288 SCIENCE PROGRESS. 



they are of such a nature as not to be capable of being 

 excited by collisions between the molecules ; and hence for 

 some reason as yet not understood they are removed from 

 the sphere of Boltzmann's theorem. 



It has been objected again that, even if the mercury 

 molecule is not capable of vibrations, it must at least be a 

 rigid body, and so have six degrees of freedom, whilst 

 Boltzmann's theorem only allows it three. Watson's 

 suggestion quoted above seems to be a sufficient answer 

 to this. Boltzmann's theorem only tells us what the dis- 

 tribution of energy is when the gas has reached a steady 

 state, and it may very well be that with the rapid change 

 of temperature caused by the passage of sound waves the 

 gas never has time to reach the steady state. 



Possibly the values of y might be very different if the 

 changes of temperatures were much slower than they are 

 in Kundt's method. The accepted value of y for carbon 

 dioxide obtained by Kundt's method is about 1*3. Reg- 

 nault's value for C^, between 15 and ioo° is '2025. Joly's 

 value for C 7 , between 12° and ioo° is *i 653, and the ratio of 

 these is 1*22. Both C p and C v are difficult to measure 

 accurately, but 7 per cent, seems to be too much to attri- 

 bute to experimental error. It would seem that we have 

 here to do with forms of vibration that only take their 

 share of the energy when given more time than is afforded 

 by sound vibrations. 



For the majority of diatomic gases 7 is near 1 "4, thus 

 pointing to five degrees of freedom. This again is less 

 than the number possessed by a single rigid body. If we 

 ascribe three to the linear motion of the centre of gravity, 

 we have left only two for the internal motions, which is not 

 enough to allow the molecule to rotate in the most general 

 way. The molecule has, in fact, the same number of 

 degrees of freedom as a finite material line moving freely 

 in space. If the two atoms were small compared with the 

 distance between them, the rotation about the line joining 

 them would probably change much more slowly than the 

 rotation about any line perpendicular to this line, and the 

 molecule would appear to have only five degrees of freedom. 



