Phlogiston Theory

Phlogiston theory was a hypothetical principle of fire, of which every combustible substance was in part composed.[1] This obsolete scientific theory from the late 17th and early 18th centuries proposed that a fire-like element, phlogiston, is released during combustion.[9] The theory became the dominant model explaining combustion and fermentation in eighteenth century chemistry and constituted the first systematic and comprehensive theory of chemistry.[2]

Origins and Development

The existence of this hypothetical substance was proposed in 1669 by Johann Becher, who called it 'combustible earth' (terra pinguis: literally 'fat earth').[3] In his Physica subterranea of 1669, he proposed the existence of three new chemical principles or "earths", one of them, terra pinguis, was thought to be a substance present in inflammable materials and given off during combustion.[11] He proposed that all matter is composed of air, water and three earths: terra lapidea related to the fusibility, terra fluida related to the fluidity and volatility, and terra pinguis related to the combustibility and flammability. According to Becher, flammable objects burned because they contained terra pinguis.

German chemist Georg Ernst Stahl developed a theory to account for these observations, the phlogiston theory. Although he had discussed this theory in earlier works, the publication of Fundamenta chymiae dogmaticae et experimentalis (1723; the fundamentals of dogmatic and experimental chemistry) represented a more complete description.[2] In the early 18th century Georg Stahl renamed the substance phlogiston (from the Greek for 'burned') and extended the theory to include the calcination (and corrosion) of metals.[3] Stahl's basic proposal was that a principle he called "phlogiston" (from the Greek phlogistos, meaning "burnt") existed, to a greater or lesser degree, in all substances. During combustion, phlogiston was freed from the burning substance and released into the air. He considered phlogiston to be an intangible "principle," rather than an element or substance with mass.[2]

Core Principles

Stahl's theory included the following ideas: All combustible substances contain phlogiston. The more phlogiston a substance contains, the better and more completely it burns. Combustion releases the phlogiston from the substance into the air.[4] In this view, the phenomena of burning, now called oxidation, was caused by the liberation of phlogiston, with the dephlogisticated substance left as an ash or residue.[1]

Metals were thought to be composed of calx (a powdery residue) and phlogiston; when a metal was heated, phlogiston was set free and the calx remained. The process could be reversed by heating the metal over charcoal (a substance believed to be rich in phlogiston, because combustion almost totally consumed it). The calx would absorb the phlogiston released by the burning charcoal and become metallic again.[3]

The phlogiston theory quickly became popular, and was very robust, explaining a wide variety of phenomena. It explained the rusting of metals. As the metal rusted, it gave off phlogiston into the air, so a metal was a combination of its rust and phlogiston. The breathing of animals could also be explained. As food was 'burned' inside the body, phlogiston was released and expelled out of the body by the lungs.[7]

The Weight Problem

This had been shown by Jean Rey in 1630, and confirmed by Robert Boyle in 1673. To reconcile this fact with the phlogiston theory, phlogiston was attributed with negative weight – or 'positive lightness' – since it, or rather its flame, tends to rise, rather than be attracted downwards by gravity.[4] When metals were strongly heated in air, the resulting calx weighed more than the original metal, not less, as would be expected if the lead had lost the phlogiston component. This inconsistency caused some phlogistonists to suggest that phlogiston might even have a negative weight.[10]

Phlogiston was often considered to have levity, that is in effect a negative weight, such that a loss of phlogiston during combustion would lead to an increase in weight.[8] By the 1730s most phlogistonists regarded their imagined substance as having an actual weight. This had dramatic consequences for the theory. If phlogiston had weight then when it left a substance during combustion or rusting the remaining material should weigh less. For example, when a metal rusted (gave off phlogiston) the material left behind often weighed considerably more than the original metal. Some scientists suggested that phlogiston had a negative weight, and so its absence made materials heavier.[16]

Scientific Context and Acceptance

By 1766, the theory of phlogiston had become the most influential in chemistry.[6] For example, experiments showed that if you burned a stick of wood in a confined space, such as a jar, after a short time the combustion would stop. This was explained by suggesting that air could only contain a certain amount of phlogiston, and once it reached its limit then no more combustion could take place.[7]

So well entrenched was the theory, that when Joseph Priestley discovered oxygen in 1774 he called it 'dephlogisticated air', believing that the mercuric oxide (or calx) that he heated with sunlight had adsorbed phlogiston, removing it from the surrounding air.[4] In 1774, Joseph Priestley – a defender of the theory who discovered several gases and invented the carbonated drink – isolated what he called 'dephlogisticated air', in which candles could burn and mice could live. Priestley believed this air supported combustion so well because it had no phlogiston in it, so could absorb the maximum amount during burning.[15]

Lavoisier's Refutation

The theory was finally demolished by Antoine Lavoisier, who showed by careful experiments with reactions in closed containers that there was no absolute gain in mass – the gain in mass of the substance was matched by a corresponding loss in mass of the air used in combustion. After experiments with Priestley's dephlogisticated air, Lavoisier realized that this gas, which he named oxygen, was taken up to form a calx (now called an oxide).[3]

The oxygen theory of combustion resulted from a demanding and sustained campaign to construct an experimentally grounded chemical theory of combustion, respiration, and calcination. The theory that emerged was in many respects a mirror image of the phlogiston theory, but gaining evidence to support the new theory involved more than merely demonstrating the errors and inadequacies of the previous theory. From the early 1770s until 1785, when the last important pieces of the theory fell into place, Lavoisier and his collaborators performed a wide range of experiments designed to advance many points on their research frontier.[18]

It was Lavoisier's obsession with weighing the reactants and products of a reaction that really turned the phlogiston theory on its head. When a substance burned in air, he found, it increased in mass rather than decreased as phlogiston theory predicted. Lavoisier coined the term 'oxygen' to describe the part of the air that combines with a combustible substance when it burns.[21]

Lavoisier began his full-scale attack on phlogiston in 1783, claiming that "Stahl's phlogiston is imaginary." Calling phlogiston "a veritable Proteus that changes its form every instant," Lavoisier asserted that it was time "to lead chemistry back to a stricter way of thinking" and "to distinguish what is fact and observation from what is system and hypothesis." As a starting point, he offered his theory of combustion, in which oxygen now played the central role.[10]

Resistance and Gradual Decline

The chemical revolution as we know it never really happened. There were numerous respectable and respected chemists who didn't jump on the Lavoisier bandwagon. In particular, Lavoisier's oxygen theory is frequently assumed to have rapidly replaced the idea of phlogiston, a hypothesized fire-like element released during combustion. But, says Chang, contrary to the widespread belief that the appearance of oxygen consigned phlogiston to the intellectual scrapheap, there were plenty of high-profile figures who retained a place for phlogiston in their chemistry long after the publication of Lavoisier's Traite Elementaire de Chemie in 1789.[21]

Lavoisier's main opponent, Priestley, outlived him, but was not able to overturn the trend to the 'new chemistry' of Lavoisier. Priestley's last book, published in 1796, still strongly supported the Phlogiston Theory, but did contain a note of surrender to the prevailing opinions of others. He wrote, "There have been few, if any, revolutions in science so great, so sudden, and so general, as the prevalence of what is now usually termed the new system of chemistry, or that of the Antiphlogistons, over the doctrine of Stahl, which was at one time thought to have been the greatest discovery that had ever been made in the science."[7]

Lavoisier did not expect his ideas to be adopted at once, because those who believed in phlogiston would "adopt new ideas only with difficulty." Lavoisier put his faith in the younger generation who would be more open to new concepts. Two years later, in 1791, the results were obvious. "All young chemists," he mused, "adopt the theory, and from that I conclude that the revolution in chemistry has come to pass."[10]

Historical Significance

The theory is important because it represented a crucial step in the development of chemical science, providing an early framework for understanding combustion and oxidation. Phlogiston theory played a significant role in shaping early chemistry by providing a framework for understanding combustion and material properties before modern concepts emerged. Its eventual rejection illustrated how scientific theories can evolve through experimentation and observation. The transition away from phlogiston theory exemplifies how scientific progress often involves discarding outdated models in favor of new evidence-based explanations, highlighting the dynamic nature of scientific inquiry and its foundational impact on modern chemical principles.[5][9]

However, despite being debunked, the theory did mark an important shift from alchemy towards an understanding of chemical elements and how they react – paving the way for modern-day chemistry.[15]