﻿Radiant 
  Heat 
  by 
  Gases 
  at 
  Varying 
  Pressures, 
  33 
  

  

  millimetres 
  of 
  gas 
  from 
  that 
  of 
  the 
  rest. 
  In 
  this 
  way 
  they 
  

   arrived 
  at 
  the 
  following 
  u 
  Sixth 
  Law 
  : 
  " 
  — 
  

  

  " 
  The 
  cooling 
  power 
  of 
  a 
  fluid 
  diminishes 
  in 
  a 
  geometrical 
  

   progression 
  when 
  its 
  tension 
  itself 
  diminishes 
  in 
  a 
  geometrical 
  

   progression. 
  If 
  the 
  ratio 
  of 
  this 
  second 
  progression 
  is 
  2, 
  the 
  

   ratio 
  of 
  the 
  first 
  is 
  1'366 
  for 
  air 
  ; 
  1*301 
  for 
  hydrogen 
  ; 
  1*431 
  

   for 
  carbonic 
  acid 
  ; 
  and 
  1*415 
  for 
  olefiant 
  gas.'* 
  

  

  My 
  own 
  observations 
  show 
  that 
  this 
  law 
  can 
  be 
  approxi- 
  

   mately 
  true 
  only 
  in 
  the 
  case 
  of 
  a 
  large 
  balloon 
  ; 
  and 
  at 
  

   pressures 
  from 
  a 
  few 
  millimetres 
  upward. 
  There 
  is 
  no 
  

   suggestion 
  of 
  it 
  when 
  a 
  small 
  balloon 
  is 
  used 
  ; 
  and 
  at 
  small 
  

   pressures 
  it 
  does 
  not 
  obtain 
  with 
  either 
  large 
  or 
  small 
  balloons. 
  

  

  I 
  find 
  that 
  in 
  a 
  small 
  balloon 
  the 
  cooling 
  effect 
  of 
  the 
  last 
  

   millimetre 
  of 
  air 
  is 
  nearly 
  ten 
  times 
  as 
  great 
  as 
  that 
  of 
  all 
  the 
  

   rest, 
  up 
  to 
  atmospheric 
  pressure, 
  combined. 
  

  

  It 
  was 
  through 
  misplaced 
  confidence 
  in 
  their 
  Sixth 
  Law 
  

   that 
  Dulong 
  and 
  Petit 
  were 
  led 
  to 
  place 
  a 
  value 
  on 
  the 
  rate 
  

   or 
  velocity 
  of 
  cooling 
  in 
  vacuo, 
  something 
  like 
  a 
  hundred 
  per 
  

   cent, 
  too 
  high, 
  as 
  I 
  shall 
  show 
  later 
  ; 
  and 
  as 
  they 
  derived 
  

   the 
  cooling 
  values 
  of 
  gases 
  by 
  deducting 
  the 
  cooling 
  effect 
  

   of 
  a 
  vacuum 
  from 
  the 
  total 
  cooling 
  observed, 
  all 
  their 
  values 
  

   for 
  gases 
  are 
  much 
  too 
  low. 
  These 
  large 
  errors 
  vitiate 
  

   much 
  of 
  their 
  otherwise 
  excellent 
  work, 
  and 
  render 
  the 
  

   numerical 
  values 
  of 
  the 
  ratios 
  given 
  in 
  the 
  Second 
  and 
  Third 
  

   Laws 
  extremely 
  doubtful. 
  

  

  Other 
  experimentalists 
  also 
  have 
  studied 
  the 
  transfer 
  of 
  

   heat 
  by 
  air 
  and 
  other 
  gases 
  at 
  various 
  pressures. 
  Kundt 
  and 
  

   Warburg 
  (Pogg. 
  Ann. 
  1874-5) 
  and 
  Winkelmann 
  (Pogg. 
  

   Ann. 
  1875-6) 
  observed 
  that 
  the 
  rate 
  of 
  heat 
  transmission 
  

   remained 
  substantially 
  constant 
  through 
  a 
  long 
  range 
  of 
  

   diminishing 
  pressure; 
  and 
  then 
  decreased 
  with 
  further 
  ex- 
  

   haustion. 
  But 
  as 
  they 
  made 
  no 
  measurements 
  of 
  pressure 
  

   below 
  one 
  millimetre 
  (1316 
  millionths 
  of 
  atmospheric 
  pres- 
  

   sure), 
  their 
  results 
  have 
  no 
  quantitative 
  value 
  for 
  low 
  pressures. 
  

  

  Crookes, 
  in 
  his 
  paper 
  " 
  On 
  Heat 
  Conduction 
  in 
  Highly 
  

   Rarefied 
  Air 
  (Proc. 
  Hoy. 
  See. 
  1880), 
  described 
  a 
  similar 
  

   experiment 
  in 
  which 
  he 
  carried 
  the 
  pressure 
  measurements 
  as 
  

   low 
  as 
  2JV1. 
  (two 
  millionths). 
  From 
  the 
  fall 
  in 
  the 
  rate 
  of 
  

   heat 
  loss 
  which 
  occurred 
  between 
  the 
  pressures 
  of 
  760 
  milli- 
  

   metres 
  and 
  1 
  millimetre, 
  and 
  5 
  M. 
  and 
  2 
  M. 
  ? 
  he 
  concludes 
  : 
  

   •• 
  We 
  may 
  legitimately 
  infer 
  that 
  each 
  additional 
  diminution 
  

   of 
  a 
  millionth 
  would 
  produce 
  a 
  still 
  greater 
  retardation 
  of 
  

   cooling, 
  so 
  that 
  in 
  such 
  high 
  vacua 
  as 
  exist 
  in 
  planetary 
  space 
  

   the 
  loss 
  of 
  heat 
  — 
  which 
  in 
  that 
  case 
  would 
  only 
  take 
  place 
  by 
  

   radiation 
  — 
  would 
  be 
  exceedingly 
  slow." 
  

  

  Phil. 
  Mag. 
  S. 
  5. 
  Vol. 
  45. 
  No. 
  272. 
  Jan. 
  1898. 
  D 
  

  

  