226 POPULAR LECTURES AND ADDRESSES. 



of Maxwell's army of sorting demons all round 

 the enclosure, with orders to push in the boundary 

 diligently everywhere, when none of the besieged 

 troops are near, and to do nothing when any of 

 them are seen approaching, and until after they 

 have turned again inwards. The result will be 

 that with exactly the same sum of kinetic and 

 potential energies of the same enclosed multitude 

 of particles, the throng has been caused to be 

 denser. Now Joule's and my own old experiments 

 on the efflux of air prove that if the crowd be 

 common air, or oxygen, or nitrogen, or carbonic 

 acid, the temperature is a little higher in the 

 denser than in the rarer condition when the 

 energies are the same. By the hypothesis, 

 equality of temperature between two different 

 gases or two portions of the same gas at different 

 densities means equality of kinetic energies in 

 the same number of molecules of the two. From 

 our observations proving the temperature to be 

 higher, it therefore follows that the potential 

 energy is smaller in the condensed crowd. This 

 always, however, under protest as to the 



