Elegant Solutions. Philip Ball
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Helmont was not the first person to propose that the world could be built from water alone. The Greek philosopher Thales, founder of the influential Ionian school, said as much in the sixth century BC, and part of his reasoning was similar – for water can be converted into ‘air’ by evaporation, while freezing transforms it into ‘earth’, which is, to say, a solid. But Thales’ idea never caught on, even among the later Ionian philosophers – and neither did van Helmont’s.
There is no compelling scientific reason why this should have been the case; rather, one might say that the circumstances were not to van Helmont’s advantage. For one thing, all-embracing ‘chemical philosophies’ were about to be eclipsed by Cartesian mechanistic science in the mid-seventeenth century: van Helmont’s writings represent their final bloom. Although he helped to place Paracelsian science on a more rational basis, he didn’t go nearly far enough; men like the Germans Andreas Libavius and Johann Rudolph Glauber were yet more ruthless in stripping chemistry of its Neoplatonic, magical trappings. At the Jardin du Roi, the royal medical and pharmaceutical school in Paris, alchemy was evolving into the academic discipline of ‘chymistry’. And the year before van Helmont’s Oriatricke appeared in England, Robert Boyle published his epoch-making critique of earlier ideas on chemistry, The Sceptical Chymist, which warned that chemists should be rigorous about how they defined an element and should not extrapolate beyond what the evidence permitted.
Besides, there were many systems of elements to choose from in the seventeenth century – several of them amalgams of Aristotle’s quartet and Paracelsus’s alchemical triumvirate of sulphur, mercury and salt – and van Helmont’s two-element scheme really did not have much more to recommend it above any other. In addition, it did not help that chemical philosophies had come to be associated with politically radical factions, such as the Bohemian rebels who denied the authority of the Holy Roman Emperor in 1619 and thereby triggered the Thirty Years’ War. In England too, Cromwell’s Puritans looked askance at such radicalism.
But van Helmont left his mark in other ways. He was interested in the ‘spirits’ that could be produced in chemical processes such as combustion, which were clearly different from ordinary air. He collected one such vapour, the ‘spirit of wood’, that was released from burning charcoal, and found that it could extinguish a flame. He was sure that these vapours were derived not from air but from water, and he decided they needed a new name. He borrowed a term that Paracelsus had used, the ancient Greek word chaos, which he transliterated as it sounded on the Flemish tongue: ‘gas’. What were these gases? That question was to set the principal research agenda of the chemists of the next century.
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