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practical, empirical science of matter, the Alexandrian alchemists sought the kind of unifying principles that Greek philosophy extolled. And so one finds tracts like Physica et Mystica (as it was known in later Latin translation) by the Egyptian sage Bolos of Mendes, who flourished around 200 BC, in which the recipes are accompanied by the cryptic comment ‘Nature triumphs over nature. Nature rejoices in nature. Nature dominates nature.’

      Not all of Hellenistic practical science took on this mystical mantle: Archimedes and Hero conducted ingenious and quantitative experiments without conjoining them to some grand theory of nature. Yet for chemistry, the pragmatic and the numinous remained wedded for centuries. When the Arabic philosophers encountered Alexandrian texts during the Islamic expansion in the seventh century AD, they embraced all aspects of its alchemical philosophy. The writings attributed to the Muslim scholar Jabir ibn Hayyan, which were most probably compiled by various members of the mystical Isma’ili sect in the late ninth and early tenth centuries, expounded the idea that all metals were composed of two fundamental ‘principles’: sulphur and mercury. These were not intended as replacements for the classical Aristotelian elements – Aristotle’s philosophy was revered by the Arabs – but they added another layer to it. ‘Philosophical’ sulphur and mercury were not the elemental substances we now recognize; rather, they were elusive, ethereal essences, more like properties than materials, which were blended in all seven of the metals that were recognized at that time.

      Despite their pseudo-theoretical veneer, the Jabirian writings are relatively clear and straightforward in so far as they provide instructions for preparing chemical substances. The great tenth-century Arabic physician Abu Bakr Muhammad ibn Zakariya al-Razi (Latinized as Rhazes) also offered recipes that were very precise in their quantities and procedures:

      Take two parts of lime that has not been slaked, and one part of yellow sulphur, and digest this with four times [the weight] of pure water until it becomes red. Filter it, and repeat the process until it becomes red. Then collect all the water, and cook it until it is decreased to half, and use it.

      This prescription produces the compound calcium polysulphide, which reacts with some metals to change their surface colour – a process that would have seemed to be related to the transmutation of one metal to another, the prime objective of later alchemists.

      These quantitative recipes, relying on careful weighing and measuring, were copied and adopted uncritically by Western alchemists and artisans in the early Middle Ages. But alchemy was not respectable science: the scientific syllabus at the universities was largely confined to geometry, astronomy and the mathematics of musical harmony. And so while alchemy propagated quantification and motivated the invention of new apparatus, it was indeed largely a kind of cookery learnt from books, and the measurement it entailed did not become a regular part of scientific enquiry. As often as not, old errors of quantification were simply retained. A medieval recipe for making the bright red pigment vermilion from sulphur and mercury – a transformation of obvious alchemical interest – specifies far too much sulphur, because it is based on the Arab alchemists’ theoretical ideas about the ‘proper’ ratio of these substances rather than on their ideal proportions for an efficient chemical reaction.

      Only a bold and extraordinary individual would have realized that one’s knowledge of the world could be increased by measuring it. The German cardinal Nicholas of Cusa (1401–1464) was such a man. He is one of the great forgotten heroes of early science, an iconoclast who was prepared to make up his own mind rather than taking all his wisdom from old books. In his book On Learned Ignorance (1440) (a title that reflected the penchant of scholars for presenting and then synthesizing opposing hypotheses) he argued, a hundred years before Copernicus, that the earth might not be at the centre of the universe. It is a sphere rotating on its axis, said Nicholas, and is larger than the moon but smaller than the sun. And it moves.

      For his investigations into natural philosophy he used fine balances and timing instruments such as sand glasses. He suggested that one might observe the rate at which objects fall by dropping them from a tall tower, and cautioned that in such an experiment one should account for air resistance. This demonstrates not only that Nicholas thought to ask quantitative questions (everyone knew that objects fell to earth, but who worried about how fast they fell?) but also that he was able to idealize an experimental test: not just to take its outcome at face value, but to think about factors that might distort the result.

      To Nicholas’s contemporaries, all manner of natural phenomena, such as the weather, were dictated by the influence of the stars. But he laughed at the astrologers, calling them ‘fools with their imaginings’, and suggested instead that the weather might be forecast not by charting the motions of the heavens but by testing the air. Just leave a piece of wool exposed to the atmosphere, he said – if wet weather looms, the increased humidity will make the wool damp. And what is more, you can put numbers to that: you can figure out how much more humid the air has become by weighing the wool to measure the moisture.

      He also had a bright idea for investigating the mystery of how plants grow. The notion of growth from a seed was a central emblem of the mystical philosophy of Neoplatonism, from which most of the medieval ideas about magic and alchemy sprung. But Nicholas saw that this was a problem that could be addressed by quantitative experiment:

      If a man should put an hundred weight of earth into a great earthen pot, and should take some Herbs, and Seeds, & weigh them, and then plant and sow them in that pot, and then should let them grow here so long, until hee had successively by little and little, gotten an hundred weight of them, hee would finde the earth but very little diminished, when he came to weigh it again, by which he might gather, that all the aforesaid herbs, had their weight from water.

      It was a fine suggestion; but the experiment was not carried out for another two hundred years.

      The troublesome recluse

      Nicholas’s heliocentrism did not incite the kind of oppression that was famously suffered by Galileo, who had the misfortune to support the idea in less tolerant times. But Galileo’s ‘martyrdom’ was of a relatively mild sort. Giordano Bruno, another heliocentric rebel, was burnt at the stake in 1600 – not, however, for his scientific views but because of his religious heresies. House arrest, to which Galileo was condemned, might seem trivial in comparison; but there was always the threat that it might turn into something worse.

      That was largely why the works of Jan Baptista van Helmont (1579–1644) went unpublished in his lifetime. Confined to Vilvoorde in the duchy of Brabant by order of the Inquisition, he did not want any more trouble with the Church. Van Helmont (Figure 1) was no rebel-rouser – in fact he chose to pursue a remarkably quiet, undemonstrative life, turning down offers for appointment as court physician from several princes. Yet this reticence belied an ambition to fashion a chemical philosophy of startling scope – the last, in fact, of its kind – and, when challenged, he did not mince his words.

      Van Helmont studied at the University of Louvain, but he felt that academic qualifications were mere vanities and he turned down the degree he had earned. Despite this independence of mind, he was at first something of a medical traditionalist; it was only after he was cured of an itch by an ointment derived from the chemical medicine of the Swiss iconoclast Paracelsus that he converted to this new kind of ‘physick’. Whereas traditional medicine throughout the Renaissance was based on the ideas of the Greek doctor Hippocrates and the Roman Galen, which held that health was governed by four bodily fluids called humours, Paracelsus (1493–1541) maintained that specific diseases should be treated with specific remedies created from nature’s pharmacopoeia by the art of alchemy. Several decades after his death, Paracelsus’s ideas gained popularity throughout Europe, and by the early seventeenth century the medical community was divided into Galenists and Paracelsians.

      Van Helmont studied the writings of Paracelsus and found much there that seemed to him to be sound advice. But he was by no means an uncritical disciple. Paracelsus tended to surround his chemical medicine with a fog of obscure terminology and overblown notions of how the world worked. Humankind, he said, was a microcosm reflected in the macrocosm of the universe, so that the disorders of the body could be compared to the disorders of nature – epilepsy, for

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