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XXIX. The Kinetic Theory of the Dissipation of Energy, 

 By Sir William Thomson*. 



IN" abstract dynamics the instantaneous reversal of the motion 

 of every moving particle of a system causes the system to 

 move backwards, each particle of it along its old path, and at 

 the same speed as before, when again in the same position. 

 That is to say, in mathematical language, any solution remains 

 a solution when t is changed into —t. In physical dynamics 

 this simple and perfect reversibility fails, on account of forces 

 depending on friction of solids ; imperfect fluidity of fluids ; 

 imperfect elasticity of solids ; inequalities of temperature, and 

 consequent conduction of heat produced by stresses in solids 

 and fluids ; imperfect magnetic retentiveness ; residual electric 

 polarization of dielectrics ; generation of heat by electric 

 currents induced by motion ; diffusion of fluids, solution of 

 solids in fluids, and other chemical changes ; and absorption 

 of radiant heat and light. Consideration of these agencies in 

 connexion with the all- pervading law of the conservation of 

 energy proved for them by Joule, led me twenty-three years 

 ago to the theory of the dissipation of energy, which I com- 

 municated first to the Royal Society of Edinburgh in 1852, 

 in a paper entitled "On a Universal Tendency in Nature to 

 the Dissipation of Mechanical Energy." 



The essence of Joule's discovery is the subjection of physical 

 phenomena to dynamical law. If, then, the motion of every 

 particle of matter in the 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 



* Communicated by the Author ; having been originally published in 

 the Proceedings of the Royal Society of Edinburgh for February, 1674. 



