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The word 'phlogiston'1 is derived from the Greek language. It is the neuter form of the word phlogistos, meaning 'inflammable'. This word is in turn derived from phlogizein meaning 'to set on fire', derived from phlox, or phlog-, meaning flame. Burning, as can be seen, is at the root of this area of scientific thought.
The theory of phlogiston arose in the late 17th Century when it was proposed by Johann Becher (who referred to it as 'inflammable earth'). It was continued in the work of Georg Stahl. The theory postulates that in all flammable materials the elusive substance phlogiston is present, a substance without colour, odour, taste or weight, which is given off when materials are burnt.
All materials could have three basic parts: phlogiston, impurities and the purest form of the material. Anything that could be burned to nothing would be considered to have been completely composed of phlogiston - such as charcoal or sulphur.
So, for example, wood when burnt leaves ash. It was, therefore, deemed that wood was composed of pure wood (ash) and phlogiston. Iron on the other hand consisted of rust, the pure form of the metal and phlogiston.
The impurities arose where the remains could not be defined as either the pure material or phlogistonated air. Gases that dissolved in water were a prime example of this kind of impurity, materials that did not meet the criteria of either pure base or phlogiston.
The theory maintained its position in scientific thought for 100 years, though throughout that period loopholes were identified and then carefully patched up with new variations and terms, like the impurities of fixed and foul air. Objections were consistently countered with new information from Phlogistonists who were not keen to see their theories blown apart.
Ultimately it was the work of French scientist Antoine Laurent Lavoiser, in the late 18th Century, which led to the systematic tearing apart of the Phlogiston Theory. His astute and systematic experimentation gave weight to gathering force of the antiphlogistonist movement. What replaced it were theories around oxygen and the reactions experienced by materials in the presence of this life-giving gas. Experiments switched from purely observational efforts to quantitative analysis that tried to qualify changes and reactions without giving in to notions of fancy expanded from what the eyes alone surveyed.
Flights of Fancy
Since its demise at the hands of scientific progress in the late 18th Century, Phlogiston has surfaced occasionally in the hands of fiction writers. A notable appearance was in the role-playing game Spelljammer (1989, out of print), a setting for the advanced Dungeons and Dragons game, that allowed participants to travel between the many worlds of the games mythos. The material Phlogiston was the chaotic nothingness that existed outside the Crystal Spheres that encapsulated each setting's world. It would occasionally coalesce into navigable rivers but otherwise posed a constant inconvenience, and occasional threat, to travellers through the void.