as applied to Gases and Vapours. 537 



ment are greatest at low temperatures, naturally induced me to 

 try in the first place the effect of a third term inversely propor- 

 tional to the square of the absolute temperature, making the en- 

 tire formula for the logarithm of the pressure of vapour in con- 

 tact with its liquid 



and the inverse formula, for calculating the 

 absolute temperature from the pressure. 



1- (37) 



^'^/^ 



7 "^ 4^rf 2r/ 



the values of the constants a, /S, 7 being determined by the or- 

 dinary methods from three experimental data for each substance. 



(34.) The agreement of those formulse with the results of ex- 

 periment proved so remarkable, that, as they are calculated to 

 De practically useful, I thought it my duty not to delay their 

 publication until I should have an opportunity of submitting my 

 theoretical researches to the Royal Society of Edinburgh. I 

 therefore communicated the formula3 to the Edinburgh New 

 Philosophical Journal for July 1849, together with the full de- 

 tails of their comparison, graphic and tabular, with the experi- 

 ments of M. Regnault upon water and mercury, and with those 

 of Dr. Ure upon alcohol, sether, turpentine and petroleum, 

 but without giving any account of the reasoning by which I had 

 been led to them. 



Without repeating those details here, I may state, that the 

 agreement between the results of the formulse and those of ob- 

 servation is in every case as close as the precision of the experi- 

 ments renders possible. This is remarkable, especially with 

 respect to the experiments of M. Regnault on the elasticity of 

 steam, which extend throughout a range of temperatures from 

 30° below zero of the Centigrade scale to 230° above it, and of 

 pressures from ggVo^^ ^^ ^^ atmosphere to 28 atmospheres, 

 and which, from the methods of observation adopted, especially 

 those of measuring temperature, necessarily surpass by far in 

 precision all other experiments of the same kind. From 20° to 

 230° Cent, the greatest discrepancy between calculation and ex- 

 periment corresponds to a difference of yg^ of a Centigrade 

 degree, and very few of the other dift'erences amount to so much 

 as ^\jth of a degree. Below 20°, where the pressure varies so 

 slowly with the temperature that its actual value is the proper 

 test of the formula, the greatest discrepancy is yo%*^^ ^^ ^ milli- 

 metre of mercury, or g^^th of an inch. If the curves repre- 

 senting the formulse were laid down on M. Regnault's diagram. 



