196 Theory of a Non-Conservative Gas [CH. vm 



235. The phenomena which give the most definite and least questionable 



, insight into the structure of matter are those of spectroscopy. The line 



v/ spectrum of a gas can be most naturally interpreted as evidence of the 



existence of a number of isochronous free vibrations in the ultimate units 



of the gas. 



It is true that other explanations of the line spectrum can be suggested ; 



these will be reserved for discussion later. At present it is proposed to 

 . examine the one just mentioned. We shall therefore assume our gas to 

 * consist of molecules of which the internal structure is such that they possess 



a number of internal vibrations, of which the frequencies of free vibrations 



are equal to those of light. 



The principal problem before us is of course to examine the rate of 

 transfer of energy in this gas. Experiment suggests that the internal 

 degrees of freedom do not receive anything like the amount of energy which 

 would be assigned to them by the Law of Equipartition. The theory 

 developed in the last two chapters indicates that this must occur if the 

 internal degrees of freedom are capable of dissipating away their energy, 

 and if also they receive energy with sufficient slowness from the translational 

 motion. The mere fact that we are aware of the existence of these internal 

 degrees of freedom shews that they are capable of dissipating their energy, 

 for it is only their dissipated energy, in the form of light-waves, which tells 

 us of their existence. As to the slowness of transfer of energy, the two 

 illustrations of the theory discussed in the present chapter both suggest that 

 for any type of structure the transfer of energy 7 will, at sufficiently low 

 temperatures, be " slow " in the sense required. 1 ^ The question before us is 

 therefore as much to determine the actual limit of temperature within 

 which this transfer may be regarded as "slow," as to determine whether such 

 a limit of temperature exists. 



