chemically as a fermentation process involving an acid under the control of a Paracelsian archeus or internal alchemist. At the same time, he was able to show that the human body secreted alkaline materials such as bile. One of his disciples, Franciscus Sylvius (1614–72), a Professor of Medicine at Leyden from 1658 until his death, and a leading exponent of iatrochemistry, extended Helmont’s digestion theory by arguing that it involved the fermentation of food, saliva, bile and pancreatic juices. For Sylvius, this was a ‘natural’ chemical process and involved no archeus, supernatural or astral mechanism of transformation. The pancreatic juices were a recent discovery of physiologists. By taste they were acidic, as was saliva; but bile was alkaline. Since it was well known that effervescence was produced when an acid and alkali reacted together, as when vinegar was poured onto chalk, Sylvius believed that digestion was a warfare, followed by neutralization, between acids and alkalis.
He did not hesitate to extend this conception of neutralization between two chemical opposites to other physiological processes. For example, by suggesting that blood contained an oily, volatile salt of bile (alkali), which reacted in the heart with blood containing acidic vital spirits, he explained how the vital animal heat was produced by effervescence. From this normal state of metabolism, pathological symptoms could be explained. All disease could be reduced to cases of super-acidity or super-alkalinity – a theory that was quickly exploited commercially by apothecaries and druggists and which is not unfamiliar from twentieth-century advertisements.
Sylvius’ theory was popularized by his Italian pupil, Otto Tachenius (1620–90), in the Hippocrates Chemicus (1666) – a title that advertised its iatrochemical approach explicitly. Amid its chemical explanations for human physiology lay a criticism that the greatest need in the 1660s was for a unifying theory of chemical classification and explanation to replace the tarnished hypotheses of the four elements and the three principles. Tachenius urged instead that physicians and chemists adopt a two-element theory that the properties and behaviour of substances lay in their acidity or alkalinity.
The fundamental problem with Tachenius’ suggestion was that there was no satisfactory definition of an acid or an alkali beyond a circular one that an acid effervesced with an alkali and vice versa.
Robert Boyle (1627–91), who was born in Ireland as the seventh son of the Earl of Cork, was educated at Eton and by means of a long continental tour from which he returned to England in 1644. In the 1650s he became associated with Samuel Hartlib and his circle of acquaintances, who sometimes referred to themselves as the ‘invisible college’. The Hartlibians were interested in exploiting chemistry both for its material usefulness in medicine and trade and for the better understanding of God and Nature. Since the group included the American alchemist George Starkey among its members, not surprisingly Boyle began to read extensively into the alchemical literature. Between 1655 and 1659 and from 1664 to 1668 Boyle lived in Oxford, where he became associated with the group of talented natural philosophers who were to form the Royal Society in 1661. Boyle was an extraordinarily devout man who, like Newton a generation later, wrote as much on theology as on natural philosophy. He paid for translations of the Bible into Malay, Turkish, Welsh and Irish, and left money in his will for the endowment of an annual series of sermons, to be preached in St Paul’s Cathedral, that would reconcile and demonstrate how science supported religion.
The generation before Boyle had seen a revival in the fortunes of the atomic theory of matter. Throughout the middle ages, as the text of Geber’s Summa perfectionis demonstrates, natural philosophers had been familiar with the Aristotelian doctrine of the minima naturalis, which they treated to all intents and purposes as ‘least chemical particles’. Lucretius’ poem, On the Nature of Things, had been rediscovered and printed in 1473. A century later, in 1575, Hero’s Pneumatica was published and disseminated an alternative non-Epicurean atomic theory in which the properties of bulk matter were explained by the presence of small vacua that were interspersed between the particles of a body. This theory, which allowed heat to be explained in terms of the agitation of particles, was exploited by, among others, Galileo, Bacon and Helmont in their search for an alternative to Aristotelianism. A century later, in 1660, the French philospher, Pierre Gassendi (1592–1655), advocated the Epicurean philosophy of atoms to replace Aristotelian physics. His work, Philosophiae Epicuri Syntagma, was a rambling summary of atomism, but its assertion of the vacuum provided an alternative to Descartes’ plenistic particle theory. Descartes’ three grades of matter, i.e. large terrestrial matter, more subtle or celestial matter that filled the interstices of the former, and still subtler particles that filled the final spaces, bore more than a passing resemblance to the elements of earth, air and fire, let alone Paracelsus’ principles of salt, mercury and sulphur. To those who have studied the matter, it is clear that Boyle was much indebted both to Gassendi and to his English disciple, Walter Charleton, whose Epicuro-Gassendo-Charletoniana (1654) had not only presented a coherent mechanical philosophy in terms of atoms or corpuscles, but placed it in an acceptable Christian context.
In 1661 Boyle published The Sceptical Chymist, a critique of peripatetic (Aristotelian), spagyric (Paracelsian and Helmontian) chemistry and the substantiation of physical and chemical properties into pre-existent substantive forms and qualities. Although designed as an argument in dialogue form between four interlocutors, Carneades (a sceptic), Themistius (an Aristotelian), Philoponus (a Paracelsian) and Eleutherius (neutral), Boyle’s rather verbose, digressive and rambling style makes it difficult for the modern reader to follow his argument. Much of the treatise becomes a monologue by Boyle’s spokesman, Carneades. Fortunately, there exists in manuscript an earlier, more straightforward, less literary, and hence more convincing, version of the essay, ‘Reflexions on the Experiments vulgarly alledged to evince the four Peripatetique Elements or the three Chymical Principles of Mixt Bodies’. Apart from one or two references to the later book, we shall follow the argument in this manuscript, which from internal evidence was written in 1658.
A typical defence of the four-element theory was to cite the familiar case of burning wood1:
The experiment commonly alledged for the common opinion of the four elements, is, that if a green stick be burned in the naked fire, there will first fly away a smoake, which argued AIRE, then will boyle out at the ends a certain liquor, which is supposed WATER, the FIRE dissolves itself by its own light, and that incombustible part it leaves at last, is nothing but the element of EARTH.
Boyle, following Helmont quite closely, raised a number of objections to this interpretation. In the first place, although four ‘elementary’ products could be extracted from wood, from other substances it was possible to extract more or fewer.
Out of some bodies, four elements cannot be extracted, as Gold, out of which not so much as any one of them hath been hitherto. The like may be said of Silver, calcined Talke, and divers other fixed bodies, which to reduce into four heterogeneal substances, is a taske that has hitherto proved too hard for Vulcan. Other bodies there be, that can be reduced into more,… as the Bloud of men and other animals, which yield, when analyzed, flegme, spirit, oile, salt and earth.
Here Boyle seems to have stumbled upon a distinction between mineral and organic substances, but he did not develop this point. Instead, he objected to the assumption that the four products of wood were truly elements. A little further chemical manipulation suggested, indeed, that the products were complex.
As for the greene sticke, the fire dos not separate it into elements, but into mixed bodies, disguised into other shapes: the Flame seems to be but the sulphurous part of the body kindled; the water boyling out at the ends, is far from being elementary water, holding much of the salt and vertu of the concrete: and therefore the ebullient juice of several plants is by physitians found effectual against several distempers, in
