The fable of phlogiston

I don’t often find myself defending archaic chemical hypotheses, let alone in front of a comedy audience. But I wasn’t angling for laughs - I was aiming to secure a place for phlogiston in the ‘Museum of Curiosities’, an institution that exists only in the ethereal realm of BBC’s Radio 4. In a forthcoming series of the same name, panellists submit an item of their choice to the museum, explaining why it deserves a place. The show will have laughs - the curator is the British comedian Bill Bailey - but the real aim is to spark entertaining discussion of the issues raised by each candidate. For phlogiston, once thought to be the cause of combustion, there are plenty of those. 

What struck me most during the recording was how strongly the old historiographic image of phlogiston still seems to hold sway. In 1930 the chemical populariser Bernard Jaffe wrote that phlogiston, which he attributed to the alchemist Johann Becher, ‘nearly destroyed the progress of chemistry’, while in 1957 the science historian John Read called it a ‘theory of unreason’. Many of us doubtless encountered phlogiston in derisive terms during our education, which is perhaps why it is forgivable that the programme’s producers wanted to know of ‘other scientific theories from the past that look silly today’. But even the esteemed science communicator, the medical doctor Jonathan Miller (one of my co-panellists), spoke of the ‘drivel’ of the alchemists and suggested that natural philosophers of earlier times got things like this wrong because they ‘didn’t think smartly enough’. 

That’s the wrong way to think about phlogiston. Yes, it had serious problems even from the outset, but that was true of just about any fundamental chemical theory of the time, Lavoisier’s oxygen included. Phlogiston also had a lot going for it, not least because it unified a wealth of observations and phenomena. Arguably it was the first overarching chemical theory with a recognisably modern character, even if aspects of ancient and alchemical theories remained. 

Fire hazard 

Phlogiston was named in 1718 by Georg Stahl, professor of medicine at the University of Halle, who derived it from the Greek phlogistos, to set on fire. But Stahl took the notion from Becher’s terra pinguis or fatty earth, one of three types of ‘earth’ that Becher designated as ‘principles’ responsible for mineral formation. Becher’s ‘earths’ were themselves a restatement of the alchemical principles sulphur, mercury and salt, proposed as the components of all things by Paracelsus. Terra pinguis was the principle of combustibility, abundant in oily or sulphurous substances. 

The idea was that phlogiston made things burn. Ignite wood or coal and their phlogiston is lost to the air, causing their mass to decrease. Combustion ceases when air is saturated in phlogiston. One key problem, noted but not explained by Stahl, was that metals don’t lose but gain weight when combusted. This is often a source of modern scorn, for it led later scientists to contorted explanations involving (for example) negative weight. Those claims prompted Lavoisier ultimately to denounce phlogiston as a ‘veritable Proteus’ that ‘adapts itself to all the explanations for which it may be required’. But actually it was not always clear whether metals did gain weight when burnt, for the powerful lenses used for heating them could sublimate the oxides. 

Fertile idea 

Yet phlogiston explained not only combustion but also acidity, respiration, chemical reactivity, and the growth and properties of plants. As Oliver Morton points out in his new book Eating the Sun, the Scottish geologist James Hutton invoked a ‘phlogiston cycle’, analogous to the carbon and energy cycles of modern earth scientists, in which phlogiston was a kind of fixed sunlight taken up by plants, some of which is buried in the deep earth as coal and which creates a ‘constant fire in the mineral regions’ that powers volcanism. 

The problem with phlogiston was not that it was plain wrong, but that it was so nearly right (it was the mirror image of the oxygen theory) that it could not easily be discredited. And indeed, that didn’t happen as cleanly and abruptly as implied in conventional accounts of the Chemical Revolution. As Hasok Chang at University College London has explained, phlogistonists persisted well into the nineteenth century, and even eminent figures such as Humphry Davy were sceptical of Lavoisier see Chemistry World website.

Phlogiston neatly demonstrates our tendency to clean up science history in retrospect, and to divide people into progressives and conservatives. It shows that the opposite of a good idea can also be a good idea. And it reminds us that science is not about being right, but about being a little less wrong. I’m sure that modern cosmologists’ dark matter and dark energy will one day look like phlogiston does now: not silly ideas, but ones that we needed until something better came along.