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:

• Sulphur (princ. of inflammability)
• Mercury (princ. of fluidity and volatility)
• Salt (princ. of fixity and inertness)

Joachim Becher (1635-82): Theory of 1669

Solid Earthy Substances contain 3 constituents:

• terra lapida (fixed earth = Salt Principle)
• terra pinguis (oily or fatty earth present in all combustible material = Sulphur = Principle of Inflammability)
• terra mercurialis (fluid earth = Mercury)

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

• - renames 'terra pinguis' or the Principle of Inflammability 'Phlogiston' (from Greek 'to set on fire') which is 'the motion of fire', 'the motion of heat', 'the sulphureous principle', 'the oily principle'.
• - Metal = compound of Calx and Phlogiston
• - Heat liberates Phlogiston and leaves the Calx
• - Phlogiston essential element of all combusitble material
• - Phlogiston escapes with burning of combustibles and enters atmosphere or joins with substances like calx to form metal.

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

• Problem: Why is it that certain substances when heated become lighter than they were before combustion (wood to ash) while others becomeheavier than they were before (metal to calx) if combustion involves a loss of phlogiston?
• Answer: Phlogiston is the spiritous aerial part of matter so that it has negative weight or positive lightness and so its loss increases the weight of the remaining calx.

Gabriel Venel (1723-75): Positive Lightness Theory

• "Phlogiston is not attracted towards the centre of the earth but tends to rise; thence comes the increase in weight in the formation of metallic calces and the diminution of weight in their reduction."
• Phlogiston is an anti-gravity material

Subsequent History of Phlogiston Theory

• Accepted by chemists Joseph Black (1728-99), Henry Cavendish (1731-1810), Joseph Priestley (1733-1804).
• Overthrow of Greek doctrine of 4 elements, Earth, Air, Fire, Water with recognition of many kinds of 'earths' (elementary substances) but retention of air, fire, water as elements.
• Phlogiston = Element of Fire or Agent Activating Fire.

Discovery of Distinct 'Airs' (Gases)

• Black isolates 'fixed air' or Carbon Dioxide (1754)
• Cavendish discusses preparation 'inflammable air' or Hydrogen (1766) as well as the production of 'sulphureous vapours' and nitreous vapours' using sulphuric and nitric acids on metals.
• Priestley in 1770s discovers ammonia, hydochloric acid gas, nitrous oxide, nitric oxide, oxygen, nitrogen, carbon monoxide, sulphur dioxide.

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

• Atmospheric Air is not an elementary substance but a mixture composed of 2 gases: 'fire air' (Oxygen) and 'foul air" (Nitrogen) in a 1 to 3 ratio by volume.
• The function of fire air or Oxygen is to take up the phlogiston given out by burning substances. The amount that could be absorbed was limited so that when oxygen in a confined space was saturated with phlogiston it could no longer support combustion [ explanation as to why a fire will go out in a confined space].

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

Work on Combustion: Oxidation Theory

• 1772 -Nonmentals like Phosphorus and Metals like Tin increase in weight when burned in air -> Increase in weight due to "absorption of air"
• Need to review past experiments about absorption and liberation of gases to settle on a consistent explanation.
• To Review Robert Boyle's (1627 - 91) contention that the increase in the weight of metals on calcination (burning) is due to the absorption of fire particles (material from the fire itself). This was proved wrong by heating tin in a sealed container, the total weight before and after heating the tin being the same; i.e., no absorbtion of anything external to the container from the fire itself.
• Discovery with Priestley that in sealed containers upon heating metals on calcination absorbed at most one fifth of the volume of air in the container. Inference that this part of the air was different from that part not absorbed. Lavoisier speculates that the special air is fixed air or Carbon Dioxide but abandons this hypothesis later.
• 1774 Lavoisier meets Priestley in Paris who shows him how to prepare Dephlogisticated air or Oxygen by heating Mercuric Oxide.
• 1780 Accepting Scheele's view, proposes that the atmosphere is 1/4 O and 3/4 N.
• 1783: The New Chemistry Announced - rejection of Stahl and Phlogiston Theory.
• Combustion and calcination involve in all cases the chemical combination of the combustible substance with Oxygen and the weight of the products formed invariably equalled the weight of the starting materials (in enclosed spaces)
• Burning and oxidation cannot be ascribed to escape of phlogiston because the old theory required that phlogiston have weight in some cases, be weightless in others, and have positive lightness (negative weight) in still others.
• Heat and Light thought of as effects of the escape of Phlogiston are in fact external to the chemistry of combustion and oxidation (e.g., rusting) since Heat and Light are imponderable (weightless).
• The change in weight of a substance during combustion or calcination was due entirely to its reaction with oxygen.
• Lavoisier makes Oxygen a general explanatory principle with unwarranted properties; e.g., O is the acidifying principle, a componenet of all acids which consist of oxygen united to a non-metallic substance: -> Refuted by Humphrey Davy in 1810 who showed that HCl contained no O.

Objection to Lavoisier's Theory

• Metals dissolve in acid: release Hydrogen and form a salt. In our terms, an example is: Fe + H2SO4 FeSO4 + H2
• The Calx (oxide) of the Metal dissolved in acid forms the same salt but does not release any gas. In our terms: FeO + H2SO4 FeSO4 + H2O
• Phlogiston Explanation: This shows that Hydrogen is Phlogiston (or Phlogiston combined with water). The Acid liberates Phlogiston from the Metal but not from the Calx, as the Metal was presumed to be a Compound of Calx + Phlogiston.
• Alternative Explanation: Lavoisier had to explain this difference in reaction and did so by accepting that Water is a Compound [see below] of Hydrogen and Oxygen. It had to be explained in particular why Hydrogen is released. In Lavoisier's view the Metal takes Oxygen from the Water present (in a dilute acid) and forms a Calx (metallic oxide). This Calx unites with the Acid to form a Salt. The Hydrogen from the Water is set free. [!]

Aftermath of Lavoisier's Theory of OXIDATION

• Britain: Blacks accepts but Cavendish and Priestley reject Oxidation. Priestley continues to regard Air as an element but eventually accepts oxidation.
• The Nature of Water
• Priestley (1781) explodes a mixture of H and O, finds gases used up, leaving dew.
• ***Experiment repeated by Cavendish: Experiment suggests that Water is a compond not an element, but Cavendish continues to believe Water is an element: Oxygen is Water deprived of Phlogiston, and Hydrogen is either Phlogiston itself or water with an excess of Phlogiston.*** [Preservation of Theory in face of Anomaly]
• Lavoisier tries to form an acid out of Hydrogen (non-metal) and Oxygen ('acidifying principle') and fails, but learns of Cavendish/Priestley experiment on Water which he repeats and concludes that Water is a Compound and not an Element.

Revolution in Chemistry

• Lavoisier's theory better explains known facts of chemistry than phlogiston theory
• Earth, air, fire, water no longer regarded as elements:
• Earth: many elements or kinds of earth recognized
• Fire resolved into heat, light, smoke
• Air composed of O and N
• Water a compound of H and O

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