while the tree had very obviously gained a lot of mass. In any case, what was the significance of the figure of 164 pounds, if the leaves were neglected? But that wasn’t the point. Numbers are hard facts; they are irrefutable. If anyone doubted the interpretation, van Helmont could demand that they kindly explain where else one hundred and sixty four pounds of material had come from (and you can imagine how absurd it would have been to suggest that this matter came out of the air!).
The experiment was beautiful because of the clarity of its concept: it was hard to see what could possibly have been overlooked, or what could have led to any error. That beauty is enhanced by the reliance on quantification, which transforms an anecdote into a scientific result. All of which makes it perhaps rather shocking that van Helmont was of course completely wrong: wood is not made from water, but from atmospheric carbon dioxide absorbed through the leaves and converted into cellulose by photosynthesis. It is hard to fault either the experimental design or the logic of the interpretation; we can’t reasonably expect van Helmont to have come to any other conclusion. There is surely a humbling message in this for scientists today: if an important part of the puzzle is missing, what seems ‘obvious’ may in fact be fundamentally fallacious.
End of an era
This was not the sole extent of van Helmont’s evidence for making water the prime matter of the world. But the rest of his argument was largely circumstantial, and lacked such quantitative exactitude. What else nourishes fish, if not water? Don’t solids of all kinds turn into water when they come into contact with it – salts, for example, which produce ‘savoury waters’ when they dissolve? Of course, there are plenty of solids that do not dissolve, but van Helmont believed this was just because the right solvent hadn’t been found (and in certain respects he was right!). He spoke of a ‘universal solvent’ that would dissolve all things, which he called the alkahest, and he spent many fruitful hours searching for it. (It’s not clear what, if he had been successful, he proposed to keep it in.)
Equally important was the evidence from Holy Scripture. Was it not made clear in Genesis that God created the world out of water, by separating ‘water from water’ and placing in the gap first the expanse of the sky and then dry ground? At the dawn of the Age of Enlightenment, theology still carried some weight in matters of science.
Yet he also adduced an ingenious piece of alchemy to support his contention. He could even turn sand into water, by melting it with an alkali to make ‘water glass’ (sodium silicate), which will liquefy as it absorbs moisture from the air. Add an acid, and the sand is regenerated in precisely the same amount. The quantities were again important here: van Helmont was convinced that matter was indestructible, so that it was conserved in any transformation of this sort.
Van 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,
