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