316 DIRECT PROPERTIES OF MOLECULES 114 



and atomic volumes are greater in the liquid state than in 

 the gaseous. The weightiest reason that he adduces for 

 this view rests on a conception, like that of our gaseous 

 theory, of the state of motion which the molecules of a 

 liquid take in consequence of their heat. 



We have to consider the atoms in lively motion not only 

 in the gaseous state, but also in the liquid and solid states. 

 The solid state seems to be characterised by the centroids of 

 the molecules being at rest while the atoms move. Dulong 

 and Petit 's law at least points to this, in so far as it 

 establishes a relation for the atomic heat of different bodies 

 into which the molecular heat does not enter at all. We 

 shall probably have to conceive of the liquid state as some- 

 thing between the other two ; so that we have to ascribe 

 to the particles of a liquid both molecular and atomic 

 motions. Of whatever kind these motions may be, 1 they in 

 any case require space for their performance. In a liquid, 

 therefore, a molecule will, under all circumstances, require a 

 larger space than if it were at rest. 



The space demanded by a molecule will presumably 

 increase, not only with the kinetic energy but also with the 

 speed itself, in such a way that, of two different kinds of 

 molecules whose energies are equal, the lighter needs a 

 larger space for its correspondingly quicker motion than the 

 heavier and therefore more slowly moving molecule. The 

 same holds good for the atoms. By this consideration 

 Lothar Meyer explains the behaviour, for instance, of 

 hydrogen, for which a much smaller atomic volume results 

 from consideration of its viscosity than Kopp had calcu- 

 lated for it from its liquid compounds ; and this was his 

 reason for assigning to hydrogen in gaseous molecules a 

 smaller volume than in molecules of a liquid. 



Similar considerations may enter into the case of other 

 atoms, even if, perhaps, they are less striking. We may 

 therefore assume it as possible, for the molecules built up 

 of atoms, that their molecular volume in the liquid state 

 is larger than in the gaseous. 



1 On this subject further explanations will be found in Clausius' memoir, 

 Pogg. Ann. 1857, c. p. 360 ; Abhandl 2. Abth. 1867, p. 236. 



