I'm fascinated by revisionist histories. I grew up in a British colony where we were systematically lied to about our own history. Events in the 1970s and 1980s forced us to begin to confront what really happened when we colonised New Zealand. At around the same time, modern histories began to appear to give us a more accurate account. James Belich's Making Peoples had a major impact on me. Michael King's Being Pakeha also struck a chord, as did Maurice Shadbolt's historical novel Monday's Warriors.
Most people who know a little bit about the history of science will have heard of phlogiston. The phlogiston theory is usually portrayed as exactly the kind of speculative metaphysics that was laid to rest by artful empiricists. Phlogiston became a symbol of the triumph of empiricism over superstition. As a student of chemistry, I imbibed this history and internalised it.
The popular history (aka science folklore) has a Whiggish feel in the sense that Lavoisier is represented as making a rational leap towards the telos of the modern view. Such, we are led to believe, is the nature of scientific progress. My favourite encyclopedia repeats the standard folklore:
The phlogiston theory was discredited by Antoine Lavoisier between 1770 and 1790. He studied the gain or loss of weight when tin, lead, phosphorus, and sulfur underwent reactions of oxidation or reduction (deoxidation); and he showed that the newly discovered element oxygen was always involved. Although a number of chemists—notably Joseph Priestley, one of the discoverers of oxygen—tried to retain some form of the phlogiston theory, by 1800 practically every chemist recognized the correctness of Lavoisier’s oxygen theory.—Encyclopedia Britannica.
Compare this remark by Hasok Chang (2012b: time 19:00) in his inaugural lecture as Hans Rausing Professor of History and Philosophy of Science, at Cambridge University:
I became a pluralist about science because I could not honestly convince myself that the phlogiston theory was simply wrong or even genuinely inferior to Lavoisier's oxygen-based chemical theory.
When I was reading about the systematic misrepresentation of the work of J. J. Thomson and Ernest Rutherford in physics folklore, Chang's lecture came to mind. I discovered Chang 4 or 5 years ago and have long wanted to review his account of phlogiston, but was caught up in other projects. In this essay, I will finally explore the basis for Chang's scepticism about the accepted history of phlogiston. While I largely rely on his book, Chang pursued this theme in two earlier articles (2009, 2010).
Setting the Scene
The story largely takes place in the mid-late eighteenth century. The two principal figures are Joseph Priestley (1733 – 1804) and Antoine-Laurent de Lavoisier (1743 – 1794).
A caveat is that while I focus on these two figures, the historical events involved dozens, if not hundreds, of scientists. Even in the 1700s, science was a communal and cooperative affair; a slow conversation amongst experts. My theme here is not "great men of history". My aim is to explore the historiography of science and reset my own beliefs. Chang's revisionist history of phlogiston is fascinating by itself, but I am intrigued by how Chang uses it as leverage in his promotion of pluralism in science. Priestley and Lavoisier are just two pegs to hang a story on. And both were, ultimately, wrong about chemistry.
Chang (2012: 2-5) introduces Priestley at some length. He refers to him as "a paragon of eighteenth-century amateur science" who "never went near a university", while noting that he was also a preacher and a "political consultant" (from what I read, Priestley was really more of a commentator and pamphleteer). As a member of a Protestant dissenting church, Priestley was barred from holding any public office or working in fields such as law or medicine. In the 1700s, British universities were still primarily concerned with training priests for the Church of England. That said, Priestley was elected a fellow of the Royal Society in 1766, which at least gained him the ears of fellow scientists. Priestley is known for his work identifying different gases in atmospheric air. He first discovered "fixed air" (i.e. carbon-dioxide) and became a minor celebrity with his invention of carbonated water. He also discovered oxygen, more on this below.
However, Chang provides no similar introduction to Lavoisier. Rather, Lavoisier appears in a piecemeal way as a foil to his main character, Priestley. The disparity seems to be rhetorical. Part of Chang's argument for plurality in science is that Priestley was on the right track and has been treated poorly by historians of science. By focusing primarily on Priestley and treating Lavoisier as secondary, Chang might be seen as rebalancing a biased story.
I'm not sure that this succeeds, because as a reviewer, I now want to introduce Lavoisier to my readers, and I have to rely on third-party sources to do that. Chang doesn't just leave the reader hanging; he misses an opportunity to put Lavoisier in context and to draw some obvious comparisons. That Priestley and Lavoisier inhabited very different worlds is apposite to any history of phlogiston.
Lavoisier was an aristoi who inherited a large fortune at the age of five (when his mother died). He attended the finest schools where he became fascinated by the sciences (such as they were at the time). This was followed by university studies, where Lavoisier qualified as a lawyer, though he never practised law (he did not need to). As an aristo, Lavoisier had access to the ruling elite, which gave him leverage in his dispute with Priestley. He was also something of a humanitarian and philanthropist, spending some of his fortune on such projects as clean drinking water, prison reform, and public education. Despite this, he was guillotined during the French Revolution after being accused of corruption in his role as a tax collector. He was later exonerated of corruption.
The contrasting social circumstances help to explain why Lavoisier was able to persuade scientists to abandon phlogiston for his oxygen theory. Lavoisier had money and class on his side in a world almost completely dominated by money and class.
Having introduced the main players, we now need to backtrack a little to put their work in its historical context. In the 1700s, the Aristotelian idea that the world is made of earth, water, fire, and air was still widely believed. To be clear, both water and air were considered to be elemental substances. 18th-century medicine was still entirely rooted in this worldview.
Alchemy still fascinated the intelligentsia of the day. On one level, alchemists pursued mundane goals, such as turning lead into gold, and on another, they sought physical immortality (i.e. immortality in this life rather than in the afterlife).
The telescope and microscope were invented in the early 1600s. With the former, Galileo observed the Moon and Jupiter's satellites, becoming the first empirical scientist to upset the existing worldview by discovering new facts about the world.
That worldview was still largely the synthesis of Christian doctrine with Aristotelian philosophy created by Thomas Aquinas (1225–1274). The microscope had also begun to reveal a level of structure to the world, and to life, that no one had previously suspected existed. The practice of alchemy began to give way to natural philosophy, i.e. the systematic investigation of properties of matter. Priestley and Lavoisier were not the only people doing this, by any means, but they were amongst the leading exponents of natural philosophy.
One of the key phenomena that captured the attention of natural philosophers, for obvious reasons, was combustion. The ancient Greeks believed that fire was elemental and that combustion released the fire element latent in the fuel. This is the precursor to the idea of phlogiston as a substance.
Phlogiston Theory
The first attempt at a systematic account of phlogiston is generally credited to Georg Ernst Stahl (1659 – 1734) in Zymotechnia fundamentalis "Fundamentals of the Art of Fermentation" (1697). The term phlogiston derives from the Greek φλόξ phlóx "flame", and was already in use when it was applied to chemistry.
The basic idea was that anything which burns contains a mixture of ash and phlogiston. Combustion is the process by which phlogiston is expelled from matter, leaving behind ash. And we see this process happening in the form of flames. And thus, a combustible substance was one that contained phlogiston. Phlogiston was "the principle of inflammability".
However, experimentation had begun to show interesting relationships between metals and metal-oxides (known at the time as calx). One could be turned into the other, and back again. For example, metallic iron gradually transforms into a reddish calx, which is a mixture of a couple of different oxides of iron. To turn iron-oxide back into iron, we mix it with charcoal or coke and heat it strongly. And this reversible reaction is common to all metals.
Chemists used phlogiston to explain this phenomenon. Metals, they conjectured, were rich in phlogiston. This is why metals have such qualities as lustre, malleability, ductility, and electrical conductivity. In becoming a calx, the metal must be losing phlogiston, and by analogy, a calx is a kind of ash. On the other hand, charcoal and coke burn readily, so they must also be rich in phlogiston. When heated together, the phlogiston must move from the charcoal back into the calx, reconstituting the metal.
This reversible reaction was striking enough for Immanuel Kant to use it, in The Critique of Pure Reason (1781), as an example of how science "began to grapple with nature in a principled way" (Chang 2012: 4).
Priestley is famous for having discovered oxygen, but as Chang emphasises, it was Lavoisier who called it that. Priestley called it dephlogisticated air, i.e. air from which phlogiston has been removed. "Air" in this context is the same as "gas" in modern parlance.
Priestley produced dephlogisticated air by heating mercury-oxide, releasing oxygen and leaving the pure metal. According to the phlogiston theory, such dephlogisticated air should readily support combustion because, being dephlogisticated, it would readily accept phlogiston from combustion. And so it proved. Combustion in dephlogisticated air was much more vigorous. Breathing the new air also made one feel invigorated. Priestley was the first human to breathe pure oxygen, though he tested it on a mouse first.
Formerly considered elemental, atmospherical air could now be divided into "fixed air" and "dephlogisticated air". A missing piece was inflammable air (hydrogen), which was discovered by Henry Cavendish in 1766, when he was observing the effects of acids on metals. Cavendish had also combusted dephlogisticated air and inflammable air to make water. And Priestley had replicated this in his own lab.
Priestley and Cavendish initially suspected that inflammable air was in fact phlogiston itself, driven from the metal by the action of acids. A calx in acid produced no inflammable air, because it was already dephlogisticated. However, the fact that dephlogisticated air and phlogiston combined to make water was suggestive and led to an important refinement of the phlogiston theory.
They settled on the idea that inflammable air (hydrogen) was phlogisticated water, and that dephlogisticated air (oxygen) was actually dephlogisticated water. And thus, the two airs combined to form water. In this view, water is still elemental.
It was Lavoisier who correctly interpreted this reaction to mean that water was not an element but a compound of hydrogen and oxygen (and it was Lavoisier who named inflammable air hydrogen, i.e. "water maker"). However, it is precisely here, Chang argues that phlogiston proves itself to be the superior theory.
Chang notes that, without the benefit of hindsight, it's difficult to say what is so wrong with the phlogiston theory. It gave us a working explanation of certain chemical phenomena, and it made testable predictions that were accurate enough to be taken seriously. For its time, phlogiston was a perfectly good scientific theory. So the question then becomes, "Why do we see it as a characteristic example of a bad scientific theory disproved by empiricism?" Was it really such a bad theory?
A Scientific Blunder?
On one hand, Chang argues that, given the times, phlogiston theory was a step in the right direction, away from alchemical views and towards seeing electricity as the flow of a fluid, which then leads towards the modern view of chemical reactions involving the flow or exchange of electrons. And on the other hand, Lavoisier's theory is far from being "correct".
If the argument is that phlogiston was an ad hoc concept that could not be observed, then why is the same criticism not levelled against Lavoisier for the role of elemental luminaire or caloric in his theory? Caloric is what we would now call "heat", and it is clearly not elemental.
The terms "oxidation" and "reduction" (and the portmanteau "redox") are generalisations from Lavoisier's explanation of metals and metal-oxides. A metal-oxide can be "reduced" to the pure metal, and a metal oxidised to form the oxide. And one can make them go back and forth by altering the conditions.
While oxidation and reduction apply to metals and their oxides, such reactions are not typical. Most redox reactions don't involve metals or oxygen. When fluorine reacts with hydrogen, for example, we say that hydrogen is "oxidised" (gives up an electron) and that fluorine is "reduced" (gains an electron). And this terminology doesn't make much sense. Even with a BSc in chemistry, I always have to stop and think carefully about which label applies because it's not intuitive.
A commonly cited reason for the collapse of the phlogiston theory is that a metal gains weight in becoming a calx. The implication is that phlogiston theory was at a loss to explain this. Superficially, the early versions of phlogiston theory argue that in becoming a calx, the metal loses phlogiston, so we would expect it to lose weight, rather than gain it. The idea that the metal combines with oxygen is correct in hindsight, and is how we see the formation of metal-oxides in the present.
However, Priestley and another phlogistonist, Richard Kirwan, did have an explanation for weight gain. I've already noted that Priestley's ideas matured and that, latterly, he had concluded that inflammable air (hydrogen) was phlogisticated water, and dephlogisticated air (oxygen) was dephlogisticated water. In Priestley's mature view, the metal formed a calx by combination with water and the loss of phlogiston. The added weight was due to the dephlogisticated water. When the calx was reduced, the metal absorbed phlogiston and gave up water.
Like Chang, when I review this explanation, keeping in mind the state of knowledge at the time, I can't see how Lavoisier's explanation is any better. Seen in the context of the times (late 18th century), there was nothing illogical about the phlogiston theory. It explained observations and made testable predictions. As Chang (2010: 50) says:
We really need to lose the habit of treating ‘phlogiston theory got X wrong’ as the end of the story; we also need to ask whether Lavoisier’s theory got X right, and whether it did not get Y and Z wrong.
Chang cites several historians of science commenting on this. For example, John McEvoy (1997) notes that...
by the end of the eighteenth century, almost every major theoretical claim that Lavoisier made about the nature and function of oxygen had been found wanting.
And Robert Siegfried (1988):
The central assumptions that had guided [Lavoisier's] work so fruitfully were proved empirically false by about 1815.
These comments are in striking contrast to the claim made by Britannica: "by 1800, practically every chemist recognized the correctness of Lavoisier’s oxygen theory". The story in Britannica is the widely accepted version of history. At the same time, Chang makes clear, the story in Britannica is simply false.
Lavoisier's theory of acids, his theory of combustion, and his theory of caloric were all clearly wrong from the viewpoint of modern chemistry. For example, Lavoisier claimed that all acids contain oxygen (the name oxygen means "acid maker"). However, hydrochloric acid (which we have in our stomachs) does not contain oxygen. Indeed, the action of acids is now thought to be because of their ability to produce hydrogen ions (aka naked protons, aka phlogisticated water), which are extremely reactive.
Moreover, as Chang (2012: 9) shows, the problems with Lavoisier's theory were well known to his contemporaries. Many scientists voiced their concerns at the time. The point is well taken. If we are judging by modern standards, then Lavoisier and Priestley were both wrong, Lavoisier no less than Priestley. Nonetheless, Lavoisier, with his fortune and his access to the French aristoi, had more leverage than dissenting Priestley.
That said, Lavoisier clearly won the argument. And the brief account of his triumph in Britannica is a classic example of the adage that the victors write history.
What We Lost
What Chang tries to do next is declared by the subtitle of section 2: "Why Phlogiston Should Have Lived" (2012: 14). The first section of the book is deliberately written relatively informally with the idea that a general reader could appreciate the argument. In this second section, however, he develops a much more philosophically rigorous approach and introduces a great deal more jargon, some of which is specific to his project.
My aim in this essay is to continue the discussion at the same level. This inevitably means losing exactly the nuances that Chang introduces and probably diverging from his intentions to some extent. I do recommend reading the rest of his argument. What follows is my, all too brief, interpretation of Chang's argument.
While his history is revisionist, Chang's point is not to promote a speculative counterfactual history (which is to say, a fictitious alternative history). Rather, he seeks to make an argument for pluralism. Where pluralism means the coexistence of different explanations for any given phenomenon, until such time as the best explanation emerges.
Chang argues that Lavoisier's view that oxygen was being exchanged in chemical reactions was clearly inferior and only applicable to metal/calx reactions. By the time this became clear, phlogiston was discredited and could not be revived. And Lavoisier's counterintuitive oxidation-reduction model became the norm in chemistry, and still is, despite its obvious disadvantages.
The idea that phlogiston was being exchanged in chemical reactions was not a bad theory (for the time). Moreover, phlogiston was already conceptually linked to electricity. Getting from redox to the exchange of electrons took another century. Chang argues that the conceptual leap from phlogiston to the exchange of electrons could have been considerably easier than it was, starting from Lavoisier's theory.
Chang's argument for pluralism is not simply based on the two theories being equally false. Indeed, he goes to some pains to explain what they both got right. The point is that the phlogiston theory had untapped potential. In prematurely killing off phlogiston and adopting Lavoisier's oxygen theory (which as we have seen was disproved a few decades later), we actually retarded the progress of science. And when Lavoisier was proven wrong, we had no alternative theory and simply retained his awkward and misleading terminology.
Had we allowed the two theories to co-exist a little longer, so that Lavoisier's explanation could be thoroughly tested and proven false before it was adopted, there is a possibility that we might have lighted on the electron exchange theory of chemical reactions a century earlier than we did. Indeed, as hinted above, phlogiston was already linked to electricity. Seen with hindsight, the rush to judgment about chemical reactions meant that scientists of the late 17th and early 18th centuries missed a huge opportunity.
Chang is a pragmatist. He knows we cannot go back. His argument is that we should be alert to this situation in the present and the future and be less eager to settle on a theory where ambiguity remains. Arguably, the temporary triumph of the various Copenhagen interpretations of Schrödinger's equation was a similar example. We settled too early, for reasons unconnected to science, only to have the chosen theory be disproved some decades later.
I don't read Chang as saying that we should hold on to pluralism no matter what. Only that, where there is room for doubt, we should allow multiple explanations to coexist, because we don't know in advance what the best answer will be. This only emerges over time. And a scientific theory can only benefit from responding to the challenges that other explanations pose.
Conclusions
Hasok Chang aims to demonstrate the value of pluralism through critiquing the history of the so-called "chemical revolution" identified with Lavoisier. And the case of phlogiston is both fascinating in its own right and a compelling study of how the lack of pluralism retarded the progress of science.
While sources like Britannica follow science folklore in insisting on the "correctness" of the oxygen theory, historians of science tell us a different story. It may be true that Lavoisier's theory was widely adopted by 1800, but historians have shown that it was also largely falsified by 1815. By this time, the phlogiston theory had been "killed", as Chang puts it.
Chang attempts to show that phlogiston was not such a bad theory and that the oxygen theory was not such a good theory. Contrary to the usual Whiggish accounts, the triumph of Lavoisier's oxygen theory was not really an example of "scientific progress". Indeed, Chang supposes that adopting the oxygen theory actually retarded the progress of science, since it pointed away from the role of electricity in chemistry. This important insight took another century to emerge.
The phlogiston theory is arguably the better of the two theories that existed in the late 1700s. Chang argues that had phlogiston persisted just a little longer, at least until Lavoisier was disproved, we might have made the leap to seeing chemical reactions in terms of the flow of electricity between elements much earlier than we eventually did. And who knows what else this might have changed?
The point is not to inaugurate some kind of neo-phlogistonist movement or to speculate about counterfactual (alternative) histories. The point is that when we have competing theories, in the present, we should allow them to coexist rather than rushing to settle on one of them.
Pluralism is a pragmatic approach to uncertainty. When different explanations are possible, we can compare and contrast the differences. Allowing such challenges is more likely to result in scientific progress than the rush to judgment or the overwhelming desire to have one right answer.
As noted at the outset, in this essay, I have largely overlooked the contributions of Priestley's and Lavoisier's contemporaries. I have emphasised the two main players, even more than Chang does, purely for narrative simplicity (and keeping this essay to a reasonable length). This might make it seem that it was something like a personal competition, when that doesn't seem to be the case. Think of this essay as a taster. My aim is to whet your appetite to go and discover Chang for yourself, or better, to go and read the original papers being published at the time. See for yourself.
Coda
The pluralism that Chang praises in the case of chemistry is not the same kind of pluralism that exists in so-called "interpretations of quantum mechanics". Chang is in favour of having multiple explanations of a phenomenon until such time as the best explanation unequivocally emerges. But he also considers that the best explanations change over time as new data comes in. Chang is a pragmatist, and this seems to be the only viable approach to science. We do not and cannot acquire metaphysical certainty because there is no epistemic privilege with respect to reality. We are all inferring facts about reality based on experience, a procedure known to be fraught with difficulties that often go unnoticed.
Generally, in science, we see competing explanations that attempt to fit a new phenomenon into our pre-existing metaphysics. In crude terms, scientific theories are made to fit into existing views about reality, and new data changes our view of reality only rarely and often incrementally. Paradigms do change, but only with great reluctance. This conservatism is generally a good thing as long as it doesn't become dogmatic.
In stark contrast to the rest of science, in quantum physics, the mathematical approximations are considered infallible and inviolable, and scientists propose different realities in which the mathematics makes sense. They have become dogmatic about their theory and refuse to consider other models. It has not gone well.
As Sabine Hossenfelder said, "Theoretical physicists used to explain what was observed. Now they try to explain why they can’t explain what was not observed."
~~Φ~~
Bibliography
Chang, Hasok. (2009) "We Have Never Been Whiggish (About Phlogiston)". Centaurus 51(4): 239-264. https://doi.org/10.1111/j.1600-0498.2009.00150.x
Chang, Hasok. (2010). "The Hidden History of Phlogiston: How Philosophical Failure Can Generate Historiographical Refinement." HYLE – International Journal for Philosophy of Chemistry 16 (2): 47-79. Online.
Chang, Hasok. (2012a). Is Water H20? Evidence, Realism and Pluralism. Springer.
Chang, Hasok. (2012b). "Scientific Pluralism and the Mission of History and Philosophy of Science." Inaugural Lecture by Professor Hasok Chang, Hans Rausing Professor of History and Philosophy of Science, 11 October 2012. https://www.youtube.com/watch?v=zGUsIf9qYw8
Stahl, Georg Ernst. (1697). Zymotechnia fundamentalis.
