﻿432 
  Mr. 
  A. 
  P. 
  Wills 
  on 
  the 
  Susceptibility 
  of 
  

  

  § 
  84. 
  Experiments 
  were 
  made 
  to 
  make 
  sure 
  that 
  the 
  result 
  

   was 
  not 
  due 
  to 
  temporary 
  electrification 
  of 
  the 
  glass. 
  For 
  

   instance 
  I 
  breathed 
  on 
  the 
  glass, 
  causing 
  a 
  conducting 
  layer 
  

   of 
  impure 
  water 
  to 
  form 
  on 
  its 
  surface. 
  Repeated 
  measure- 
  

   ments, 
  made 
  by 
  the 
  usual 
  method, 
  showed 
  that 
  the 
  potential 
  

   at 
  once 
  fell 
  to 
  a 
  small 
  fraction 
  of 
  a 
  volt, 
  but 
  slowly 
  rose 
  again 
  

   to 
  nearly 
  its 
  original 
  value 
  as 
  the 
  film 
  evaporated. 
  This 
  shows 
  

   that 
  the 
  electrification 
  was 
  not 
  a 
  temporary 
  one 
  of 
  the 
  glass 
  

   surface, 
  for 
  that 
  would 
  not 
  have 
  returned 
  to 
  a 
  definite 
  value. 
  

   Heating 
  the 
  plate 
  by 
  radiation 
  or 
  washing 
  the 
  glass 
  with 
  

   benzol 
  caused 
  the 
  potential 
  to 
  rise, 
  further, 
  but 
  in 
  no 
  case 
  was 
  

   the 
  potential 
  quite 
  so 
  high 
  as 
  when 
  the 
  plate 
  was 
  first 
  formed. 
  

   An 
  even 
  more 
  convincing 
  proof 
  that 
  the 
  potential 
  measured 
  

   was 
  really 
  that 
  of 
  the 
  sodium 
  was 
  found 
  in 
  the 
  fact 
  that 
  the 
  

   sensibility 
  of 
  the 
  apparatus 
  was 
  such 
  as 
  would 
  be 
  given 
  by 
  a 
  

   plate 
  the 
  size 
  of 
  the 
  sodium. 
  If 
  the 
  electrification 
  had 
  been 
  

   on 
  the 
  whole 
  surface 
  of 
  the 
  glass, 
  the 
  sensibility, 
  on 
  account 
  

   of 
  the 
  larger 
  surface, 
  would 
  have 
  b^en 
  at 
  least 
  ten 
  times 
  as 
  

   great 
  as 
  that 
  obs 
  n 
  rved. 
  

  

  § 
  85. 
  The 
  experiments 
  described 
  in 
  this 
  chapter 
  show 
  that 
  

   (i.) 
  when 
  two 
  metals 
  are 
  coated 
  with 
  the 
  same 
  non-conductor, 
  

   such 
  as 
  wax 
  or 
  glass, 
  their 
  potential 
  is 
  not 
  sensibly 
  different 
  

   from 
  that 
  of 
  the 
  bare 
  metals 
  in 
  air 
  ; 
  (ii.) 
  that 
  temperature- 
  

   variation 
  still 
  takes 
  place, 
  though 
  air 
  be 
  excluded. 
  These 
  

   results 
  seem 
  to 
  prove 
  that 
  gaseous 
  films 
  play 
  no 
  essential 
  

   part 
  in 
  the 
  phenomenon. 
  

  

  XLIV. 
  On 
  the 
  Susceptibility 
  of 
  Diamagnetic 
  and 
  Weakly 
  

   Magnetic 
  Substances. 
  By 
  Albert 
  P. 
  Wills*. 
  

  

  T^ARADAY 
  (1845) 
  showed 
  that 
  all 
  substances, 
  whether 
  

   -L 
  solid, 
  liquid, 
  or 
  gaseous, 
  are 
  either 
  diamagnetic 
  or 
  

   magnetic. 
  Previously, 
  however, 
  Brugmans 
  (1778) 
  and 
  

   Becquerel 
  (1827) 
  observed 
  certain 
  diamagnetic 
  phenomena. 
  

  

  By 
  way 
  of 
  relative 
  measurement 
  of 
  the 
  coefficient 
  of 
  

   susceptibility, 
  a 
  great 
  deal 
  has 
  been 
  done 
  by 
  various 
  

   physicists, 
  among 
  them 
  Pliicker 
  and 
  Faraday. 
  For 
  bismuth 
  

   there 
  have 
  been 
  a 
  number 
  of 
  determinations 
  in 
  absolute 
  

   measure. 
  It 
  will 
  suffice 
  to 
  mention 
  here 
  four 
  of 
  the 
  best 
  

   methods 
  used 
  : 
  — 
  1. 
  The 
  method 
  used 
  by 
  Christie 
  f 
  (1858) 
  

   depends 
  upon 
  the 
  comparison 
  of 
  the 
  inductive 
  action 
  of 
  the 
  

   magnetized 
  bismuth 
  with 
  that 
  of 
  a 
  solenoid 
  through 
  which 
  a 
  

   known 
  current 
  is 
  flowing. 
  2. 
  The 
  method 
  used 
  by 
  Topler 
  

  

  * 
  Communicated 
  by 
  Prof. 
  A. 
  G. 
  Webster. 
  

   ■f 
  Pog-g. 
  Ann. 
  ciii. 
  p. 
  577 
  (1858). 
  

  

  