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  XXXVIIL— 
  On 
  the 
  Production 
  of 
  Crystalline 
  Structure 
  in 
  Crystallised 
  Powders, 
  by 
  

   Compression 
  and 
  Traction. 
  By 
  Sir 
  David 
  Brewster, 
  K.H., 
  D.C.L., 
  F.R.S., 
  

   V.P.R.S. 
  Edin., 
  and 
  Associate 
  of 
  the 
  Institute 
  of 
  France. 
  

  

  (Read 
  7th 
  March 
  1353.) 
  

  

  The 
  influence 
  of 
  compression 
  and 
  dilatation 
  in 
  producing 
  the 
  doubly 
  refract- 
  

   ing 
  structure 
  in 
  solids 
  of 
  all 
  kinds, 
  whether 
  crystallised 
  or 
  uncrystallised, 
  which 
  

   do 
  not 
  possess 
  it, 
  and 
  in 
  modifying 
  that 
  structure 
  in 
  all 
  crystals 
  which 
  do 
  possess 
  

   it, 
  has 
  been 
  long 
  known 
  ; 
  but 
  with 
  this 
  class 
  of 
  phenomena, 
  those 
  which 
  I 
  am 
  

   about 
  to 
  describe 
  have 
  no 
  connection 
  whatever. 
  

  

  In 
  the 
  course 
  of 
  experiments 
  on 
  the 
  double 
  reflexion 
  and 
  polarisation 
  of 
  light 
  

   which 
  I 
  discovered 
  in 
  the 
  ctirysammates 
  of 
  potash 
  and 
  magnesia, 
  murexide, 
  and 
  

   other 
  crystals, 
  I 
  was 
  surprised 
  to 
  find 
  that 
  these 
  substances 
  could 
  be 
  spread 
  out 
  

   upon 
  glass 
  by 
  hard 
  pressure, 
  like 
  grease 
  or 
  soft 
  wax, 
  and 
  that 
  in 
  the 
  case 
  of 
  chry- 
  

   sammate 
  of 
  potash 
  and 
  other 
  bodies, 
  when 
  the 
  powder 
  could 
  scarcely 
  be 
  distin- 
  

   guished 
  from 
  snuff, 
  I 
  obtained 
  a 
  transparent 
  film, 
  exhibiting 
  the 
  phenomena 
  of 
  

   double 
  reflexion 
  and 
  polarisation 
  from 
  its 
  surface, 
  as 
  perfectly 
  as 
  if 
  I 
  had 
  been 
  

   using 
  a 
  large 
  crystal. 
  

  

  In 
  subsequently 
  repeating 
  these 
  experiments, 
  and 
  examining, 
  under 
  polarised 
  

   light, 
  the 
  film 
  thus 
  produced 
  by 
  compression 
  and 
  traction, 
  I 
  was 
  surprised 
  to 
  ob- 
  

   serve 
  that 
  the 
  streaks 
  and 
  separate 
  lines 
  of 
  the 
  film, 
  as 
  well 
  as 
  the 
  film 
  itself, 
  had 
  

   regular 
  axes 
  of 
  double 
  refraction, 
  as 
  if 
  they 
  were 
  regularly 
  crystallised 
  portions 
  of 
  

   the 
  substance 
  under 
  examination. 
  These 
  streaks 
  and 
  capillary 
  lines, 
  which 
  were 
  

   often 
  of 
  extreme 
  minuteness, 
  did 
  not 
  appear 
  to 
  consist 
  of 
  insulated 
  particles 
  merely 
  

   dragged 
  into 
  a 
  line, 
  but 
  when 
  the 
  substance 
  possessed 
  the 
  new 
  property 
  in 
  per- 
  

   fection, 
  the 
  lines 
  of 
  polarised 
  light 
  were 
  continuous, 
  and 
  the 
  crystallographic 
  as 
  

   well 
  as 
  the 
  optical 
  axes 
  of 
  the 
  particles 
  were 
  placed 
  in 
  that 
  line. 
  In 
  other 
  cases, 
  

   where 
  the 
  experiment 
  was 
  less 
  successful, 
  the 
  insulation 
  of 
  the 
  particles 
  was 
  

   easily 
  recognised, 
  though 
  the 
  general 
  mass 
  of 
  them 
  was 
  crystallographically 
  

   arranged. 
  

  

  In 
  making 
  these 
  experiments, 
  the 
  natural 
  crystalline 
  powder, 
  or 
  the 
  particles 
  

   of 
  the 
  crushed 
  crystal, 
  may 
  be 
  placed, 
  either 
  upon 
  a 
  polished 
  glass 
  surface, 
  or 
  upon 
  

   a 
  piece 
  of 
  glass 
  ground 
  on 
  one 
  side. 
  In 
  those 
  cases 
  where 
  the 
  substance 
  is 
  soft, 
  the 
  

   polished 
  surface 
  is 
  preferable, 
  but 
  when 
  the 
  powder 
  is 
  hard 
  and 
  considerable 
  pres- 
  

   sure 
  necessary, 
  it 
  is 
  better 
  to 
  place 
  it 
  upon 
  the 
  ground 
  surface 
  of 
  a 
  piece 
  of 
  glass, 
  

  

  VOL. 
  XX. 
  PART 
  IV. 
  7 
  L 
  

  

  