﻿61G 
  SIR 
  DAVID 
  BREWSTER 
  ON 
  CIRCULAR 
  CRYSTALS. 
  

  

  The 
  solutions 
  of 
  this 
  substance 
  in 
  muriatic 
  and 
  acetic 
  acids 
  gave 
  no 
  circular 
  

   crystals. 
  

  

  14. 
  Sulphate 
  of 
  Ammonia 
  and 
  Magnesia. 
  — 
  When 
  an 
  aqueous 
  solution 
  of 
  this 
  

   salt 
  is 
  brought 
  into 
  a 
  viscous 
  state 
  by 
  heat, 
  and 
  slowly 
  cooled, 
  very 
  beautiful 
  cir- 
  

   cular 
  crystals 
  are 
  formed, 
  sometimes 
  large, 
  and 
  sometimes 
  smaller 
  than 
  the 
  

   1000th 
  of 
  an 
  inch. 
  Round 
  some 
  of 
  them 
  there 
  is 
  formed 
  a 
  radiant 
  crystalline 
  

   halo, 
  separated 
  by 
  a 
  black 
  circle 
  from 
  the 
  disc 
  which 
  gives 
  the 
  four 
  luminous 
  

   sectors. 
  The 
  polarisation 
  of 
  the 
  crystals 
  is 
  positive, 
  and 
  they 
  are 
  perfect 
  at 
  the 
  

   centre, 
  unless 
  when 
  large 
  and 
  badly 
  formed. 
  

  

  15. 
  Hatchetine, 
  Cacao 
  Butter, 
  White 
  Wax, 
  Tallow, 
  Adipocire, 
  and 
  all 
  Soaps 
  

   and 
  different 
  kinds 
  of 
  Fat, 
  produce 
  circular 
  crystals 
  like 
  Oil 
  of 
  Mace, 
  and 
  give 
  the 
  

   same 
  halos, 
  which, 
  in 
  the 
  polariscope, 
  are 
  divided 
  into 
  four 
  sectors. 
  

  

  16. 
  Borax 
  in 
  Phosphoric 
  Acid. 
  — 
  The 
  crystals 
  produced 
  by 
  this 
  combination 
  

   are 
  those 
  discovered 
  by 
  Mr 
  Talbot, 
  and 
  have 
  been 
  already 
  described. 
  I 
  mention 
  

   them 
  again, 
  in 
  order 
  to 
  notice 
  the 
  interesting 
  hemispherical 
  bells 
  which 
  I 
  have 
  

   observed 
  when 
  an 
  aqueous 
  solution 
  is 
  raised 
  into 
  froth 
  by 
  heat. 
  These 
  bells 
  or 
  

   bubbles 
  indurate 
  and 
  polarise 
  the 
  light 
  by 
  refraction, 
  as 
  in 
  the 
  case 
  of 
  nitrate 
  of 
  

   uranium 
  already 
  mentioned. 
  They 
  are 
  traversed 
  by 
  the 
  black 
  cross, 
  and 
  exhibit 
  

   rings 
  of 
  colour, 
  which 
  are 
  green 
  at 
  the 
  centre, 
  then 
  red, 
  then 
  green 
  and 
  red 
  again. 
  

   Some 
  of 
  these 
  bells 
  contained 
  smaller 
  ones 
  within 
  them, 
  in 
  one 
  case 
  no 
  fewer 
  

   than 
  eight 
  ; 
  and 
  in 
  one 
  of 
  them 
  I 
  distinctly 
  observed 
  a 
  crystalline 
  structure, 
  the 
  

   minute 
  crystals 
  radiating 
  from 
  the 
  apex 
  of 
  the 
  bell. 
  

  

  17. 
  Mannite. 
  — 
  This 
  substance 
  gives 
  circular 
  crystals 
  with 
  more 
  facility 
  and 
  

   certainty 
  than 
  any 
  other 
  which 
  I 
  have 
  examined. 
  When 
  Mannite 
  is 
  melted 
  by 
  

   heat, 
  it 
  gives 
  beautiful 
  circular 
  crystals. 
  When 
  dissolved 
  in 
  water 
  they 
  are 
  very 
  

   good. 
  They 
  are 
  not 
  good 
  in 
  alcohol 
  or 
  ether 
  ; 
  but 
  in 
  acetic 
  acid 
  the 
  finest 
  circular 
  

   crystals 
  are 
  formed. 
  The 
  black 
  concentric 
  circles 
  are 
  peculiarly 
  fine, 
  and 
  are, 
  so 
  

   far 
  as 
  the 
  microscope 
  can 
  shew 
  it, 
  entirely 
  free 
  of 
  matter. 
  In 
  the 
  crystals 
  from 
  

   acetic 
  acid, 
  the 
  sectors 
  shade 
  off 
  into 
  the 
  arms 
  of 
  the 
  black 
  cross 
  with 
  such 
  per- 
  

   fection, 
  that 
  the 
  circular 
  disc 
  loses 
  its 
  flat 
  appearance, 
  and 
  seems 
  to 
  be 
  composed 
  

   of 
  four 
  solid 
  cones, 
  whose 
  apices 
  meet 
  in 
  the 
  centre. 
  In 
  place 
  of 
  being 
  circular 
  

   the 
  crystals 
  are 
  sometimes 
  drawn 
  out, 
  as 
  it 
  were, 
  into 
  long 
  cones, 
  as 
  shewn 
  in 
  

   Fig. 
  19, 
  rounded 
  at 
  their 
  summits, 
  and 
  having 
  the 
  appearance 
  of 
  solids 
  of 
  that 
  shape. 
  

   The 
  black 
  cross 
  appears 
  at 
  the 
  summit 
  of 
  the 
  rounded 
  cones, 
  one 
  of 
  its 
  arms, 
  and 
  

   sometimes 
  two, 
  according 
  to 
  the 
  position 
  of 
  the 
  plane 
  of 
  primitive 
  polarisation, 
  

   stretching 
  out 
  to 
  the 
  termination 
  of 
  the 
  rounded 
  cone. 
  These 
  cones 
  are 
  often 
  

   crossed 
  by 
  two, 
  three, 
  or 
  /our 
  concentric 
  arches, 
  perfectly 
  black. 
  In 
  these 
  elon- 
  

  

  