In praise of wrong ideas
[This is my latest column for Chemistry World, and explains what I got up to on Monday night. I’m not sure when the series is being broadcast – this was the first to be recorded. It’s an odd format, and I’m not entirely sure it works, or at least, not yet. Along with Jonathan Miller, my fellow guest was mathematician Marcus de Sautoy. Jonathan chose to submit the Nottingham Alabasters (look it up – interesting stuff), and Marcus the odd symmetry group called the Monster.]


I can’t say that I’d expected to find myself defending phlogiston, 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 actually it needn’t. The real aim is to spark discussion of the issues that each guest’s choice raises. For phlogiston, 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 popularizer 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 (who was 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’.

I feel this isn’t the right 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 recognizably modern character, even if the debts to ancient and alchemical theories of the elements remained clear.

Phlogiston was in fact 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 – it was abundant in oily or sulphurous substances.

The idea, then, was that phlogiston made things burn. When wood or coal was ignited, its phlogiston was lost to the air, which was why its mass decreases. 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 such as that phlogiston buoyed up heavier substances, or (sometimes) had 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.

In any event, 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 (Fourth Estate), 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.

So phlogiston was an astonishingly fertile idea. The problem 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.

This is one of the reasons I chose phlogiston for the museum – it reminds us of our ahistorical tendency to clean up science history in retrospect, and to divide people facilely into progressives and conservatives. It also 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 being a little less wrong. I’m sure that one day the dark matter and dark energy of cosmologists will look like phlogiston does now: not silly ideas, but ones that we needed until something better came along.