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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

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