﻿32 
  Mr. 
  C. 
  F. 
  Brush 
  on 
  the 
  r 
  Transmission 
  of 
  

  

  In 
  their 
  researches 
  on 
  the 
  " 
  Communication 
  of 
  Heat," 
  

   Dulong 
  and 
  Petit 
  used 
  as 
  the 
  cooling 
  body 
  a 
  very 
  large 
  

   thermometer 
  bulb 
  filled 
  with 
  mercury 
  ; 
  and 
  as 
  the 
  recipient 
  

   of 
  the 
  heat, 
  a 
  large 
  copper 
  bulb 
  or 
  " 
  balloon 
  " 
  about 
  three 
  

   decimetres 
  in 
  diameter, 
  in 
  the 
  centre 
  of 
  which 
  the 
  ther- 
  

   mometer 
  bulb 
  was 
  placed. 
  The 
  copper 
  balloon 
  was 
  coated 
  

   with 
  lamp-black 
  on 
  the 
  inside, 
  and 
  kept 
  at 
  any 
  desired 
  con- 
  

   stant 
  temperature 
  by 
  means 
  of 
  a 
  water-bath 
  or 
  melting 
  ice. 
  

   The 
  thermometer 
  tube 
  was 
  of 
  such 
  length 
  as 
  to 
  bring 
  the 
  

   zero 
  of 
  the 
  scale 
  outside 
  the 
  balloon 
  ; 
  and 
  the 
  thermometer 
  

   was 
  adapted 
  to 
  be 
  removed, 
  heated, 
  and 
  quickly 
  replaced, 
  

   air-tight. 
  The 
  balloon 
  was 
  connected 
  with 
  an 
  air-pump 
  

   capable 
  of 
  rapidly 
  exhausting 
  it 
  down 
  to 
  about 
  two 
  millimetres 
  

   pressure 
  ; 
  and 
  also 
  with 
  a 
  gas-holder 
  from 
  which 
  it 
  could 
  be 
  

   quickly 
  filled 
  with 
  the 
  gas 
  whose 
  cooling 
  properties 
  were 
  to 
  

   be 
  determined. 
  The 
  rate 
  or 
  " 
  velocity 
  " 
  of 
  cooling 
  of 
  the 
  

   thermometer 
  bulb 
  was 
  deduced 
  from 
  observations 
  of 
  the 
  

   falling 
  temperature 
  at 
  equal 
  intervals 
  of 
  time. 
  

  

  With 
  this 
  apparatus 
  Dulong 
  and 
  Petit 
  made 
  many 
  carefully 
  

   conducted 
  experiments 
  at 
  differences 
  of 
  temperature 
  between 
  

   the 
  thermometer 
  and 
  balloon 
  ranging 
  as 
  high 
  as 
  300 
  degrees 
  ; 
  

   and 
  with 
  several 
  different 
  gases 
  . 
  besides 
  air, 
  ranging 
  in 
  

   pressure 
  from 
  atmospheric 
  to 
  two 
  millimetres. 
  From 
  the 
  

   results 
  of 
  these 
  experiments 
  they 
  deduced 
  several 
  laws 
  of 
  

   cooling 
  which 
  they 
  held 
  to 
  be 
  general 
  in 
  their 
  application. 
  

   They 
  sharply 
  divided 
  the 
  cooling 
  into 
  two 
  parts 
  ; 
  that 
  due 
  to 
  

   convection 
  — 
  the 
  actual 
  contact 
  of 
  the 
  surrounding 
  cooler 
  gas 
  

   renewed 
  by 
  its 
  own 
  currents, 
  and 
  that 
  due 
  purely 
  to 
  radiation 
  — 
  

   the 
  same 
  as 
  would 
  occur 
  in 
  an 
  " 
  absolute 
  vacuum." 
  They 
  

   derived 
  a 
  constant 
  value 
  for 
  the 
  latter, 
  and 
  values 
  for 
  the 
  

   former 
  varying 
  with 
  different 
  gases 
  and 
  different 
  pressures. 
  

   They 
  generally 
  used 
  the 
  thermometer 
  bulb 
  naked, 
  with 
  its 
  

   natural 
  vitreous 
  surface, 
  but 
  sometimes 
  they 
  silvered 
  it. 
  

   While 
  this 
  radical 
  change 
  in 
  the 
  character 
  of 
  surface 
  greatly 
  

   changed 
  the 
  loss 
  of 
  heat 
  due 
  to 
  radiation, 
  it 
  apparently 
  had 
  

   no 
  effect 
  on 
  that 
  due 
  to 
  convection. 
  They 
  also 
  tried 
  ther- 
  

   mometers 
  with 
  different-sized 
  bulbs 
  ; 
  and 
  again, 
  various 
  

   shaped 
  vessels, 
  filled 
  with 
  various 
  liquids 
  surrounding 
  a 
  small 
  

   thermometer 
  bulb, 
  were 
  tried 
  as 
  the 
  cooling 
  body. 
  None 
  of 
  

   these 
  changes 
  affected 
  the 
  general 
  laws 
  of 
  cooling 
  which 
  they 
  

   had 
  deduced. 
  But 
  they 
  did 
  not 
  try 
  a 
  smaller 
  copper 
  balloon. 
  

   Had 
  they 
  tried 
  a 
  very 
  small 
  one 
  they 
  would 
  have 
  found 
  some 
  

   of 
  their 
  general 
  deductions 
  untenable. 
  

  

  MM. 
  Dulong 
  and 
  Petit 
  devoted 
  a 
  lengthy 
  chapter 
  to 
  the 
  

   discussion 
  of 
  the 
  cooling 
  of 
  bodies 
  in 
  vacuo. 
  But 
  they 
  fell 
  

   into 
  the 
  grave 
  error 
  of 
  deducing 
  the 
  behaviour 
  of 
  the 
  last 
  few 
  

  

  