﻿40 
  Mr. 
  C. 
  F. 
  Brush 
  on 
  the 
  Transmission 
  of 
  

  

  PL 
  VII. 
  gives 
  the 
  curve 
  for 
  air, 
  with 
  the 
  small 
  bulb. 
  It 
  

   differs 
  from 
  that 
  with 
  the 
  large 
  bulb 
  quite 
  as 
  much 
  as 
  did 
  the 
  

   hydrogen 
  curve. 
  Section 
  A 
  droops 
  slightly, 
  and 
  then 
  regains 
  

   almost 
  its 
  full 
  atmospheric 
  value 
  at 
  one 
  per 
  cent, 
  pressure. 
  

   Section 
  B 
  has 
  the 
  same 
  form 
  as 
  with 
  the 
  large 
  bulb 
  (PI. 
  II.), 
  

   but 
  more 
  than 
  double 
  its 
  value 
  ; 
  and 
  section 
  C 
  also 
  has 
  a 
  

   much 
  higher 
  value 
  throughout. 
  The 
  curves 
  a, 
  a 
  a, 
  aaa 
  have 
  

   small 
  ratio 
  values 
  at 
  the 
  beginning, 
  and 
  are 
  absorbed 
  into 
  

   section 
  A 
  the 
  same 
  as 
  with 
  hydrogen. 
  But 
  a 
  a 
  and 
  aaa 
  

   coalesce 
  when 
  they 
  reappear, 
  and 
  coincide 
  to 
  the 
  end 
  ; 
  while 
  

   the 
  ratio 
  between 
  a 
  and 
  a 
  a 
  remains 
  constant 
  at 
  a 
  very 
  small 
  

   value. 
  

  

  PI. 
  VIII. 
  is 
  the 
  curve 
  for 
  carbon 
  dioxide, 
  with 
  the 
  small 
  

   bulb. 
  It 
  closely 
  resembles 
  the 
  air-curve 
  in 
  form, 
  but 
  has 
  a 
  

   very 
  much 
  smaller 
  value 
  throughout. 
  While 
  the 
  curves 
  a 
  a 
  

   and 
  aaa 
  are 
  soon 
  united, 
  and 
  remain 
  so 
  to 
  the 
  end, 
  a 
  and 
  a 
  a 
  

   never 
  disappear 
  as 
  they 
  did 
  in 
  the 
  cases 
  of 
  hydrogen 
  and 
  air. 
  

  

  With 
  the 
  small 
  bulb, 
  as 
  with 
  the 
  large, 
  no 
  change 
  in 
  the 
  

   character 
  of 
  the 
  phenomena 
  was 
  observable 
  when 
  the 
  ex- 
  

   haustion 
  had 
  reached 
  the 
  point 
  at 
  which 
  the 
  mean 
  free 
  path 
  

   of 
  the 
  molecules 
  equalled 
  the 
  space 
  through 
  which 
  the 
  heat 
  

   was 
  conducted. 
  This 
  point 
  was 
  reached 
  in 
  the 
  small 
  bulb 
  at 
  

   a 
  pressure 
  of 
  about 
  fourteen 
  millionths. 
  

  

  It 
  seems 
  reasonable 
  to 
  assume 
  that 
  the 
  radical 
  difference 
  

   between 
  sections 
  A 
  of 
  the 
  curves 
  obtained 
  with 
  the 
  large 
  and 
  

   small 
  bulbs 
  respectively 
  was 
  due 
  to 
  an 
  almost 
  complete 
  sup- 
  

   pression 
  of 
  convection-currents 
  in 
  the 
  latter 
  case. 
  In 
  the 
  

   absence 
  of 
  convection-currents, 
  that 
  part 
  of 
  the 
  heat 
  trans- 
  

   mitted 
  by 
  the 
  gas 
  was 
  probably 
  carried 
  by 
  a 
  process 
  analogous 
  

   to 
  conduction 
  in 
  solids. 
  The 
  shortness 
  of 
  conductor 
  in 
  the 
  

   case 
  of 
  the 
  small 
  bulb 
  may 
  account 
  for 
  the 
  greatly 
  increased 
  

   rate 
  of 
  conduction. 
  But 
  why 
  the 
  conductivity 
  of 
  a 
  gas 
  

   remains 
  nearly 
  constant 
  through 
  a 
  very 
  wide 
  range 
  of 
  pres- 
  

   sure 
  is 
  not 
  clear. 
  Sir 
  Wm. 
  Crookes's 
  explanation 
  of 
  this 
  

   phenomenon 
  seems 
  to 
  me 
  very 
  unsatisfactory. 
  

  

  It 
  will 
  be 
  noticed 
  that 
  the 
  " 
  sether-line 
  " 
  is 
  about 
  four 
  per 
  

   cent, 
  lower 
  with 
  the 
  small 
  bulb 
  than 
  with 
  the 
  large 
  one. 
  This 
  

   may 
  be 
  due 
  to 
  the 
  greatly 
  decreased 
  amount 
  of 
  surface 
  pre- 
  

   sented 
  by 
  the 
  small 
  bulb 
  for 
  absorption 
  of 
  the 
  radiant 
  heat. 
  

  

  The 
  enormous 
  heat-conducting 
  capacity 
  of 
  gases 
  at 
  very 
  

   small 
  pressures 
  is 
  strikingly 
  shown 
  in 
  all 
  the 
  curves. 
  But 
  

   hydrogen 
  is 
  preeminent 
  in 
  this 
  respect. 
  Thus, 
  in 
  the 
  large 
  

   bulb, 
  hydrogen 
  at 
  a 
  pressure 
  of 
  only 
  twenty-six 
  millionths 
  of 
  

   an 
  atmosphere 
  transmits 
  heat 
  as 
  rapidly 
  as 
  the 
  aether 
  ! 
  At 
  

   seventy-sis 
  millionths 
  it 
  equals 
  air 
  at 
  atmospheric 
  pressure 
  ; 
  

   that 
  is 
  to 
  say, 
  it 
  does 
  the 
  work 
  of 
  nearly 
  two 
  hundred 
  thousand 
  

   times 
  its 
  weight 
  of 
  air 
  ! 
  

  

  