Table. 
  

  

  When 
  the 
  crystals 
  are 
  thicker, 
  they 
  exhibit 
  a 
  singular 
  variety 
  of 
  forms, 
  of 
  

   which 
  I 
  have 
  given 
  a 
  specimen 
  in 
  Figs. 
  15, 
  16, 
  17, 
  and 
  18, 
  the 
  relation 
  of 
  which 
  

   to 
  Fig. 
  14, 
  will 
  be 
  easily 
  recognised. 
  The 
  crystals 
  shewn 
  in 
  Figs. 
  16 
  and 
  18 
  

   were 
  obtained 
  from 
  a 
  weak 
  solution 
  of 
  the 
  salt, 
  and 
  are 
  very 
  interesting. 
  In 
  the 
  

   dark 
  field 
  of 
  the 
  microscope, 
  we 
  see 
  only 
  the 
  brilliant 
  golden-yellow 
  border, 
  and 
  

   it 
  requires 
  a 
  strong 
  light 
  and 
  a 
  very 
  high 
  power 
  to 
  discover, 
  in 
  the 
  black 
  interior 
  

   of 
  the 
  square, 
  minute 
  specks 
  of 
  light 
  equally 
  diffused 
  over 
  its 
  surface. 
  By 
  a 
  

   slight 
  turn 
  of 
  the 
  analyser, 
  we 
  perceive 
  the 
  slightly 
  darker 
  diagonal 
  cross 
  shewn 
  

   in 
  Fig. 
  16. 
  These 
  squares 
  are 
  often 
  wholly 
  and 
  uniformly 
  filled 
  up 
  with 
  crystals 
  

   of 
  the 
  same 
  tint 
  as 
  their 
  outline 
  ; 
  and 
  occasionally 
  only 
  part 
  of 
  the 
  square 
  is 
  thus 
  

   occupied. 
  The 
  small 
  and 
  often 
  shapeless 
  crystals 
  (occasionally 
  oval 
  and 
  pear- 
  

   shaped), 
  which 
  form 
  the 
  outline 
  of 
  the 
  square 
  in 
  Fig. 
  16, 
  and 
  of 
  the 
  cross 
  in 
  Fig. 
  

  

  * 
  Treatise 
  on 
  Light, 
  § 
  770, 
  Figs. 
  155, 
  156, 
  157, 
  Plate 
  IX. 
  

  

  