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CHAPTER 2

An Element Compounded

Cavendish’s Water and the Beauty of Detail

London, 1781—The eccentric aristocrat Henry Cavendish, one of the wealthiest men in England, ignites two kinds of ‘air’ in a glass vessel and finds that they combine to form water. It is an experiment that has been performed before, and one that will be repeated subsequently by several other scientists. But Cavendish subjects the process to greater scrutiny than anyone previously, making careful measurements of all the quantities concerned, and his results point the way to a more definitive and remarkable conclusion: that these ‘airs’ are the very constituents of water, previously considered to be an irreducible element.

But is that what Cavendish himself thought? The issue, and with it Cavendish’s claim to the discovery that water is a compound, were hotly contested in the nineteenth century. This ‘water controversy’ is further clouded by Cavendish’s gentlemanly disregard for acclaim, which meant that he did not hurry into print but examined his findings for a further three years before publishing them. In the meantime, others scented the same trail, and the result was a priority dispute that historians are still debating today.

Even though van Helmont’s belief in water as the fundamental stuff of all creation was not taken seriously by the late eighteenth century, nonetheless there seemed little reason to doubt that water was an element – the last, perhaps, of the Aristotelian elements to remain unchallenged. The problem is that when everyone believes something, no one bothers to check it. When he performed his famous experiment, Henry Cavendish was not setting out to investigate the nature of water. Like many of his contemporaries, he was more interested in that other ancient element: air.

This was the age of ‘pneumatic chemistry’, when researchers devoted themselves to collecting the ‘vapours’ that bubbled from chemical processes. Once considered inert and therefore uninteresting, ‘air’ was now found to come in several varieties. The English clergyman Stephen Hales showed in 1727 that ‘airs’ could be collected by bubbling them through water to ‘wash’ them, and then collecting them in a submerged, inverted glass vessel. The ‘Hales trough’ allowed one to quantify the amount of ‘air’ collected by observing the volume of water it displaced.

The Scotsman Joseph Black used the technique to study an ‘air’ produced by heating limestone or magnesia: this vapour seemed to be miraculously ‘fixed’ in the minerals until heat drove it out, and Black called it ‘fixed air’. It was not like ‘common air’: substances wouldn’t burn in fixed air, and it had the signature property of turning lime water (a solution of calcium hydroxide, then known as slaked lime) cloudy. And then there was the deathly ‘mephitic air’ identified by Black’s student Daniel Rutherford, a residue of common air that remained after combustion was carried out in a sealed vessel. The ‘chymists’ of the seventeenth century had known about another vapour produced when acids acted on certain metals: the Swedish apothecary Carl Wilhellm Scheele collected this gas in 1770 and observed that it burnt explosively in common air. Scheele called it ‘inflammable air’.

The chemistry of airs had a theory, and it was based around the substance called phlogiston. In 1703 the German chemist Georg Stahl named this mercurial substance after the Greek word phlogistos, ‘to set on fire’. Phlogiston was what made things burn. Some substance, said eighteenth-century scientists, was being transferred between the air and a combusting material – and that substance was phlogiston.

Materials were considered to lose phlogiston when they burnt.* When common air was saturated with phlogiston, burning ceased: that was why a candle inside a sealed vessel would eventually go out. For the English Nonconformist minister Joseph Priestley, this explained the character of Rutherford’s mephitic air: it was nothing but normal air mixed with a sufficiency of phlogiston. In 1774 Priestley discovered how to make the opposite of this lifeless, smothering substance: how to create an ‘air’ that was ‘dephlogisticated’ and thus wonderfully conducive to combustion. He made it by heating mercury oxide, something that others (including Scheele) had done before.

In the same year, Priestley’s friend John Warltire looked carefully at the explosive combustion of Scheele’s inflammable air. Warltire seized on the contemporary fad for investigating electricity by using an electrical spark to ignite a mixture of common air and inflammable air, and he found that after the explosion there was less ‘air’ than before, and that the walls of his vessel were coated with dew. In Paris, Pierre Joseph Macquer found much the same thing: inflammable air burnt with a smokeless flame, and when a porcelain plate was placed over the flame, it was moistened with drops ‘which appeared to be nothing else but pure water’.

And so what? Everyone knew that water could condense out of common air to mist a window with droplets or to make the pages of books curl up in dank cellars. Warltire did not much concern himself with the water, and neither did Priestley when he repeated the experiment in 1781. They were more interested in what was happening to the ‘airs’, and what this meant for phlogiston theory. Inflammable air was clearly rich in phlogiston – indeed, some scientists, including Scheele and Cavendish himself, suspected that it might be pure phlogiston – and Priestley figured that this phlogiston caused common air to release the water it contained: ‘common air’, he said, ‘deposits its moisture by phlogistication’.

Then Henry Cavendish decided to take a look too.

A queer fellow

More than any other science, chemistry has traditionally told its history through a progression of colourful characters. Empedocles, drunk on dreams of immortality, throws himself into Mount Etna; Paracelsus staggers foul-mouthed and drunken through Renaissance Europe; Johann Becher, the wily alchemist who started the whole phlogiston business, swindles the princes of the Nertherlands with promises of alchemical gold; Dmitri Mendeleyev, who drew up the periodic table, is the wild and shaggy prophet of Siberia. Most of these tales contain a strong element of hearsay, if not outright invention. And Cavendish can be relied upon for a gloriously odd comic turn. In Bernard Jaffe’s Crucibles, the archetype for this kind of history, Cavendish was ‘gripped by an almost insane interest in the secrets of nature, . . . not giving a moment’s thought to his health or appearance’. The son of Lord Charles Cavendish and heir to a fortune, he ‘never owned but one suit of clothes at a time and continued to dress in the habiliments of a previous century, and shabby ones, to boot’ (Figure 2).

Figure 2 Henry Cavendish: this ink-and-wash sketch by William Alexander, the only known portrait of the reclusive scientist, was prepared by the artist with not a little subterfuge

(Reproduced Courtesy of the Library and Information Centre, Royal Society of Chemistry)

If this Cavendish is a stage character, however, there is no denying that he is more than just invention. The Honourable Henry Cavendish was genuinely strange and difficult to know; his own colleagues make that clear enough. Charles Blagden, Cavendish’s associate and the only person with whom he seems to have had anything approaching a close relationship, calls him sulky, melancholy, forbidding, odd and dry. The scientist and politician Lord Brougham, 35 years after Cavendish’s death, says that he ‘uttered fewer words in the course of his life than any man who ever lived to fourscore years, not at all excepting the monks of La Trappe’. He recalls how Cavendish would shuffle quickly from room to room at the Royal Society, occasionally uttering a ‘shrill cry’ and ‘seeming to be annoyed if looked at’.

Even the usually generous Humphry Davy, who said on Cavendish’s death that since the demise of Isaac Newton England had suffered ‘no scientific loss so great’, found the man himself ‘cold and selfish’ (he made the same charge of Blagden). Davy admitted that Cavendish was ‘afraid of strangers, and seemed, when embarrassed, even to articulate with difficulty’. The chemist Thomas Thomson called him ‘shy and bashful to a degree bordering on disease’.

That seems indeed to be the true measure of the man. Contrary to what Jaffe suggests, Cavendish may not have been exactly misanthropic but, rather, painfully shy to the point where he was barely able to interact at all with his fellows. If he seemed ‘cold’, it is likely that this was simply the appearance conveyed by his extreme diffidence. Perhaps the most telling image we have is that of Cavendish hovering on the doorstep of the house of Joseph Banks, the Royal Society’s president, unable to bring himself to knock on the door and face the crowds within.

On the basis of the biography of Cavendish published in 1851 by chemist George Wilson, Oliver Sacks has made a tentative diagnosis of the subject’s social dysfunction:

Many of the characteristics that distinguished Cavendish are almost pathognomic of Asperger’s syndrome: a striking literalness and directness of mind, extreme single-mindedness, a passion for calculation and quantitative exactitude, unconventional, stubbornly held views, and a disposition to use rigorously exact (rather than figurative) language – even in his rare non-scientific communication – coupled with a virtual incomprehension of social behaviours and human relationships.

There seems to be sufficient consensus among contemporary descriptions of Cavendish’s behaviour to make such a conclusion likely. But Wilson’s biography, while often taken at face value, was not a dispassionate account of the man; it had an agenda, as we shall see.

Yet for all his reticence, Cavendish scarcely ever missed the weekly dinner of the Royal Society Club at the Crown and Anchor on the Strand, nor was he often absent from the Monday Club at the George & Vulture coffee house. Although conversation seemed an agony to him, he forced himself into society, because in the end he wanted to mix with his learned colleagues and share with them the adventure of science.

For that was the life Cavendish chose. Like his father, he could have followed the conventional political career of an aristocrat; but like Charles he turned instead to science. He had only just been elected a member of the Royal Society when, in 1766, he published a stunning paper in the society’s Philosophical Translations (he never published anywhere else) on the chemistry of airs. ‘Three Papers, Containing Experiments on Factitious Air’ won him the Royal Society’s prestigious Copley Medal.

‘Factitious’ meant any air that was somehow contained within other materials ‘in an unelastic state, and is produced from thence by art’. Black’s fixed air was such a gas, and inflammable air was another. Cavendish was not content with noting that this latter air went pop when ignited; he reported careful measurements showing that it was 8700 times lighter than water and capable of holding ‘1/9 its weight of moisture’.

This kind of detail reveals the way Cavendish thought about experiments. His laboratory, housed within the grounds of his ample townhouse in Great Marlborough Street, near Piccadilly in London, was filled with measuring devices. The caricature presented by Wilson, and more or less uncritically repeated ever since, shows Cavendish as a calculating machine, obsessed with quantification; but the fact was that he understood this was now the only reliable way to do science. We’ve seen that van Helmont recognized the value of measurement in the seventeenth century; but Cavendish’s vision penetrated further than that. He understood the meaning of accuracy and precision, and realised that all experiments have a finite and unavoidable margin of error. He estimated the accuracy of his determinations, making distinctions between the errors introduced by the experimenter and the limitations of the instrumentation. To reduce such sources of error, he would repeat experiments and take averages of the results. And he would quote numerical results only to the appropriate number of significant figures. The great French scientist Pierre-Simon Laplace, who pioneered statistical techniques for handling errors in experiment (and of whom more later), remarked to Blagden that Cavendish’s work was conducted with the ‘precision and finesse that distinguish that excellent physicist’. This is arguably Cavendish’s greatest contribution to experimental science: an attention to numerical detail that keeps the experimenters’ claims in proportion to what their methods justify.

And numbers have power. By putting numbers on the low weight of this vapour relative to common air, Cavendish excited speculations about whether it might enable a man to ‘fly’ by means of the buoyancy of a balloon filled with it. And so it did: the physicist Jacques Charles took to the air in 1783 in Paris, prompting Antoine Lavoisier to scale up his method of producing ‘inflammable air’ while Joseph Banks covered up his nationalistic chagrin with sniffy remarks about the flighty French.

In the early 1780s, Cavendish decided to explore ‘the diminution which common air is well known to suffer by all the various ways in which it is phlogisticated’. In other words, he was keen to examine the process that Warltire and Priestley had described, in which common air is reduced in volume by igniting it with inflammable air (which might or might not be phlogiston itself). Thus he was not, in a sense, proposing to do anything new; rather, he saw that sometimes an experiment yields its secrets only when you start to look at the details. ‘As the experiment seemed likely to throw great light on the subject I had in view’, he explained in the report of his studies, presented to the Royal Society in 1784, ‘I thought it well worth examining more closely’.

Anatomy of an explosion

Like the others before him, Cavendish made inflammable air by dissolving zinc or iron with acids, and he set off the detonation with a spark. ‘The bulk of the air remaining after the explosion’, he wrote,

is then very little more than four-fifths of the common air employed; so that as common air cannot be reduced to a much less bulk than that by any method of phlogistication, we may safely conclude that when they are mixed in this proportion, and exploded, almost all the inflammable air, and about one-fifth part of the common air, lose their elasticity, and are condensed into the dew which lines the glass.

Every detail was carefully checked out; nothing was taken for granted. The dew, he said ‘had no taste nor smell, and . . . left no sensible sediment when evaporated to dryness; neither did it yield any pungent smell during the evaporation; in short, it seemed pure water’. In some experiments he noticed that the explosion produced a little ‘sooty matter’, but he concluded that this was probably a residue from the putty (‘luting’) with which the glass apparatus was sealed; and indeed ‘in another experiment, in which it was contrived so that the luting should not be much heated, scarce any sooty tinge could be perceived’.

Was the dew truly pure water? Cavendish found in some initial experiments that it was in fact slightly acidic, and he spent long hours tracking down where the acid came from. Although he did not put it quite this way himself, the acidity stems from reactions between oxygen in the air and a little of the nitrogen that makes up the ‘inert’ four-fifths of the remaining gas, creating nitrogen oxides, which are acidic when dissolved in water. Such pursuit of anomalies was one reason why Cavendish was so slow to publish his findings, which he did some three years after the experiments were begun. But the fact is that Cavendish was in no hurry in any case. For him, publication was not the objective, and he seems blithely unconcerned about securing any claims to priority. He seems to have adopted the approach advocated by his colleague William Heberden, who said that the happiest writer wrote ‘always with a view to publishing, though without ever doing so’.

So what was this experiment telling him? In retrospect it seems obvious: inflammable air and the ‘active’ constituent of common air – or hydrogen and oxygen, as Lavoisier was already calling these substances – unite to form water. But Cavendish was at that stage still in thrall to the phlogiston theory, and so things were by no means so clear to him. He ascertained that the lost one-fifth of common air could be identified as the ‘dephlogisticated air’ that Priestley had described: indeed, when he used this air instead of common air, it was all used up if ignited with twice its volume of inflammable air. But this interpretation meant that phlogiston had to appear somewhere in the balance. Cavendish concluded that dephlogisticated air was ‘in reality nothing but dephlogisticated water, or water deprived of its phlogiston’. In other words, water was not being made from two constitutive parts but was appearing through the combination of phlogiston-poor water with the phlogiston contained in inflammable air. Alternatively, if the inflammable air were not phlogiston itself, then it was ‘phlogisticated water’, or ‘water united to phlogiston’. The phlogiston effectively cancelled out:

[water – phlogiston] + [water + phlogiston] = water

How we are to understand Cavendish’s conclusions has been a matter of great debate, because to some extent the issue of whether or not he made a genuine ‘discovery’ about the nature of water hinges on it. The truth is that there is nothing in what Cavendish wrote about his experiment that indicates unambiguously that he questioned the elemental status of water. That is to say, it remains unclear whether he decided that water somehow pre-existed in his airs and was simply being condensed in the explosion (which is pretty evidently what Priestley believed) or whether he had some inkling that water was being created from its constituents in a chemical process. Traditional historical accounts of Cavendish’s experiment tend to imply that he made more or less the correct interpretation, even if he couched it in the archaic terms of phlogiston theory. But historian of science David Philip Miller has argued fairly persuasively that Cavendish’s thoughts were closer to Priestley’s. In any event, for an explicit and decisive statement of water’s compound nature, we must look across the English Channel.

A new kind of chemistry

In Paris, Antoine Lavoisier was on the same path: familiar with Macquer’s work, he too was looking more closely at what happened when the two airs were united. But he had a different hypothesis. In the mid-1770s he had concluded that Priestley’s dephlogisticated air was in fact a substance in its own right: an element, which he proposed to call oxygen. The name means ‘acid-former’, for Lavoisier had the (misguided) notion that this element was the ‘principle of acidity’, the substance that creates all acids.

Cavendish knew of Lavoisier’s oxygen, but he did not much care for it. He pointed out, quite correctly, that there was at least one acid – marine acid, now called hydrochloric acid – that did not appear to contain this putative element. (Lavoisier admitted in 1783 that there were some difficulties in that regard which he was still working on.) But while some of Cavendish’s contemporaries, Priestley in particular, were trenchantly opposed to Lavoisier’s theory because of an innate conservatism, Cavendish was more pragmatic – he argued simply that no one could at that stage know the truth of the matter. His objections were directed more at the way Lavoisier sought to impose the oxygen theory on chemical science by a relabelling exercise: in 1787 the French chemist proposed a new system of nomenclature in his magisterial Traité elementaire de chimie, the adoption of which would make it virtually impossible to practice chemistry without implicity endorsing oxygen. Imagine what would happen, Cavendish complained, if everyone who came up with a new theory concocted a new terminology to go along with it. In the end, chemistry would become a veritable Tower of Babel in which no one could understand anyone else. He derided the ‘rage of name-making’ and dismissed Lavoisier’s Traité as a mere ‘fashion’. Until there were experimental results that could settle such disputes, he said, it was better to stick with the tried-and-tested terminology, since new names inevitably prejudice the very terms within which theoretical questions can be framed.

That Cavendish’s opposition was not motivated by mere traditionalism is clear from the fact that he gradually abandoned phlogiston and accepted Lavoisier’s oxygen as the evidence stacked up in the French chemist’s favour. Even in 1785 he was prepared to concede that phlogiston was a ‘doubtful point’, and by early 1787 the phlogistonist Richard Kirwan in England wrote to Louis Bernard Guyton de Morveau, a colleague of Lavoisier’s, saying that ‘Mr Cavendish has renounced phlogiston.’ By the turn of the century, Cavendish was prepared even to use Lavoisier’s terms: dephlogisticated air became oxygen, and inflammable air was hydrogen – the gas which, thanks to the researches of Warltire, Priestley and Cavendish as well as his own, Lavoisier saw fit to call the ‘water former’.

But that is rather leaping ahead of the matter. In the late 1770s Lavoisier decided that, since his oxygen was the principle of acidity, its combination with hydrogen should produce an acid. In 1781–2 he looked for it in experiments along the same lines as Priestley and Warltire, but saw none. Working with Laplace, he combined oxygen and hydrogen in a glass vessel and found that their combined weight was more or less equal to that of the resulting water.

They were not the only French scientists to try it. When Joseph Priestley conducted further experiments of this kind in March 1783, the French scientist Edmond Charles Genet in London wrote a letter describing the work to the French Académie des Sciences, the equivalent of the Royal Society. Genet’s letter was read to the academicians in early May. Lavoisier was there to hear it, and so was the mathematician Gaspard Monge from the military school of Mézières, who promptly repeated the experiment in June.

Lavoisier and Laplace did likewise – but by then they knew of Cavendish’s results too, for Charles Blagden told them about his colleague’s investigations in early June while on a trip to Paris. Lavoisier, as ambitious as Cavendish was diffident, quickly repeated the measurements on 24 June (forgoing, in haste, his usual quantitative precision) and presented them soon after to the Académie. He referred to the earlier work by both Monge and Cavendish, magisterially indicating that he ‘proposed to confirm’ Cavendish’s observations ‘in order to give it greater authority’.

Curiously, Lavoisier continued at this point to call oxygen ‘dephlogisticated air’ – but for him this was more or less just a conventional label, and did not oblige him to fit phlogiston into his explanations. That enabled him to see through to the proper conclusion with far more directness and insight than Cavendish. ‘It is difficult to refuse to recognize’, he said, ‘that in this experiment, water is made artificially and from scratch.’

And, in a master stroke, he verified that this was so by showing how water might be split into its two constituents. Lavoisier felt that the right way to investigate the composition of matter was to come at it from both directions: synthesis, or making a substance from elemental components, and analysis, which meant separating the substance into those fundamental ingredients. He described how, in collaboration with the engineer Jean Baptiste Meusnier, he ‘analysed’ water by placing it together with iron filings in an environment free of air, held in an inverted bowl under a pool of mercury. The iron, he reported, was converted into rust, just as it is when it absorbs dephlogisticated air (that is, oxygen) from common air; and ‘at the same time it released a quantity of inflammable air in proportion to the quantity of dephlogisticated air which had been absorbed by the iron.’ ‘Thus’, he concluded, “water, in this experiment, is decomposed into two distinct substances, dephlogisticated air . . . and inflammable air. Water is not a simple substance at all, not properly called an element, as had always been thought.”

Cavendish’s experiment was beautiful because of his attention to detail, a characteristic that redirected attention towards the formation of water and pointed clearly to the conclusion that Lavoisier subsequently drew. But Lavoisier’s follow-up studies surely deserve a share of that beauty, because of the way he found the right interpretation and then went on to make it irrefutable.

Needless to say, not everyone saw it quite like that. The shroud of phlogiston that made Cavendish’s explanation of his experiment somewhat ambiguous also helped to protect him from the kind of reactionary responses that Lavoisier’s starker message attracted. An English chemist named William Ford Stevenson showed how reluctant some scientists were to abandon the ancient elemental status of water when he called Lavoisier’s claims a kind of ‘deception’. How on Earth could water, which puts out fires and was for that reason ‘the most powerful antiphlogistic we possess’, how could this substance truly be compounded from an air ‘which surpasses all other substances in its inflammability’? Cavendish betrayed that he had not quite grasped the true implications of his results when he too expressed doubts about Lavoisier’s conclusions. Priestley, a staunch believer in phlogiston, had no time for them. Blagden, meanwhile, was more angered (and with some justification) by Lavoisier’s failure to give sufficient credit to what Cavendish had already achieved – although at that point Cavendish had still not submitted his report to the Royal Society.

Water wars

Even that was not the full extent of the controversy. No sooner had Cavendish’s paper finally been read to the Royal Society in January 1784 than it awakened a new dispute. The Swiss scientist Jean André De Luc heard about the report and asked Cavendish for a copy, whereupon he wrote to his friend James Watt, suggesting that Cavendish was a plagiarist who had copied Watt’s ideas ‘word for word’. For Watt had repeated Warltire’s experiment several years earlier while he was still a university technician at Edinburgh, working under Joseph Black. It was not so much the experiment itself that incited De Luc’s charges, but Cavendish’s interpretation in terms of ‘dephlogisticated water’, which seemed very much along the lines of what Watt had deduced: he had claimed that water was a compound of pure air and phlogiston.

At least, that is what some historical accounts indicate; but again, there is ambiguity about whether Watt truly identified water as a substance produced by the chemical reaction of two ‘elements’. Drawing on Joseph Priestley’s experiments in early 1783 on the spark ignition of dephlogisticated and inflammable air (which were themselves stimulated by Cavendish’s still unpublished work), Watt suggested in April of that year that ‘water is composed of dephlogisticated and inflammable air, or phlogiston, deprived of part of their latent heat.’ Is this a statement that water is a compound substance? Latent heat is the heat a gas releases when it condenses into a liquid – and so Watt’s conclusion seems to blend notions about both the combination of two gases and the condensation of water. It’s hard to know quite what to make of it.

At that time, Watt expressed his ideas about water in letters to Priestley, De Luc and Joseph Black. He had intended that they be read out formally to the Royal Society, but then withdrew his formal communication after learning that Priestley’s further investigations seemed to point to some inconsistencies with other ideas that Watt’s letter contained. Yet when De Luc saw Cavendish’s report, he decided that it had appropriated Watt’s ‘theory’ without attribution.

Watt was annoyed, although unable to conclude for sure that intellectual theft was involved. ‘I by no means wish’, he wrote to De Luc,

to make any illiberal attack on Mr C. It is barely possible he may have heard nothing of my theory; but as the Frenchman said when he found a man in bed with his wife, ‘I suspect something’.

All the same, Watt conceded that Cavendish’s interpretations were not identical to his own, and even admitted that ‘his is more likely to be [right], as he has made many more experiments, and, consequently has more facts to argue upon’. There is a trace of envy at Cavendish’s riches and status in comparison to Watt’s own humble origins (he was the son of a Clydeside shipbuilder) when he tells De Luc that he ‘could despise the united power of the illustrious house of Cavendish’. Yet Watt seems to have put aside his bitterness soon enough. He wrote a paper that same year describing his own experiments and ideas on water, in which he graciously noted that ‘I believe that Mr Cavendish was the first who discovered that the combination of dephlogisticated and inflammable air produced moisture on the sides of the glass in which they were fired.’ The unworldly Cavendish probably never knew about Watt’s initial anger; in 1785 he recommended Watt for a fellowship of the Royal Society.

This apparent conciliation did not prevent others from arguing over who discovered that water was a compound. Cavendish, Watt, Lavoisier and Monge have all been put forward as candidates. The debate raged heatedly in the mid-nineteenth century, when it centred on Watt’s rival claim. His case was argued forcefully by François Arago, secretary of the French Académie des Sciences, in his Eloge de James Watt, and Lord Brougham and Watt’s son James Watt Jr added their voices to this appeal. In response, William Vernon Harcourt, in his address as president-elect to the British Association for the Advancement of Science in Birmingham in August 1839, vehemently asserted Cavendish’s priority – a speech that left some members of the audience bristling, for Watt the engineer was a hero in the industrial Midlands of England.

David Philip Miller has shown that this ‘water controversy’ was fuelled by broader agendas. Watt Jr was no doubt motivated by filial concern for his father’s reputation, but Arago and Watt’s other supporters hoped that their protagonist’s claim to this discovery in fundamental science would lend weight to their belief in an intimate link between pure and applied science. Harcourt’s camp, meanwhile, consisted of an academic élite that was keen to promote the image of the ‘gentleman of science’ who sought knowledge for its own sake and remained aloof from the practical concerns of the engineer. The reclusive, high-born and disinterested Cavendish was its ideal exemplar. George Wilson’s biography of Cavendish was a product of this controversy – a polemic that aimed to establishing its subject’s priority and honourable conduct, it gave disproportionate attention to his experiments on water. But in other respects Wilson’s researches left him with a rather poor impression of Cavendish’s character, and his portrait set the template for the subsequent descriptions of a peculiar, asocial man, ‘the personification and embodiment of a cold, unimpassioned intellectuality’ as the editor of a collection of Cavendish’s papers put it.

There is, however, a postscript to Cavendish’s compulsive attention to detail that illustrates just how valuable to science pedantry can be. In 1783 he looked at the other component of common air, the ‘phlogisticated air’ that would not support combustion. This is, of course, nitrogen, which, as Cavendish showed, is converted into nitrous acid via reactions with oxygen. ‘Acid in aerial form’ was how Blagden summarized Cavendish’s conclusions about phlogisticated air, and both he and Priestley felt that these studies represented a more important contribution than Cavendish’s work on water – a reflection of the ‘pneumatic’ preoccupation of chemists at that time.

But while Cavendish was able to eliminate nearly all of the phlogisticated component of common air, he remarked that there always seemed to be a tiny bit of ‘air’ left, which appeared as a recalcitrant bubble in his experiments. This seemed to make up just part of common air, and Cavendish suspected that it was just the consequence of his experimental inadequacies. All the same, he wrote down his observations and Wilson mentioned them in the biography.

Some years later, an English chemistry student named William Ramsay bought a second-hand copy of Wilson’s book and read about the mysterious bubble. That reference lodged in his remarkable mind, and he recalled it in the 1890s when he was a professor of chemistry at University College, London. Ramsay was at that time corresponding with the English physicist Lord Rayleigh, who suspected that nitrogen extracted from air might have a small impurity of some unreactive substance. They repeated Cavendish’s experiments on nitrogen, and in 1894 they announced that they had discovered a new element, one that did not seem to react with any other. They named it after the Greek world for ‘idle’: argon. Within several years, Ramsay had found three other, similarly inert, gases and had unearthed an entirely new group of the periodic table of elements. That was the start of another story, and in Chapter 8 we shall hear its conclusion.

* That was why wood got lighter as it was consumed by flames. But, inconveniently, metals got heavier when they were heated (calcined) in air, even though they were supposed to be losing phlogiston. No problem, said the advocates of phlogiston theory: apparently this volatile ‘principle of combustion’ can sometimes have negative weight.