MOLECULAR ENERGY 293 



we know that, although it is small, it is finite in diam- 

 eter. Helium and mercury vapor, which are both 

 monatomic gases, also have values of molecular heat 

 of about 3 calories and behave as if there was no energy 

 of rotation. 



In the case of a diatomic gas there are five degrees 

 of freedom, if the distance between the centers of the 

 atoms remains fixed. The position of one atom is 

 then determined with reference to the other by two 

 angles which are formed by the connecting line and 

 two reference planes, one angle giving the bearing or 

 " azimuth" and the other the height or " elevation" as 

 measured from a plane perpendicular to the first 

 reference plane. Such a system, therefore, has five 

 degrees of freedom, and we should expect its specific 

 heat to be 5 calories. 



Many of the diatomic gases have molecular heats 

 of about this value. For example, oxygen has the 

 following values: C v is 5.17 at 300 C., 5.35 at 500, 

 and 6.00 at 2000 C. It will be noticed that although 

 in one instance C v has about the expected value, it 

 does vary and is higher at the higher temperatures. 

 At the higher temperatures oxygen seems to be more 

 nearly in the condition of the gases which we know 

 dissociate. For such gases we find the value of the 

 molecular heat about 6 at ordinary temperatures and 

 higher than that at still higher temperatures. It 

 seems as though the bonds which maintain the dis- 

 tance between the two atoms were gradually weakening, 

 allowing an oscillation of the atoms in the case of more 

 and more molecules as higher temperatures are reached. 



If there is a motion of one atom relative to the 



