﻿216 
  Dr. 
  C. 
  Chree's 
  Notes 
  on 
  Thermometry. 
  

  

  of 
  the 
  formula 
  to 
  the 
  temperatures 
  —38° 
  C. 
  and 
  200° 
  C. 
  is 
  

   somewhat 
  of 
  an 
  extrapolation 
  from 
  the 
  experimental 
  point 
  of 
  

   view. 
  

  

  Table 
  II. 
  

   Value 
  of 
  a:— 
  y 
  (in 
  centigrade 
  degrees) 
  from 
  (25). 
  

  

  Temperature 
  y 
  ^ 
  

   centigrade. 
  

  

  French 
  crystal 
  

   (hard). 
  

  

  Jena 
  glass 
  16 
  m 
  . 
  

  

  o 
  

  

  -38 
  

  

  +50 
  

  

  +200 
  

  

  -0-006 
  

   +0003 
  

   -0-023 
  

  

  -0°173 
  

  

  +0-082 
  

   -0-658 
  

  

  +0°004 
  

   -0002 
  

  

  +0016 
  

  

  

  

  Obviously 
  in 
  the 
  case 
  of 
  French 
  crystal 
  the 
  distinction 
  

   between 
  the 
  fixed 
  and 
  movable 
  zero 
  scales 
  must 
  be 
  taken 
  

   into 
  account, 
  even 
  for 
  accuracy 
  of 
  the 
  order 
  o, 
  l 
  C. 
  

  

  Generally 
  speaking, 
  English 
  thermometer 
  glass 
  resembles 
  

   more 
  nearly 
  French 
  crystal 
  than 
  the 
  other 
  two 
  glasses, 
  in 
  so 
  

   far 
  as 
  at 
  temperatures 
  above 
  50° 
  C. 
  the 
  term 
  d 
  2 
  t 
  2 
  tends 
  to 
  

   predominate. 
  

  

  Some 
  experiments, 
  however, 
  at 
  Kew 
  Observatory, 
  still 
  

   incomplete, 
  show 
  notable 
  differences 
  in 
  the 
  behaviour 
  of 
  

  

  X 
  J 
  

  

  different 
  English 
  thermometers, 
  all 
  nominally 
  of 
  the 
  same 
  

   glass. 
  In 
  some 
  the 
  depression 
  of 
  zero 
  after 
  exposure 
  to 
  

   100° 
  C. 
  is 
  only 
  a 
  shade 
  larger 
  than 
  in 
  Jena 
  glass 
  16 
  IU 
  , 
  in 
  

   others 
  the 
  depression 
  is 
  twice, 
  or 
  even 
  thrice, 
  as 
  great. 
  If 
  

   the 
  cause 
  is 
  difference 
  as 
  regards 
  annealing 
  or 
  in 
  treatment, 
  

   one 
  would 
  expect 
  to 
  hear 
  of 
  like 
  differences 
  in 
  verre 
  dur 
  and 
  

   Jena 
  glass 
  thermometers. 
  

  

  In 
  all 
  the 
  English 
  glass 
  thermometers 
  I 
  have 
  met 
  with, 
  d 
  2 
  

   was 
  positive. 
  This 
  implies, 
  it 
  will 
  be 
  noticed, 
  that 
  they 
  would 
  

   read 
  lower 
  at 
  —38° 
  C. 
  on 
  the 
  fixed 
  zero 
  than 
  on 
  the 
  movable 
  

   zero 
  scale. 
  

  

  Relations 
  of 
  different 
  Temperature 
  Scales. 
  

   Experimental 
  Results. 
  

  

  § 
  11. 
  Of 
  the 
  kinds 
  of 
  glass 
  used 
  in 
  the 
  higher 
  thermometry 
  

   the 
  most 
  notable 
  are 
  probably 
  verre 
  dur 
  and 
  the 
  two 
  Jena 
  

   glasses 
  16 
  m 
  and 
  59 
  m 
  . 
  The 
  following 
  particulars 
  as 
  to 
  their 
  

   chemical 
  composition 
  are 
  derived 
  from 
  p. 
  10 
  of 
  vol. 
  i. 
  of 
  the 
  

   Reich 
  san 
  start's 
  Wiss. 
  Abhandlungen. 
  

  

  