﻿Radiant 
  Heat 
  by 
  Gases 
  at 
  Varying 
  Pressures. 
  41 
  

  

  It 
  is 
  remarkable 
  that 
  at 
  pressures 
  up 
  to 
  a 
  fe»v 
  miiiioiitlis 
  all 
  

   of 
  the 
  curves 
  are 
  nearly 
  straight 
  lines. 
  This 
  is 
  especially 
  

   noticeable 
  in 
  the 
  small-bulb 
  curves, 
  showing 
  that 
  at 
  these 
  

   small 
  pressures 
  the 
  heat-transmitting 
  power 
  of 
  a 
  gas 
  varies 
  

   directly 
  with 
  its 
  amount. 
  Hence 
  it 
  seems 
  reasonably 
  certain 
  

   that 
  if 
  the 
  very 
  small 
  fraction 
  of 
  a 
  millionth 
  of 
  the 
  gas 
  

   examined, 
  which 
  remained 
  at 
  the 
  end 
  of 
  each 
  experiment, 
  

   could 
  have 
  been 
  entirely 
  removed, 
  the 
  heat- 
  transmitting- 
  

   power 
  of 
  the 
  vacuum 
  would 
  not 
  have 
  been 
  materially 
  dimi- 
  

   nished, 
  It 
  was 
  customary 
  at 
  the 
  end 
  of 
  the 
  experiments 
  with 
  

   each 
  gas 
  to 
  close 
  the 
  gauge 
  permanently 
  when 
  the 
  pressure 
  

   had 
  fallen 
  to 
  a 
  tenth 
  of 
  a 
  millionth 
  or 
  so, 
  and 
  with 
  the 
  capacity 
  

   of 
  the 
  whole 
  apparatus 
  thus 
  reduced 
  run 
  the 
  pump 
  con- 
  

   tinuously 
  from 
  one 
  to 
  two 
  hours. 
  Several 
  sets 
  of 
  observa- 
  

   tions 
  were 
  always 
  made 
  during 
  this 
  extreme 
  exhaustion, 
  and 
  

   while 
  the 
  change 
  in 
  the 
  rate 
  of 
  cooling 
  of 
  the 
  thermometer 
  

   was 
  generally 
  appreciable 
  it 
  was 
  always 
  very 
  small 
  indeed. 
  

   In 
  my 
  earlier 
  experiments 
  I 
  took 
  the 
  greatest 
  care 
  to 
  ensure 
  

   the 
  absence 
  of 
  mercury-vapour 
  in 
  the 
  final 
  vacuum. 
  But 
  the 
  

   presence 
  or 
  absence 
  of 
  mercury-vapour 
  made 
  no 
  difference 
  

   distinguishable 
  from 
  the 
  errors 
  of 
  observation. 
  

  

  Of 
  course 
  the 
  best 
  vacuum 
  producible 
  by 
  a 
  Sprengel-pump 
  

   still 
  contains 
  many 
  thousands 
  of 
  millions 
  of 
  gas-molecules 
  per 
  

   cubic 
  centimetre. 
  This 
  may 
  be 
  regarded 
  as 
  a 
  prodigiously 
  large 
  

   or 
  exceedingly 
  small 
  quantity 
  of 
  gas, 
  according 
  to 
  our 
  point 
  of 
  

   view. 
  While 
  it 
  has 
  no 
  apparent 
  effect 
  on 
  the 
  general 
  heat- 
  

   transmitting 
  capacity 
  of 
  the 
  vacuum, 
  it 
  does 
  seem 
  to 
  interfere 
  

   with 
  or 
  modify 
  some 
  function 
  of 
  the 
  aether. 
  This 
  is 
  the 
  only 
  

   explanation 
  of 
  certain 
  phenomena 
  that 
  I 
  can 
  offer. 
  I 
  refer 
  

   to 
  the 
  different 
  behaviour 
  of 
  the 
  vacua 
  with 
  different 
  residual 
  

   gases, 
  and 
  in 
  different- 
  sized 
  bulbs, 
  in 
  the 
  matter 
  of 
  adherence 
  

   to, 
  or 
  departure 
  from, 
  Newton's 
  simple 
  law 
  of 
  cooling. 
  The 
  

   curves 
  a,aa,aaa 
  illustrate 
  these 
  differences 
  in 
  the 
  several 
  

   cases 
  at 
  the 
  extreme 
  end 
  of 
  section 
  C 
  of 
  the 
  principal 
  curves. 
  

   These 
  differences 
  are 
  too 
  large 
  to 
  be 
  attributed 
  to 
  errors 
  of 
  

   observation. 
  This 
  is 
  one 
  of 
  several 
  reasons 
  which 
  lead 
  me 
  to 
  

   suspect 
  that 
  at 
  higher 
  pressures 
  all 
  the 
  gases 
  examined 
  inter- 
  

   fere 
  materially 
  with 
  and 
  retard 
  the 
  transmission 
  of 
  heat 
  by 
  

   the 
  sether. 
  In 
  other 
  words, 
  I 
  suspect 
  that 
  the 
  dotted 
  aether 
  

   line 
  of 
  my 
  curve 
  sheets 
  should 
  not 
  be 
  drawn 
  parallel 
  with 
  the 
  

   base, 
  and 
  have 
  a 
  constant 
  value 
  at 
  all 
  gaseous 
  pressures 
  as 
  

   shown, 
  but 
  should 
  have 
  a 
  decreasing 
  value 
  as 
  the 
  gas 
  pressure 
  

   rises 
  from 
  zero. 
  On 
  this 
  interesting 
  phase 
  of 
  my 
  subject 
  I 
  

   hope 
  to 
  have 
  more 
  to 
  say 
  at 
  a 
  future 
  date. 
  

  

  Before 
  closing 
  I 
  will 
  call 
  attention 
  to 
  two 
  more 
  curve 
  

   sheets. 
  PI. 
  IX. 
  is 
  an 
  air-curve 
  plotted 
  from 
  figures 
  given 
  in 
  

   Dulong 
  and 
  Petit's 
  paper. 
  It 
  is 
  drawn 
  to 
  such 
  a 
  scale 
  that 
  

  

  