In science, equilibration refers to an interaction type of state or process between a system and its surroundings, such that there is a progression of equilibrium states. The term is similar to thermalization, the exchange of energy between two systems.

The term "equilibration" seems to have been introduced by English philosopher scientist Herbert Spencer, employed greatly in his noted treatise First Principles (where he outlined four types of equilibration), to account for evolution in relation to society and changes in its surroundings. [1]

In circa 1858, Spencer was confounded when he was told by Irish physicist John Tyndall that his ideas on equilibration in relation to the end states of processes was incongruous with the second law and the logic of heat death. In a letter to Tyndall, Spencer declares: [2]

“Regarding, as I had done, equilibration as the ultimate and highest state of society, I had assumed it to be not only the ultimate but also the highest state of the universe … and your assertion that when equilibrium was reached life must cease, staggered me. Indeed, not seeing my way out of the conclusion, I remember being out of spirits for some days afterwards.”

The issue of what then happens to a society after equilibration is reached stayed on Spencer’s mind for the next forty years. In his 1899 autobiographical “The Filliation of Ideas”, Spencer concludes that when equilibration is completed, “dissolution” occurs. [3]

1. Spencer, Herbert (1897). First Principles (ch XXII: Equilibration, pgs. 496-530). D. Appleton & Co.
2. Duncan, David. (1908). Life and Letters of Herbert Spencer (vol I, pg. 136). Two volumes. New York: Appleton.
3. Bailey, Kenneth D. Social Entropy Theory (pg. 58). State University of New York Press.

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