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The Case of Phlogiston and the Dephlogisticated Air. James Ladyman University of Bristol. Outline. A brief introduction to phlogiston Phlogiston in philosophy of science: 2.1 Classic views of phlogiston

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The case of phlogiston and the dephlogisticated air

The Case of Phlogiston and the Dephlogisticated Air

James Ladyman

University of Bristol


  • A brief introduction to phlogiston

  • Phlogiston in philosophy of science:

    2.1 Classic views of phlogiston

    2.2. Phlogiston and the contemporary scientific realism debate and the arguments from theory change

    2.3 The flight to reference and recent accounts of the phlogiston case

  • Structural Realism and Phlogiston

1 the theory of phlogiston heyday 1700 1790
1. The Theory of Phlogiston (heyday 1700-1790)

  • Becher (1635-1682) proposed combustible earth (as one of three earths composing ordinary substances).

  • Stahl (1660-1734) proposed phlogiston in 1697? (also year the world’s first engine was built): parallels between metals heated in air (calcination) and ordinary combustion – creation of dust and light often given off, lots of air needed. Phlogiston given off in combustion - flames

  • Wood turns to ash when burnt (phlogiston must therefore have mass).

  • Iron rusts to calx also giving off phlogiston.

  • Charcoal (and sulphur) combust almost completely so charcoal is very nearly pure phlogiston.

  • Phlogiston was supposed to have a metallic quality.Metal calx burnt in charcoal becomes ordinary metal - adding phlogiston adds the metallic quality to the true form (calx) of the metal (practical application to copper mining).

  • When something burns it uses up the oxygen in the air until eventually the fire goes out. This was explained in terms of the saturation of the air with phlogiston.

  • Combustion, respiration and calcification are all the same kind of reaction (oxidisation). (Animals in a sealed chamber phlogisticate the air (Boyle).)

The theory of phlogiston
The Theory of Phlogiston

  • Basic explanatory entity - the material substance phlogiston

  • Basic form of explanation - the movement in space of phlogiston

    (Gale calls this the basic form of Democritean explanation)

  • Unification of generalizations and relations among the phenomena by description in terms of the release or absorption of phlogiston. (Oxidation/Reduction)

  • The properties of phlogiston were purely descriptive and qualitative.

  • The domain of application was that of chemical reactions involving qualitative change.


  • Since Rey (1630) it was known that the calx of a metal could be heavier than its metallic form.

  • Some Stahlians posited negative weight for metallic phlogiston but most believed that impurities caused the weight increase.

Priestley 1733 1804
Priestley (1733-1804)

  • Plants dephlogisticate the air (cycle between plants and animals).

  • Air without any phlogiston is air whose potential to be burnt is maximal.

  • Dephlogisticated air by heating a calx (red mercury) (Priestley 1774)

  • Scheele (1742-1786): ‘fire air’ (1771-2, published 1777)

  • There is a clear sense in which the dephlogisticated air that Priestley describes breathing is oxygen.

  • Phlogisticated air is air saturated with phlogiston.

  • Inflammable air [hydrogen (Cavendish 1766)] is pure phlogiston not charcoal according to Cavendish.

  • Priestley burned metal oxide in inflammable air to make pure metal (and water) - reduction inverse of oxidisation.

  • But inflammable air is obviously not just ordinary air with phlogiston in higher concentration, since ordinary air becomes saturated with phlogiston during combustion and eventually the fire goes out.

  • Some of the phlogisticated air dissolves in water (carbon dioxide) and some does not (mostly nitrogen). Neither supports ordinary combustion (like Oxygen) or reduction (like hydrogen).

Lavoiser 1743 1794
Lavoiser (1743-1794)

  • Worked quantitatively and precisely

  • Detected mass increase of some residues from combustion

  • Introduced the conservation of mass

  • Lavoisier defined a chemical element (1789) as the endpoint of the chemical process of analysis.

  • Stahl – metals not elements since had to have phlogiston added to them.

  • Cavendish (1731-1810) produced dephlogisticated air and inflammable air from water (1783) (and vice versa)

  • Oxygen (dephlogisticated air) and hydrogen (phlogisticated air) are both elements making up compound ordinary air.

  • Burning, respiration and rusting of iron are all oxidisation.

  • Problems with carbon monoxide and acids not containing oxygen.

2 phlogiston in philosophy of science 2 1 classic views of phlogiston
2 Phlogiston in philosophy of science2.1 Classic views of phlogiston

  • Phlogiston theory contained terms (‘phlogiston’, ‘principle’) that fail to refer to the world. It was replaced by the successfully referring theory (‘oxygen’, ‘element’) of Lavoiser.

  • Many people think of the demise of phlogiston theory as the start of properly scientific chemistry - modern chemical apparatus including scales, and precise measurement of masses and volumes.

  • Phlogiston theory was qualitative, and still related to the Renaissance and Aristotelian ideas of principles, qualities and virtues.

  • Phlogiston theory is not mature science.

  • Note that phlogiston theory is not mathematicised.

  • It was falsified by the fact that oxides are heavier than pure sample of metal.

  • Classic ad hoc modification - negative mass for metallic phlogiston (but then magnesium oxide is heavier than magnesium).

Kuhn on phlogiston
Kuhn on Phlogiston

  • The transition from Priestley to Lavoisier is a classic example of a scientific revolution.

  • Contrary to the Whiggish view, phlogiston theory had a lot going for it.

  • Priestley and Lavoisier are living in different worlds - so ‘phlogiston’ referred for Priestley?

  • But note that this revolution violates Planck’s maxim about the old guard dying out not converting. Chemists converted almost on mass and almost overnight when Lavoisier’s theory was complete.

Neo popperian lakatosian post kuhnian view
Neo-Popperian/Lakatosian/Post-Kuhnian View

Noretta Koertege (1968):

phlogiston theory as an example supporting Post’s ‘general correspondence principle’ according to which the well-confirmed empirical generalizations of old theories are retained by their successors: there are no ‘Kuhn losses’.

George Gale:

  • ‘phlogiston theory was an extremely adequate explanatory theory’

  • Explains loss of weight of wood, coal and ordinary substance when burnt.

  • Charcoal leaves hardly any ash because it is almost pure phlogiston.

  • Air saturated with phlogiston cannot support respiration.

  • Metals are alike because they all contain phlogiston (Lavoiser’s theory discouraged nineteenth century chemists from seeking explanations of this fact.)

  • But the theory is wrong because phlogiston is nonexistent.

2 2 phlogiston and the contemporary scientific realism debate and the arguments from theory change
2.2 Phlogiston and the contemporary scientific realism debate and the arguments from theory change

The pessimistic meta induction versus the argument from theory change
The Pessimistic Meta-Induction versus the Argument from Theory Change

The Pessimistic Meta-Induction:

  • There have been many empirically successful theories in the history of science which have subsequently been rejected, and whose theoretical terms do not refer according to our best current theories.

  • Our best current theories are no different in kind from those discarded theories, and so we have no reason to think they will not ultimately be replaced as well.

    So, by induction we have positive reason to expect that our best current theories will be replaced by new theories according to which some of the central theoretical terms of our best current theories do not refer, and hence, we should not believe in the approximate truth or the successful reference of the theoretical terms of our best current theories.

Responding to the pmi
Responding to the PMI Theory Change

  • Respond to PMI by reducing inductive base ruling out theories on the basis of some criteria:

  • Maturity - reliance on well-entrenched background theories

  • Mathematicization

  • Strong empirical success:

    Quantitative precision

    Novel predictive success

Contemporary science is different
Contemporary science is different Theory Change

It is quite plausible to claim this:

  • Hugely mathematicised

  • Hugely integrated and unified especially with respect to the macrosciences and the chemistry of the periodic table

  • Hugely quantitatively accurate. QED accurate to 13 significant figures.

    (have to count Newtonian gravitation - accurate to one part in 107 although based on data accurate to one part in 103)

  • Exponential growth (Fahrbach)

The argument from theory change
The Argument from Theory Change Theory Change

  • We do not need to form an inductive argument based on Laudan’s list to undermine the no-miracles argument for realism.

  • Laudan's paper was also intended to show that the successful reference of its theoretical terms is not a necessary condition for the novel predictive success of a theory (1981, 45), and there are counter-examples to the no-miracles argument.

  • Psillos (1999) p. 108: the divide and conquer strategy is needed because even if there are only a couple of examples of false and non-referring, but mature and strongly successful theories, then the “explanatory connection between empirical success and truth-likeness is still undermined”.

  • Lewis (2001), Lange (2002) and Magnus and Callender (2004) regard the pessimistic meta-induction as another instance of the base rate fallacy (see 2.1.2 above). Here we sidestep their concerns by reformulating the argument from theory change so that it is not a probabilistic or inductive argument.

The argument from theory change1
The Argument from Theory-Change Theory Change

  • There are cases of mature theories that enjoyed novel predictive success by anyone’s standards, namely the ether theory of light and the caloric theory of heat.

  • “If their central theoretical terms do not refer, the realist's claim that approximate truth explains empirical success will no longer be enough to establish realism, because we will need some other explanation for success of the caloric and ether theories. If this will do for these theories then it ought to do for others where we happened to have retained the central theoretical terms, and then we do not need the realist’s preferred explanation that such theories are true and successfully refer to unobservable entities.” (Ladyman and Ross (2007), p. 84, Ladyman (2002))

  • Psillos’ solution: ‘caloric’ - non-central, ‘ether’ - refers after all

The argument from theory change2
The Argument from Theory Change Theory Change

  • Successful reference of its central theoretical terms is a necessary condition for the approximate truth of a theory.

  • There are examples of theories that were mature and had novel predictive success but whose central theoretical terms do not refer.

  • So there are examples of theories that were mature and had novel predictive success but which are not approximately true.

  • Approximate truth and successful reference of central theoretical terms is not a necessary condition for the novel-predictive success of scientific theories

    So, the no-miracles argument is undermined since, if approximate truth and successful reference are not available to be part of the explanation of some theories’ novel predictive success, there is no reason to think that the novel predictive success of other theories has to be explained by realism. (Ladyman and Ross 2007, p.84-85)

Realism truth and reference
Realism, Truth and Reference Theory Change

Truth explains empirical success and successful reference is a necessary condition for approximate truth.

Laudan (1981) on the first abductive argument for scientific realism:

  • approximate truth implies empirical success

  • successful reference implies empirical success

    However, both these claims are contestable.

    1 is likely only true modulo the assumption of accurate background theories and auxiliary hypotheses.

    2 is only plausible assuming some form of descriptivism about reference.

Realists on phlogiston
Realists on Phlogiston Theory Change

  • Putnam (1978): ‘phlogiston’ is a prime example of a non-referring term

  • Lewis (1970): ‘phlogiston’ does not refer

  • Hardin and Rosenberg (1982):

    Phlogiston theory is pre-paradigmatic science

    Approximate truth does not require successful reference

  • Psillos (1999):

    “a phlogiston-based taxonomy is wrong because no natural kind has the kind-constitutive properties attributed to phlogiston” (288)

    “phlogiston refers to nothing” (291)

    ‘Phlogiston’ is a counterexample to the simple causal theory since then it would refer to oxygen (whatever is involved in combustion).

Phlogiston s success
Phlogiston’s Success Theory Change

  • Metal + heat = calx [metal oxide]

  • Calx + charcoal (source of phlogiston) = metal (+ fixed air [carbon dioxide (Joseph Black (1728-1799) 1754)])

  • So Metal = calx + phlogiston (explaining what metals have in common)

  • Charcoal = calx (fixed air) + phlogiston

  • Metal + acid = salt + inflammable air (note Lavoisier thought acids had to contain oxygen)

  • Metal + water = calx + inflammable air

  • (Water = inflammable air + dephlogisticated air)

  • Dephlogisticated marine acid (Scheele) = chlorine (Davy).

  • novel prediction: heat calx in inflammable air to get pure metal

  • novel prediction of new acids by Scheele (formic acid, lactic acid)

  • Animals and plants have opposite effects on the air - the former phlogisticate and the latter dephlogisticate

  • Metals are alike (lost by intermediary science between Priestley and electronic chemistry).

  • Phlogistication and dephlogistication are inverse chemical reactions (reduction and oxygenation) (Carrier and Schurtz) - but saturation of the air with phlogiston is not reduction.

Securing reference
Securing Reference Theory Change

  • Kitcher (1978, 1993): some tokens of ‘dephologisticated air’ refer.

  • Theory-laden terms have ‘heterogeneous reference potential’.

  • Depends whether reference fixed by baptism, descriptivist or conformist.

Macleish 2005 2006
MacLeish (2005, 2006) Theory Change

  • Kitcher faces the ‘discrimination problem’.

  • Three possible solutions:

    • what we would say

    • what was really the case

    • what Priestley would have said

  • disjunctive partial denotation (c.f. Field (1973))

Schurz Theory Change

  • Phlogistication - assimilation of phlogiston

  • Dephlogistication - release of phlogiston

  • Oxidation (in general sense) of X = formation of ionic bond with with electronegative substance

  • Reduction is regaining of electrons

  • If oxidising agent is oxygen, and X is a source of carbon then product is carbon dioxide ie ordinary phlogisicated air.

  • If oxidising agent is an acid, then hydrogen is emitted.

    We could go further and allow that ‘phlogiston rich’ and ‘phlogiston deficient’ refer too

Underdetermination Theory Change

  • Phlogiston equals antioxygen

  • Phlogiston equals spare electrons

Comparing phlogiston and other cases
Comparing phlogiston and other cases Theory Change

  • The case of Phlogiston is like that of caloric:

    Both were supposed to be a material substance, both were involved in explanations in terms of their movement in space, both were ideas from earlier science that were eliminated.

    Psillos says ‘caloric’ was not really central to the empirical success of the caloric theory of heat. Similarly, one could argue that ‘phlogiston’ was not central to the empirical success of phlogiston theory.

    Compare with ‘mass’ and ‘atom’

    The Democritean style of explanation is not preserved by fundamental physics.

3 structural realism
3 Structural Realism Theory Change

  • (i) ESR and OSR both involve commitment to the claim that science is progressive and cumulative and that the growth in our structural knowledge of the world goes beyond knowledge of empirical regularities.

  • (ii) ESR and OSR both depart from standard scientific realism in rejecting term by term reference of theories, and hence standard referential semantics, and any account of approximate truth based on it.

  • (iii) According to both OSR and ESR, scientific theories do not give us knowledge of the intrinsic natures of unobservable individual objects.

  • Two versions of ESR can be contemplated. According to ESR1 there are such objects but we cannot know them, and according to ESR2 there may or may not be such objects, but we cannot know either way, and if there are such objects we cannot know them. (It seems that Worrall now advocates ESR2.)

Phlogiston and structural realism
Phlogiston and Structural Realism Theory Change

  • (i) is supported by the case of phlogiston theory. The empirical success of the theory was retained in subsequent chemistry:

    Combustion, calcification and respiration are all the same kind of reaction.

    The reaction has an inverse reaction and there is a cycle between plants and animals such that animals change the properties of the air in one way and plants in the opposite way.

    Inflammable air (Hydrogen) really is metallic.

  • (ii) is also supported by the case of phlogiston theory.

    Solutions to the phlogiston problem based on defending the reference of the theoretical term phlogiston and dephlogisticated air are not plausible. Schurz view is better since reference of the names of the processes of ‘phlogistication’ and ‘dephlogistication’ is more plausible.

  • (iii) is also supported by the case of phlogiston theory.

    The phlogiston case further supports the induction that science will replace material explanations with structural ones.

Phlogiston and osr
Phlogiston and OSR Theory Change

  • Real Patterns (Dennett, Ross, Ladyman and Ross)

  • Not merely retention of empirical generalizations between phlogiston theory and subsequent science so more than structural empiricism is warranted.

Whewell history of the inductive sciences
Whewell, Theory ChangeHistory of the Inductive Sciences

  • “But we must not forget how natural it was to suppose that some part of a body was destroyed or removed by combustion…It would be easy to show, from the writings of phlogistic chemists, what important and extensive truths their theory enabled them to express simply and clearly.” (p. 270)