434 Dr. P. Chappuis on Gas-Thermometry . 



The values deduced from this function for a few points are 



Temp. 



Coeff. 



0° 



0-00-367 698 



20 



560 



40 



461 



50 



427 



60 



400 



70 



384 



80 



378 



90 



381 



100 



0-00367 393 



From this expression it appears that the coefficient of dilata- 

 tion of nitrogen diminishes gradually up to about 80°, then 

 increases very slightly. This increase, which is of the same 

 order of magnitude as the probable errors, is not in agree- 

 ment with our knowledge of the variations of the coefficients 

 of gases. It is very probable that the coefficient approaches 

 a definite limiting value for each initial pressure, which in 

 this case seems to be attained below 100°. In fact, nitrogen 

 at 100° behaves like hydrogen at the ordinary temperature, 

 its compressibility being less than is required by the Boyle- 

 Mariotte law. ]t may be seen from the table just given that 

 this limiting value of the coefficient is approximately 



a 2 = 0-00367 380. 



Assuming this value, it is possible to calculate the fictitious 

 initial pressure which should have been observed if the 

 nitrogen had retained down to 0° the properties of a perfect 

 gas. 



We thus obtain the following relations: 



p 



Tt = P (l + at) ; whence P = — — - . 



1 + at 



But from direct observation we have 



Metre Metre 



P =l, and P 100 = 1367466, 



whence we obtain for the mean coefficient of dilatation atj 

 between 0° and 100° 



ai = 0-003674 66. 



The fictitious initial pressure of the nitrogen thermometer 

 supposed perfect may be deduced from the expression above : 



p m. 



« £l°° =1-000063. 



p ° 1 -f 0-00367380 x 100 



