﻿Radiant 
  Heat 
  by 
  Gases 
  at 
  Varying 
  Pressures, 
  35 
  

  

  small 
  pressures, 
  and 
  H 
  is 
  a 
  drying-bulb 
  containing 
  phosphorus 
  

   pentoxide. 
  The 
  glass 
  stopcock 
  I 
  serves 
  to 
  admit 
  other 
  gases 
  

   than 
  air, 
  and 
  is 
  of 
  course 
  made 
  absolutely 
  tight 
  when 
  closed. 
  

   The 
  mercury 
  valve 
  K 
  prevents 
  any 
  leakage 
  backward 
  from 
  

   the 
  pump 
  when 
  the 
  latter 
  is 
  stopped 
  during 
  observations. 
  

   Exhaustion 
  is 
  effected 
  by 
  an 
  automatic 
  Sprengel 
  pump 
  

   having 
  five 
  fall 
  tubes. 
  L 
  is 
  a 
  fine 
  cathetometer 
  placed 
  in 
  

   front 
  of 
  the 
  whole 
  apparatus, 
  and 
  by 
  rotation 
  on 
  its 
  vertical 
  

   axis 
  is 
  adapted 
  to 
  read 
  the 
  McLeod 
  gauge, 
  both 
  barometers, 
  

   and 
  the 
  thermometer. 
  It 
  has 
  a 
  vertically 
  divided 
  scale 
  with 
  

   vernier 
  and 
  microscope, 
  for 
  reading 
  the 
  barometers, 
  and 
  a 
  

   micrometer 
  for 
  reading 
  the 
  gauge. 
  A 
  watch 
  N 
  is 
  mounted 
  

   close 
  beside 
  the 
  thermometer 
  on 
  a 
  sliding 
  frame, 
  so 
  as 
  to 
  be 
  

   easily 
  kept 
  in 
  the 
  field 
  of 
  view 
  of 
  the 
  cathetometer 
  telescope 
  

   when 
  the 
  latter 
  is 
  used 
  to 
  observe 
  the 
  falling 
  temperature. 
  

  

  Before 
  using 
  this 
  apparatus 
  I 
  always 
  exhausted 
  to 
  a 
  good 
  

   vacuum, 
  and 
  heated 
  the 
  bulb 
  JB 
  by 
  means 
  of 
  a 
  water-bath, 
  and 
  

   all 
  other 
  vacuous 
  parts 
  by 
  means 
  of 
  an 
  air-bath, 
  to 
  100 
  degrees 
  

   for 
  several 
  hours. 
  This 
  was 
  found 
  necessary 
  in 
  the 
  first 
  

   instance 
  with 
  air, 
  in 
  order 
  to 
  divest 
  the 
  inner 
  glass 
  surfaces 
  

   of 
  that 
  portion 
  of 
  their 
  coating 
  of 
  adherent 
  gas 
  most 
  easily 
  

   given 
  off 
  in 
  a 
  vacuum. 
  This 
  gas 
  was 
  pumped 
  out 
  ; 
  and 
  not 
  

   being 
  principally 
  air, 
  was 
  not 
  largely 
  re-absorbed 
  when 
  air 
  

   was 
  admitted. 
  Without 
  this 
  precaution 
  I 
  was 
  unable 
  to 
  

   obtain 
  constant 
  results 
  at 
  very 
  low 
  pressures. 
  When 
  other 
  

   gases 
  were 
  tried 
  successively, 
  the 
  preliminary 
  heating 
  pre- 
  

   vented 
  gas 
  from 
  one 
  operation 
  attaching 
  itself 
  to 
  the 
  glass 
  

   and 
  remaining 
  to 
  contaminate 
  the 
  succeeding 
  gas 
  at 
  very 
  

   low 
  pressures. 
  

  

  I 
  next 
  introduced 
  the 
  proper 
  gas 
  up 
  to 
  atmospheric 
  pres- 
  

   sure, 
  and 
  made 
  a 
  preliminary 
  cooling 
  of 
  the 
  thermometer 
  by 
  

   raising 
  the 
  ice- 
  tank 
  C. 
  This 
  preliminary 
  cooling 
  was 
  found 
  

   to 
  have 
  a 
  slight 
  effect 
  on 
  the 
  readings 
  next 
  following, 
  and 
  

   was 
  done 
  to 
  make 
  the 
  first 
  set 
  of 
  readings 
  on 
  any 
  day 
  

   entirely 
  comparable 
  with 
  the 
  others. 
  I 
  then 
  lowered 
  the 
  

   ice-tank, 
  and 
  when 
  the 
  temperature 
  had 
  risen 
  to 
  18 
  degrees, 
  

   stirred 
  the 
  ice 
  and 
  water 
  thoroughly, 
  raised 
  the 
  tank 
  again, 
  

   and 
  observed 
  the 
  thermometer 
  through 
  the 
  telescope 
  — 
  noting 
  

   by 
  the 
  watch 
  N 
  the 
  instant 
  when 
  the 
  falling 
  mercury 
  passed 
  

   each 
  degree 
  of 
  the 
  scale. 
  Then, 
  with 
  the 
  ice-tank 
  still 
  up, 
  I 
  

   noted 
  the 
  pressure 
  by 
  measuring 
  with 
  the 
  cathetometer 
  the 
  

   difference 
  in 
  height 
  of 
  the 
  barometer 
  columns 
  in 
  D 
  and 
  E. 
  

   The 
  barometer 
  D 
  showed 
  that 
  the 
  gas 
  in 
  the 
  radiation 
  bulb 
  

   cooled 
  nearly 
  to 
  zero 
  with 
  very 
  great 
  rapidity 
  when 
  the 
  ice- 
  

   tank 
  was 
  raised. 
  I 
  always 
  measured 
  pressures 
  with 
  the 
  

   radiation 
  bulb 
  cold. 
  It 
  was 
  usual 
  to 
  repeat 
  the 
  whole 
  

  

  D2 
  

  

  