In thermodynamics, the law of dissipation states that in the material world there exists a universal tendency to the dissipation of mechanical energy and that any restoration of mechanical energy, without more than an equivalent of dissipation, is impossible. [1]

Overview
The law of dissipation was stated by Irish physicist William Thomson in his April 1852 paper “On a Universal Tendency in Nature to the Dissipation of Mechanical Energy”. [2] The following simplified original statement of the law of dissipation of energy, made by Kelvin at the conclusion of his paper, is oft-quoted summary of the law of dissipation: [3]

Law of Dissipation (parts)
1. There is at present in the material world a universal tendency to the dissipation of mechanical energy.
2. Any restoration of mechanical energy, without more than an equivalent of dissipation, is impossible in inanimate material processes, and is probably never effected by means of organized matter, either endowed with vegetable life or subject to the will of an animated creature.
3. Within a finite period of time past, the earth must have been, and within a finite period of time to come the earth must again be, unfit for the habitation of man as at present constituted, unless operations have been, or are to be performed, which are impossible under the laws to which the known operations going on at present in the material world are subject.

	Law of Dissipation (parts)
1.	There is at present in the material world a universal tendency to the dissipation of mechanical energy.
2.	Any restoration of mechanical energy, without more than an equivalent of dissipation, is impossible in inanimate material processes, and is probably never effected by means of organized matter, either endowed with vegetable life or subject to the will of an animated creature.
3.	Within a finite period of time past, the earth must have been, and within a finite period of time to come the earth must again be, unfit for the habitation of man as at present constituted, unless operations have been, or are to be performed, which are impossible under the laws to which the known operations going on at present in the material world are subject.

The law of dissipation was an early verbal prototype version of the second law of thermodynamics made more rigorous in the years to follow by German physicist Rudolf Clausius in his mechanical theory of heat.

References
1. Thomson, William. (1852). "On a Universal Tendency in Nature to the Dissipation of Mechanical Energy" (Google Books) (URL), Proceedings of the Royal Society of Edinburgh for April 19, 1852, also Philosophical Magazine, Oct. 1852, also Mathematical and Physical Papers, vol. i, art. 59, pp. 511.
2. Anon. (1908). “Obituary Notices of Fellows Deceased: Lord Kelvin” Proceedings of the Royal Society of London (pgs. iii-lxxvi, keyword: “law of dissipation”, pg. xxxvi). Vol 81.
3. Thayer, William R. (1921). “Vagaries of Historians”. Annual Report of the American Historical Association (pgs. 80-84, “law of dissipation”, pg. 81). G.P.O.