﻿302 
  Dr. 
  C. 
  Chree's 
  Notes 
  on 
  Thermometry. 
  

  

  owing 
  to 
  the 
  slowness 
  with 
  which 
  the 
  spirit 
  drains 
  down 
  the 
  

   tube 
  after 
  a 
  fall 
  of 
  temperature. 
  

  

  On 
  the 
  fixed 
  zero 
  method, 
  the 
  thermometer 
  ought 
  theoreti- 
  

   cally 
  to 
  be 
  kept 
  at 
  0° 
  G. 
  for 
  an 
  indefinite 
  time. 
  In 
  practice, 
  

   however, 
  prolonged 
  exposure 
  to 
  a 
  temperature 
  of 
  about 
  15° 
  C. 
  

   is 
  a 
  much 
  more 
  usual 
  preliminary. 
  Now 
  a 
  verve 
  dur 
  thermo- 
  

   meter 
  after 
  long 
  exposure 
  to 
  15° 
  C. 
  would 
  give, 
  even 
  after 
  

   10 
  or 
  15 
  minutes 
  immersion 
  in 
  ice. 
  a 
  zero 
  reading 
  about 
  

   0°*015 
  C. 
  below 
  the 
  true 
  fixed 
  zero, 
  and 
  this, 
  of 
  course, 
  is 
  a 
  

   very 
  serious 
  difference 
  in 
  thermometry 
  aiming 
  at 
  accuracy 
  of 
  

   the 
  order 
  0°*001 
  C. 
  Even 
  with 
  English 
  glass, 
  where 
  the 
  

   depression 
  of 
  zero 
  after 
  ordinary 
  atmospheric 
  temperatures 
  is 
  

   considerably 
  less 
  than 
  with 
  verre 
  dur, 
  it 
  is 
  hopeless 
  to 
  think 
  

   of 
  accuracy 
  above 
  o, 
  01 
  C. 
  unless 
  attention 
  is 
  paid 
  to 
  the 
  

   previous 
  temperature. 
  

  

  For 
  work 
  of 
  the 
  highest 
  accuracy, 
  the 
  pressure 
  to 
  which 
  

   the 
  ice 
  is 
  exposed 
  may 
  become 
  of 
  importance. 
  According 
  to 
  

   the 
  theory 
  of 
  Prof. 
  James 
  Thomson, 
  and 
  the 
  experiments 
  of 
  

   Lord 
  Kelvin, 
  an 
  increase 
  of 
  one 
  atmosphere 
  in 
  the 
  pressure 
  

   lowers 
  the 
  freezing-point 
  about 
  o, 
  0075 
  C* 
  Thus 
  in 
  vacuum- 
  

   tubes 
  where 
  the 
  pressure 
  is 
  practically 
  nil, 
  or 
  in 
  laboratory 
  

   experiments 
  with 
  compressed 
  gases, 
  a 
  very 
  sensible 
  correction 
  

   might 
  be 
  necessary. 
  Even 
  under 
  natural 
  conditions 
  a 
  cor- 
  

   rection 
  of 
  the 
  order 
  o, 
  003 
  0. 
  might 
  be 
  required 
  at 
  lofty 
  

   stations. 
  

  

  It 
  must 
  not 
  be 
  overlooked 
  that 
  such 
  variations 
  in 
  external 
  

   pressure 
  affect 
  the 
  reading 
  in 
  another 
  totally 
  different 
  way, 
  

   through 
  their 
  direct 
  influence 
  on 
  the 
  glass 
  of 
  the 
  thermo- 
  

   meter 
  (see 
  § 
  26). 
  In 
  fact, 
  care 
  is 
  required 
  to 
  ensure 
  that 
  no 
  

   external 
  pressure 
  effect 
  is 
  produced 
  by 
  too 
  tight 
  packing 
  of 
  

   the 
  ice, 
  or 
  by 
  allowing 
  the 
  bulb 
  to 
  rest 
  unsupported 
  on 
  largish 
  

   ice 
  -crystals. 
  

  

  In 
  determining 
  freezing-points 
  of 
  metals 
  or 
  solutions 
  it 
  is 
  

   customary 
  to 
  cool 
  down 
  the 
  liquid 
  and 
  observe 
  the 
  stationary 
  

   point 
  on 
  an 
  immersed 
  thermometer. 
  This 
  method, 
  though 
  

   not 
  usually 
  adopted, 
  is 
  of 
  course 
  applicable 
  to 
  the 
  freezing- 
  

   point 
  of 
  water. 
  A 
  discussion 
  of 
  it 
  by 
  Dr. 
  M. 
  Wildermann 
  

   has 
  appeared 
  recently 
  in 
  the 
  Phil. 
  Mag. 
  |> 
  in 
  which 
  will 
  be 
  

   found 
  references 
  to 
  recent 
  work 
  on 
  the 
  subject. 
  

  

  * 
  Lord 
  Kelvin's 
  ' 
  Mathematical 
  and 
  Physical 
  Papers,' 
  vol. 
  i. 
  pp. 
  156- 
  

   169 
  ; 
  or 
  Bavnes' 
  ' 
  Thermodynamics/ 
  art. 
  lxxxix. 
  

   t 
  December, 
  1897, 
  p. 
  459. 
  

  

  