327 
of Edinburgh, Session 1873 - 74 . 
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 he 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 molecules, 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 communication of energy from one molecule 
to another. Fourier’s exquisite mathematical analysis expresses 
perfectly the statistics 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 liquid), which Fick showed to be subject to Fourier’s for- 
mulas. Now, suppose the weapon of the ideal army to be a club, 
or, as it were, a molecular cricket bat; and suppose, for convenience, 
the mass of each demon with his weapon to be several times 
greater than that of a molecule. Every time he strikes a molecule 
he is to send it away with the same energy as it had immediately 
before. Each demon is to keep as nearly as possible to a certain 
station, making only such excursions from it as the execution of 
his orders requires. He is to experience no forces except such as 
result from collisions with molecules, and mutual forces between 
parts of his own mass, including his weapon. Thus his voluntary 
movements cannot influence the position of his centre of gravity, 
otherwise than by producing collision with molecules. 
The whole interface between hot and cold is to be divided into 
small areas, each allotted to a single demon. The duty of each 
demon is to guard his allotment, turning molecules back, or allow- 
ing them to pass through from either side, according to certain 
