THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



DECEMBER 1881. 



XLIX. On the Theoretic Determination of Vapour-pressure and 

 the Volumes of Vapour and Liquid. By Prof. R. Clausius*. 



§ 1. "TTTHEN a gas is more and more compressed at con- 

 ▼ ' stant temperature, at a certain pressure, as is 

 known, condensation commences, and is completed without 

 any increase of the pressure ; and only when it is finished is 

 an augmentation of the pressure required for a still further 

 diminution of the volume, which pressure must then be in- 

 creased in proportion very greatly. Besides this actual course 

 of the thing, J. Thomson, as is well known, has imagined an- 

 other process, which, it is true, cannot in reality take place, 

 because the states of equilibrium occurring in it are in part 

 unstable, but is yet theoretically conceivable — namely, an alte- 

 ration of volume in which the entire mass is supposed to be 

 constantly homogeneous, and the pressure accordingly to 

 change continuously. The curve which represents the change 

 of pressure corresponding to the change of volume for this 

 last process may be named briefly the theoretic isothermal. 

 The real isothermal differs from it by this — that for a certain 

 length, which in the case of compression corresponds to the 

 process of condensation, and, conversely, in that of expansion 

 corresponds to the process of vaporization, the curved line is 

 replaced by a straight line parallel to the axis of abscissas. 

 This straight line must, as can be demonstrated from the 

 second proposition of the mechanical theory of heat, be situated 



* Translated from Wiedemann's Annalen, vol. xiv. pp. 279-290. 

 Phil. Mag. S. 5. Vol. 12. No. 77. Dec. 1881. 2 G 



