Stahl combustion theory
A depiction of phlogiston theory, the supposition that when wood burned, phlogiston and calx were products.
In chemistry, phlogiston theory argued that heat was a fire-like element, having mass, called “phlogiston”, contained within combustible bodies and released during combustion.

In 1697 to 1703, German chemist Georg Stahl (1659-1734) introduced and developed phlogiston theory, as an improved modification of Johann Becher’s 1669 terra pinguis theory of combustion. [1]

Stahl explained natural phenomena by affirming that “heat matter” (or matter of heat), a hypothetical entity to which he assigned the name “phlogiston”, was contained in all combustible bodies and was made to appear when such bodies were burned. [2]

In 1774 to 1786, English polymath Joseph Priestley, in his six-volume Experiments and Observations on Different Kinds of Air, published the results of a number of experiments he conducted on air, that worked to repudiated the last vestiges of four element theory, in which he attempted to situate his own variant of phlogiston theory. [3] Herein, according to English chemical thermodynamicist and chemistry historian Leslie Woodcock, Priestley posited phlogiston as the driving force for chemical reactions; thereby, supposedly, presaging Gibbs by some 200-years, a precursor to Gibbs free energy, in the following sense, as supposed by Woodcock: [4]

“Every thermodynamic material does indeed have a constitutive state function, (‘phlogiston’?), which can be given a definition: ‘minus the Gibbs chemical potential of oxygen within the material.’ It has the dimensions of (free) energy per mole of oxygen, and measures its oxidation propensity. While the antiphlogistonists may have been the first inorganic chemists, Priestley’s conceptual interpretation of phlogiston was the first attempt at the physical chemistry of reactions. If phlogiston is regarded as an alternative description of ‘Gibbs free energy,’ the theory appears to be an intuitively accurate description, as could reasonably be expected at that time, and remains essentially correct today given its precise thermodynamic definition.”

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1. Partington, James R. (1957). A Short History of Chemistry (section: The Theory of Phlogiston, pgs. 85-89). Dover.
2. Hemmeter, John C. (1918). “Lavoisier and the History of the Physiology of Respiration and Metabolism: Contemporary Views of the Life Processes”, Bulletin of the Johns Hopkins Hospital (pg. 262), 29: 254-64, Nov.
3. Experiments and Observations on Different Kinds of Air – Wikipedia.
4. Woodcock, Leslie V. (2005). “Phlogiston Theory and Chemical Revolutions” (Ѻ), Bulletin of the History of Chemistry, 30(2):63-69.

Further reading
● Golinski, Jan. (1999). Science as Public Culture: Chemistry and Enlightenment in Britain, 1760-1820 (phlogiston, pgs. 120, 139, 181; as a principle of combustion, 23, 134; phlogiston theory, pgs. 118, 131-36, 146, 151, 154, 213, 258; overthrow of, pgs. 131-35, 140). Cambridge University Press.

External links
Phlogiston theory – Wikipedia.

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