﻿Radiant 
  Heat 
  by 
  Gases 
  at 
  Varying 
  Pressures. 
  37 
  

  

  of 
  B, 
  it 
  would 
  be 
  50 
  metres 
  long 
  ; 
  and 
  if 
  completed 
  on 
  the 
  

   scale 
  of 
  C, 
  it 
  would 
  be 
  five 
  kilometres 
  long. 
  This 
  magnih'ca- 
  

   tion 
  of 
  the 
  abscissae 
  without 
  change 
  of 
  the 
  ordinates 
  enables 
  

   us 
  to 
  study 
  every 
  part 
  of 
  the 
  curve 
  with 
  ease. 
  The 
  small 
  

   circles 
  represent 
  the 
  points 
  in 
  the 
  curve 
  established 
  by 
  ob- 
  

   servation. 
  These 
  points 
  are 
  shown 
  exactly 
  as 
  found, 
  without 
  

   any 
  attempt 
  to 
  smooth 
  out 
  rough 
  places 
  in 
  the 
  curve. 
  The 
  

   same 
  is 
  true 
  of 
  the 
  curves 
  of 
  other 
  gases. 
  The 
  heavy 
  dotted 
  

   line 
  parallel 
  with 
  the 
  base 
  represents 
  that 
  portion 
  of 
  the 
  total 
  

   heat 
  transmission 
  due 
  to 
  the 
  aether 
  ; 
  while 
  all 
  above 
  it 
  repre- 
  

   sents 
  that 
  due 
  to 
  the 
  air. 
  

  

  Starting 
  at 
  the 
  left-hand 
  end 
  of 
  section 
  A, 
  representing 
  

   the 
  rate 
  of 
  heat 
  transmission 
  at 
  atmospheric 
  pressure, 
  we 
  

   observe 
  that 
  the 
  curve 
  drops 
  regularly 
  at 
  a 
  rate 
  faster 
  than 
  

   the 
  diminution 
  of 
  pressure, 
  during 
  95 
  per 
  cent, 
  of 
  the 
  whole 
  

   range 
  of 
  pressure 
  from 
  atmospheric 
  to 
  zero. 
  Beyond 
  this 
  

   point 
  the 
  rate 
  of 
  heat 
  transmission 
  remains 
  substantially 
  

   constant, 
  as 
  shown 
  by 
  section 
  B 
  and 
  the 
  latter 
  part 
  of 
  A, 
  

   down 
  to 
  a 
  pressure 
  of 
  about 
  "0003 
  — 
  a 
  range 
  of 
  nearly 
  ninety- 
  

   nine 
  and 
  a 
  half 
  per 
  cent, 
  of 
  that 
  remaining. 
  Here 
  the 
  curve 
  

   suddenly 
  begins 
  to 
  drop 
  again, 
  and 
  falls 
  steadily, 
  as 
  shown 
  

   by 
  section 
  C 
  and 
  the 
  latter 
  part 
  of 
  B, 
  until 
  it 
  meets 
  the 
  aether 
  

   line 
  at 
  the 
  zero 
  of 
  pressure. 
  

  

  Under 
  the 
  curve 
  A 
  I 
  have 
  drawn 
  curves 
  with 
  finer 
  lines 
  

   representing 
  the 
  rate 
  of 
  heat 
  transmission 
  at 
  smaller 
  differ- 
  

   ences 
  of 
  temperature 
  between 
  the 
  thermometer 
  and 
  ice-bath. 
  

   As 
  before 
  stated, 
  A 
  represents 
  the 
  cooling 
  from 
  15 
  degrees 
  to 
  

   10 
  degrees. 
  On 
  the 
  same 
  scale 
  a 
  represents 
  the 
  cooling 
  

   from 
  9 
  degrees 
  to 
  6 
  degrees; 
  a 
  a 
  from 
  6 
  degrees 
  to 
  4 
  degrees; 
  

   and 
  aaa 
  from 
  3 
  degrees 
  to 
  2 
  degrees. 
  Now, 
  Newton's 
  law 
  

   of 
  cooling 
  requires 
  that 
  the 
  rate 
  shall 
  vary 
  directly 
  with 
  the 
  

   difference 
  of 
  temperature 
  between 
  the 
  cooling 
  body 
  and 
  the 
  

   surrounding 
  medium. 
  While 
  this 
  law 
  is 
  known 
  to 
  be 
  incor- 
  

   rect 
  for 
  large 
  differences 
  of 
  temperature, 
  it 
  is 
  generally 
  

   accepted 
  for 
  very 
  small 
  differences. 
  If 
  it 
  were 
  correct 
  under 
  

   the 
  conditions 
  of 
  the 
  present 
  experiment, 
  then 
  the 
  ratios 
  of 
  

   the 
  times 
  required 
  for 
  the 
  temperature 
  to 
  fall 
  through 
  the 
  

   several 
  ranges 
  above 
  indicated, 
  would 
  all 
  equal 
  unity, 
  and 
  the 
  

   curves 
  A, 
  a, 
  a 
  a, 
  aaa 
  would 
  coalesce. 
  But 
  they 
  are 
  very 
  

   far 
  from 
  doing 
  this. 
  It 
  will 
  be 
  observed 
  that 
  all 
  of 
  these 
  

   curves 
  preserve 
  their 
  relative 
  values 
  very 
  closely 
  indeed, 
  

   until 
  they 
  approach 
  the 
  point 
  of 
  pressure 
  where 
  the 
  curve 
  A 
  

   reverses 
  itself 
  ; 
  then 
  they 
  begin 
  to 
  bunch 
  themselves 
  very 
  

   much 
  closer 
  together, 
  especially 
  the 
  lower 
  ones, 
  and 
  shortly 
  

   reach 
  a 
  greatly 
  reduced, 
  as 
  well 
  as 
  varied 
  ratio 
  of 
  values 
  

   which 
  they 
  retain 
  substantially 
  unchanged 
  to 
  the 
  end, 
  as 
  

  

  