﻿556 
  

  

  SIR 
  DAVID 
  BREWSTER 
  ON 
  THE 
  PRODUCTION 
  OF 
  

  

  as 
  the 
  particles 
  are 
  detained 
  between 
  its 
  minute 
  elevations, 
  and 
  submit 
  more 
  

   readily 
  to 
  the 
  combined 
  force 
  of 
  pressure 
  and 
  tractioo. 
  When 
  the 
  powder 
  is 
  thus 
  

   placed, 
  I 
  take 
  a 
  polished 
  and 
  elastic 
  knife, 
  and 
  with 
  its 
  broad 
  point 
  I 
  compress 
  

   and 
  drag 
  the 
  powder 
  in 
  a 
  given 
  direction, 
  till 
  there 
  is 
  the 
  appearance 
  of 
  a 
  polished 
  

   surface 
  on 
  the 
  compressed 
  substance. 
  In 
  general. 
  I 
  have 
  used 
  both 
  the 
  smooth 
  

   and 
  the 
  rough 
  glass, 
  and 
  have 
  frequently 
  obtained 
  results 
  with 
  the 
  one, 
  which 
  

   were 
  not 
  given 
  by 
  the 
  other. 
  

  

  If 
  we 
  now 
  place 
  the 
  plate 
  of 
  glass 
  in 
  a 
  polarising 
  microscope, 
  with 
  the 
  field 
  

   dark, 
  we 
  shall 
  find 
  that 
  the 
  streaks 
  and 
  lines 
  produced 
  by 
  traction 
  have, 
  in 
  cer- 
  

   tain 
  substances, 
  regular 
  neutral 
  and 
  depolarising 
  axes, 
  as 
  if 
  they 
  were 
  prismatic 
  

   crystals 
  of 
  the 
  substance 
  under 
  examination. 
  With 
  the 
  clirysammate 
  of 
  magnesia, 
  

   a 
  red 
  powder 
  with 
  specks 
  of 
  yellow 
  reflected 
  light, 
  the 
  phenomena 
  are 
  peculiarly 
  

   splendid 
  ; 
  the 
  natural 
  colours 
  of 
  the 
  substance, 
  which 
  vary 
  greatly 
  with 
  the 
  thick- 
  

   ness 
  of 
  the 
  streaks 
  and 
  films, 
  being 
  combined 
  with 
  the 
  different 
  tints 
  which 
  they 
  

   polarise. 
  As 
  the 
  crystals 
  of 
  this 
  substance 
  possess 
  unusual 
  reflexion, 
  this 
  pro- 
  

   perty 
  is 
  displayed 
  in 
  the 
  crystallised 
  streaks 
  produced 
  by 
  traction 
  ; 
  and 
  the 
  

   superficial 
  colours 
  which 
  they 
  reflect, 
  vary 
  with 
  the 
  azimuth 
  which 
  the 
  plane 
  of 
  

   incidence 
  forms 
  with 
  the 
  plane 
  passing 
  through 
  the 
  axis 
  of 
  the 
  prism. 
  

  

  The 
  remarkable 
  property 
  which 
  I 
  have 
  now 
  described, 
  I 
  have 
  found, 
  in 
  a 
  

   greater 
  or 
  a 
  less 
  degree, 
  in 
  the 
  following 
  crystals 
  : 
  — 
  

  

  Clirysammate 
  of 
  magnesia. 
  

  

  of 
  potash. 
  

   Hydro-chrysammid. 
  

   Murexide. 
  

  

  Aloetinate 
  of 
  potash. 
  

   Aloetinic 
  acid. 
  

   Oxamide. 
  

   Palmine. 
  

   Palmic 
  acid. 
  

   Amygdaline. 
  

   Tannin, 
  pure. 
  

   Quinine, 
  pure. 
  

  

  acetate 
  of. 
  

  

  sulphate 
  of. 
  

  

  muriate 
  of. 
  

  

  phosphate 
  of. 
  

  

  citrate 
  of. 
  

   Cacao 
  butter. 
  

   Veratric 
  acid. 
  

   Esculine. 
  

   Theine. 
  

  

  Silver, 
  cyanide 
  of. 
  

   acetate 
  of. 
  

  

  Platina 
  and 
  magnesia, 
  cyanuret 
  of. 
  

  

  . 
  . 
  . 
  and 
  barytes, 
  cyanuret 
  of. 
  

   potassium, 
  cyanuret 
  of. 
  

  

  ... 
  ammonia, 
  chloride 
  of. 
  

   Potash, 
  oxymuriate 
  of. 
  

  

  chromate 
  of. 
  

   Urea, 
  nitrate 
  of. 
  

   Sulphur. 
  

   Camphor. 
  

   Cinchonine. 
  

  

  sulphate 
  of. 
  

   Meconic 
  acid. 
  

   Brucine, 
  sulphate 
  of. 
  

   Morphia, 
  acetate 
  of. 
  

   Tin, 
  iodide 
  of. 
  

   Cerium, 
  oxide 
  of. 
  

   Parmeline. 
  

   Lecanorine. 
  

   Indigo, 
  red. 
  

   Ammonia, 
  oxalate 
  of. 
  

   sulphate 
  of. 
  

   Soda, 
  chromate 
  of. 
  

   Lead, 
  iodide 
  of. 
  

  

  