﻿1875.] 
  Repulsion 
  resulting 
  from 
  Radiation. 
  377 
  

  

  perfectly 
  exhausted 
  apparatus, 
  viz. 
  it 
  was 
  repelled 
  by 
  heat 
  of 
  low 
  inten- 
  

   sity 
  and 
  attracted 
  by 
  cold. 
  A 
  similar 
  experiment 
  was 
  next 
  tried, 
  only 
  

   water 
  was 
  placed 
  in 
  the 
  bulb 
  before 
  exhaustion. 
  The 
  water 
  was 
  then 
  

   boiled 
  away 
  in 
  vacuo, 
  and 
  the 
  exhaustion 
  continued, 
  with 
  frequent 
  

   heating 
  of 
  the 
  apparatus 
  to 
  dull 
  redness, 
  for 
  about 
  48 
  hours. 
  At 
  

   the 
  end 
  of 
  this 
  time 
  the 
  bar 
  of 
  aluminium 
  was 
  found 
  to 
  behave 
  

   exactly 
  the 
  same 
  as 
  the 
  one 
  in 
  the 
  former 
  experiment, 
  being 
  repelled 
  by 
  

   radiation. 
  

  

  It 
  is 
  impossible 
  to 
  conceive 
  that 
  in 
  these 
  experiments 
  sufficient 
  con- 
  

   densable 
  gas 
  or 
  vapour 
  was 
  present 
  to 
  produce 
  the 
  effects 
  Prof. 
  Osborne 
  

   Reynolds 
  ascribes 
  to 
  it. 
  After 
  the 
  repeated 
  heating 
  to 
  redness 
  at 
  the 
  

   highest 
  attainable 
  exhaustion, 
  it 
  is 
  impossible 
  that 
  sufficient 
  vapour 
  or 
  

   gas 
  should 
  condense 
  on 
  the 
  movable 
  index 
  to 
  be 
  instantly 
  driven 
  off 
  by 
  

   the 
  warmth 
  of 
  the 
  finger 
  with 
  recoil 
  enough 
  to 
  drive 
  backwards 
  a 
  heavy 
  

   piece 
  of 
  metal. 
  

  

  While 
  objecting 
  to 
  the 
  theories 
  already 
  advanced 
  as 
  not 
  accounting 
  

   for 
  all 
  the 
  facts 
  of 
  the 
  case, 
  the 
  author 
  confesses 
  that 
  he 
  is 
  not 
  as 
  yet 
  

   prepared 
  with 
  one 
  to 
  put 
  in 
  their 
  place. 
  He 
  wishes 
  to 
  avoid 
  g^ing 
  any 
  

   theory 
  on 
  the 
  subject 
  until 
  a 
  sufficient 
  number 
  of 
  facts 
  have 
  been 
  accumu- 
  

   lated. 
  The 
  facts 
  will 
  then 
  tell 
  their 
  own 
  tale. 
  The 
  conditions 
  under 
  

   which 
  they 
  invariably 
  occur 
  will 
  give 
  the 
  laws, 
  and 
  the 
  theory 
  will 
  follow 
  

   without 
  much 
  difficulty. 
  

  

  Supplement. 
  Received 
  April 
  20, 
  1875. 
  

  

  Since 
  the 
  experiments 
  mentioned 
  in 
  the 
  foregoing 
  Abstract 
  were 
  con- 
  

   cluded, 
  the 
  author 
  has 
  examined 
  more 
  fully 
  the 
  action 
  of 
  radiation 
  on 
  

   black 
  and 
  white 
  surfaces. 
  At 
  the 
  highest 
  exhaustion 
  heat 
  appears 
  to 
  

   act 
  almost 
  equally 
  on 
  white 
  and 
  on 
  lampblacked 
  pith, 
  repelling 
  them 
  in 
  

   about 
  the 
  same 
  degree. 
  

  

  The 
  action 
  of 
  the 
  luminous 
  rays, 
  however, 
  is 
  different. 
  These 
  repel 
  

   the 
  black 
  surface 
  more 
  energetically 
  than 
  they 
  do 
  the 
  white 
  surface. 
  

   Taking 
  ad-vantage 
  of 
  this 
  fact, 
  the 
  author 
  has 
  constructed 
  an 
  instrument 
  

   which 
  he 
  calls 
  a 
  radiometer. 
  This 
  consists 
  of 
  four 
  arms, 
  suspended 
  on 
  

   a 
  steel 
  point 
  resting 
  on 
  a 
  cup, 
  so 
  that 
  it 
  is 
  capable 
  of 
  revolving 
  hori- 
  

   zontally. 
  To 
  the 
  extremity 
  of 
  each 
  arm 
  is 
  fastened 
  a 
  thin 
  disk 
  of 
  pith, 
  

   lampblacked 
  on 
  one 
  side, 
  the 
  black 
  surfaces 
  facing 
  the 
  same 
  way. 
  The 
  

   whole 
  is 
  enclosed 
  in 
  a 
  glass 
  globe, 
  which 
  is 
  then 
  exhausted 
  to 
  the 
  highest 
  

   attainable 
  point 
  and 
  hermetically 
  sealed. 
  

  

  The 
  author 
  finds 
  that 
  this 
  instrument 
  revolves 
  under 
  the 
  influence 
  of 
  

   radiation, 
  the 
  rapidity 
  of 
  revolution 
  being 
  in 
  proportion 
  to 
  the 
  intensity 
  

   of 
  the 
  incident 
  rays. 
  

  

  Several 
  radiometers, 
  of 
  various 
  constructions 
  as 
  regards 
  details, 
  but 
  all 
  

   depending 
  on 
  the 
  above-named 
  discovery, 
  were 
  exhibited 
  by 
  the 
  author 
  at 
  

   the 
  Soiree 
  of 
  the 
  Royal 
  Society 
  on 
  the 
  7th 
  inst., 
  and 
  numerous 
  experiments 
  

  

  