o f Edinburgh , Session 1873-74. 325 
Brought forward, 356 
Deduct Deceased — Rev. Dr Guthrie; Prof. John 
Hunter; Very Rev. Dean Ramsay; Prof. 
Macquorn Rankine ; Arch. Smith, Esq. ; 
Rev. Prof. Stevenson ; Dr J. L. Stewart ; R. 
W. Thomson, Esq., ..... 8 
Resigned — J. E. M’Lennan, Esq. ; Dr Alex. Wood, 2 
Cancelled — Dr Richardson, Dr Foulerton, . . 2 
12 
Total number of Ordinary Fellows at Nov. 1873, . 344 
The following Communications were read 
9. 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 - 1. 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 polarisation of dielectrics; genera- 
tion 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 connection with the all-pervading law of the conserva- 
tion of energy proved for them by Joule, led me twenty-three years 
ago to the theory of the dissipation of energy, which I communi- 
cated 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 par- 
