﻿Theory 
  of 
  Gravity. 
  667 
  

  

  the 
  velocity 
  un 
  of 
  any 
  point 
  on 
  the 
  surface 
  the 
  equation 
  of 
  

   the 
  surface 
  in 
  the 
  neighbourhood 
  of 
  that 
  point 
  can 
  be 
  written 
  

  

  dx 
  = 
  0, 
  

  

  The 
  axis 
  of 
  x 
  being 
  taken 
  along 
  the 
  normal. 
  

   The 
  conditions 
  at 
  the 
  material 
  surface 
  are 
  now 
  

  

  since 
  referred 
  to 
  the 
  local 
  axes 
  the 
  surface 
  has 
  zero 
  velocity. 
  

   And 
  hence 
  it 
  can 
  be 
  shown 
  that 
  all 
  stresses 
  across 
  the 
  

   surface 
  of 
  matter, 
  forming 
  the 
  second 
  line 
  of 
  (83), 
  vanish. 
  

  

  § 
  8. 
  Positive 
  and 
  Negative 
  Charge, 
  Magneton 
  

   and 
  Electron. 
  

  

  In 
  the 
  paper 
  already 
  referred 
  to 
  I 
  suggested 
  a 
  distinction 
  

   between 
  positive 
  and 
  negative 
  electricity. 
  At 
  that 
  time 
  I 
  

   was 
  content 
  to 
  retain 
  the 
  continuous 
  electric 
  fluid 
  ; 
  this 
  

   gave 
  one 
  type 
  of 
  electricity. 
  But 
  I 
  also 
  contemplated 
  

   the 
  existence 
  of 
  closed 
  surfaces, 
  such 
  as 
  are 
  now 
  taken 
  to 
  be 
  

   the 
  boundaries 
  of 
  matter. 
  The 
  electric 
  induction 
  over 
  such 
  

   a 
  surface 
  is 
  constant. 
  Here, 
  then, 
  we 
  have 
  another 
  type 
  of 
  

   electric 
  charge. 
  

  

  I 
  have 
  abandoned 
  the 
  electric 
  fluid. 
  It 
  might 
  be 
  possible 
  

   to 
  construct 
  a 
  second 
  type 
  of 
  electric 
  charge 
  by 
  making 
  the 
  

   conducting 
  surface 
  merely 
  a 
  cavity 
  in 
  the 
  aether. 
  I 
  prefer 
  

   now 
  to 
  identify 
  provisionally 
  positive 
  and 
  negative 
  charges 
  

   as 
  magnetons 
  and 
  electrons. 
  The 
  theory 
  is 
  simple. 
  Maxwell's 
  

   equations 
  give 
  

  

  d 
  

  

  

  (84) 
  

  

  Here 
  E^E 
  + 
  c" 
  1 
  [uH] 
  and 
  H'^H-c-^uE], 
  

  

  dS 
  is 
  an 
  element 
  of 
  area 
  moving 
  with 
  the 
  velocity 
  u, 
  and 
  dv 
  

   is 
  an 
  element 
  of 
  length 
  of 
  its 
  boundary. 
  (84) 
  shows 
  that 
  over 
  

   any 
  closed 
  surface 
  whatever 
  

  

  is 
  a 
  constant, 
  or 
  the 
  electric 
  induction 
  is 
  a 
  constant. 
  This 
  

   defines 
  the 
  electric 
  charge 
  on 
  the 
  surface. 
  

  

  Now 
  suppose 
  the 
  given 
  surface 
  is 
  multiply 
  connected 
  and 
  

   cZS 
  is 
  an 
  element 
  of 
  area 
  of 
  any 
  barrier 
  drawn 
  across 
  one 
  of 
  

   its 
  apertures, 
  dx 
  is 
  then 
  an 
  element 
  lying 
  iin 
  the 
  original 
  

  

  