﻿376 
  

  

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

  

  glass 
  cell 
  containing 
  8 
  millims. 
  of 
  water 
  ; 
  a 
  plate 
  of 
  alum 
  5 
  millims. 
  thick; 
  

   calc-spar, 
  27 
  millims. 
  thick 
  ; 
  ammonio-sulphate 
  of 
  copper, 
  opaque 
  to 
  rays 
  

   below 
  E 
  ; 
  ditto, 
  opaque 
  to 
  rays 
  below 
  Gr. 
  

  

  The 
  author 
  considers 
  that 
  these 
  experiments 
  show 
  that 
  the 
  repulsion 
  

   is 
  not 
  entirely 
  due 
  to 
  the 
  rays 
  usually 
  called 
  heat, 
  i. 
  e. 
  to 
  the 
  extreme 
  

   and 
  ultra 
  red 
  of 
  the 
  spectrum. 
  Experiments 
  have 
  been 
  tried 
  with 
  the 
  

   electric 
  and 
  the 
  solar 
  spectrum 
  formed 
  with 
  a 
  quartz 
  train, 
  which 
  prove 
  

   the 
  action 
  to 
  be 
  also 
  exerted 
  by 
  the 
  luminous 
  and 
  ultra 
  violet 
  rays. 
  Some 
  

   numerical 
  data 
  have 
  been 
  obtained 
  ; 
  but 
  unfavourable 
  weather 
  has 
  pre- 
  

   vented 
  many 
  observations 
  being 
  made 
  with 
  the 
  solar 
  spectrum. 
  

  

  The 
  barometric 
  position 
  of 
  the 
  neutral 
  point 
  dividing 
  attraction 
  from 
  

   repulsion 
  is 
  next 
  discussed. 
  The 
  position 
  of 
  this 
  point 
  varies 
  with 
  the 
  

   density 
  of 
  the 
  substance 
  on 
  which 
  radiation 
  falls, 
  the 
  ratio 
  of 
  its 
  mass 
  to 
  

   its 
  surface, 
  its 
  radiating 
  and 
  conducting-power 
  for 
  heat, 
  the 
  physical 
  con- 
  

   dition 
  of 
  its 
  surface, 
  the 
  kind 
  of 
  gas 
  filling 
  the 
  apparatus, 
  the 
  intensity 
  

   of 
  radiation, 
  and 
  the 
  temperature 
  of 
  the 
  surrounding 
  atmosphere. 
  The 
  

   author 
  is 
  inclined 
  to 
  believe 
  that 
  the 
  true 
  action 
  of 
  radiation 
  is 
  repulsion 
  

   at 
  any 
  pressure, 
  and 
  that 
  the 
  attraction 
  observed 
  when 
  the 
  rarefaction 
  is 
  

   below 
  the 
  neutral 
  point 
  is 
  caused 
  by 
  some 
  modifying 
  circumstances 
  

   connected 
  with 
  the 
  surrounding 
  gas, 
  but 
  not 
  being 
  of 
  the 
  nature 
  of 
  air- 
  

   currents. 
  

  

  The 
  neutral 
  point 
  for 
  a 
  thin 
  surface 
  of 
  pith 
  being 
  low, 
  and 
  that 
  for 
  

   a 
  moderately 
  thick 
  piece 
  of 
  platinum 
  being 
  high, 
  it 
  follows 
  that 
  at 
  a 
  rare- 
  

   faction 
  intermediate 
  between 
  these 
  two 
  points 
  pith 
  will 
  be 
  repelled, 
  

   and 
  that 
  platinum 
  will 
  be 
  attracted 
  by 
  the 
  same 
  beam 
  of 
  radiation. 
  

   This 
  is 
  proved 
  experimentally 
  ; 
  and 
  an 
  apparatus 
  showing 
  simultaneous 
  

   attraction 
  and 
  repulsion 
  by 
  the 
  same 
  ray 
  of 
  light 
  is 
  described 
  and 
  illus- 
  

   trated 
  in 
  the 
  paper. 
  

  

  The 
  paper 
  concludes 
  with 
  a 
  discussion 
  of 
  the 
  various 
  theories 
  which 
  

   have 
  been 
  adduced 
  in 
  explanation 
  of 
  these 
  phenomena. 
  The 
  air-current 
  

   and 
  electrical 
  theory 
  are 
  considered 
  to 
  have 
  been 
  abundantly 
  disproved. 
  

   The 
  following 
  experiment 
  is 
  given 
  by 
  the 
  author 
  to 
  show 
  that 
  Prof. 
  

   Osborne 
  Reynolds's 
  hypothesis 
  of 
  the 
  movements 
  due 
  to 
  evapora- 
  

   tion 
  and 
  condensation 
  at 
  the 
  surface 
  will 
  not 
  account 
  for 
  all 
  the 
  facts 
  of 
  

   the 
  case, 
  and 
  that 
  therefore 
  he 
  has 
  not 
  hit 
  upon 
  the 
  true 
  explanation. 
  

   A 
  thick 
  and 
  strong 
  bulb 
  was 
  blown 
  at 
  the 
  end 
  of 
  a 
  piece 
  of 
  very 
  diffi- 
  

   cultly 
  fusible 
  green 
  glass, 
  specially 
  made 
  for 
  steam-boiler 
  gauges. 
  In 
  it 
  

   was 
  supported 
  a 
  thin 
  bar 
  of 
  aluminium 
  at 
  the 
  end 
  of 
  a 
  long 
  platinum 
  

   wire. 
  The 
  upper 
  end 
  of 
  the 
  wire 
  was 
  passed 
  through 
  the 
  top 
  of 
  the 
  tube 
  

   and 
  well 
  sealed 
  in, 
  for 
  electrical 
  purposes. 
  The 
  apparatus 
  was 
  sealed 
  by 
  

   fusion 
  to 
  the 
  Sprengel 
  pump, 
  and 
  exhaustion 
  was 
  kept 
  going 
  on 
  for 
  two 
  

   days, 
  until 
  an 
  induction-spark 
  refused 
  to 
  pass 
  across 
  the 
  vacuum. 
  During 
  

   this 
  time 
  the 
  bulb 
  and 
  its 
  contents 
  were 
  several 
  times 
  raised 
  to 
  a 
  dull 
  red 
  

   heat. 
  At 
  the 
  end 
  of 
  two 
  days' 
  exhaustion 
  the 
  tube 
  was 
  found 
  to 
  behave 
  

   in 
  the 
  same 
  manner 
  as, 
  but 
  in 
  a 
  stronger 
  degree 
  than, 
  it 
  would 
  in 
  a 
  less 
  

  

  