﻿374 
  Mr. 
  W. 
  Crookes 
  on 
  Attraction 
  and 
  [Apr. 
  22, 
  

  

  piece 
  of 
  ice 
  as 
  a 
  needle 
  follows 
  a 
  magnet. 
  With 
  a 
  large 
  bulb, 
  very 
  well 
  

   exhausted 
  and 
  containing 
  a 
  suspended 
  bar 
  of 
  pith, 
  a 
  somewhat 
  striking 
  

   effect 
  is 
  produced 
  when 
  a 
  lighted 
  candle 
  is 
  placed 
  about 
  2 
  inches 
  from 
  the 
  

   globe. 
  The 
  pith 
  bar 
  commences 
  to 
  oscillate 
  to 
  and 
  fro, 
  the 
  swing 
  gradually 
  

   increasing 
  in 
  amplitude 
  until 
  the 
  dead 
  centre 
  is 
  passed 
  over, 
  when 
  

   several 
  complete 
  revolutions 
  are 
  made. 
  The 
  torsion 
  of 
  the 
  suspending 
  

   fibre 
  now 
  offers 
  resistance 
  to 
  the 
  revolutions, 
  and 
  the 
  bar 
  commences 
  to 
  

   turn 
  in 
  the 
  opposite 
  direction. 
  This 
  movement 
  is 
  kept 
  up 
  with 
  great 
  

   energy 
  and 
  regularity 
  as 
  long 
  as 
  the 
  candle 
  burns. 
  

  

  The 
  author 
  discusses 
  the 
  action 
  of 
  ice, 
  or 
  a 
  cold 
  substance, 
  on 
  the 
  sus- 
  

   pended 
  index. 
  Cold 
  being 
  simply 
  negative 
  heat, 
  it 
  is 
  not 
  at 
  first 
  sight 
  

   obvious 
  how 
  it 
  can 
  produce 
  the 
  opposite 
  effect 
  to 
  heat. 
  The 
  author, 
  

   however, 
  explains 
  this 
  by 
  the 
  law 
  of 
  exchanges, 
  and 
  shows 
  that 
  attraction 
  

   by 
  a 
  cold 
  body 
  is 
  really 
  repulsion 
  by 
  radiation 
  falling 
  on 
  the 
  opposite 
  

   side. 
  According 
  to 
  the 
  same 
  law 
  r 
  , 
  it 
  is 
  not 
  difficult 
  to 
  foresee 
  what 
  

   will 
  be 
  the 
  action 
  of 
  two 
  bodies, 
  each 
  free 
  to 
  move, 
  if 
  they 
  are 
  

   brought 
  near 
  to 
  one 
  another 
  in 
  space, 
  and 
  if 
  they 
  differ 
  in 
  temperature 
  

   either 
  from 
  each 
  other 
  or 
  from 
  the 
  limiting 
  walls 
  of 
  the 
  space. 
  The 
  

   author 
  gives 
  four 
  typical 
  cases, 
  with 
  experiments, 
  which 
  prove 
  his 
  rea- 
  

   soning 
  to 
  be 
  correct. 
  

  

  Experiments 
  are 
  described 
  with 
  the 
  object 
  of 
  ascertaining 
  whether 
  the 
  

   attraction 
  by 
  heat, 
  which, 
  commencing 
  at 
  the 
  neutral 
  point, 
  increases 
  

   with 
  the 
  density 
  of 
  the 
  enclosed 
  air, 
  will 
  be 
  continued 
  in 
  the 
  same 
  ratio 
  

   if 
  the 
  apparatus 
  is 
  filled 
  with 
  air 
  above 
  the 
  atmospheric 
  pressure. 
  This 
  

   is 
  found 
  to 
  be 
  the 
  case. 
  

  

  Various 
  experiments 
  are 
  described 
  with 
  bulb-apparatus, 
  in 
  which 
  the 
  

   bulb 
  is 
  surrounded 
  with 
  a 
  shell 
  containing 
  various 
  adiathermous 
  liquids 
  

   and 
  also 
  with 
  a 
  shell 
  of 
  vacuum. 
  In 
  all 
  cases 
  radiation 
  passed 
  through, 
  

   producing 
  the 
  normal 
  action 
  of 
  attraction 
  in 
  air 
  and 
  repulsion 
  in 
  a 
  

   vacuum. 
  

  

  The 
  author 
  next 
  describes 
  a 
  form 
  of 
  apparatus 
  by 
  which 
  measurable 
  

   results 
  are 
  attainable. 
  It 
  consists 
  of 
  a 
  long 
  glass 
  tube, 
  with 
  a 
  wider 
  

   piece 
  at 
  the 
  end. 
  In 
  it 
  is 
  suspended 
  a 
  lump 
  of 
  magnesium 
  by 
  a 
  very 
  fine 
  

   platinum 
  wire, 
  the 
  distance 
  between 
  the 
  point 
  of 
  suspension 
  and 
  the 
  

   centre 
  of 
  gravity 
  of 
  the 
  magnesium 
  bob 
  being 
  39*14 
  inches. 
  Near 
  the 
  

   magnesium 
  is 
  a 
  platinum 
  spiral, 
  capable 
  of 
  being 
  ignited 
  by 
  a 
  voltaic 
  

   battery. 
  Observations 
  of 
  the 
  movement 
  of 
  the 
  pendulum 
  are 
  made 
  

   with 
  a 
  telescope 
  with 
  micrometer 
  eyepiece. 
  With 
  this 
  apparatus 
  a 
  large 
  

   series 
  of 
  experiments 
  are 
  described, 
  starting 
  from 
  air 
  of 
  normal 
  density, 
  

   and 
  working 
  at 
  intermediate 
  pressures 
  up 
  to 
  the 
  best 
  attainable 
  vacuum. 
  

   The 
  results 
  are 
  given 
  in 
  two 
  tables. 
  

  

  With 
  this 
  apparatus 
  it 
  was 
  found 
  that 
  a 
  candle-flame 
  brought 
  within 
  

   a 
  few 
  inches 
  of 
  the 
  magnesium 
  weight, 
  or 
  its 
  image 
  focused 
  on 
  the 
  weight 
  

   and 
  alternately 
  obscured 
  and 
  exposed 
  by 
  a 
  piece 
  of 
  card 
  at 
  intervals 
  of 
  

   one 
  second, 
  will 
  soon 
  set 
  the 
  pendulum 
  in 
  vibration 
  when 
  the 
  vacuum 
  is 
  

  

  