54 MOLECULAR AND ATOMIC ENERGY 121 



This excludes the idea that a rotation of the molecules 

 about their centroids can occur ; or, at least, if such a rota- 

 tion be present, it cannot be altered by addition of heat ; _\jk 

 for, though we need not put H K, we must at least y 

 put H H = K K Q . The rotation of the particles has 

 therefore to be independent of their rectilinear motion, 

 and has not to be altered by collision between two particles. 

 This is difficult to understand if the laws of elastic collision 

 are to hold good for the collisions of atoms. We may not 

 therefore figure the atoms as small elastic bodies occupying 

 spaces, like spheres for instance, but as vanishingly small 

 massive points, if we do not make other assumptions as to 

 their nature, to which we shall return in Chapter X., 123 

 and 124. 



That monatomic gaseous molecules also may be capable 

 of oscillatory motions in their interior we may look upon 

 as probable, since in their spectra whole series of different 

 lines are found. But these motions, as we may assume in 

 accordance with E^ Wiedemann ' s l observations, require 

 so small an expenditure of energy that its amount does not 

 come at all into account in comparison with the kinetic 

 energy of the molecular motion. 



Hence monatomic molecules need in no way be rigid 

 massive points ; it is only necessary that they should be 

 very small particles in whose interior only such motions 

 can come into play as demand but very little energy. It 

 therefore does not appear impossible that the ratio 

 C/c = 1-67 should be found in the case of chemically com- 

 pound molecules also, if the connection of the atoms is so 

 firm that internal motions are excluded. 



55. Polyatomic Molecules 



In the case of a gas whose molecules are made up of 

 several atoms, the ratio of the kinetic to total energy, K : J3", 

 is indeed not directly known ; but in spite of this we can 

 here too test by observation the correctness of the theory 



1 Wied. Ann. 1889, xxxvii. pp. 241, 248. Comp. 123. 



