thumb|upright|right|The alchemist and physician [[J. J. Becher proposed the phlogiston theory.]]
The phlogiston theory, a superseded scientific theory, postulated the existence of a fire-like element dubbed phlogiston () contained within combustible bodies and released during combustion. The name comes from the Ancient Greek (burning up), from (flame). The idea of a substance was first proposed in 1669 by Johann Joachim Becher and later put together more formally in 1697 by Georg Ernst Stahl. Phlogiston theory attempted to explain chemical processes such as combustion and rusting, 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 by Antoine Lavoisier in the 1770s and by other scientists. Phlogiston theory led to experiments that ultimately resulted in the identification (), and naming (1777), of oxygen by Joseph Priestley and Antoine Lavoisier, respectively.
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. This method of accounting for combustion was inverse to the oxygen theory by Antoine Lavoisier.
<blockquote>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 a calx; nor could phlogisticated air support life. Breathing was thought to take phlogiston out of the body.</blockquote>
Joseph Black's Scottish student Daniel Rutherford discovered nitrogen in 1772, and the pair used the theory to explain his results. The residue of air left after burning, a mixture of nitrogen and carbon dioxide, was sometimes referred to as phlogisticated air, having taken up all of the phlogiston. Conversely, when Joseph Priestley discovered oxygen, he believed it to be dephlogisticated air, capable of combining with more phlogiston and thus supporting combustion for longer than ordinary air.
History
Empedocles had formulated the classical theory that there were four elements—water, earth, fire, and air—and Aristotle 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 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.
Terra pinguis
In 1667, Johann Joachim Becher 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 the earth: , , and . was the element that imparted oily, sulfurous, or combustible properties. Becher believed that was a key feature of combustion and was released when combustible substances were burned. Becher's idea was that combustible substances contain an ignitable matter, the .
Georg Ernst Stahl
thumb|upright|[[Georg Ernst Stahl]]
In 1703, Georg Ernst Stahl, a 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. 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., 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.]]
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.
Challenge and demise
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, like Robert Boyle, explained this by concluding that phlogiston has negative mass; others, such as Louis-Bernard Guyton de Morveau, gave the more conventional argument that it is lighter than air. However, a more detailed analysis based on Archimedes' principle, the densities of magnesium and its combustion product showed that just being lighter than air could not account for the increase in mass. 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. By the end of the eighteenth century, for the few chemists who still used the term phlogiston, the concept was linked to hydrogen. 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 a calx, 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". 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 mass (specifically, oxygen) and could be measured by means of weighing closed vessels. The use of closed vessels by Lavoisier and earlier by the Russian scientist Mikhail Lomonosov also negated the buoyancy that had disguised the mass 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.
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. 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.