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Notes on The Phlogiston Theory

[From Stephen Mason, A History of the Sciences: pp. 303-13]

Iatrochemistry (17th - 18th centuries)

Original View

Chemical Substances contain 3 essences or principles:

Joachim Becher (1635-82): Theory of 1669

Solid Earthy Substances contain 3 constituents:

Process of Burning and Calcination

Calcination is the process of converting Limestone (or marble or chalk) CaCO3 to Lime CaO by means of heat. In fact, Carbon dioxide is driven off in the process.

- involves decomposition of compound body into constituent parts; viz., sulphureous terra pinguis and fixed terra lapida in simplest cases.

- simple bodies cannot undergo combustion; only those with terra pinguis and another earth,

Georg Ernst Stahl (1660-1734): Theory of 1703

Phlogiston Theory : Theory of Calcination & Combustion

Phlogiston is an explanatory Principle, a theoretical mechanism to explain change.

Combustible objects are rich in Phlogiston. The process of burning involves loss of Phlogiston to the air. What is left over after combustion is without Phlogiston and so cannot burn. Wood has Phlogiston but not ash. Rusting of metals is analogous to the burning of wood; so metals possess Phlogiston while their rust (or 'calx') does not. (The difference in the presence and absence of fire in combustion is explained by speed at which Phlogiston leaves the combustible.)

Explanation of conversion of rocky ores to metals: Rocky ore is poor in Phlogiston while Charcoal is rich in Phlogiston. When you heat the Ore and Charcoal together, Phlogiston passes from the charcoal to the ore. The Charcoal is turned to Phlogiston-poor ash while the Phlogiston-poor ore is turned to Phlogiston-rich metal.

Air is incidentally useful to combustion. It is a carrier or medium holding Phlogiston as it leaves the wood or metal and passing in on to something else (if something were available).

Theory of Phlogiston was universally accepted in 1780.

A Problem with the Nature of Phlogiston

Gabriel Venel (1723-75): Positive Lightness Theory

Subsequent History of Phlogiston Theory

Discovery of Distinct 'Airs' (Gases)

Carl Scheele (1742 - 86): on Oxygen (1777)

Antoine Lavoisier (1743 - 94): Critique of Phlogiston Theory: Oxidation

Work on Combustion: Oxidation Theory

Objection to Lavoisier's Theory

Aftermath of Lavoisier's Theory of OXIDATION

Revolution in Chemistry

New notion of Chemical Element [Lavoisier (1789) lists 23 elementary substances including 'caloric, the imponderable matter of Heat]. Lavoisier, an aristocrat, is executed in 1794 by guillotine during the Terror after the French Revolution.

Conservatism in Chemistry

Priestley never entirely abandoned Phlogiston theory. Accepts that air not an element but a mixture. Water he eventually thinks is an element after considering it might be a compound. Explains the fact that water is formed by the action of hydrogen (or Phlogiston to some) by assuming that Hydrogen was a Compound of Phlogiston and Water and not Phlogiston itself. Dies in America (which he left for in 1794) and writes a treatise defending Phlogiston.

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Philosophical Relevance of Phlogiston Theory

In Kuhnian terms, Stahl's Phlogiston Theory (PT) and Lavoisier's Oxidation Theory (OT) belong to different chemical paradigms. The former belongs to Iatrochemistry, the latter to so-called modern Chemistry.

They differ in fundamental ways about certain fundamental chemical phenomena; viz., the processes of Combustion and Calcination. The explanations they offer of what these processes are and their underlying mechanisms are dramatically different.

Those both theories accept that many substances are in fact compounds of others, PT retains the old doctrine of a limited number of basic 'elements' as the fundamental building blocks while OT allows a multiplicity of fundamental elements. None of the elements of PT are elements of OT. OT rejects not only that the basic stuff in PT is not elemental, but more radically asserts that one of the so-called elements of PT - Phlogiston or terra pinguis - doesn't even exist.

In Kuhnian terms, PT and OT are incommensurable because their central concepts like 'element' and 'Phlogiston' and 'compound' do not overlap in meaning. In fact, they seem to mean completely different things in each theory. Furthermore, the so-called observations of the real world possible are in fact determined by the conceptual resources of each theory. For the proponent of PT, the release of Phlogiston is observed in combustion and calcination, and the measure of its actual negative weight or positive lightness in a substance is attainable by weighing that substance before and after combustion or calcination.

Now, if the very basic observations of each theory are themselves made possible only by already accepting the conceptual fundamentals of each theory, then nothing can count as a relevant observation which is neutral between the theories. If so, then within each theory possible observations are a direct function of the theory's basic components. An experimenter 'sees' Phlogiston released only by accepting the fundamentals of PT; an observer 'sees' the chemical trapping of oxygen by a heated metal only by working within the fundamentals of OT. But this means that no observation from OT is sufficient to refute or falsify any theoretical claim within PT and vice versa. There is no neutrally rational way of determining which of these views represents reality better.

For a critique of this position, see O'Hear p. 88 on the difference between the "grand theoretical level" and the "less elevated level recording and discovering effects of a more mundane sort".

Copyright © Stan Godlovitch

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Last modified: Tue May 22 02:57:01 PDT 2007