The Fontana History of Chemistry. William Brock J.

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.