The first segment of a ten page science cartoon on fugacity by Lucas Landherr. [4]
In science, fugacity, or “escaping tendency”, symbol Ψ (Psi), is a quantity that captures the tendency of a molecular species to escape from the phase in which it is in, when that phase is brought into contact with any other phase not containing that species, defined by the following two conditions: [1]

a. The fugacity of a molecular species is the same in two phases when these phases are in equilibrium as regards the distribution of that species.
b. The fugacity of a gas approaches the gas pressure as a limiting value if the gas is indefinitely rarefied, i.e. the escaping tendency of a perfect gas is equal to its gas pressure.


See main: Social fugacity
In 1934, American sociologist George Homans, in his An Introduction to Pareto: His Sociology, in reference to Harvard Pareto circle, in an attempt to clarify Vilfredo Pareto’s physico-chemical based definition of socio-economic equilibrium, cited, via footnote, the following 1915 statement by American physical chemist Edward Washburn (Ѻ): [2]

“The effect of any physico-chemical equilibrium, produced by an attempt to alter any one of the factors which influence it, can be qualitatively predicted by means of a theorem formulated by Le Chatelier which may be stated as follows: If an attempt is made to alter any one of the factors (e.g. the temperature or pressure of the system or the fugacity of any constituent of the system) which influence any physico-chemical equilibrium, then a shift in the equilibrium will take place in such a direction as to decrease the magnitude of the alteration which would otherwise occur in that factor.”

It does not seem, however, that Homans here had any idea what the term “fugacity” meant, as it is one of the more veiled terms in chemical thermodynamics, possibly second only to that of entropy.

In 1987, Mirza Beg, in his New Dimensions in Sociology: a Physico-Chemical Approach to Human Behavior, employed the fugacity concept in an attempt to quantify or describe the tendency of a human, conceptualized as a fluid-like molecule, to escape his or her society; the following are examples: [3]

“If the study of fugacity is applicable to the escaping tendency of a set of organisms from a certain environment, it could be extended to the process of urbanization. It might be possible to calculate the rate of migration from the rural to the urban areas and later on, the rate of emigration from the country.”
— Mirza Beg (1987), New Dimensions in Sociology (pg. 3)


1. Lewis, Gilbert. (1901). “The Law of Physico-Chemical Change” (abs) (fugacity, pg. 54-56), Proceedings of the American Academy of Arts and Sciences, 37(3):49-69.
2. (a) Washburn, Edward W. (1915). An Introduction to the Principles of Physical Chemistry (pg.s 306-07). McGraw-Hill.
(b) Homans, George C. and Curtis, Charles P. (1934). An Introduction to Pareto: His Sociology (fugacity, pg. 273). Alfred A. Knopf.
(c) Homans, George C. (1951). The Human Group (fugacity, pg. 302). Transaction Publishers, 2003.
3. Beg, Mirza Arshad Ali. (1987). New Dimensions in Sociology: a Physico-Chemical Approach to Human Behavior (abs) (intro) (fugacity, pg. 25). Karachi: The Hamdard Foundation.
4. Shepherd, Dante. (2019). “Fugacity” (artist: Joan Cooke), Science Comic, North Eastern University.

External links
Fugacity – Wikipedia.

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