The phlogiston theory is a

superseded scientific theory This list catalogs well-accepted theories in science and pre-scientific natural philosophy and natural history which have since been superseded by scientific theories. Many discarded explanations were once supported by a scientific consensus, b ...

that postulated the existence of a fire-like element called phlogiston () contained within combustible bodies and released during

combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combus ...

. The name comes from the

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...

(''burning up''), from (''flame''). The idea was first proposed in 1667 by

Johann Joachim Becher Johann Joachim Becher (; 6 May 1635 – October 1682) was a German physician, alchemist, precursor of chemistry, scholar and adventurer, best known for his development of the phlogiston theory of combustion, and his advancement of Austrian cameral ...

and later put together more formally by

Georg Ernst Stahl Georg Ernst Stahl (22 October 1659 – 24 May 1734) was a German chemist, physician and philosopher. He was a supporter of vitalism, and until the late 18th century his works on phlogiston were accepted as an explanation for chemical processes.K ...

. Phlogiston theory attempted to explain chemical processes such as

and

rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO( ...

ing, now collectively known as

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

. It was challenged by the concomitant weight increase, and was abandoned before the end of the 18th century following experiments by

Antoine Lavoisier Antoine-Laurent de Lavoisier ( , ; ; 26 August 17438 May 1794),
CNRS (

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

Theory

Phlogiston theory states that ''phlogisticated'' substances contain phlogiston and that they ''dephlogisticate'' when burned, releasing stored phlogiston which is absorbed by the air. Growing plants then absorb this phlogiston, which is why air does not spontaneously combust and also why plant matter burns as well as it does. Thus phlogiston accounted for combustion via a process that was opposite to that of the oxygen theory.

In general, substances that burned in air were said to be rich in phlogiston; the fact that combustion soon ceased in an enclosed space was taken as clear-cut evidence that air had the capacity to absorb only a finite amount of phlogiston. When air had become completely phlogisticated it would no longer serve to support combustion of any material, nor would a metal heated in it yield a
calx Calx is a substance formed from an ore or mineral that has been heated. Calx, especially of a metal, is now known as an oxide. According to the obsolete phlogiston theory, the calx was the true elemental substance, having lost its phlogiston in t ...
; nor could phlogisticated air support life. Breathing was thought to take phlogiston out of the body.

Joseph Black Joseph Black (16 April 1728 – 6 December 1799) was a Scottish physicist and chemist, known for his discoveries of magnesium, latent heat, specific heat, and carbon dioxide. He was Professor of Anatomy and Chemistry at the University of Glas ...

's Scottish student

Daniel Rutherford Daniel Rutherford (3 November 1749 – 15 December 1819) was a Scottish physician, chemist and botanist who is known for the isolation of nitrogen in 1772. Life Rutherford was born on 3 November 1749, the son of Anne Mackay and Professor John ...

discovered

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

in 1772, and the pair used the theory to explain his results. The residue of air left after burning, in fact a mixture of nitrogen and

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

, was sometimes referred to as ''phlogisticated air,'' having taken up all of the phlogiston. Conversely, when

Joseph Priestley Joseph Priestley (; 24 March 1733 – 6 February 1804) was an English chemist, natural philosopher, separatist theologian, grammarian, multi-subject educator, and liberal political theorist. He published over 150 works, and conducted exp ...

discovered

, he believed it to be ''dephlogisticated air,'' capable of combining with more phlogiston and thus supporting combustion for longer than ordinary air.

History

Empedocles Empedocles (; grc-gre, Ἐμπεδοκλῆς; , 444–443 BC) was a Greek pre-Socratic philosopher and a native citizen of Akragas, a Greek city in Sicily. Empedocles' philosophy is best known for originating the cosmogonic theory of the ...

had formulated the classical theory that there were four elements—water, earth, fire and air—and

Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ph ...

reinforced this idea by characterising them as moist, dry, hot and cold. Fire was thus thought of as a substance, and burning was seen as a process of decomposition which applied only to compounds. Experience had shown that burning was not always accompanied by a loss of material, and a better theory was needed to account for this.

Johann Joachim Becher

In 1667,

published his book , which contained the first instance of what would become the phlogiston theory. In his book, Becher eliminated fire and air from the classical element model and replaced them with three forms of earth: , , and . was the element that imparted

oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...

y, sulphurous, or combustible properties. Becher believed that was a key feature of combustion and was released when combustible substances were burned. Becher did not have much to do with phlogiston theory as we know it now, but he had a large influence on his student Stahl. Becher's main contribution was the start of the theory itself, however much of it was changed after him. Becher's idea was that combustible substances contain an ignitable matter, the .

Georg Ernst Stahl

In 1703

, professor of medicine and chemistry at Halle, proposed a variant of the theory in which he renamed Becher's to ''phlogiston'', and it was in this form that the theory probably had its greatest influence.Mason, Stephen F., (1962). ''A History of the Sciences'' (revised edition). New York: Collier Books. Ch. 26. The term 'phlogiston' itself was not something that Stahl invented. There is evidence that the word was used as early as 1606, and in a way that was very similar to what Stahl was using it for. The term was derived from a Greek word meaning to inflame. The following paragraph describes Stahl's view of phlogiston: Stahl's first definition of phlogiston first appeared in his , published in 1697. His most quoted definition was found in the treatise on chemistry entitled in 1723. According to Stahl, phlogiston was a substance that was not able to be put into a bottle, but could be transferred nonetheless. To him, wood was just a combination of ash and phlogiston, and making a metal was as simple as getting a metal

calx Calx is a substance formed from an ore or mineral that has been heated. Calx, especially of a metal, is now known as an oxide. According to the obsolete phlogiston theory, the calx was the true elemental substance, having lost its phlogiston in t ...

and adding phlogiston.

Soot Soot ( ) is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolysed ...

was almost pure phlogiston, which is why heating it with a metallic calx transforms the calx into the metal and Stahl attempted to prove that the phlogiston in soot and sulphur were identical by converting

sulphate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ar ...

s to

liver of sulphur Liver of sulfur is a loosely defined mixture of potassium sulfide, potassium polysulfide, potassium thiosulfate, and likely potassium bisulfide. Synonyms include hepar sulfuris, sulfur, sulfurated potash and sulfurated potassa. There are two di ...

using charcoal. He did not account for the increase in weight on combustion of tin and lead that were known at the time.

J. H. Pott

Johann Heinrich Pott Johann Heinrich Pott (6 October 1692 – 29 March 1777) was a Prussian physician and chemist. He is considered a pioneer of pyrochemistry. He examined the elements bismuth and manganese apart from attempting improvements to glass and porcelain prod ...

, a student of one of Stahl's students, expanded the theory and attempted to make it much more understandable to a general audience. He compared phlogiston to light or fire, saying that all three were substances whose natures were widely understood but not easily defined. He thought that phlogiston should not be considered as a particle but as an essence that permeates substances, arguing that in a pound of any substance one could not simply pick out the particles of phlogiston. Pott also observed the fact that when certain substances are burned they increase in mass instead of losing the mass of the phlogiston as it escapes; according to him, phlogiston was the basic fire principle and could not be obtained by itself. Flames were considered to be a mix of phlogiston and water, while a phlogiston-and-earthy mixture could not burn properly. Phlogiston permeating everything in the universe, it could be released as heat when combined with acid. Pott proposed the following properties: # The form of phlogiston consists of a circular movement around its axis. # When homogeneous it cannot be consumed or dissipated in fire. # The reason it causes expansion in most bodies is unknown, but not accidental. It is proportional to the compactness of the texture of the bodies or to the intimacy of their constitution. # The increase of weight during

calcination Calcination refers to thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), gener ...

is evident only after a long time, and is due either to the fact that the particles of the body become more compact, decrease the volume and hence increase the density as in the case of lead; or that little heavy particles of air become lodged in the substance as in the case of powdered zinc oxide. # Air attracts the phlogiston of bodies. # When set in motion, phlogiston is the chief active principle in nature of all inanimate bodies. # It is the basis of colors. # It is the principal agent in fermentation. Pott's formulations proposed little new theory; he merely supplied further details and rendered existing theory more approachable to the common man.

Others

Johann Juncker Johann Juncker (23 December 1679 in Londorf, Hesse – 25 October 1759 in Halle) was a German physician and chemist. Juncker was a leader in the Pietist reform movement as it applied to medicine. He directed the Francke Foundations and initiated ...

also created a very complete picture of phlogiston. When reading Stahl's work, he assumed that phlogiston was in fact very material. He therefore came to the conclusion that phlogiston has the property of levity, or that it makes the compound that it is in much lighter than it would be without the phlogiston. He also showed that air was needed for combustion by putting substances in a sealed flask and trying to burn them. Guillaume-François Rouelle brought the theory of phlogiston to France, and he was a very influential scientist and teacher so it gained quite a strong foothold very quickly. Many of his students became very influential scientists in their own right, Lavoisier included. The French viewed phlogiston as a very subtle principle that vanishes in all analysis, yet it is in all bodies. Essentially they followed straight from Stahl's theory.

Giovanni Antonio Giobert Giovanni Antonio Giobert also known as Jean-Antoine Giobert (27 October 1761 in Mongardino - 14 September 1834 in Millefiori) was an Italian chemist and mineralogist who studied magnetism, galvanism, and agricultural chemistry. He introduced Anto ...

introduced Lavoisier's work in Italy. Giobert won a prize competition from the Academy of Letters and Sciences of Mantua in 1792 for his work refuting phlogiston theory. He presented a paper at the of Turin on March 18, 1792, entitled ("Chemical examination of the doctrine of phlogiston and the doctrine of pneumatists in relation to the nature of water"), which is considered the most original defense of Lavoisier's theory of water composition to appear in Italy.

Challenge and demise

Eventually, quantitative

experiment An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into Causality, cause-and-effect by demonstrating what outcome oc ...

s revealed problems, including the fact that some metals gained weight after they burned, even though they were supposed to have lost phlogiston. Some phlogiston proponents, like

Robert Boyle Robert Boyle (; 25 January 1627 – 31 December 1691) was an Anglo-Irish natural philosopher, chemist, physicist, alchemist and inventor. Boyle is largely regarded today as the first modern chemist, and therefore one of the founders of ...

, explained this by concluding that phlogiston has negative weight; others, such as

Louis-Bernard Guyton de Morveau Louis-Bernard Guyton, Baron de Morveau (also Louis-Bernard Guyton-Morveau after the French Revolution; 4 January 1737 – 2 January 1816) was a French chemist, politician, and aeronaut. He is credited with producing the first systematic method o ...

, gave the more conventional argument that it is lighter than air. However, a more detailed analysis based on

Archimedes' principle Archimedes' principle (also spelled Archimedes's principle) states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially, is equal to the weight of the fluid that the body displaces. Archimedes' ...

, the densities of magnesium and its combustion product showed that just being lighter than air could not account for the increase in weight. Stahl himself did not address the problem of the metals that burn gaining weight, but those who followed his school of thought were the ones that worked on this problem. During the eighteenth century, as it became clear that metals gained weight after they were oxidized, phlogiston was increasingly regarded as a ''principle'' rather than a material substance. By the end of the eighteenth century, for the few chemists who still used the term phlogiston, the concept was linked to

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

, for example, in referring to the reaction of steam on iron, while fully acknowledging that the iron gains weight after it binds with oxygen to form a

, iron oxide, iron also loses "the basis of inflammable air (

), and this is the substance or principle, to which we give the name phlogiston". Following Lavoisier's description of oxygen as the ''oxidizing principle'' (hence its name, from Ancient Greek: , "sharp"; , "birth" referring to oxygen's supposed role in the formation of acids), Priestley described phlogiston as the ''alkaline principle.'' Phlogiston remained the dominant theory until the 1770s when Antoine-Laurent de Lavoisier showed that combustion requires a gas that has weight (specifically,

) and could be measured by means of weighing closed vessels. The use of closed vessels by de Lavoisier and earlier, by the Russian scientist Mikhail Lomonosov, also negated the buoyancy that had disguised the weight of the gases of combustion and culminated in the principle of mass conservation. These observations solved the mass paradox and set the stage for the new oxygen theory of combustion. The British chemist

Elizabeth Fulhame Elizabeth Fulhame (fl. 1794) was an early British chemist who invented the concept of catalysis and discovered photoreduction. She describes catalysis as a process at length in her 1794 book ''An Essay On Combustion with a View to a New Art of ...

demonstrated through experiment that many oxidation reactions occur only in the presence of

water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

, that they directly involve water, and that water is regenerated and is detectable at the end of the reaction. Based on her experiments, she disagreed with some of the conclusions of Lavoisier as well as with the phlogiston theorists that he critiqued. Her book on the subject appeared in print soon after Lavoisier's execution for Farm-General membership during the

French Revolution The French Revolution ( ) was a period of radical political and societal change in France that began with the Estates General of 1789 and ended with the formation of the French Consulate in coup of 18 Brumaire, November 1799. Many of its ...

. Experienced chemists who supported Stahl's phlogiston theory attempted to respond to the challenges suggested by Lavoisier and the newer chemists. In doing so, phlogiston theory became more complicated and assumed too much, contributing to the overall demise of the theory. Many people tried to remodel their theories on phlogiston in order to have the theory work with what Lavoisier was doing in his experiments. Pierre Macquer reworded his theory many times, and even though he is said to have thought the theory of phlogiston was doomed, he stood by phlogiston and tried to make the theory work.

References

External links

* {{Authority control Combustion History of chemistry Misidentified chemical elements Obsolete theories in physics