[ 374 ] 



XXXT. 



THE YAPOUR-PRESSUEES, SPECIFIC VOLUMES, HEATS OF 

 VAPORISATION, AND CRITICAL CONSTANTS OF THIRTY 

 PURE SUBSTANCES. 



By SYDNEY YOUNG, D.Sc, F.R.S., Trinity College, Dublin. 

 [Read Januahy 25. Ordered for Publication Febeuahy 8. Published June 10, 1910.] 



The kinetic theory in its original form is applicable only to theoretically 

 perfect gases, the molecules, so far as their dimensions are concerned, being 

 regarded as mathematical points. It is assumed that they are endowed with 

 perfect elasticity, and that they do not attract one another. 



The extension of the theory to cover the case of real gases under high 

 pressure and of liquids is due, in the first place, to Van der Waals,' who 

 proposed the fundamental equation of state : — 



p + -^V-y -h) = ST. 



The researches of Andrews, Amagat, and Ramsay and Young, liowever, 

 led to the conclusion that the equation of Van der Waals, althougli 

 reproducing the general form of the isotliermals, did not give results in 

 sufficiently close agreement with the experimentally determined data ; and 

 numerous modifications of the formula have been proposed by physicists, but 

 no completely satisfactory equation has yet been devised. 



On the other hand, Van der Waals pointed out that by expressing the 

 pressures as fractions of tlie critical pressure, the absolute temperatures as 

 fractions of the absolute critical temperature, and the volumes of liquid and 

 of vapour as fractions of tlie critical volume, a " reduced " equation could 

 be derived from the original formula which should be applicable to all 

 substances. 



Applying the reduced equation to the special case of liquids at their 

 boiling-points and saturated vapours, it would follow that if the pressures of 

 any two substances are proportional to their critical pressures, tlieir boiling- 

 points (expressed as absolute temperatures) should be proportional to their 

 critical temperatures, and their volumes, both as liquid and as saturated 

 vapour, should be proportional to their critical volumes. 



' Continuitat der gasformigen und fliissigen Zusfandes. 



