384 SCIENCE PROGRESS 



rotation, it has to be regarded as a massive point. One would 

 expect this to be true of any atom . 



In dealing with a diatomic molecule, Bjerrum ascribes to it 

 three degrees of freedom in virtue of translation, two in virtue 

 of rotation, and one in virtue of vibration, i.e. vibration along 

 the line joining the two atoms. As our present purpose is to 

 consider more especially the question of rotation of gas molecules, 

 we are not further concerned with the treatment of vibrations 

 which Bjerrum gives in terms of the quantum theory. Rota- 

 tions are regarded in the first place as being governed by the 

 equipartition principle of classical statistical mechanics, i.e. 

 the amount of kinetic energy per degree of freedom is 1/2 RT, 

 and, as rotational energy is regarded as being entirely kinetic, 1 

 the amount of energy in the rotational form in a diatomic 

 molecule is RT. The rotation of this type of molecule is analo- 

 gous to that of a cylinder rotating around its major axis. In 

 spite of the analogy, one can scarcely avoid the conclusion that 

 rotational energy is here attributed to each atom, a conclusion 

 which it is difficult to reconcile with the results obtained in the 

 case of argon. 



In molecules containing three or more atoms, Bjerrum 

 considers that there are three degrees of freedom in respect of 

 rotation. He is obviously treating such molecules in exactly 

 the same way as we would treat a solid sphere. On summing 

 the various energy terms due to the different types of motion, 

 Bjerrum has succeeded in accounting, more or less satisfactorily, 

 for the observed molecular heats of a number of gases. We 

 are rather inclined to think, however, that the number of degrees 

 of freedom chosen is a little arbitrary. Whilst recognising the 

 value of Bjerrum's contribution to the subject, it is obvious, 

 at the same time, that the problem is by no means solved. 

 This will be apparent when we come to consider the views 

 recently put forward by Kriiger. 



Kriiger (Ann. Physik, 191 6, [4], 50, 346 ; ib. 51, 450) takes 



1 As a matter of fact, Bjerrum has shown that the potential energy of rotation is 

 a negligibly small fraction of the kinetic energy. In a later paper (Nernst Fest- 

 schrift, 1912, p. 90) Bjerrum regards rotational energy as a quantity which has 

 to be treated from the standpoint of the quantum theory. He there indicates how 

 this accounts for the broadening of certain lines in the shorter end of the infra-red 

 spectrum, such broadening being due to a number of fine lines lying on either side 

 of the principal line itself. 



