Manchester Memoirs, Vol. xliii. (1899), No. 4. 7 



of freedom. But a molecule consisting of three atoms 

 possesses six degrees of freedom, for in addition to the 

 other five, it may rotate in the third plane. In these cases, 

 some of the energy given to a gas when it is heated will 

 be expended in causing such rotational motions, and the 

 specific heat at constant volume of such a gas will be greater 

 than if no such rotational motions were possible. But the 

 heat corresponding to the work done during the expansion 

 of the gas will not be altered thereby ; and hence the 

 ratio between the two specific heats of the gas, that at 

 constant volume and that at constant pressure, will be 

 smaller. I need not enter into the details of this theorem, 

 but will merely state that for a gas consisting of di-atomic 

 molecules, the ratio of the specific heats is calculated 

 by Boltzmann to be i'4, and that of tri-atomic gases 

 1-3. With gases whose molecules contain a still larger 

 number of atoms, and indeed for some of those with three, 

 the inter-atomic motions become more complex, and 

 absorb energy in the form of heat to a still greater extent. 



It will be noticed that these ideas of Boltzmann's 

 stand on a very different footing from the first conception 

 by which it is shown that the specific heat ratio for a 

 mono-atomic gas should be 1.6. The problem is more 

 complex, and an entirely different set of considerations is 

 introduced to deal with it. Yet it is very ingenious, and 

 affords a plausible explanation of the found ratio of 

 specific heats of di-atomic gases. 



I have ventured to draw your attention to these con- 

 siderations, because the recently discovered gases of the 

 atmosphere throw some light on the kinetic theory of 

 gases, and support the views which I have endeavoured 

 to make clear to you. I will ask your permission, however, 

 to interpolate here a short account of the discovery of 

 these gases. 



