﻿SIR 
  DAVID 
  BREWSTER 
  ON 
  CIRCULAR 
  CRYSTALS. 
  615 
  

  

  composed 
  glass. 
  After 
  the 
  deliquescence 
  of 
  the 
  crystals, 
  I 
  attempted 
  to 
  make 
  

   another 
  crop, 
  but 
  having 
  failed, 
  I 
  set 
  the 
  piece 
  of 
  glass 
  aside. 
  In 
  the 
  course 
  of 
  

   half-an-hour, 
  however, 
  I 
  found 
  it 
  covered 
  with 
  a 
  fine 
  and 
  splendidly-coloured 
  set 
  

   of 
  circular 
  crystals, 
  which 
  dissolved 
  wholly 
  when 
  placed 
  in 
  castor-oil 
  with 
  the 
  

   view 
  of 
  preserving 
  them. 
  The 
  light 
  polarised 
  by 
  the 
  bells 
  above 
  mentioned, 
  

   formed 
  a 
  double 
  ring, 
  red 
  on 
  one 
  side, 
  and 
  green 
  on 
  the 
  other, 
  with 
  a 
  black 
  space 
  

   between. 
  

  

  Upon 
  examining 
  the 
  solution 
  in 
  castor-oil, 
  after 
  having 
  stood 
  upwards 
  of 
  four 
  

   years, 
  I 
  find 
  that 
  circular 
  crystals 
  of 
  three 
  different 
  kinds 
  have 
  been 
  formed, 
  

   some 
  small 
  and 
  very 
  perfect, 
  with 
  four 
  sectors 
  and 
  no 
  rim 
  ; 
  others 
  with 
  broad 
  

   rims, 
  with 
  quaquaversus 
  polarisation 
  ; 
  and 
  a 
  third 
  set 
  in 
  which 
  the 
  structure 
  

   has 
  been 
  entirely 
  decomposed, 
  and 
  the 
  circular 
  form 
  of 
  the 
  disc 
  preserved. 
  

  

  10. 
  Palmine. 
  — 
  This 
  substance 
  melts 
  like 
  tallow 
  into 
  a 
  uniformly 
  luminous 
  

   film, 
  apparently 
  with 
  quaquaversus 
  polarisation 
  ; 
  but 
  upon 
  examining 
  it 
  with 
  a 
  

   high 
  power 
  in 
  the 
  polarising 
  microscope, 
  it 
  exhibits 
  millions 
  of 
  circular 
  crystals, 
  

   each 
  bearing 
  its 
  little 
  black 
  cross. 
  These 
  crystals 
  are 
  so 
  minute 
  as 
  to 
  produce 
  

   splendid 
  halos, 
  which, 
  in 
  the 
  polariscope, 
  give 
  four 
  luminous 
  sectors 
  exactly 
  like 
  

   those 
  in 
  oil 
  of 
  mace. 
  

  

  11. 
  Chromic 
  Acid. 
  — 
  The 
  circular 
  crystals 
  of 
  this 
  substance, 
  dissolved 
  in 
  water, 
  

   are 
  of 
  a 
  very 
  peculiar 
  kind. 
  They 
  are 
  negative, 
  and 
  are 
  very 
  imperfectly 
  repre- 
  

   sented 
  in 
  Fig. 
  11, 
  where 
  the 
  circular 
  disc 
  is 
  composed 
  of 
  a 
  great 
  number 
  of 
  con- 
  

   centric 
  circles, 
  whose 
  tint 
  is 
  the 
  blue 
  of 
  the 
  first 
  order, 
  rising, 
  in 
  some 
  cases, 
  to 
  

   the 
  yellow 
  of 
  the 
  same 
  order. 
  These 
  circles 
  may 
  be 
  described 
  as 
  rippled 
  lines 
  con- 
  

   sisting 
  of 
  minute 
  crystals, 
  separated 
  by 
  others 
  still 
  more 
  minute, 
  and 
  incapable 
  of 
  

   polarising 
  the 
  light. 
  The 
  system 
  of 
  concentric 
  rings 
  is 
  traversed 
  by 
  the 
  usual 
  black 
  

   cross. 
  This 
  salt 
  gives 
  another 
  kind 
  of 
  crystals, 
  in 
  which 
  are 
  separate 
  concentric 
  

   rings 
  without 
  the 
  black 
  cross, 
  and 
  consequently 
  with 
  quaquaversus 
  polarisation. 
  

  

  12. 
  Berberine. 
  — 
  This 
  salt 
  gives 
  very 
  fine 
  circular 
  crystals 
  which 
  are 
  negative, 
  

   and 
  form 
  beautiful 
  halos 
  like 
  those 
  in 
  oil 
  of 
  mace. 
  The 
  ordinary 
  crystals 
  often 
  

   form 
  a 
  number 
  of 
  crystalline 
  rings 
  in 
  contact, 
  each 
  of 
  which 
  contains 
  circular 
  

   crystals 
  of 
  different 
  sizes, 
  and 
  occasionally 
  prismatic 
  crystals 
  along 
  with 
  them. 
  

  

  13. 
  Sulphate 
  of 
  Cadmium. 
  — 
  The 
  sulphuret 
  of 
  cadmium, 
  dissolved 
  in 
  nitric 
  

   acid, 
  is 
  converted 
  into 
  sulphate, 
  which 
  gives 
  beautiful 
  negative 
  circular 
  crystals, 
  

   varying 
  from 
  the 
  800th 
  of 
  an 
  inch 
  to 
  the 
  3000th. 
  After 
  the 
  sulphuret 
  is 
  melted, 
  

   and 
  the 
  acid 
  driven 
  off, 
  no 
  crystallisation 
  is 
  seen, 
  but 
  in 
  an 
  hour 
  or 
  two 
  a 
  deli- 
  

   quescence 
  takes 
  place, 
  and 
  the 
  circular 
  crystals 
  gradually 
  appear. 
  There 
  are 
  

   many 
  of 
  them 
  so 
  small 
  and 
  thin, 
  that 
  they 
  have 
  no 
  action 
  on 
  polarised 
  light. 
  

  

  VOL. 
  XX. 
  PART 
  IV. 
  8 
  D 
  

  

  