6 MOLECULAR MOTION AND ITS ENEEGY 2 



rations which the atoms can individually execute without 

 /breaking up the molecule. Atomic motion includes, there- 

 fore, not only the oscillations that take place within the 

 molecule, but also the rotation of the atoms about the 

 centroid of the molecule. 



This division of the whole heat-motion corresponds to 

 the division of physical science into physics and chemistry 

 not, indeed, in every respect, but in so far as chemistry 

 deals chiefly with the equilibrium of atoms, while physics 

 treats more of the mechanics of molecules. 



Chemical equilibrium, or the unchanged existence of 

 molecules, is attained when the affinity which holds the 

 atoms together is in equilibrium with the forces that tend 

 to break up the molecule ; such forces arise from the 

 motion of the atoms, partly from the collisions of those 

 which vibrate, and partly from the centrifugal tendency of 

 those which rotate. Since then, in a chemically stable 

 body, the atomic motions are kept in continuous dynamical 

 equilibrium with the chemical forces, and their action is 

 overcome by the latter, only the molecular motion comes 

 primarily into account in the investigation of purely physical 

 forces and phenomena, and we therefore limit the range of 

 our discussion in the first place to the latter. 



Just as the atomic motion tends to break up a molecule, 

 so the molecular motion tends to loosen the connection 

 between the parts of a body, partly in consequence of 

 collisions between the molecules, and partly from their 

 centrifugal tendency ; and equilibrium is maintained at 

 when there is no external pressure by cohesion, a 

 in which we need see nothing different from affinity. 

 It seems enough to account for cohesion in an excess of 

 affinity over the dissociating action of atomic motion, which 

 is not large enough to attract an atom into the pale of 

 a molecule and to keep it there, but is sufficient to 

 bind together neighbouring molecules in a much less close 

 bond. 



The problem of discovering the laws of molecular motion 

 is therefore identical with that of determining the laws of 

 cohesion, since when the medium is in equilibrium the 



