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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 which simple water is altogether unavailable. The smoake is so far from being aire, that it is as yet a very mixt body, by distillation yielding an oile, which leaves an earthe behind it; that it abounds in salt, may appear by its aptness to fertilise land, and by its bitterness, and by its making the eyes water (which the smoake of common water will not doe) and beyond all dispute, by the pure salt that may be easily extracted from it, of which I lately made some, exceeding white, volatile and penetrant.

This criticism clearly shows how carefully Boyle had studied the products of the destructive distillation of wood – an experiment that used to be one of the introductory lessons in British secondary school chemistry syllabuses in the twentieth century.

Finally, Boyle turned his penetrating criticism to the method of fire analysis itself. Why was it, he asked, that if the conditions of fire analysis were slightly altered or a different method of analysis was used, the products of analysis were different? Thus, if a Guajacum log was burned in an open grate, ashes and soot resulted; but if it was distilled in a retort, ‘oile, spirit, vinegar, water and charcoale’ resulted. And whereas aqua fortis (concentrated nitric acid) separated silver and gold by dissolving the silver, fire would, on the contrary, fuse the two metals together. Moreover, the degree of fire (the temperature) could make the results of analysis vary enormously.

Thus lead with one degree of fire, will be turned into minium [lead oxide], and with another be vitrified, and in neither of these will suffer any separation of elements. And if it be lawful for an Aristotelian, to make ashes (which he mistakes for Earthe) passe for an element, why may not a Chymist upon the same principle, argue that glas is one of the elements of many bodies, because by only a further degree of fire, their ashes may be vitrified?

Boyle concluded, therefore, that fire analysis was totally unsuited to demonstrating that substances are all composed of the same number of elements. To do this was like affirming ‘that all words consist of the same letters’. Such a critique of the Aristotelian elements was by no means unique to Boyle. Indeed, there is considerable evidence that, apart from his own original experiments, he drew the main thrust of the critique from the writings of Gassendi, who had made similar points when reviving the atomic philosophy of Epicurus and Lucretius.

Once this is realized, the point of his objections to the three principles of Paracelsus becomes plain. Lying in the background to the ‘Reflexions’, and made explicit in The Sceptical Chymist, was a corpuscular philosophy. Boyle’s argument was that, even if there were three principles or elements inside a material, it did not necessarily follow that an analysis into these three parts was possible, or that they were the ultimate parts. Oddly enough, nineteenth-century organic chemists were to be faced by exactly the same problem: what guarantee was there that the products of a reaction told one anything about the original substance?

It is not altogether unquestionable that if three principles be separated from bodies, they were pre-existent in them; for, perhaps, when fire dos sever the parts of bodies, the igneous atoms doe variously associate themselves with the disjoined particles of the dissolved body, or else make severall combinations of the freed principles of the same body betwixt themselves, and by that union, or at least cohesion, there may result mixts of a new sort.

As Laurent discovered in the 1830s, such scepticism is valuable; but if taken too literally, it would prevent any use of reactions as evidence of composition.

Boyle therefore concluded of the Paracelsian principles that, until such time as someone analysed gold and similar substances into three consistent parts, ‘I will not deny it to be possible absolutely … yet must I suspend my belief, till either experience or competent testimony hath convinced me of it’.

There was one further card up Boyle’s sleeve; he was able to use the Helmontian theory of one element as an argument against the alternative three- and four-element theories. He appears at first to have had strong doubts concerning the truth of Helmont’s water hypothesis; but after experiments of his own he had to admit that it seemed plausible. In both the ‘Reflexions’ and The Sceptical Chymist, Helmont’s work appears in a favourable light. Nearly a third of the ‘Reflexions’ is devoted to a discussion of Helmont’s work. Some of Boyle’s own experiments seemed to support the water theory, though he remained agnostic on the question whether or not water was truly elementary. Indeed, in The Sceptical Chymist he argued that water itself was probably an agglomeration of particles.

Boyle’s experiments were very similar to those of Helmont:

I have not without some wonder in the analysis of bodies, marvelled how great a share of water goes to the making up of divers, whose disguise promises nothing neer so much. Some hard and solid woods yield more of water alone than all the other elements. The distillation of eels, though it yields some oile, and spirit, and volatile salt, besides the caput mortum, yet were all these so disproportionate to the water that came from them … that they seemed to have been nothing, but coagulated phlegme.

Boyle’s own astute version of the willow tree experiment, after verification with a squash or marrow seed left to grow in a pot for five months, involved growing mint in water alone, for, as he reasoned, if the plant drew its substance entirely from water, the presence of earth in which to grow the seed or shoot was irrelevant.

Helmont’s position, based upon a thorough experimental foundation, seemed on the face of things very attractive. But Boyle could find no evidence for the growth of metals or minerals from water; neither could he see how plant perfumes and nectars arose from water alone. There was no evidence that an alcahest existed and, in any case, the mechanical philosophy saw no ultimate physical difference between a solvent and a solute. Thus although Helmont’s experiments were a useful stick with which to beat the Aristotelian and Paracelsian theories of elements, Boyle was no partisan of Helmont’s alternative interpretation.

On the other hand, Helmont’s theory appealed to Boyle’s Biblical literalism, for the world, according to Genesis and Hebrew mythologies, had emerged ‘by the operation of the Spirit of God,… moving Himself as hatching females do … upon the face of the water’. This original water could never have been elementary, but must have consisted ‘of a great variety of seminal principles and rudiments, and of other corpuscles fit to be subdued and fashioned by them’. Possibly, then, common water had retained some of this original creative power.

Boyle’s advice on the whole question of the evidence for the existence of elements was to keep an open mind and a sceptical front.

The surest way is to learne by particular experiments what heterogeneous parts particular bodies do consist of, and by what wayes, either actual or potential fire, they may best and most conveniently be separated without fruitlessly contending to force bodies into more elements than Nature made them up of, or strip the severed principles so naked, as by making them exquisitely elementary, to make them laboriously uselesse.

There was irony in that final remark, for through his adherence to the corpuscular philosophy Boyle proceeded to make the concept of the element ‘laboriously uselesse’. Before pursuing this point, however, what sceptical mischief did Boyle wreak on the acid – alkali theory?

This theory was not discussed in either The Sceptical Chymist or its manuscript draft version. Instead, Boyle criticized Sylvius’ and Tachenius’ views in 1675 in Reflections upon the Hypothesis of Alcali and Acidium. Ten years previously, in his Experimental History of Colours (see chapter 5), Boyle had made an important contribution to acid – base chemistry with the development of indicators. He had found that a blue vegetable substance, syrup of violets, turned red with acids and green with alkalis. The test was applicable to all the known acids and could be used confidently to give a working definition of an acid: namely, that an acid was a substance that turned syrup of violets red. The test was also quantitative in a rough-and-ready way, since neutral points could be determined.

When Boyle came to consider the Sylvius – Tachenius theory in 1675, he was able to object to the vagueness of the terms ‘acid’ and ‘alkali’ as commonly used in the theory. Effervescence, he pointed out, was not a good test of acidity, since it was also the test for alkalinity; it also created difficulties with the metals, which effervesced when added to acids. Were metals alkalis? If zinc was reacted with the alkali called soda (sodium carbonate), it was dissolved. Was zinc, therefore, an acid?

Whereas in The Sceptical Chymist Boyle had only played the critic and not put forward any concrete proposal to replace the Aristotelian and Paracelsian theories, in the case of his criticism of the acid – alkali theory, he was able to offer an alternative, experimentally based classification of acidic, alkaline and neutral solutions, which could be used helpfully in chemical analysis. By building on this experimental work, succeeding chemists were able to develop the theory of salts, which proved one of the starting points for Lavoisier’s revision of chemical composition in the eighteenth century.

There was also a second important criticism of the acid – alkali theory. In its vague metaphorical talk of ‘strife’ between acidic and alkaline solutions, the theory possessed a decidedly unmechanical, indeed, anti-mechanical, air about it. To a corpuscular philosopher like Boyle, the theory was occult, in the seventeenth-century sense that it appealed to explanations that could not be reduced to the mechanical geometrical principles of size, shape and motion with which God had originally endowed them. Even so, it is doubtful whether Boyle subscribed fully to the reduction of chemical properties to geometrical qualities, as early eighteenth-century philosophers were to do. The most Boyle was prepared to argue was that chemical properties depended on the way the particles that composed one body were disposed to react with those of others.

He was, no doubt, acutely aware of the fact that, by abolishing Aristotelian formal causes, an explanation of the distinction between chemical species was lost. Gassendi’s solution, which Boyle followed, had been to introduce ‘seminal virtues’ or seeds, ‘which fit the corpuscles together … into little masses [which] shapes them uniformly’. Boyle’s experiments on variable crystalline shapes produced when the same acid was reacted with different metals enabled him to argue that each acid, alkali and metal had its own specific internal form or virtue, which could be modified in the presence of others. Here Boyle found the earlier idea of medieval minima and mixtion useful since, unlike physical atomism, it tried to explain combination by more than physical cohesion alone. As previously noted, another way forward, represented by Descartes, was to explain form geometrically by attributing chemical significance to the shapes of the ultimate physical particles. Descartes’ three elements came in three shapes, irregular, massive and solid, and long and thin. Although there was an obvious analogy with Paracelsian sulphur, salt and mercury, Norma Emerton has also noted the parallel with contemporary Dutch land drainage schemes in which a framework of sticks interleaved with branches was covered with stones to form a terra firma. For Descartes, therefore, composition (mixtion) and the new form was caused by simple entanglement.