[From Nature, Vol. x.] 



LXIX. Van der Wacds on the Continuity of the Gaseous and Liquid States*. 



THAT the same substance at the same temperature and pressure can exist 

 in two very different states, as a liquid and as a gas, is a fact of the highest 

 scientific importance, for it is only by the careful study of the difference be- 

 tween these two states, the conditions of the substance passing from one to the 

 other, and the phenomena which occur at the surface which separates a liquid 

 from its vapour, that we can expect to obtain a dynamical theory of liquids. 

 A dynamical theory of " perfect " gases is already in existence ; that is to say, 

 we can explain many of the physical properties of bodies when in an extremely 

 rarefied state by supposing their molecules to be in rapid motion, and that 

 they act on one another only when they come very near one another. A 

 molecule of a gas, according to this theory, exists in two very different states 

 during alternate intervals of time. During its encounter with another molecule, 

 an intense force is acting between the two molecules, and producing changes 

 in the motion of both. During the time of describing its free path, the mole- 

 cule is at such a distance from other molecules that no sensible force acts 

 between them, and the centre of mass of the molecule is therefore moving with 

 constant velocity and in a straight line. 



If we define as a perfect gas a system of molecules so sparsely scattered 

 that the aggregate of the time which a molecule spends in its encounters with 

 other molecules is exceedingly small compared with the aggregate of the time 

 which it spends in describing its free paths, it is not difficult to work out the 

 dynamical theory of such a system. For in this case the vast majority of the 

 molecules at any given instant are describing their free paths, and only a small 



* Over de Continniteit van den Gas en Vloeistofloestand. Academisch pro?fsclirift. Door Johannes 

 Diderik van der Waals. (Leiden: A. W. Sijthoff, 1873.) 



