Young — Vapou?-- Pressures, <^"c., of Thirty Pure Substances. 875 



Moreover, these geueralisatioDS regarding " correspondiug " pressures, 

 temperatures, and volumes, might be true, even though the original equation 

 required considerable modification. 



It was with the object of testing tlie correctness of these generalisations 

 that I undertook a series of determinations of vapour-pressures, specific 

 volumes, and critical constants of pure liquids. 



The data have been published in a series of papers in the Transactions of 

 the Chemical Society, the Proceedings of the Physical Society, and the 

 Philosophical Magazine' ; but, for the following reasons, complete data for 

 any one substance are not to be found in a single paper : — 



1. New methods have from time to time been devised, chiefly for the 

 determination of the specific volumes of liquid and saturated vapour. 



2. When the earliest ])apers were published, no satisfactory method of 

 determining the critical volume of a substance was known ; and it was shown 

 by Gouy that, owing to the extreme compressibility of a substance in the 

 immediate neighbourhood of the critical point, the direct method at first 

 employed could not be regarded as accurate. 



It was, however, discovered by Cailletet and Mathias that the mean 

 densities of liquid and saturated vapour, when plotted against the 

 temperatures, appear to fall on a straight line (fig. 1, p. 376)''; and they 

 showed that the point of intersection, 0, of this rectilinear diameter, AC, 

 with the closed curve of orthobarie volumes, VCL, gives the critical density. 



Slight deviations from the law of Cailletet and Mathias were observed as 

 a rule ; but they ajipeared to be insignificant, and were attributed to errors of 

 experiment; and the critical densities of the great majority of the substances 

 were determined by this method. 



Subsequently, however, on close investigation of the whole of the results, 

 it appeared that the deviations were too regular to be attributable to 

 experimental error, and that the diameter must in most cases be regarded as 

 slightly curved. A redetermination of the critical densities, taking tlie 

 slight curvature of the diameter into account, was therefore necessary. 



3. The methods employed for the determination of the volumes of a gram 

 of the saturated vapours of most of the substances are of such a nature that 

 the results increase in accuracy as the temperature rises. When the papers 

 were first published, no special importance was attached to the data at the 

 lowest temperatures (generally not far from the boiling-points under normal 

 pressure) ; and it was considered sufficient to plot the logarithms of the 



' The expeiimeiits on ethyl ether, methyl alcohol, ethyl alcohol, propyl alcohol, and acetic acid 

 (up to 290°) were carried out by Sir William Eamsay and the author at an earlier date. 

 '' In this figure the actual data for normal pentaue are reproduced. 



3f2 



