﻿558 
  SIR 
  DAVID 
  BREWSTER 
  ON 
  THE 
  PRODUCTION 
  OF 
  

  

  Without 
  expecting 
  any 
  very 
  interesting 
  result, 
  T 
  submitted 
  to 
  examination 
  

   several 
  of 
  the 
  soft 
  solids 
  which 
  possess 
  double 
  refraction, 
  such 
  as 
  bees' 
  wax, 
  oil 
  of 
  

   mace, 
  tallow, 
  and 
  almond 
  soap. 
  The 
  last 
  of 
  these 
  substances, 
  though 
  in 
  common 
  

   use, 
  is 
  a 
  very 
  remarkable 
  one. 
  Owing 
  to 
  its 
  particles 
  not 
  being 
  in 
  optical 
  contact, 
  

   it 
  has 
  a 
  fine 
  pearly 
  lustre, 
  and 
  may 
  be 
  drawn 
  out 
  into 
  long 
  and 
  slender 
  strings. 
  

   Upon 
  laying 
  a 
  portion 
  of 
  it 
  on 
  glass, 
  it 
  has 
  a 
  quaquaversus 
  polarising 
  structure, 
  

   with 
  a 
  tendency 
  to 
  form 
  circular 
  crystals, 
  but 
  when 
  it 
  is 
  drawn 
  out 
  into 
  strings, 
  

   and 
  laid 
  upon 
  glass, 
  these 
  strings 
  have 
  neutral 
  and 
  depolarising 
  axes, 
  like 
  the 
  

   streaks 
  formed 
  by 
  compression 
  and 
  traction. 
  In 
  the 
  present 
  case, 
  it 
  is 
  by 
  traction 
  

   alone, 
  that 
  this 
  crystalline 
  arrangement 
  of 
  the 
  particles 
  is 
  produced. 
  

  

  In 
  oil 
  of 
  mace 
  and 
  tallow, 
  a 
  similar 
  effect 
  is 
  produced 
  by 
  compression 
  and 
  

   traction. 
  With 
  bees' 
  1 
  wax, 
  the 
  depolarising 
  lines 
  are 
  still 
  better 
  displayed, 
  and 
  the 
  

   effect 
  is 
  considerably 
  increased 
  by 
  mixing 
  the 
  bees' 
  wax 
  with 
  a 
  small 
  quantity 
  

   of 
  rosin. 
  

  

  As 
  the 
  preceding 
  experiments 
  place 
  it 
  beyond 
  a 
  doubt, 
  that 
  the 
  optical 
  or 
  

   crystallographic 
  axes 
  of 
  a 
  number 
  of 
  minute 
  particles 
  are 
  dragged 
  by 
  pressure 
  

   and 
  traction 
  into 
  the 
  same 
  direction, 
  so 
  as 
  to 
  act 
  upon 
  light 
  like 
  regular 
  crystals, 
  

   it 
  became 
  interesting 
  to 
  discover 
  the 
  cause 
  of 
  phenomena 
  which 
  certainly 
  could 
  

   not 
  have 
  been 
  anticipated 
  from 
  any 
  theoretical 
  principle 
  with 
  which 
  we 
  are 
  

   acquainted. 
  The 
  primary 
  force, 
  and 
  indeed 
  the 
  only 
  apparent 
  one 
  exerted 
  in 
  

   these 
  experiments, 
  is 
  a 
  mechanical 
  force 
  ; 
  but 
  it 
  is 
  not 
  improbable 
  that 
  a 
  secondary 
  

   force, 
  namely, 
  that 
  of 
  electricity, 
  may 
  be 
  generated 
  by 
  the 
  friction 
  which 
  accom- 
  

   panies 
  the 
  forces 
  of 
  pressure 
  and 
  traction. 
  That 
  such 
  a 
  force 
  is 
  excited 
  with 
  

   certain 
  crystals 
  will 
  not 
  admit 
  of 
  a 
  doubt 
  ; 
  but 
  even 
  if 
  it 
  were 
  developed 
  in 
  every 
  

   case, 
  this 
  would 
  not 
  prove 
  that 
  electricity 
  was 
  the 
  agent 
  in 
  producing 
  the 
  pheno- 
  

   mena 
  under 
  consideration. 
  In 
  subjecting 
  asparagine 
  to 
  compression 
  and 
  traction, 
  

   I 
  observed, 
  upon 
  placing 
  it 
  in 
  the 
  polarising 
  microscope, 
  that 
  its 
  particles 
  were 
  

   moving 
  about 
  under 
  an 
  electrical 
  influence, 
  but 
  in 
  no 
  other 
  case 
  did 
  the 
  same 
  

   phenomenon 
  present 
  itself 
  to 
  me. 
  

  

  The 
  experiments 
  with 
  soft 
  solids, 
  but 
  especially 
  those 
  made 
  with 
  the 
  almond 
  

   soap, 
  exclude 
  the 
  supposition 
  that 
  the 
  electricity 
  of 
  friction 
  is 
  the 
  cause 
  of 
  the 
  

   crystalline 
  arrangement 
  of 
  its 
  particles 
  ; 
  though 
  it 
  is 
  not 
  improbable 
  that 
  the 
  

   sliding 
  of 
  the 
  particles 
  upon 
  one 
  another, 
  as 
  produced 
  by 
  traction, 
  and 
  their 
  

   mutual 
  separation, 
  as 
  in 
  the 
  case 
  of 
  tearing 
  asunder 
  mica 
  or 
  paper, 
  may 
  produce 
  

   enough 
  of 
  electricity 
  to 
  have 
  some 
  share 
  in 
  giving 
  the 
  same 
  direction 
  to 
  the 
  axes 
  

   of 
  the 
  particles. 
  

  

  When 
  a 
  portion 
  of 
  almond 
  soap 
  is 
  placed 
  upon 
  glass, 
  the 
  axes 
  of 
  its 
  particles 
  

   lie 
  in 
  every 
  direction, 
  and 
  have 
  no 
  tendency 
  to 
  assume 
  the 
  crystalline 
  arrange- 
  

   ment. 
  The 
  forces 
  of 
  aggregation 
  emanating 
  from 
  three 
  rectangular 
  axes, 
  are 
  not 
  

   strong 
  enough 
  to 
  overcome 
  the 
  inertia, 
  as 
  we 
  may 
  call 
  it, 
  arising 
  from 
  the 
  natural 
  

   quaquaversus 
  adhesiveness 
  of 
  the 
  substance, 
  and 
  from 
  the 
  water 
  interposed 
  be- 
  

  

  