132 MOLECULAR MOTION AND ITS ENERGY 57 



of normal combination or combustion. This presumption 

 is confirmed by EL B. Dixon's 1 observations on mixtures 

 of oxygen and hydrogen. 



Before a complete breaking up of a compound molecule 

 occurs the addition of heat produces a loosening of the bonds 

 of the atoms in the molecule. In this process, which with 

 Clausius we may term disgregation, the heat acts in two 

 ways : it increases the kinetic energy of the atoms and over- 

 comes a part of their affinity. The sum of both actions 

 requires the expenditure of energy, which we will denote by 

 the letter e. 



Disgregation becomes dissociation, i.e. the molecules 

 break up into their atoms, when the energy of the atomic 

 motion is able to overcome the remaining part of the affinity. 



For this to be produced the energy of motion must be at 

 least equal in magnitude to the total amount of the energy 

 of chemical affinity. The value therefore of the atomic 

 energy which is attained at the temperature of commence- 

 ment of complete dissociation is the mechanical measure 

 of the maximum energy to which the chemical affinity of 

 an atom is capable of giving rise. 



58. Dependence of the Specific Heats on 

 Temperature 



The foregoing discussions show that the molecular and 

 atomic energies are by no means magnitudes of the same 

 kind. Now that we know this, it seems doubtful if both 

 kinds of energy will increase in equal measure when the 

 temperature rises. Hitherto we have assumed this, since 

 the theory had given the law that the kinetic energy of 

 the molecules bears a constant ratio to the total energy 

 contained in a gas. The proof of this law, however, rests 

 on the assumption, which is not in general true, that the 

 specific heat of gases at constant volume is independent of 

 the temperature. 



We cannot well test by direct observation of the speci- 

 fic heat at constant volume whether this assumption is 



1 Nature, xxxii. 1885, p. 535. 



