ON THE KINETIC THEORY OF GASES. 219 



ing to that law, either T or x 2 + y 2 + z 2 , being always in the 

 same ratio to one another, may be taken as the measure. 

 But if we admit, as I think we must, that deviations from 

 this law, small or great, must be always occurring, thanks to 

 the ubiquitous ether or other disturbers of the peace, the 

 question can no longer be evaded. On the whole, the ten- 

 dency has been still to regard energy of translation as 

 measuring temperature. 



31. But to this Tait objects strenuously. I will 

 endeavour to give his reasons. When (he says) there is 

 molecular attraction, the mean kinetic energy increases with 

 the density (which must be admitted on any theory), and so 

 the "sorting demons," as Lord Kelvin calls them, might, 

 by advancing from time to time those portions of an elastic 

 boundary on which no impact is impending, diminish the 

 volume, and so (if kinetic energy is temperature) increase 

 the temperature, without doing any work. But why should 

 they not ? The increase of temperature is consistent with 

 the conservation of energy, because it takes place at the 

 expense of potential. And if it be inconsistent with any 

 other law, e.g., the second law of thermodynamics, is 

 not that exactly the object for which Maxwell created the 

 demons, viz., to violate the second law without violating the 

 conservation of energy? Is it not the law of their being? 

 Violation of the second law is only malum prohibitum. 



32. Tait again says, and I here quote his own 

 words : " Let the contents of equal volumes at different 

 parts of a tall column of gas be compared. In each the 

 pressure may be regarded, so far as it is due to the external 

 potential, as being applied by bounding walls. But the 

 temperature is the same in each, and the only other quan- 

 tity which is the same in each is E (i.e., R/). For as the 

 particles are free to travel from point to point throughout 

 the whole extent of the group, the average value of E must 

 be the same for all, and therefore in regions where the 

 density is small, it must be that of free particles, i.e., 

 absolute temperature." 



I confess to having some difficulty in following this 

 reasoning. I understand E or R/ to be the mean kinetic 

 energy of a molecule during free path, and therefore in case 



