326 Proceedings of the Poyal Society 
tide of matter in tlie universe were precisely reversed at any 
instant, the course of nature would be simply reversed for ever 
after. The bursting bubble of foam at the foot of a waterfall 
would reunite and descend into the water ; the thermal motions 
would reconcentrate their energy, and throw the mass up the fall 
in drops re-forming into a close column of ascending water. Heat 
which had been generated by the friction of solids and dissipated by 
conduction, and radiation with absorption, would come again to the 
place of contact, and throw the moving body back against the force to 
which it had previously yielded. Boulders would recover from the 
mud the materials required to rebuild them into their previous 
jagged forms, and would become reunited to the mountain peak 
from which they had formerly broken away. And if also the 
materialistic hypothesis of life were true, living creatures would 
grow backwards, with conscious knowledge of the future, but no 
memory of the past, and would become again unborn. But the 
real phenomena of life infinitely transcend human science, and 
speculation regarding consequences 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 commences, and the distribution of temperature becomes 
continuously 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 prevented 
by an army of Maxwell’s “ intelligent demons,” * stationed at the 
* 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 organisation to give him. the faculty of observing and 
influencing individual molecules of matter. 
