130 MOLECULAR MOTION AND ITS ENERGY 56 



of translatory motion E of the molecule. The values of this 

 last ratio, which are placed in the last column, are in nearly 

 all cases less than 1, and, indeed, are in general less than J. 

 The number of exceptions, including perhaps ethyl ether 1 

 and possibly seven l chlorine compounds, is so small that we 

 shall be inclined to consider the rule, that in gases with 

 ^^^V polyatomic molecules the mean energy of an atom is smaller 

 than the translatory energy of a molecule, to be a veritable 

 law of nature, which, like Boyle's and the other laws of 

 gases, admits exceptions under certain circumstances. 



The possible grounds for such exceptions will appear 

 from the following considerations. 



57. Dissociation and Disgregation 



By our theory and by experiment, so far as the theory 

 has up to the present been confirmed by observation, the 

 molecular energy E consists only in that of the linear to- 

 and-fro motion of the molecules, that is, in kinetic energy, 

 or, in the older nomenclature, in vis viva. We may not, 

 however, assert this of the energy Q* of the atoms nor of the 

 mean energy e of a single atom. For the atoms do not move 

 freely like the molecules, which in the gaseous state exert 

 no cohesion towards each other, but they are bound to 

 each other by chemical affinity, and are, consequently, con- 

 strained to a certain extent in their motion. The energy 

 of the atoms, therefore, does not consist, as that of the mole- 

 cules, entirely in kinetic energy, but also in the potential 

 energy of the affinity which holds the atoms together ; the 

 magnitude ( is the sum of the amounts of both kinds of 

 energy which are present within the molecule, or, according 

 to Leibniz's terminology, it is the sum of the vis viva and 

 vis mortua of the components of the molecule. In the same 

 way the mean energy e of an atom is made up of its kinetic 

 energy and its share of the potential energy of the chemical 

 forces of affinity. 



From this it first of all follows that we are not entitled 

 to infer the magnitude of the speed of the atoms from the 



1 Eefer also to the table of 55. 



