﻿271 
  Lord 
  Kelvin: 
  

  

  and 
  16° 
  ; 
  the 
  temperature 
  of 
  melting 
  solid 
  hydrogen, 
  both 
  

   determined 
  by 
  Dewar 
  with 
  his 
  helium 
  thermometer. 
  There 
  

   is 
  no 
  difficulty 
  in 
  believing 
  that 
  the 
  electrions 
  in 
  each 
  

   of 
  the 
  metallic 
  atoms 
  are 
  so 
  numerous 
  that, 
  though 
  they 
  rest 
  

   in 
  stable 
  equilibrium 
  within 
  the 
  atoms, 
  closely 
  packed 
  to 
  

   constitute 
  the 
  solid 
  metal 
  at 
  0° 
  absolute, 
  and 
  may 
  move 
  about 
  

   within 
  the 
  atom 
  with 
  their 
  wildly 
  irregular 
  thermal 
  motions 
  

   at 
  1° 
  of 
  absolute 
  temperature, 
  they 
  may 
  between 
  1° 
  and 
  

   2° 
  begin 
  to 
  spill 
  from 
  atom 
  to 
  atom. 
  Thus, 
  like 
  glass 
  or 
  

   a 
  Nernst 
  filament 
  below 
  300° 
  absolute, 
  a 
  metal 
  may 
  be 
  an 
  

   almost 
  perfect 
  insulator 
  of 
  electricity 
  below 
  1° 
  absolute: 
  

   may, 
  like 
  glass 
  at 
  333° 
  absolute, 
  show 
  very 
  notable 
  conductivity 
  

   at 
  2° 
  absolute 
  : 
  and, 
  like 
  glass 
  at 
  473° 
  absolute 
  as 
  compared 
  

   with 
  glass 
  at 
  333° 
  absolute, 
  may 
  show 
  8000 
  times 
  as 
  much 
  

   electric 
  conductivity 
  at 
  2°*8 
  as 
  at 
  2°. 
  And, 
  like 
  the 
  Nernst 
  

   filament 
  at 
  1800° 
  or 
  2000° 
  absolute, 
  our 
  hypothetical 
  metal 
  

   may 
  at 
  6° 
  absolute 
  show 
  r 
  high 
  conductivity, 
  comparable 
  with 
  

   that 
  of 
  lead 
  or 
  copper 
  at 
  ordinary 
  temperatures. 
  The 
  electric 
  

   conductivity 
  in 
  the 
  Nernst 
  filament 
  goes 
  on 
  increasing 
  as 
  

   the 
  temperature 
  rises 
  till 
  the 
  filament 
  melts 
  or 
  evaporates. 
  

   Nevertheless 
  it 
  is 
  quite 
  conceivable 
  that 
  in 
  our 
  hypothetical 
  

   metal 
  with 
  rising 
  temperature 
  from 
  2° 
  to 
  16° 
  absolute 
  the 
  

   electric 
  conductivity 
  may 
  come 
  to 
  a 
  maximum 
  and 
  decrease 
  

   with 
  further 
  rise 
  of 
  temperature 
  up 
  to 
  and 
  beyond 
  ordinary 
  

   atmospheric 
  temperatures. 
  In 
  fact, 
  while 
  some 
  extent 
  of 
  

   thermal 
  motions 
  is 
  necessary 
  for 
  electric 
  conductivity, 
  too 
  

   much 
  of 
  these 
  motions 
  must 
  mar 
  the 
  freedom 
  with 
  which 
  

   an 
  electrion 
  can 
  thread 
  its 
  way 
  through 
  the 
  crowd 
  of 
  atoms 
  

   to 
  perform 
  the 
  function 
  of 
  electric 
  conduction. 
  It 
  seems 
  

   certain 
  that 
  this 
  is 
  the 
  matter-of-fact 
  explanation 
  of 
  the 
  

   diminution 
  of 
  electric 
  conductivity 
  in 
  metals 
  with 
  rise 
  of 
  

   temperature. 
  

  

  § 
  31. 
  Regretting 
  much 
  not 
  to 
  be 
  able 
  (for 
  want 
  of 
  time) 
  

   to 
  include 
  estimates 
  of 
  absolute 
  magnitudes 
  in 
  the 
  present 
  

   communication, 
  I 
  end 
  it 
  with 
  applications 
  of 
  our 
  hypothesis 
  

   to 
  the 
  pyro-electricity 
  and 
  piezo-electricity 
  of 
  crystals, 
  A 
  

   crystal 
  is 
  a 
  homogeneous 
  assemblage 
  of 
  bodies. 
  Conversely, 
  

   a 
  homogeneous 
  assemblage 
  of 
  bodies 
  is 
  not 
  a 
  crystal 
  if 
  the 
  

   distance 
  between 
  centres 
  of 
  nearest 
  neighbours 
  is 
  a 
  centi- 
  

   metre 
  or 
  more 
  ; 
  it 
  is 
  a 
  crystal 
  if 
  the 
  distance 
  between 
  nearest 
  

   neighbours 
  is 
  10 
  -8 
  of 
  a 
  cm. 
  or 
  less. 
  Pyro-electricity 
  and 
  

   piezo-electricity 
  are 
  developments 
  of 
  vitreous 
  and 
  resinous 
  

   electric 
  forces 
  such 
  as 
  would 
  result 
  from 
  vitreous 
  and 
  resinous 
  

   electrification 
  on 
  different 
  parts 
  of 
  the 
  surface 
  of 
  a 
  crystal, 
  

   produced 
  respectively 
  by 
  change 
  of 
  temperature 
  and 
  by 
  stress 
  

   due 
  to 
  balancing 
  forces 
  applied 
  to 
  the 
  surfaces. 
  

  

  § 
  32. 
  To 
  see 
  how 
  such 
  properties 
  can 
  or 
  must 
  exist 
  in 
  

  

  