Thephlogiston theory, asuperseded scientific theory, postulated the existence of a fire-like element dubbedphlogiston (/flɒˈdʒɪstən,floʊ-,-ɒn/)[1][2] contained within combustible bodies and released duringcombustion. The name comes from theAncient Greekφλογιστόνphlogistón (burning up), fromφλόξphlóx (flame). The idea of aphlogistic substance was first proposed in 1669 byJohann Joachim Becher and later put together more formally in 1697 byGeorg Ernst Stahl. Phlogiston theory attempted to explain chemical processes such ascombustion andrusting, now collectively known as oxidation. The theory was challenged by the concomitant mass increase and was abandoned before the end of the 18th century following experiments byAntoine Lavoisier in the 1770s and by other scientists. Phlogiston theory led to experiments that ultimately resulted in the identification (c. 1771), and naming (1777), ofoxygen byJoseph Priestley andAntoine Lavoisier, respectively.
Phlogiston theory states thatphlogisticated substances contain phlogiston and that theydephlogisticate 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. This method of accounting for combustion was inverse to theoxygen theory by Antoine Lavoisier.
In general, substances that burned in the 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 the air had become completely phlogisticated it would no longer serve to support the combustion of any material, nor would a metal heated in it yield acalx; nor could phlogisticated air support life. Breathing was thought to take phlogiston out of the body.[3]
Joseph Black's Scottish studentDaniel Rutherford discoverednitrogen in 1772, and the pair used the theory to explain his results. The residue of air left after burning, a mixture of nitrogen andcarbon dioxide, was sometimes referred to asphlogisticated air, having taken up all of the phlogiston. Conversely, whenJoseph Priestley discoveredoxygen, he believed it to bedephlogisticated air, capable of combining with more phlogiston and thus supporting combustion for longer than ordinary air.[4]
Empedocles had formulated theclassical theory that there were four elements—water, earth, fire, and air—andAristotle 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 ofdecomposition that 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.[5]: 4
In 1667,Johann Joachim Becher published his bookPhysica subterranea, 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 the earth:terra lapidea,terra fluida, andterra pinguis.[6][7]Terra pinguis was the element that imparted oily,sulfurous, or combustible properties.[8] Becher believed thatterra pinguis was a key feature of combustion and was released when combustible substances were burned.[6] Becher did not have much to do with phlogiston theory as it is known 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.[9] Becher's idea was that combustible substances contain an ignitable matter, theterra pinguis.[10]
In 1703,Georg Ernst Stahl, a professor of medicine and chemistry atHalle, proposed a variant of the theory in which he renamed Becher'sterra pinguis tophlogiston, and it was in this form that the theory probably had its greatest influence.[11] 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.[9] The term was derived from a Greek word meaning inflame. The following paragraph describes Stahl's view of phlogiston:
To Stahl, metals were compounds containing phlogiston in combination with metallic oxides (calces); when ignited, the phlogiston was freed from the metal leaving the oxide behind. When the oxide was heated with a substance rich in phlogiston, such as charcoal, the calx again took up phlogiston and regenerated the metal. Phlogiston was a definite substance, the same in all its combinations.[10]
Stahl's first definition of phlogiston first appeared in hisZymotechnia fundamentalis, published in 1697. His most quoted definition was found in the treatise on chemistry entitledFundamenta chymiae in 1723.[9] 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 metalcalx and adding phlogiston.[10]Soot 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 andsulfur were identical by convertingsulfates toliver of sulfur usingcharcoal. He did not account for the increase in mass on combustion of tin and lead that were known at the time.[5]: 6–7
Johann Heinrich Pott, a student of Stahl[12], expanded the theory and attempted tomake it much more understandable to a general audience. He compared phlogiston to light or fire, saying that all three were substances whose natures werewidely 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.[9] 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 permeates everything in the universe, it could be released as heat when combined with an acid. Pott proposed the following properties:
Pott's formulations proposed little new theory; he merely supplied further details and rendered existing theory more approachable to the common man.
Johann Juncker 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, i.e., 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.[9]
Guillaume-François Rouelle brought the theory of phlogiston to France, where he was an influential scientist and teacher, popularizing the theory very quickly. Many of his students became influential scientists in their own right, Lavoisier included.[10] 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.[9]
Giovanni Antonio Giobert introduced Lavoisier's work in Italy. Giobert won a prize competition from the Academy of Letters and Sciences ofMantua in 1792 for his work refuting phlogiston theory. He presented a paper at theAcadémie royale des Sciences of Turin on 18 March 1792, entitledExamen chimique de la doctrine du phlogistique et de la doctrine des pneumatistes par rapport à la nature de l'eau ("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 defence of Lavoisier's theory of water composition to appear in Italy.[14]
Eventually, quantitative experiments revealed problems, including the fact that some metals gained mass after they burned, even though they were supposed to have lost phlogiston.Some phlogiston proponents, likeRobert Boyle,[15] explained this by concluding that phlogiston has negative mass; others, such asLouis-Bernard Guyton de Morveau, gave the more conventional argument that it is lighter than air. However, a more detailed analysis based onArchimedes' principle, the densities of magnesium and its combustion product showed that just being lighter than air could not account for the increase in mass.[citation needed] Stahl himself did not address the problem of the metals that burn gaining mass, but those who followed his school of thought worked on this problem.[9]
During the eighteenth century, as it became clear that metals gained mass after they were oxidized, phlogiston was increasingly regarded as aprinciple rather than a material substance.[16] By the end of the eighteenth century, for the few chemists who still used the term phlogiston, the concept was linked tohydrogen.Joseph Priestley, for example, in referring to the reaction of steam on iron, while fully acknowledging that the iron gains mass after it binds with oxygen to form acalx, iron oxide, iron also loses "the basis of inflammable air (hydrogen), and this is the substance or principle, to which we give the name phlogiston".[17] FollowingLavoisier's description of oxygen as theoxidizing principle (hence its name, from Ancient Greek:oksús, "sharp";génos, "birth" referring to oxygen's supposed role in the formation of acids), Priestley described phlogiston as thealkaline principle.[18]
Phlogiston remained the dominant theory until the 1770s whenAntoine-Laurent de Lavoisier showed that combustion requires a gas that has mass (specifically,oxygen) and could be measured by means of weighing closed vessels.[19] The use of closed vessels by Lavoisier and earlier by the Russian scientistMikhail Lomonosov also negated thebuoyancy that had disguised the mass of the gases of combustion, and culminated in theprinciple of mass conservation. These observations solved the mass paradox and set the stage for the newoxygen theory of combustion.[20] The British chemistElizabeth Fulhame demonstrated through experiment that manyoxidation reactions occur only in the presence of water, 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 forFarm-General membership during theFrench Revolution.[21][22]
Experienced chemists who supported Stahl's phlogiston theory attempted to respond to the challenges suggested by Lavoisier and the newer chemists. In doing so, the theory became more complicated and assumed too much, contributing to its overall demise.[20] Many people tried to remodel their theories on phlogiston 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.[23]
Quotations related toPhlogiston theory at Wikiquote