﻿Radiant 
  Heat 
  by 
  Gases 
  at 
  Varying 
  Pressures. 
  39 
  

  

  seven 
  times 
  as 
  fast 
  through 
  a 
  long 
  range 
  of 
  intermediate 
  

   pressures. 
  

  

  As 
  evidence 
  of 
  the 
  accuracy 
  of 
  the 
  observations 
  on 
  which 
  

   the 
  curves 
  thus 
  far 
  described 
  are 
  based, 
  it 
  is 
  gratifying 
  to 
  

   note 
  that 
  the 
  vacuum, 
  or 
  sether 
  line, 
  locates 
  itself 
  exactly 
  the 
  

   same 
  in 
  all. 
  

  

  In 
  making 
  the 
  above-described 
  observations 
  I 
  looked 
  for 
  

   some 
  change 
  in 
  the 
  phenomena 
  when 
  the 
  exhaustion 
  reached 
  

   the 
  point 
  at 
  which 
  the 
  mean 
  free 
  path 
  of 
  the 
  gas 
  molecules 
  

   equalled 
  the 
  distance 
  between 
  the 
  thermometer 
  bulb 
  and 
  the 
  

   cold 
  walls 
  of 
  the 
  enclosing 
  globe. 
  This 
  should 
  have 
  been 
  at 
  

   a 
  pressure 
  of 
  about 
  two 
  millionths. 
  No 
  such 
  change 
  was 
  

   observable, 
  however, 
  in 
  any 
  case. 
  Partly 
  in 
  pursuance 
  of 
  the 
  

   same 
  idea, 
  I 
  resolved 
  to 
  repeat 
  some 
  of 
  my 
  experiments, 
  

   using 
  a 
  very 
  much 
  smaller 
  radiation 
  bulb. 
  This 
  I 
  expected 
  

   would 
  also 
  reduce 
  that 
  portion 
  of 
  the 
  total 
  cooling 
  effect 
  due 
  

   to 
  convection 
  currents. 
  I 
  accordingly 
  employed 
  the 
  bulb 
  or 
  

   tube 
  P 
  (PI. 
  I.) 
  in 
  my 
  further 
  experiments. 
  This 
  is 
  made 
  

   from 
  a 
  thin 
  glass 
  tube 
  slightly 
  less 
  than 
  20 
  millimetres 
  internal 
  

   diameter, 
  and 
  in 
  it 
  hangs 
  the 
  same 
  thermometer 
  A 
  which 
  

   was 
  used 
  before. 
  In 
  transferring 
  the 
  thermometer, 
  great 
  

   care 
  was 
  taken 
  to 
  avoid 
  any 
  disturbance 
  of 
  the 
  coating 
  of 
  

   lampblack 
  on 
  its 
  bulb. 
  At 
  b 
  is 
  a 
  contraction 
  of 
  the 
  tube 
  P 
  

   to 
  prevent 
  the 
  thermometer 
  bulb 
  swinging 
  against 
  the 
  inside 
  

   of 
  the 
  tube. 
  The 
  contraction 
  b 
  is, 
  however, 
  much 
  larger 
  

   than 
  the 
  thermometer 
  stem, 
  so 
  that 
  normally 
  the 
  latter 
  does 
  

   not 
  touch 
  it. 
  The 
  thermometer 
  bulb 
  hangs 
  exactly 
  in 
  

   the 
  centre 
  of 
  P, 
  near 
  its 
  bottom, 
  and 
  is 
  separated 
  from 
  it 
  by 
  

   a 
  space 
  of 
  a 
  trifle 
  more 
  than 
  six 
  millimetres 
  — 
  almost 
  exactly 
  

   a 
  quarter 
  of 
  an 
  inch 
  ; 
  instead 
  of 
  two 
  inches, 
  as 
  in 
  the 
  case 
  of 
  the 
  

   " 
  large 
  bulb/'' 
  The 
  tube 
  or 
  bulb 
  P, 
  I 
  shall 
  hereafter 
  designate 
  

   the 
  " 
  small 
  radiation 
  bulb," 
  or 
  simply 
  u 
  small 
  bulb," 
  to 
  dis- 
  

   tinguish 
  it 
  from 
  the 
  large 
  one. 
  

  

  PL 
  VI. 
  shows 
  the 
  curve 
  for 
  hydrogen, 
  with 
  the 
  small 
  bulb. 
  

   It 
  differs 
  radically 
  in 
  size 
  and 
  form 
  from 
  that 
  obtained 
  with 
  

   the 
  large 
  bulb. 
  Section 
  A, 
  instead 
  of 
  drooping 
  rapidly 
  with 
  

   decreasing 
  pressure, 
  maintains 
  almost 
  its 
  full 
  value 
  through- 
  

   out. 
  Section 
  B 
  starts 
  with 
  nearly 
  double 
  its 
  old 
  value, 
  but 
  

   breaks 
  down 
  much 
  earlier. 
  Section 
  C 
  starts 
  with 
  a 
  little 
  

   higher 
  value, 
  but 
  is 
  much 
  straighter, 
  and 
  consequently 
  has 
  a 
  

   lower 
  value 
  throughout 
  most 
  of 
  its 
  length. 
  The 
  curves 
  

   a, 
  a 
  a, 
  aaa 
  are 
  very 
  peculiar. 
  They 
  start 
  at 
  atmospheric 
  

   pressure 
  with 
  much 
  smaller 
  total, 
  and 
  very 
  different 
  relative 
  

   ratios 
  than 
  in 
  PL 
  V., 
  and 
  are 
  successively 
  absorbed 
  into 
  A. 
  

   They 
  reappear 
  later, 
  however, 
  as 
  shown 
  in 
  section 
  C, 
  but 
  

   with 
  smaller 
  ratios 
  than 
  in 
  PL 
  V. 
  

  

  