﻿CRYSTALLINE 
  STRUCTURE 
  BY 
  COMPRESSION 
  AND 
  TRACTION. 
  559 
  

  

  tween 
  its 
  particles 
  ; 
  but 
  when 
  the 
  portion 
  of 
  soap 
  is 
  drawn 
  out 
  into 
  a 
  thread, 
  

   these 
  resistances 
  to 
  crystalline 
  arrangement 
  are 
  diminished 
  ; 
  elementary 
  prisms, 
  

   or 
  crystals 
  whose 
  length 
  is 
  greater 
  than 
  their 
  breadth, 
  will 
  have 
  a 
  tendency 
  to 
  

   place 
  their 
  greatest 
  length 
  in 
  the 
  line 
  of 
  traction, 
  and 
  all 
  lateral 
  obstruction 
  

   to 
  the 
  play 
  of 
  its 
  natural 
  polarities 
  being 
  to 
  a 
  great 
  extent 
  removed, 
  when 
  the 
  

   substance 
  is 
  drawn 
  into 
  a 
  capillary 
  thread 
  the 
  molecules 
  will 
  have 
  free 
  scope 
  to 
  

   assume 
  their 
  natural 
  crystalline 
  arrangement. 
  

  

  The 
  application 
  of 
  these 
  views 
  to 
  the 
  powders 
  and 
  particles 
  of 
  hard 
  crystals, 
  

   is 
  not 
  so 
  readily 
  apprehended 
  ; 
  but 
  when 
  we 
  consider 
  that 
  the 
  pressure 
  brings 
  

   the 
  molecules 
  of 
  the 
  substance 
  within 
  the 
  sphere 
  of 
  their 
  polarities, 
  and 
  that 
  the 
  

   force 
  of 
  traction 
  reduces 
  the 
  compressed 
  film 
  into 
  separate 
  streaks 
  and 
  lines, 
  like 
  

   the 
  threads 
  of 
  the 
  almond 
  soap, 
  we 
  have 
  reason 
  to 
  conclude, 
  that 
  even 
  in 
  hard 
  

   substances 
  the 
  atoms 
  ; 
  when 
  released 
  from 
  their 
  lateral 
  adhesions, 
  and 
  brought 
  

   into 
  narrow 
  lines, 
  will 
  assume 
  the 
  crystalline 
  arrangement. 
  

  

  In 
  the 
  course 
  of 
  these 
  experiments, 
  I 
  have 
  observed, 
  in 
  some 
  cases 
  where 
  the 
  

   crystalline 
  arrangement 
  was 
  very 
  imperfectly 
  effected, 
  a 
  tendency 
  in 
  the 
  atoms 
  to 
  

   quit 
  their 
  position, 
  as 
  if 
  they 
  were 
  in 
  a 
  state 
  of 
  unnatural 
  constraint, 
  like 
  the 
  par- 
  

   ticles 
  of 
  silex 
  and 
  manganese 
  in 
  certain 
  kinds 
  of 
  glass 
  which 
  experience 
  a 
  slow 
  de- 
  

   composition. 
  If 
  this 
  should 
  prove 
  to 
  be 
  the 
  case, 
  either 
  partially 
  or 
  generally, 
  

   which 
  time 
  only 
  can 
  shew, 
  it 
  will 
  doubtless 
  arise 
  from 
  the 
  non-homologous 
  sides 
  

   of 
  the 
  elementary 
  atoms 
  having 
  come 
  into 
  contact, 
  a 
  condition 
  of 
  the 
  crystalline 
  

   lines 
  perfectly 
  compatible 
  with 
  the 
  existence 
  of 
  neutral 
  and 
  depolarising 
  axes, 
  

   and 
  of 
  the 
  colours 
  of 
  polarised 
  light, 
  provided 
  that 
  the 
  non-homologous 
  sides 
  

   in 
  contact 
  deviate 
  from 
  their 
  proper 
  position, 
  either 
  90° 
  or 
  180°. 
  If 
  we 
  cut 
  a 
  plate 
  

   of 
  mica, 
  for 
  example, 
  into 
  two 
  pieces, 
  and 
  combine 
  them 
  by 
  turning 
  one 
  of 
  them 
  

   round 
  90° 
  or 
  180°, 
  polarised 
  light 
  transmitted 
  through 
  them 
  perpendicularly, 
  will 
  

   exhibit 
  the 
  same 
  colours 
  as 
  when 
  they 
  were 
  in 
  their 
  natural 
  position, 
  and 
  also 
  

   the 
  same 
  neutral 
  and 
  depolarising 
  axes. 
  If 
  the 
  polarised 
  light 
  is 
  transmitted 
  

   obliquely, 
  the 
  hemitropism 
  of 
  the 
  combination, 
  as 
  we 
  may 
  call 
  it, 
  will 
  be 
  in- 
  

   stantly 
  discovered 
  by 
  the 
  difference 
  of 
  colour 
  of 
  the 
  two 
  plates. 
  

  

  St 
  Leonard's 
  College, 
  St 
  Andrews, 
  

   February 
  25, 
  1853. 
  

  

  VOL. 
  XX. 
  PART 
  IV. 
  7 
  M 
  

  

  