76 SIR WILLIAM RAMSAY 



to hold accurately, where c is a very small constant, and t r and t 

 are the temperatures of one of the two substances at the two 

 pressures p' and p. 



The first generalisation is closely connected with the formula 

 of Clausius and Clapeyron, 



L = dp_T 



v : - v 2 dt J ' 



Ramsay had previously studied the first term of the formula, 

 - > chiefly at atmospheric pressure, and had communicated 

 his results to the Chemical Section of the Philosophical Society 

 of Glasgow in 1877, but the generalisation in the form ^~ 



= constant at atmospheric pressure for different substances was 

 first actually published by Trouton [Phil. Mag. 18, 54 (1884)], 

 and is known as Trout on' s law. 



Finally these and another allied generalisation were published 

 by us in a series of five papers termed ' Some Thermodynamical 

 Relations/ Pts. I. to V., Phil Mag. 20, 515 (1885) ; 21, 33 

 and 135, 22 ; 32 and 37 (1886). 



I found that Regnault's vapour pressures of mercury below 

 the boiling point, when compared with those of one of the other 

 substances, gave results which did not agree with the formula 

 R'=R+ c(t f -t), and on reading all his papers bearing on this 

 subject I found that Regnault at an early date had made rough 

 determinations of the vapour pressures of mercury below the 

 boiling point and that, at a considerably later date, when he made 

 his complete investigation over a wide range of temperature, he 

 evidently forgot that his early experiments made no claim to 

 great accuracy, and he adopted them without further verification. 

 We therefore found it necessary to redetermine these vapour 

 pressures, and we carried out experiments with special apparatus 

 between 220 and the boiling point of sulphur. We employed 

 the equation R'=R -fc('-t), taking water as the standard 



