The Phlogiston Theory of Combustion

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Phlogiston does not exist and never did. However, during much of the 17th and 18th centuries many scientists and philosophers believed that it did, and that it solved the age-old problem of explaining what happened when things burned.

Robert Boyle (1627-91) had discovered that fire depends on air. If you deprive a burning substance of air, the fire is extinguished. He also found that air was necessary for enabling animals to breathe and for producing metal oxides when they were heated at a temperature below their melting point. However, he did not use the word “oxide” (which rather gives the game away about what was really happening), instead calling the resulting compound a “calx”, with the process of forming the calx termed “calcination”.

However, in Germany the chemist Johann Joachim Becher (1635-82) came up with a theory that did not require air to be part of the combustion process. Instead, the material being burned possessed a substance that was released during combustion, and the flammability of materials depended on how much of this substance they had to start with.

This idea was later popularised by another German chemist, Georg Stahl (1659-1734) who gave the name “phlogiston” to this substance, it being a Greek word meaning “burning up”.

Phlogiston also applied to the reverse process of producing a metal from a calx by burning charcoal alongside it. Charcoal burned well because it was rich in phlogiston, which then flowed into the calx.

However, if this was so, why did some substances (such as metals) gain weight when heat was applied, rather than losing it? Surely the opposite should have been the case? The proponents of phlogiston, rather than realising that the theory was false, came up with various explanations, including the idea that the substances had “negative weight”, whatever that might mean.

One believer in phlogiston was Joseph Priestley (1733-1804). His experiments with heating mercury calx led to him discovering that it produced a different kind of air. This had the property of making candles burn much more vigorously and a mouse would live longer in a jar containing only this air than in a jar with ordinary air. What Priestley had discovered, of course, was oxygen, but his reluctance to change his original views led to him calling it “dephlogistonated air” as opposed to air that has phlogiston added to it by the processes of burning and breathing.

Priestley had visited Antoine Lavoisier in Paris and shown him some of his experiments. Lavoisier (1743-94) carried out some experiments of his own and concluded that phlogiston was unnecessary as an explanation of what went on during combustion. In 1772 he discovered that when phosphorus and sulphur are burned they absorb air rather than expelling something. Also, when a calx is reduced to a metal, that is when something is expelled, namely an “air” that supports neither life nor combustion. This is of course carbon dioxide.

The phlogiston theory was therefore completely wrong in that it proposed the opposite of what was actually the case. Combustion requires the presence of one particular gas, namely oxygen, and it produces another, namely carbon dioxide. The theory is yet another example of how some scientists can blind themselves to the truth by sticking to prejudices rather than approaching matters with an open mind.

More about this author: John Welford

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