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  XL 
  On 
  the 
  Necessity 
  for 
  Postulating 
  an 
  Mthev. 
  

   By 
  B. 
  Hopkinson 
  *. 
  

  

  THE 
  meeting 
  of 
  the 
  British 
  Association 
  generally 
  brings 
  

   into 
  prominence 
  the 
  more 
  controversial 
  parts 
  of 
  science, 
  

   and 
  that 
  just 
  concluded 
  was 
  no 
  exception 
  to 
  the 
  rule. 
  

   Professor 
  Riicker's 
  eloquent 
  defence 
  of 
  the 
  atomic 
  theory 
  

   was 
  perhaps 
  even 
  more 
  striking 
  as 
  an 
  acknowledgment 
  of 
  a 
  

   strong 
  body 
  of 
  opinion 
  for 
  which 
  atoms 
  are 
  by 
  no 
  means 
  

   physical 
  realities 
  ; 
  and 
  though 
  he 
  did 
  not 
  deal 
  with 
  it 
  in 
  any 
  

   detail, 
  the 
  hypothesis 
  of 
  an 
  aether 
  was 
  mentioned 
  along 
  with 
  

   that 
  which 
  was 
  more 
  immediately 
  the 
  subject 
  of 
  the 
  address, 
  

   as 
  a 
  matter 
  regarding 
  which 
  doubts 
  were 
  rife. 
  In 
  fact 
  

   there 
  are 
  representatives 
  of 
  every 
  shade 
  of 
  opinion 
  as 
  to 
  the 
  

   pether, 
  from 
  those 
  who 
  regard 
  it 
  as 
  a 
  wholly 
  unnecessary 
  

   conception, 
  to 
  those 
  who 
  wish 
  to 
  invest 
  it 
  with 
  every 
  material 
  

   property 
  that 
  it 
  will 
  support. 
  

  

  It 
  is 
  not 
  my 
  intention 
  to 
  enter 
  into 
  the 
  merits 
  of 
  this 
  

   controversy, 
  but 
  I 
  wish 
  to 
  discuss 
  shortly 
  the 
  nature 
  of 
  the 
  

   difference 
  between 
  the 
  parties 
  to 
  it. 
  How 
  far 
  is 
  the 
  postulate 
  

   of 
  an 
  sether 
  founded 
  on 
  metaphysical 
  grounds, 
  and 
  therefore 
  

   matter 
  of 
  dispute? 
  how 
  far 
  has 
  it 
  what 
  may 
  be 
  called 
  a 
  

   logical 
  basis, 
  such 
  as 
  all 
  must 
  admit 
  ? 
  

  

  To 
  illustrate 
  what 
  is 
  meant 
  by 
  these 
  somewhat 
  vague 
  

   terms, 
  take 
  the 
  phenomenon 
  of 
  gravitation. 
  All 
  we 
  know 
  of 
  

   gravitation 
  is 
  that 
  if 
  there 
  is 
  matter 
  at 
  a 
  place 
  A, 
  then 
  

   another 
  piece 
  of 
  matter 
  at 
  P 
  distant 
  from 
  A 
  will 
  experience 
  

   a 
  force 
  of 
  attraction 
  towards 
  A. 
  We 
  may 
  further 
  analyse 
  

   and 
  express 
  the 
  phenomenon 
  by 
  attributing 
  to 
  every 
  point 
  of 
  

   space 
  a 
  force, 
  depending 
  according 
  to 
  known 
  laws 
  on 
  the 
  

   nature 
  of* 
  A, 
  and 
  the 
  position 
  of 
  the 
  point 
  relative 
  thereto, 
  

   which 
  force, 
  however, 
  can 
  only 
  be 
  made 
  manifest 
  to 
  the 
  senses 
  

   by 
  the 
  use 
  of 
  matter 
  as 
  a 
  detector. 
  Any 
  further 
  analysis 
  

   than 
  this 
  leads 
  us 
  into 
  the 
  realms 
  of 
  speculation, 
  and 
  difference 
  

   of 
  opinion, 
  hitherto 
  out 
  of 
  the 
  question, 
  becomes 
  possible. 
  

   Thus 
  Newton 
  said 
  that 
  one 
  who 
  believed 
  in 
  the 
  possibility 
  of 
  

   attraction 
  between 
  distant 
  bodies 
  without 
  an 
  intervening 
  

   medium 
  must 
  be 
  a 
  fool 
  ; 
  but 
  J. 
  S. 
  Mill 
  cheerfully 
  avowed 
  his 
  

   sympathy 
  with 
  the 
  fools. 
  In 
  fact 
  the 
  question 
  of 
  medium 
  or 
  

   no 
  medium 
  so 
  far 
  as 
  gravitation 
  is 
  concerned 
  may 
  be 
  called 
  

   purely 
  metaphysical 
  ; 
  it 
  is 
  similar 
  in 
  kind 
  to 
  the 
  question 
  

   whether 
  the 
  gravitational 
  force 
  exerted 
  by 
  A 
  has 
  any 
  

   existence 
  when 
  there 
  is 
  no 
  other 
  body 
  to 
  be 
  attracted. 
  Those 
  

  

  * 
  This 
  paper 
  is 
  an 
  expansion 
  of 
  some 
  observations 
  communicated 
  to 
  

   the 
  British 
  Association 
  at 
  Glasgow, 
  1901. 
  Communicated 
  by 
  the 
  

   Author. 
  

  

  