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
