294 THE REALITIES OF MODERN SCIENCE 



other there are six degrees of freedom, namely, three of 

 translation, two of rotation, and one of oscillation. 

 In an oscillation, however, there is a continuous change 

 of kinetic energy into potential and then of potential 

 into kinetic. According to the generalization of the 

 equipartition of energy the kinetic energy of this 

 oscillation is on the average 1 calorie. Now the 

 average value of the potential energy must be the 

 same. 1 There should then be required an average of 

 1 calorie of kinetic and 1 calorie of potential energy, 

 or a total of two calories for this degree of freedom. 

 The molecular heat should then be 7 calories and not 

 6. We, therefore, expect the molecular heat of a dia- 

 tomic molecule to be either 5 or 7 calories, depending 

 upon whether or not there is freedom of oscillation, 

 but we do not expect intermediate values. Such, 

 however, are found in actual experiment. 



The idea of an equipartition of energy among the 

 degrees of translation was seen in Chapter XIII to lead 

 to a correct expression of the kinetic relations for gas 

 molecules. An equipartition between translation and 

 rotation (or internal oscillations) would seem from 

 page 291 to be physically necessary for granular atoms 

 composed of electrons. As a matter of fact, one of the 

 methods employed by Perrin in determining the value 



1 For convenience in this relative motion consider that one atom 

 oscillates about a mean position with reference to the other. The 

 oscillating atom has its maximum k.e. while passing through this 

 unstressed position. At the turning points, both nearer and farther 

 from the other atom, there is zero k.e., but in these positions it has 

 a potential energy equal to its maximum k.e. The result is that the 

 k.e. varies from a maximum to zero while the p.e. is varying from 

 zero to an equal maximum. The average values of the p.e. and the 

 k.e. are then equal. 



