BRIDGMAN. — THERMODYNAMIC PROPERTIES OF LIQUIDS. Ill 



i;ional to the kinetic energy of translation of the molecules is not new. 

 For instance, there is a recent paper by Brillouin,^® in which he dis- 

 cusses at length the possible necessity of a change in the usual defini- 

 nition of temperature as the volume changes. 



One other very important consideration which we shall probably 

 be obliged to introduce into a theory valid for high pressures has 

 already been mentioned several times in the discussion of the various 

 thermodynamic properties. It is this; in gases we think of the 

 molecules as perfect spheres, but at high pressures we shall undoubt- 

 edly have to recognize that they possess more complicated shapes. 

 It is inconceivable that a molecule should not have a characteristic 

 shape; we cannot well imagine the possibility of so fitting together 

 the atoms of a complicated organic compound as not to produce some 

 irregularities in the molecule. Shape becomes increasingly important 

 at high pressures where the molecules are forced together and con- 

 strained to adapt themselves as best they can to each others irregu- 

 larities. Beside the possibility of shape, there is the possibility that 

 there are local centers of force in the molecule ; that we cannot regard 

 the molecule as a homogeneous sphere exerting a force towards its own 

 center, but that when we approach too close to the molecule, there are 

 individualized centers of force that begin to act of their own account. 

 This again, seems by no means a forced conception. 



Along with the idea of molecules M'ith shape goes the conception 

 that at high pressures these shapes must be forced to more or less 

 adapt themselves to each other; in other words, the molecules must 

 begin to show traces of regular arrangement. The regularity is by 

 no means the thorough going regularity of a crystal in which the 

 molecules are permanently moored to certain mean positions: the 

 molecules of the liquid still circulate about among each other, but as 

 they slide past each other there may be a growing tendency at higher 

 pressures to point the long axes in the direction of relative motion, 

 for example. Just as at a crowded ball room, there is a tendency for 

 the throng of }'oung men making their way to and from the refreshment 

 room to hold their plates out from them in the direction of motion. 

 This increasing order of arrangement seems not only natural, but 

 inevitable at high pressures. It may ultimately terminate in crystal- 

 lization. We should expect furthermore, at equal volumes, there 

 should be nearer approach to order at lower temperatures where the 

 violence of temperature agitation is less. 



59 Brillouin, Ann. Chim. et phys., 18, 387-400 (1909). 



