Phlogiston Theory

Phlogiston arose at the end of the seventeenth century when the dominant theory of matter was still the four-element theory of the ancient Greeks. This had been supplemented by the tria prima concept of the Paracelcians. On top of the four Greek elements of earth, water, fire and air Paracelsus had added the principles of mercury, sulphur and salt. It is important to realise that these are principles involved in the composition of substances rather than substances themselves. In an analogy based on the combustion of a piece of wood Paracelsus compared the smoke to mercury, the flame to sulphur and the ash to salt. This analogy is important, as combustion alongside distillation was one of the two principle methods of chemical investigation available to alchemists in the Early Modern Period.
Whilst rejecting Paracelcian alchemy the German physician Johann Joachim Becher (1635 – 1682) borrowed the tria prima replacing the mercury, sulphur and salt with three forms of the element earth:
terra fluida or mercurious earth, which contributed fluidity, subtility, volatility and metallicity to substances.
terra pinguis or fatty earth, which produced oily, sulphureous and combustible properties.
terra lapidea or vitreous earth, which was the principle of fusibilty.
Becher published this theory in his Physica Subterranea in 1667. For Becher histerra pinguis played an essential role in combustion.
Another German physician Georg Ernst Stahl (1659 – 1734) took up Becher’s theory, in 1718, renaming terra pinguis, phlogiston, from the Greek meaning inflammable, using this principle to explain both combustion and corrosion (rusting). Hypothesising that all inflammable materials contain phlogiston, which is consumed or used up during combustion. The important point is that the phlogiston theory as developed by Stahl readily explained the known facts of combustion.

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