Phlogiston theory (Becher, Stahl)

c. 1700 Fire and matter theory theory refuted

A chemical theory that explained combustion, calcination of metals and respiration by a single 'principle of combustibility': an invisible substance that leaves a body during burning. A metal was regarded as a compound of its calx and phlogiston.

The system was built by the German chemist Georg Ernst Stahl at the turn of the 18th century; it was accepted by most chemists of the 18th century.

Its main anomaly: the calx of a metal weighs more than the metal itself, although according to the theory the metal loses something in calcination.

Why this date

Stahl, building on Becher 1669. approximate: Britannica dates it to "about the beginning of the 18th century"

What grew out of it

Weight gain of metals in calcination as a general fact (Guyton de Morveau)

influenced · Confidence: high

Guyton's weighings were made within the phlogiston programme: he continued the work of the phlogistonist Chardenon and explained the weight gain he found by phlogiston being lighter than air, so the question was posed and answered in the terms of Stahl's theory.

Link sources
- Blumenthal G., Ladyman J. «The development of problems within the phlogiston theories, 1766-1791», Foundations of Chemistry 19 (2017) Verified 20 Sep 2026
  Following on from Chardenon's work and the associated controversies, Guyton (1772) was the first to show clearly that weight gain during calcination was a general phenomenon.
- Blumenthal G., Ladyman J. «The development of problems within the phlogiston theories, 1766-1791», Foundations of Chemistry 19 (2017) Verified 20 Sep 2026
  Guyton produced the revised proposal that the addition of phlogiston to a metallic calx made the metal lighter, because it was specifically lighter than air, but this explanation was no more workable than Chardenon's and was not adopted.
- Smeaton W. A. «Guyton de Morveau, Louis Bernard», Dictionary of Scientific Biography (via Encyclopedia.com) (1972) Verified 20 Sep 2026
  In order to explain why a metal containing phlogiston weighs less than its calx, he modified Chardenon's theory, believing that phlogiston was specifically lighter than all other substances.
- Blumenthal G., Ladyman J. «Theory comparison and choice in chemistry, 1766-1791», Foundations of Chemistry 20(3) (2018) Verified 20 Sep 2026
  the work which had been done to try to perfect the system of phlogiston had insensibly prepared its ruin
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.

Node sources

Britannica, «Phlogiston» (2024) Verified 20 Sep 2026, quotation assembled from fragments
Johann Joachim Becher in 1669 set forth his view that substances contained three kinds of earth, which he called the vitrifiable, the mercurial, and the combustible. He supposed that, when a substance burned, combustible earth (Latin terra pinguis, meaning “fat earth”) was liberated. ... To this hypothetical substance Georg Ernst Stahl, at about the beginning of the 18th century, applied the name phlogiston (from Greek, meaning “burned”). ... when a metal was converted to its calx, or metallic ash (its oxide, in modern terms), phlogiston was lost. Therefore, metals were composed of calx and phlogiston.
Britannica, «Phlogiston» (2024) Verified 20 Sep 2026
The major objection to the theory, that the ash of organic substances weighed less than the original while the calx was heavier than the metal, was of little significance to Stahl, who thought of phlogiston as an immaterial “principle” rather than as an actual substance. As chemistry advanced, phlogiston was considered a true substance, and much effort was expended in accounting for the weight changes observed.
Stanford Encyclopedia of Philosophy, «Philosophy of Chemistry», §3.2 (2023) Verified 20 Sep 2026, quotation assembled from fragments
Another substance concept from roughly the same period is phlogiston, which served as the basis for 18th century theories of processes that came to be called oxidation and reduction. Georg Ernst Stahl (1660–1734) introduced the theory, drawing on older theoretical ideas. ... Johann Joackim Becher (1635–82) modified these ideas at the end of the 17th century, arguing that the calcination of metals is a kind of combustion involving the loss of what he called the principle of flammability. Stahl subsequently renamed this principle “phlogiston”.
Stanford Encyclopedia of Philosophy, «Philosophy of Chemistry», §3.2 (2023) Verified 20 Sep 2026
The phlogiston theory suggested that reactions could involve the replacement of one part of a substance with another, where previously all reactions were thought to be simple associations or dissociations.
Stanford Encyclopedia of Philosophy, «Philosophy of Chemistry», §3.2 (2023) Verified 20 Sep 2026
As reactants and products came to be routinely weighed, it became clear that metals gain weight when they become a calx. But according to the phlogiston theory, the calx involves the loss of phlogiston. Although the idea that a process involving the loss of a substance could involve the gain of weight seems strange to us, phlogiston theorists were not immediately worried. Some phlogiston theorists proposed explanations based on the “levitation” properties of phlogiston, what Priestly later referred to as phlogiston’s “negative weight.”
Blumenthal G., Ladyman J. «The development of problems within the phlogiston theories, 1766-1791», Foundations of Chemistry 19 (2017) Verified 20 Sep 2026
There was a rapid proliferation of late phlogistic theories, particularly from 1784, and the accounts of composition and important implications of the main theories are set out and their issues analysed. Each of them either reached impasses due to internal problems, or included features that made them unacceptable even to other phlogistians.

"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.

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Phlogiston theory (Becher, Stahl)

Why this date

What grew out of it

Weight gain of metals in calcination as a general fact (Guyton de Morveau)

Other links

Oxygen theory of combustion and "Réflexions sur le phlogistique"

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