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