﻿SIR 
  DAVID 
  BREWSTER 
  ON 
  CIRCULAR 
  CRYSTALS. 
  60.9 
  

  

  Borax. 
  I 
  was 
  thus 
  enabled 
  to 
  study 
  the 
  phenomena 
  which 
  they 
  exhibit 
  in 
  their 
  

   formation, 
  their 
  structure, 
  and 
  their 
  subsequent 
  decomposition. 
  

  

  In 
  the 
  expectation 
  of 
  obtaining 
  a 
  greater 
  variety 
  of 
  structure, 
  and 
  discovering 
  

   new 
  phenomena, 
  I 
  submitted 
  to 
  examination 
  about 
  300 
  doubly-refracting 
  sub- 
  

   stances, 
  and 
  among 
  these 
  I 
  discovered 
  nearly 
  seventy 
  that 
  give 
  circular 
  crystals, 
  

   about 
  thirty 
  of 
  which 
  are 
  positive, 
  like 
  Zircon, 
  and 
  forty 
  negative, 
  like 
  Calcareous 
  

   spar. 
  

  

  In 
  the 
  course 
  of 
  these 
  experiments, 
  which 
  have 
  occupied 
  much 
  of 
  my 
  time, 
  I 
  

   have 
  observed 
  many 
  new 
  and 
  splendid 
  phenomena, 
  which 
  lay 
  open 
  an 
  extensive 
  

   field 
  of 
  research, 
  and 
  promise 
  to 
  throw 
  much 
  light 
  on 
  those 
  abnormal 
  crystallisa- 
  

   tions 
  which 
  take 
  place 
  under 
  the 
  constraining 
  influences 
  of 
  heat 
  and 
  pressure, 
  

   and 
  also 
  on 
  their 
  subsequent 
  decomposition 
  and 
  return 
  to 
  their 
  molecular 
  state. 
  

  

  In 
  submitting 
  to 
  the 
  Society 
  an 
  account 
  of 
  these 
  experiments, 
  I 
  shall 
  begin 
  

   with 
  the 
  Lithoxanthate 
  of 
  Ammonia, 
  as 
  it 
  exhibits 
  a 
  greater 
  variety 
  of 
  phenomena, 
  

   and 
  is 
  more 
  easily 
  converted 
  into 
  circular 
  crystals 
  than 
  any 
  other 
  salts 
  with 
  

   which 
  I 
  am 
  acquainted. 
  

  

  1. 
  Lithoxanthate 
  of 
  Ammonia. 
  — 
  This 
  substance, 
  under 
  ordinary 
  circumstances, 
  

   crystallises 
  in 
  minute 
  prisms, 
  often 
  in 
  beautiful 
  dendritic 
  forms, 
  and 
  in 
  spherical 
  

   groups 
  of 
  crystals 
  in 
  which 
  the 
  prisms 
  are 
  not 
  in 
  optical 
  contact, 
  and 
  yet 
  suffi- 
  

   ciently 
  united 
  to 
  exhibit 
  the 
  black 
  cross 
  at 
  the 
  centre 
  of 
  the 
  sphere. 
  

  

  When 
  the 
  circular 
  crystals 
  are 
  produced, 
  and 
  are 
  transparent, 
  they 
  have 
  very 
  

   different 
  aspects 
  in 
  different 
  specimens. 
  In 
  their 
  simplest 
  form, 
  they 
  are 
  united 
  

   in 
  a 
  continuous 
  film, 
  each 
  circular 
  crystal 
  exhibiting 
  four 
  luminous 
  sectors 
  sepa- 
  

   rated 
  by 
  a 
  black 
  cross, 
  the 
  arms 
  of 
  which 
  are, 
  of 
  course, 
  always 
  parallel 
  and 
  

   perpendicular 
  to 
  the 
  plane 
  of 
  primitive 
  polarisation. 
  The 
  light 
  polarised 
  by 
  the 
  

   sector 
  is 
  the 
  blue 
  of 
  the 
  first 
  order, 
  often 
  rising 
  to 
  the 
  white, 
  and 
  sometimes 
  to 
  the 
  

   yellow, 
  of 
  the 
  same 
  order. 
  

  

  When 
  we 
  look 
  at 
  a 
  small 
  and 
  bright 
  luminous 
  disc 
  through 
  a 
  film 
  of 
  such 
  

   crystals, 
  we 
  see 
  a 
  halo, 
  and 
  sometimes 
  two 
  halos, 
  the 
  diameter 
  of 
  the 
  halo 
  dimi- 
  

   nishing 
  as 
  the 
  circular 
  crystals 
  increase 
  in 
  size. 
  When 
  the 
  film 
  is 
  placed 
  in 
  the 
  

   polariscope, 
  the 
  halo 
  is 
  converted 
  into 
  four 
  luminous 
  sectors, 
  and 
  into 
  eight 
  when 
  

   it 
  is 
  double, 
  exactly 
  the 
  same 
  as 
  those 
  produced 
  by 
  oil 
  of 
  mace, 
  and 
  shewn 
  in 
  

   Figs. 
  1 
  and 
  2. 
  

  

  When 
  the 
  circular 
  crystals 
  are 
  separate, 
  their 
  structure 
  is 
  more 
  complex, 
  and 
  

   their 
  appearance 
  more 
  beautiful. 
  In 
  one 
  of 
  these, 
  shewn 
  in 
  Fig. 
  3, 
  I 
  have 
  ob- 
  

   served, 
  but 
  only 
  once, 
  the 
  three 
  first 
  orders 
  of 
  colours 
  of 
  thin 
  plates, 
  exactly 
  like 
  

   the 
  uniaxal 
  system 
  of 
  rings 
  in 
  regular 
  crystals; 
  and 
  consequently, 
  the 
  thick- 
  

   ness 
  of 
  the 
  spicular 
  crystals 
  which 
  composed 
  them 
  must 
  have 
  increased 
  from 
  the 
  

   centre 
  outwards, 
  according 
  to 
  the 
  law 
  in 
  Newton's 
  Table 
  of 
  Periodical 
  Colours. 
  

   This 
  result 
  was 
  so 
  remarkable, 
  that 
  I 
  determined 
  the 
  character 
  of 
  the 
  three 
  

  

  