﻿276 
  Lord 
  Kelvin 
  : 
  

  

  and 
  as 
  in 
  accordance 
  with 
  the 
  free 
  electro-polarities 
  of 
  fractured 
  

   surfaces 
  of 
  tourmaline 
  discovered 
  by 
  Canton*. 
  If 
  a 
  crystal, 
  

   which 
  we 
  may 
  imagine 
  as 
  given 
  with 
  the 
  electrions 
  wholly 
  

   undisturbed 
  from 
  their 
  positions 
  according 
  to 
  § 
  32, 
  is 
  dipped 
  

   in 
  water 
  and 
  then 
  allowed 
  to 
  dry, 
  electrions 
  would 
  by 
  this 
  

   process 
  be 
  removed 
  from 
  one 
  part 
  of 
  its 
  surface 
  and 
  dis- 
  

   tributed 
  over 
  the 
  remainder 
  so 
  as 
  to 
  wholly 
  annul 
  its 
  external 
  

   manifestation 
  of 
  electric 
  quality. 
  If 
  now 
  either 
  by 
  change 
  

   of 
  temperature 
  or 
  by 
  mechanical 
  stress 
  the 
  distances 
  between 
  

   the 
  atoms 
  are 
  altered, 
  the 
  interior 
  electro-polarization 
  becomes 
  

   necessarily 
  altered 
  ; 
  and 
  the 
  masking 
  superficial 
  electrification 
  

   got 
  by 
  the 
  dipping 
  in 
  water 
  and 
  drying 
  will 
  now 
  not 
  exactly 
  

   annul 
  the 
  electrostatic 
  force 
  in 
  the 
  air 
  around 
  the 
  solid. 
  If 
  

   at 
  the 
  altered 
  temperature 
  or 
  under 
  the 
  supposed 
  stress 
  the 
  

   solid 
  is 
  again 
  dipped 
  in 
  water 
  and 
  dried, 
  the 
  external 
  electric 
  

   force 
  will 
  be 
  again 
  annulled. 
  Thus 
  is 
  explained 
  the 
  pyro- 
  

   electricity 
  of 
  tourmaline 
  discovered 
  by 
  Aepinus. 
  

  

  § 
  34. 
  But 
  a 
  merely 
  di-polar 
  electric 
  crystal 
  with 
  its 
  single 
  

   axis 
  presents 
  to 
  us 
  only 
  a 
  small, 
  and 
  the 
  very 
  simplest, 
  part 
  

   of 
  the 
  whole 
  subject 
  of 
  electro 
  -crystallography. 
  In 
  boracite,, 
  

   a 
  crystal 
  of 
  the 
  cubic 
  class, 
  Haiiy 
  found 
  in 
  the 
  four 
  diagonals 
  

   of 
  the 
  cube, 
  or 
  the 
  perpendiculars 
  to 
  the 
  four 
  faces 
  of 
  the 
  

   regular 
  octahedron, 
  four 
  di-polar 
  axes 
  : 
  the 
  crystal 
  on 
  being 
  

   irregularly 
  heated 
  or 
  cooled 
  showed 
  as 
  it 
  were 
  opposite 
  elec- 
  

   tricities 
  on 
  the 
  surfaces 
  in 
  the 
  neighbourhood 
  of 
  opposite 
  

   pairs 
  of 
  corners 
  of 
  the 
  cube, 
  or 
  around 
  the 
  centres 
  of 
  the 
  

   opposite 
  pairs 
  of 
  triangular 
  faces 
  of 
  the 
  octahedron. 
  His 
  

   discoveries 
  allow 
  us 
  to 
  conclude 
  that 
  in 
  general 
  the 
  electric 
  

   geolotropy 
  of 
  crystals 
  is 
  octopolar 
  with 
  four 
  axes, 
  not 
  merely 
  

   dipolar 
  as 
  in 
  the 
  old-known 
  electricity 
  of 
  the 
  tourmaline. 
  

   The 
  intensities 
  of 
  the 
  electric 
  virtue 
  are 
  generally 
  different 
  

   for 
  the 
  four 
  axes, 
  and 
  the 
  directions 
  of 
  the 
  axes 
  are 
  in 
  general 
  

   unsymmetrically 
  oriented 
  for 
  crystals 
  of 
  the 
  unsymmetrical 
  

   classes. 
  For 
  crystals 
  of 
  the 
  optically 
  uniaxal 
  class, 
  one 
  of 
  the 
  

   electro-polar 
  axes 
  must 
  generally 
  coincide 
  with 
  the 
  optic 
  axis, 
  

   and 
  the 
  other 
  three 
  may 
  be 
  perpendicular 
  to 
  it. 
  The 
  intensities 
  

   of 
  the 
  electro-polar 
  virtue 
  are 
  essentially 
  equal 
  for 
  these 
  three 
  

  

  * 
  Wiedemann 
  (Die 
  Lehre 
  von 
  der 
  Elektricitdt, 
  Second 
  Edition, 
  1894, 
  

   vol. 
  ii. 
  § 
  378) 
  mentions 
  an 
  experiment 
  without 
  fully 
  describing 
  it 
  by 
  

   which 
  a 
  null 
  result, 
  seemingly 
  at 
  variance 
  with 
  Canton's 
  experimental 
  

   discovery 
  and 
  condemnatory 
  of 
  my 
  suggested 
  theory, 
  was 
  found. 
  

   Interesting 
  experiments 
  might 
  be 
  made 
  by 
  pressing 
  together 
  and 
  

   reseparating 
  fractured 
  surfaces 
  of 
  tourmaline, 
  or 
  by 
  pressing 
  and 
  rubbing- 
  

   polished 
  surfaces 
  together 
  and 
  separating 
  them. 
  It 
  would 
  be 
  very 
  

   difficult 
  to 
  get 
  trustworthy 
  results 
  by 
  breakages, 
  because 
  it 
  would 
  be 
  

   almost 
  impossible 
  to 
  avoid 
  irregular 
  electrifications 
  by 
  the 
  appliances 
  

   used 
  for 
  making 
  the 
  breakage. 
  The 
  mode 
  of 
  electric 
  measurement 
  

   followed 
  in 
  the 
  experiment 
  referred 
  to 
  by 
  Wiedemann 
  is 
  not 
  described. 
  

  

  