292 Sir William Thomson on the Kinetic 



transcend human science ; and speculation regarding conse- 

 quences of their imagined reversal is utterly unprofitable. 

 Far otherwise, however, is it in respect to the reversal of the 

 motions of matter uninfluenced by life, a very elementary 

 consideration of which leads to the full explanation of the 

 theory of dissipation of energy. 



To take one of the simplest cases of the dissipation of energy, 

 the conduction of heat through a solid — consider a bar of 

 metal warmer at one end than the other, and left to itself. 

 To avoid all needless complication of taking loss or gain of 

 heat into account, imagine the bar to be varnished with a 

 substance impermeable to heat. For the sake of definiteness, 

 imagine the bar to be first given with one-half of it at one 

 uniform temperature, and the other half of it at another 

 uniform temperature. Instantly a diffusing of heat com- 

 mences, and the distribution of temperature becomes con- 

 tinuously less and less unequal, tending to perfect uniformity, 

 but never in any finite time attaining perfectly to this ultimate 

 condition. This process of diffusion could be perfectly pre- 

 vented by an army of Maxwell's "intelligent demons,"* 

 stationed at the surface, or interface as we may call it with 

 Professor James Thomson, separating the hot from the cold 

 part of the bar. To see precisely how this is to be done, 

 consider rather a gas than a solid, because we have much 

 knowledge regarding the molecular motions of a gas, and little 

 or no knowledge of the molecular motions of a solid. Take 

 a jar with the lower half occupied by cold air or gas, and the 

 upper half occupied with air or gas of the same kind, but at 

 a higher temperature ; and let the mouth of the jar be closed 

 by an air-tight lid. If the containing vessel were perfectly 

 impermeable to heat, the diffusion of heat would follow the 

 same law in the gas as in the solid, though in the gas the 

 diffusion of heat takes place chiefly by the diffusion of mole- 

 cules, each taking its energy with it, and only to a small 

 proportion of its whole amount by the interchange of energy 

 between molecule and molecule ; whereas in the solid there 

 is little or no diffusion of substance, and the diffusion of heat 

 takes place entirely, or almost entirely, through the com- 

 munication of energy from one molecule to another. Fourier's 

 exquisite mathematical analysis expresses perfectly the sta- 

 tistics of the process of diffusion in each case, whether it be 

 " conduction of heat/' as Fourier and his followers have called 

 it, or the diffusion of substance in fluid masses (gaseous or 



* The definition of a demon, according- to the use of this word by 

 Maxwell, is an intelligent being endowed with free-will and fine enough 

 tactile and perceptive organization to give him the faculty of observing 

 and influencing individual molecules of matter. 



