Temperature and the Tension of Vapours. 9 



founding it on Dulong and Arago' s original observations at tem- 

 peratures above 100°. It corresponds with the experimental 

 results of the above-named French academicians better than the 

 formula which they themselves have used, and it may therefore 

 be regarded as the best general expression of their experiments. 



It will be seen that Dulong and Arago throughout their de- 

 terminations found somewhat higher temperatures than Reg- 

 nault ; and this difference, constantly but very slowly increasing, 

 amounts in the case of the greater tensions to even 1° C. This 

 disparity, however, immediately appears far less considerable 

 when it is remembered that Regnault's results refer to the air- 

 thermometer, and those of Dulong and Arago, on the other 

 hand, to the mercurial thermometer ; moreover the results con- 

 tained in the column to which D. and A. are affixed were made 

 with a thermometer the bulb of which dipped into the boiling 

 liquid, whilst the thermometer employed for the observations 

 under E. was only surrounded by the vapour. 



It is accordingly self-evident that the numbers in both columns 

 must have been somewhat greater than the corresponding results 

 referred to the air-thermometer, and indeed in about the ratio in 

 which they have actually been found. Regnault, during his very 

 numerous experiments on the tension of aqueous vapour at high 

 temperatures, took the precaution to employ the air-thermometer 

 side by side with the mercurial thermometer. 



Some of his observations allow of a good comparison with 

 similar ones of Dulong and Arago, made at nearly the same 

 temperatures, and taken from their original papers. They are 

 as follow : — 



mo 



D.and A. 

 P millims. 



TO 



R. 



P millims. 



Millims. 



149-6 



3475-9 



149-46 



3479-31 



9-5 



168-4 



5605-4 



16801 



5554-83 



140 



1695 



5773'7 



16936 



5757-34 



14-2 



1807 



7500-1 



180-50 



745912 



17-5 



1937 



9998-9 



193-88 

 19379 



10001-64 

 9979-79 



21-8 



209-1 



137690 



209-61 



13789-33 



28-2 



218-4 



16381-4 



218-67 



16364-59 



32-2 



220-6 



17182-6 



220-21 



16886-00 



33-2 



The numbers in the last column indicate the effect of a change 

 in temperature of o, l on the height of the column of pressure. 

 It will be perceived that the differences in these columns of 

 figures do not exceed the limits of unavoidable errors of obser- 

 vation with the mercurial thermometer. Regnault's observa- 

 tions not only, in consequence of their great number, control and 

 confirm one another, but are also, from their very close agree- 



