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The 'phlogiston theory' (from the Ancient Greek φλογιστόν ''phlŏgistón'' "burnt up," from φλόξ ''phlóx'' "fire") is an obsolete scientific theory, stated initially in 1667 by Johann Joachim Becher, whereby in addition to the classical four elements of the Greeks, there was an additional fire-like element called “phlogiston” that was contained within combustible bodies, and released, to lesser or greater degrees, during combustion. The theory was an attempt to explain oxidation processes, such as combustion and the rusting of metals.

Contents

History

Theory

Challenge and demise

Enduring aspects

References

History

In 1667, Johann Joachim Becher published his ''Physica Subterranea'', which was the first mention of what would become the phlogiston theory. Traditionally, alchemists considered that there were four classical elements: fire, water, air, and earth. In his book, Becher eliminated fire and air from the classical element model and replaced them with three forms of earth: ''terra lapidea'', ''terra mercurialis'', and ''terra pinguis''.^[1][2]
In Becher's theory, presence of ''terra lapidea'', represented the degree of fusibility. ''Terra mercurialis'', also ''terra fluida'', indicated the degree of fluidity, subtility, volatility, and metallicity. ''Terra pinguis'' was the element which imparted oily, sulphureous, or combustible properties.^[3] Becher believed that ''terra pinguis'' was a key feature of combustion and was released when combustible substances were burned.
Georg Ernst Stahl, a German chemist, was a student of Becher's who expanded on his theories with several publications in the period between 1703 and 1731. In a 1718 work, Stahl was the first to rename ''terra pinguis'' as phlogiston from the Ancient Greek ''phlogios'' for "fiery". Stahl's work analyzed the role of phlogiston in combustion and calcination, the 17th century term for oxidation.

Theory

The theory holds that all flammable materials contain phlogiston, a substance without color, odor, taste, or weight that is liberated in burning. Once burned, the "dephlogisticated" substance was held to be in its "true" form, the calx.
"Phlogisticated" substances are those that contain phlogiston and are "dephlogisticated" when burned. Since any substance could be observed to burn for only a limited time with limited air (for instance in a sealed container), air was thought to have a specific capacity for phlogiston.
Joseph Black's student Daniel Rutherford discovered nitrogen 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, was sometimes referred to as "phlogisticated air", having taken up all of the phlogiston.
Conversely, when oxygen was first discovered it was thought to be "dephlogisticated air", capable of combining with more phlogiston and thus supporting combustion for longer than ordinary air.

Challenge and demise

Eventually, quantitative experiments revealed problems, including the fact that some metals, such as magnesium, gained weight when they burned, even though they were supposed to have lost phlogiston. Mikhail Lomonosov attempted to repeat Robert Boyle's celebrated experiment in 1753 and concluded that the phlogiston theory was false. He wrote in his diary: "Today I made an experiment in hermetic glass vessels in order to determine whether the mass of metals increases from the action of pure heat. The experiment demonstrated that the famous Robert Boyle was deluded, for without access of air from outside, the mass of the burnt metal remains the same."
Some phlogiston proponents explained this by concluding that phlogiston had "negative weight"; others, such as Louis-Bernard Guyton de Morveau, gave the more conventional argument that it was lighter than air. However, a more detailed analysis based on the Archimedean principle and the densities of magnesium and its combustion product shows that just being lighter than air cannot account for the increase in mass.
Still, phlogiston remained the dominant theory until Antoine-Laurent Lavoisier showed that combustion requires oxygen, solving the weight paradox and setting the stage for the new caloric theory of combustion.
In some respects, the phlogiston theory can be seen as the opposite of the modern "oxygen theory". The phlogiston theory states that all flammable materials contain phlogiston that is liberated in burning, leaving the "dephlogisticated" substance in its "true" calx form. In the modern theory, on the other hand, flammable materials (and unrusted metals) are "deoxygenated" when in their pure form and become oxygenated when burned.

Enduring aspects

Phlogiston theory allowed chemists to bring explanation of apparently different phenomena into a coherent structure: combustion, metabolism, and formation of rust. The recognition of the relation between combustion and metabolism was a forerunner of the recognition that the metabolism of living creatures and combustion can be understood in terms of fundamentally related chemical processes.

References

1. The last sorcerers: The path from alchemy to the periodic table, , Richard, Morris, Joseph Henry Press, 2003,
2. Becher, ''Physica Subterranea'' p. 256 ''et seq.''
3. The Norton history of chemistry, , William Hodson, Brock, W. W. Norton, 1993,