Mechanism and Chemical Medicine in Seventeenth-Century England: Boyle’s Investigation of Ferments and Fermentation

Abstract

In this paper I take into account Boyle’s explanation of vital phenomena, paying special attention to his work on fermentation. Boyle never published a specific work on ferments and fermentation, yet, this subject played a central part in his medical agenda. He pointed out that the understanding of ferments and fermentation would throw new light on physiological phenomena, notably on digestion. He was not isolated in his quest for the knowledge of fermentation: most early modern natural philosophers and physicians thoroughly investigated this topic providing different accounts of the fermentative process. The research on fermentation became an integrant part of the Oxford physiologists’ work on blood and respiration. In the first part of the paper, I examine the alchemical and Paracelsian roots of early modern research on fermentation, in the second, I investigate the chemical and medical work on fermentation carried out by the English physiologists (including Thomas Willis and the English Helmontians), as well as by Boyle and Newton.

Parts of this paper have been presented at a range of seminars and conferences. I would like to thank all the audiences for their questions and suggestions, as well as the anonymous reviewers for their comments.

1 Introduction

In A Free Inquiry into the Vulgarly Receiv’d Notion of Nature (1686), Boyle summed up his view of the human body along the following lines:

…I look not on a human body as on a watch or a hand-mill, i.e., as a machine made up only of solid, or at least consistent parts; but as an hydraulical, or rather hydraulo-pneumatical engine, that consists not only of solid, and stable parts; but of fluids, and those in organical motion. And not only so, but I consider that these fluids, and the liquors and spirits, are in a living man so constituted, that in certain circumstances the liquors are dispos’d to be put into a fermentation or commotion, whereby either some depuration of themselves, or some discharge of hurtful matter by excretion, or both, are produc’d…¹

Boyle included the above statement in a work he sent to the press at the end of his career, several years after the publication of The usefulness of experimental philosophy (1663), where he adopted a range of iatrochemical theories.² The quotation from Notion of Nature, testifying to Boyle’s continual commitment to some central themes of iatrochemistry, deserves special attention, notably the statement that body fluids are “in organical motion.” He was not explicit about the meaning of “organical motion,” yet one can argue that he referred to a kind of motion that he saw as not reducible to the mechanical laws. Boyle by no means jettisoned mechanical explanation s; he argued that in the investigation of living organism they were to be integrated by taking into account agents and processes following rules other than those of the impact of corpuscles.³

In order to assess Boyle’s view of human body it seems worthwhile to provide a brief account of his mechanical philosophy – a topic that is still matter of contention among Boyle scholars. My first point is that a variety of versions of the ‘mechanical philosophy’ were to be found in seventeenth-century science. Those who followed Descartes aimed at reducing all natural phenomena to the motion and impact of particles of inert matter. Those who adopted Epicurus’ atomism, like Pierre Gassendi, Walter Charleton and Robert Hooke, explained the physical world by means of matter and motion, yet they maintained that matter was not inert, stating that motion was a property of corpuscles.⁴ Boyle was concerned about the renaissance of Epicurean philosophy and criticized the modern atomists’ theory of the origin of motion. He rejected their tenet that motion was innate to matter: God bestowed motion to corpuscles and guided their movements. For Boyle, the origin and the determination of motion (velocity and direction) depend on God.⁵ In Boyle’s view, matter and motion cannot “constitute this beautiful and orderly world” if they are not directed by an intelligent agent. To Boyle, matter was homogeneous and inert: “The great mass of lazy matter was created by God at the beginning, and by him put into a swift and various motion, whereby it was actually divided into small parts of several sizes and figures, whose motion and crossing of each other were so guided by God, as to constitute, by their occursions and coalitions, the great inanimate parts of the universe.”⁶ Although he subscribed to Descartes’ view of inert matter, Boyle differed with Descartes over the contents and method of the mechanical philosophy. I am here focusing on the classification of corpuscles, a relevant aspect of Boyle’s matter theory as it enabled him to provide corpuscular explanations of chemical change. Simple corpuscles, which are endowed with mechanical properties (i.e., shape, bulk, motion, or rest), form compound corpuscles that Boyle called “corpuscles of the second order.” The latter are very seldom broken as their texture remains unchanged in several chemical reactions. Boyle wrote:

That there are multitudes of corpuscles, which are made up of the coalition of of several of the former minima naturalia, and whose bulk is so small, and their adhesion so close and strict, that each of these little primitive concretions or clusters (if I may so call them) of particles is singly below the discernment of sense , and though not absolutely indivisible by nature into the prima naturalia that composed it, or perhaps into other little fragments, yet, for the reasons freshly intimated, they very rarely happen to be actually dissolved or broken, but remain entire in a great variety of sensible bodies, and under various forms or disguises. As not to repeat what we lately mentioned of the undestroyed purging corpuscles of milk, we see that even grosser and more compounded corpuscles may have such a permanent texture: for quicksilver, for instance, may be turned into a red powder for a fusible and malleable body, or a fugitive smoke, and disguised I know not how many other ways, and yet remain true and recoverable mercury.⁷

This topic crops up again in the 1685 tract on specific medicines, where Boyle argues: “the particles of divers bodies may retain their nature in all the digestion and strainers they pass through.”⁸ Compound corpuscles are endowed with mechanical properties as any parcel of matter, but in addition to the more fundamental properties, they have specific chemical ones, ultimately depending on their texture. What is to be stressed here is that for Boyle compound corpuscles operate according to their chemical properties. Boyle’s effort to reform the chemists’ classification of chemical substances is in fact grounded on experimental arguments as well as on the notion of chemical corpuscles.⁹ The importance of chemical corpuscles can hardly be overestimated, as it bridged the gap between Boyle’s corpuscular theory of matter and the chemical and medical investigations. Indeed, Boyle’s classification of corpuscles according to their complexity marks a sharp difference between Boyle’s and Descartes ’ version of the mechanical philosophy, in that Boyle refrained from adopting a reductionist approach to chemical and medical phenomena.¹⁰ Boyle did not have recourse to imaginary sizes and shapes of corpuscles and was very cautious in framing hypotheses about micro-mechanisms operating in the human body.

In his recent book on Malpighi, Bertoloni Meli has thoroughly investigated the importance of mechanical views in early modern medicine, providing a broad definition of the term ‘mechanical,’ more inclusive than the strict version adopted by Descartes and his followers.

Bertoloni Meli argued that “By mechanical he [e.g. Steno] and other anatomists understood ‘machine-like’ rather than based on the laws of mechanics: this interpretation goes hand in hand with a view of seventeenth-century mechanics according to which objects take center stage and embody more abstract relations. As in mechanics, in anatomy too understanding a complex structure meant decomposing it and recognizing in it elements associated with simpler, known objects that could be understood and handled separately. The notion of machine at the time was quite a complex and heterogeneous one…” ¹¹ This is a very clarifying statement, especially as it points to the importance of considering complex machines for understanding early modern anatomists’ cognitive practice. As it is apparent from the quotation at the beginning of this paper, Boyle saw the human body as a complex machine, i.e., as a “hydraulo-pneumatical engine.” The analogy between living bodies and machines occurs in a number of writings, including The Christian Virtuoso, where Boyle described the egg as “a wonderful mechanical contrivance… so fine and difficult a piece of mechanism , that the most skillful artists, (whether mathematicians or chemists) that have attempted to detain one liquor in the midst of another, every way ambient, have found their industry defeated.”¹² It is to be noticed that Boyle imposed some restrictions to the use of the mechanical analogies. He maintained that perfect devices as living organisms “cannot be satisfactory explicated after the manner of the acting of meer corporeal agents,” as they are produced under the “superintendence and guidance” of God’s wisdom.¹³ He therefore stressed the gap between human artifacts and the works of the divine workman.

Despite Boyle’s use of mechanisms for understanding organisms, it is apparent that his view of living bodies was more nuanced than the mechanical one, as he often merged chemical theories and experiments with mechanical notions. I believe that Boyle subscribed to the view that “understanding a complex structure meant decomposing it and recognizing in it elements associated with simpler, known objects that could be understood and handled separately,” yet such a knowledge, for Boyle, was useful as far as the structure of parts was concerned, but could not explain per se the functions of living bodies.¹⁴According to Boyle, the study of size, shape and structure of body parts is useful to anatomy , but is inadequate for the understanding of living organisms. This is spelled out in The Christian Virtuoso, where Boyle aimed at showing “the religious use a Christian Virtuoso may make of the contemplation of the Microcosm.”¹⁵ He split the section devoted to the study of man into two sub-sections, the first dealing with the anatomy of a dead man, the second with the living human body. A living human body performs functions that cannot be reduced to the number, size, shape, fabrick of parts and to the “contrivance and symmetry of the whole machine as such.”¹⁶ He listed a series of topics to be taken into account for the understanding of living organisms, including “the chemical operations that nature exercises in a living body, especially in the liquors and other fluids it contains,” notably, the invisible fluids (i.e. spirits), made of “active corpuscles,” and “the ferments, or principles analogous to them, that are supposed to be lodged in particular parts.”¹⁷ He articulated his view of anatomy as follows:

For all anatomy can do, is to manifest or display the structure of the consistent parts, such as the bones, cartilages, nerves, arteries, veins etc., and expose to our senses the visible liquors of the body, such as blood, gall, the concreted juices, urine, etc. But it cannot show us either of the two sorts of invisible parts, viz. the animal and other spirits, and the ferments, (or principles analogous to them) that may reasonably be supposed to lodge in the stomach, kidneys, and other particular parts. And yet the influences and operations of these are so considerable, that I am apt to think, that most of the parts of the grosser body seem intended by nature, but as a kind of kitchens to dress the aliment, and make its finer parts pure and subtil enough to become animal; or if you please, hormetick or impulsive spirits, fit to actuate the brain and nerves, and thereby to become the grand instruments of sense , motion, and imagination.¹⁸

As attested by this quotation, Boyle adopted a relevant notion of Paracelsian and Helmontian medicine, namely, the so-called living anatomy – which was evidently at odds with the strict version of the mechanical view of human body. Unlike most Paracelsians and Helmontians, Boyle (who had practiced anatomy throughout his career), did not rule out anatomical knowledge, he rather aimed at integrating it with the chemical analysis of body fluids.¹⁹

The study of ferments and fermentation was meant to fulfil such a task. Boyle maintained that fermentation was responsible “for some depuration of themselves [e.g., body fluids],” and it produced “some discharge of hurtful matter by excretion.”²⁰ Boyle maintained that physiological phenomena such as digestion, assimilation and excretion were not the outcome of mechanism only (as in Cartesian medicine), being the result of mechanisms and chemical reactions – the latter being activated by ferments.

In this paper I set out to investigate Boyle’s explanation of vital phenomena, paying special attention to his work on fermentation , in the context of seventeenth-century English medicine. An important part of Boyle’s study of fermentation was aimed at exploring its practical uses, i.e., the preparation of medicines and food preservation.²¹ As we shall see, most seventeenth-century physicians had recourse to fermentation to account for physiological phenomena, notably, respiration, digestion, hematopoiesis and the motion of blood, as well as for a range of diseases.

In order to locate Boyle’s and the English physiologists’ research on fermentation in historical perspective, in the following section I take into account the alchemical and Paracelsian views of ferments and fermentation. As we shall see, they paved the way to a research tradition that reached a peak in the second half of the seventeenth century.²²

2 Fermentation in Alchemy and in Paracelsian Medicine (to 1650)

The recent reassessment of the history of alchemy has highlighted the experimental contributions the alchemists gave to modern science and has rightly stressed the importance of quantitative approaches in alchemy, long deemed spiritual and esoteric, and having little or no impact on early modern science.²³ Yet, the so-called new historiography of alchemy has often downplayed a significant aspect of alchemy, i.e., the interpretation of the opus (i.e., the preparation of the philosophers’ stone) as a qualitative process involving a series of changes expressed in terms of purification, fermentation , digestion, maturation. It is my cotention that alchemical operations were often described by means of analogies with processes occurring in living organisms. The alchemical opus was described as a process triggered by active principles and formative powers, not as the outcome of mechanical change, namely, aggregation and separation of corpuscles of matter. Metals were supposed to undergo generation , vegetation and ripening – processes to be reproduced by the alchemist in the laboratory in order to achieve transmutation.

Volatile substances and formative powers played a central part in alchemy: for most alchemists, spirits were not just the volatile particles of bodies, they were the powers hidden in objects – powers responsible for the generation and growth of a range of substances, including metals. This is apparent in two influential alchemical texts, i.e., pseudo-Lull’s Testamentum (fourteenth century) and Pietro Bono’s Pretiosa margarita novella (New Pearl of Great Price, fourteenth century). The author of Testamentum employs much of the terminology and concepts related to living organisms and calls the multiplicativa virtus “spirit,” because it has the power of giving life to the dead body (vivificare corpus mortuum).²⁴

Most alchemists maintained that the philosophers’ stone transmuted metals by means of fermentation . According to the author of Testamentum, the stone needs to undergo a process of fermentation in order to become a perfect elixir. Such a process is a purification produced by digestion, requiring the action of the ferment. The latter has the power of converting matter into its own nature.²⁵ Pietro Bono paid special attention to fermentation, devoting a section of his work to ferment, to its properties and to its transmuting power. He started by saying that the opus is not complete if it does not include the work of ferments.²⁶ For Pietro, the stone is the same as the ferment, as it transforms the mass of the body it acts upon, but is not converted into the new substance.²⁷ These views are summed up in Martin Ruland’s Lexicon of Alchemy (1612) where we read: “For even as a small modicum of ferment, or yeast, can leaven a large mass of flour, so does the chemical ferment assimilate itself to the thing to be fermented. […] The Stone itself is the ferment; Gold and Mercury are also called Ferment.”²⁸

Alchemists often used theories based on the notions of semina, spirits and ferments to explain the generation and transmutation of metals.²⁹ The legacy of alchemy is apparent in Paracelsus and in his followers, notably in the medical works of Petrus Severinus and of Oswald Croll. They saw human physiology as the outcome of chemical reactions triggered by spiritual principles. Paracelsians saw spirits as the cause of metal generation and transmutation. Moreover, in his medical tracts he explained digestion and growth as the outcome of fermentation .³⁰ In the macrocosm, Paracelsus argued, salt and sulfur produce fermentation in the bowels of the earth, whence minerals and rocks are generated. In addition, he saw the elixir as a medicine fermented from the seven metals.³¹ Fermentation was often explored in connection with the alchemical transmutation and with the preparation of chemical medicines. Like pseudo-Lull and Pietro Bono, Andreas Libavius claimed that a small quantity of ferment brings about the transformation of a given substance into its nature. The ferment brings about the transmutation by means of spirits penetrating the bulk of the body (“totam penetrat massam”).³²

In the early decades of the seventeenth century, an increasing number of physicians and naturalists had recourse to fermentation to account for a variety of phenomena, as attested by Pietro Castelli, Daniel Sennert, Angelo Sala, Edward Jorden, among others. A physician and professor of medicine, first in Rome and subsequently in Messina, Pietro Castelli (c. 1575–1661) adopted chemical theories and experiments in his medical work. He dealt with digestion in his Epistolae medicinales (1626) and explained it as the action of an acid ferment found in the stomach. He ruled out the traditional medical views of digestion as concoctio produced by heat, since the latter, he claimed, played only an indirect role in digestion. Digestion was the outcome of fermentation, activated by an acid spirit in the stomach that he described by employing a typical Paracelsian parlance. Such a spirit—for Castelli—was “the alchemist of the stomach.”³³ Fermentation occurs at various stages during digestion, and chyle is purged of its feces by fermentation. Daniel Sennert (1572–1637), an influential professor of medicine at the University of Wittenberg, referred to fermentation in his explanation of digestion. Sennert was a critic of the Paracelsians and did not rule out the action of heat in the digestion (notably in chylifaction). When he dealt with scurvy (De scorbuto), he argued that by means of a process analogous to fermentation the gross and fixed parts of food are made volatile and perfected, i.e., free of their excrement.³⁴

A Calvinist apothecary from Vicenza, Angelo Sala (1576–1637) spent most of his life in Protestant countries (in German States and in the Low Countries). His chemical works are mostly practical, focusing on the preparation of chemical medicines, on vitriol, and on antimony.³⁵ A follower of Paracelsian iatrochemistry, Sala adopted a corpuscular matter theory and dealt with fermentation in his Hydrelaeologia (1633), a work devoted to distillation and to the extraction of spirits. Sala defined fermentation as the internal motion of the constituent particles of bodies. Fermentation is generated by heat in the presence of moisture and brings about a new and nobler substance.³⁶ In Saccharologia (1637) he paid special attention to the fermentation of sugar, and described the resulting production of “phlogistic spirit.”³⁷

A physician from Kent, Edward Jorden (c. 1565–1633), studied in Padua and practiced in London and in Bath. He studied mineral waters and explained the generation of metals as fermentation . Following the Paracelsian outlook (he often referred to Paracelsus , Dorn and Croll), Jorden claimed that spirit was the plastic agent operating in the earth as a ferment: “There is a Seminarie Spirit of all minerals in the bowels of the earth, which meeting with convenient matter, and adiuvant causes, is not idle, but doth proceed to produce minerals, according to the nature of it, and the matter which it meets withall: which matter it workes upon like a ferment, and by his motion procures an actuall heate, as an instrument to further his work; which actuall heate is increased by the fermentation of the matter.”³⁸

Anthon Günther Billich (1598–1640), Sala’s son in law and physician to the Count of Oldenburg, wrote an erudite work (Anatome Fermentationis Platonicae, 1639) containing a survey of philosophical and medical doctrines related to fermentation . He maintained that the origin of life and every physiological process were the outcome of fermentation.³⁹ A critic of the Paracelsian chemical principles, Billich, as Pagel put it, “allows the classical elements a prominent role in bringing about fermentation, although he regarded fire as the leader.”⁴⁰ Billich maintained that chemists could not claim priority in the discovery of fermentation, which was well-known in the antiquity, as attested by Plato’s Timaeus.

Jan Baptista van Helmont assigned a prominent role to fermentation both in natural philosophy and in medicine. In his influential Ortus medicinae (1648) he maintained that generation and growth, as well as transmutations were not the outcome of purely material change, but required also the action of ferments.⁴¹ As Walter Pagel pointed out: “In according ferments a central position in his natural philosophy, van Helmont was indebted to alchemy in the first place and to Paracelsianism in the second. […] The soul -like ferment vanquishes matter. Its luminous quality enables ferments to enter bodies which are thereby exalted and sublimated, reaching a level intermediate between the elemental and the spiritual, a mercurial existence.”⁴² For van Helmont, ferments are psycho-physical agents, principles of activity, God’s gifts and the roots of activity in nature, prior to semina.⁴³ Ferments operate according to a kind of program established by God, they are endowed with the power of shaping the prime matter, i.e., water. Ferments dispose matter to receive the idea, or the first shape of individual objects. Van Helmont’s ferments transform the mass into something akin to the image contained in the ferment. He compared this process with pregnancy, by stating that the “Image of the Ferment makes the mass pregnant with semen.” Ferments operate by means of “odours” having a penetrative power that impregnates and disposes matter. In the human body, ferments are responsible for major physiological processes, notably, digestion and the origin and motion of blood. Food is assimilated by the action of a specific agent, a spiritual driving force immanent to the human body. Chemical reactions produced by acid juice in the stomach are directed by the ferment, since acids do not have any digestive power in themselves.⁴⁴ Digestion is a qualitative transformation of matter, not a change in bulk and size, a transmutation like the alchemical opus. So, he argues, digestion occurs by means of a true fermentative metamorphosis (‘per veram fermentalem metamorphosin’).⁴⁵

3 Fermentation, Blood and Animal Heat

The young Robert Boyle ’s medical writings betray a strong Helmontian outlook, though they show no trace of the spiritual agents—that, as we have seen, played a central part in van Helmont’s medicine. It is no surprise that Boyle stressed the importance of ferments and fermentation in The Usefulnesse of Experimental Natural Philosophy (1663), largely written in the 1640s and 1650s and extolling the importance of chemistry to improve the knowledge of the human body as well as to renew man’s dominion over nature. Boyle wrote:

And let me add, that he that thoroughly understands the nature of ferments and fermentation , shall probably be much better able then he that ignores them to give a fair account of divers phaenomena of severall diseases … which will perhaps be never thoroughly understood without an insight into the doctrine of fermentation, in order to which, for that and other reasons, I design’d my historicall notes touching that subject.⁴⁶

Boyle’s work on fermentation does not survive, but there are fragments of his projected essay in his manuscripts. His investigation of fermentation started at an early stage of his career, under the influence of van Helmont, and was spurred by members of the Hartlib Circle, like George Starkey, Benjamin Worsley, and Frederick Clodius. Hartlib’s associates explored fermentation as part of a wide range of chemical and medical investigations, dealing with nutrition, pharmacology and alchemy. Starkey wrote to Boyle about ferments in a letter of April-May 1651 dealing with transmutation. Echoing van Helmont, Starkey wrote: “we want a formal principle which is a fermentum archeale, which is the invisible seed ....” On 3 February 1652 Starkey insisted on this point by claiming that “every transmutation presupposes a corruptive ferment.”⁴⁷ As attested by Boyle’s workdiaries of 1655, Starkey and Clodius sent Boyle recipes for the confection of liquors, the extraction of oils from vegetables and of spirit and fixed salt from urine. All these operations involved fermentation.⁴⁸

Boyle followed with the utmost attention Worsley’s experimental investigations of nitre.⁴⁹ In an undated paper on saltpeter, Worsley came to the conclusion that he had “found out by Experience a ferment, which mixt among fit Matter, will cause the whole at length to turne into the nature of nitrum.”⁵⁰ Boyle explored the practical uses of fermentation , notably the new perspectives it opened to pharmacology and nutrition. For Boyle, fermentation can produce such change in vegetables as to make them fit to perform extraordinary cures, as attested by the preparation of medicines to cure diseases of the kidneys, and the confection of opiates.⁵¹ It offers great potentiality to correct the taste and other qualities of food and drinks and to elaborate new techniques for the preservation of food. As he put it in The usefulnesse, “the distinct knowledge of the true nature and particular phaenomena of fermentation would enable men to prepare a great variety of drinks… to correct and meliorate both hard and liquid aliments....” This can be achieved by means of chemistry, since—he stated—“chemistry can enable us to confer a very grateful taste on very many of the things we eat, barely by a skillful and moderate untying the formerly clogged spirits, and other sapid parts contained in them.” The fermentation of some aliments—he believed—could help keep them “long uncorrupted.”⁵² As one gathers from The usefulnesse, Boyle planned to write a natural history of fermentation, of which a list of headings is to be found among his papers, possibly written in the 1650s. It is apparent that in the 1650s Boyle focused on fermentation and on its medical uses, as attested by scattered notes on fermentation to be found among his manuscripts of mid-1650s and sections of The usefulnesse, written in the late 1650s – dealing with the fermentation of body fluids, notably urine and blood.⁵³ Boyle set out to explore the different stages of fermentation, the changes in color, odor and other properties of bodies fermented; which animals, vegetables and mineral bodies are capable of fermentation; “what things promote fermentation” and “what things oppose fermentation”; finally “A corollary of the difference betwixt fermentation and putrefaction.”⁵⁴ Boyle scrutinized a number of fermentative processes paying special attention to the conspicuous alterations in the bulk of fermenting liquors and to the chemical change produced by fermentation: “ardent spirit of fermented sugar and highly rectified Spirit (sal ammoniac), being shaken together in the cold did presently begin and in a short time made a coagulum, that seem’d at first to take up the greatest part of the space possess’d by the two mingled Liquors, but afterwards subsided in the form of a white saline powder which I judg’d not to fill much above a quarter of that space.”⁵⁵ As attested by a note to be found in his workdiary of late 1660s, Boyle tried to provide a corpuscular explanation of fermentation, framing the hypothesis that “energetic particles” activated the fermentative process.⁵⁶

In the mid-seventeenth century there was a growth of interest in ferments and fermentation among physicians. This is shown in a wide range of medical works published in England and in Oxford university medical disputations too.⁵⁷ In propositions disputed in 1651, Ralph Bathurst rejected the Galenic view of digestion, arguing that digestion was performed by an acid ferment, which was secreted by the walls of the stomach.⁵⁸ Eminent English physiologists like Thomas Willis, Francis Glisson and Walter Charleton engaged in the study of fermentation in relation to digestion, respiration, the origin and the circulation of blood. In 1659, Thomas Willis published Diatribae duae medico-philosophicae, the first tract devoted to fermentation (De fermentatione), the second to fevers (De febribus). The former was meant to provide a theoretical introduction for the work on fevers. Willis’ work on fermentation deals with the composition of mixed bodies by adopting the five chemical principles (e.g., spirit, salt, sulfur, water and earth). He defined fermentation in chemical and corpuscular terms, stressing that it brings about the transformation and perfection of bodies. For Willis, fermentation occurs in both natural and artificial bodies, “in all which is found an heterogeneity of parts or particles, to wit, there are in them some substances light, and alwaies endeavouring to fly away: and also there are others thick, earthy, and more fixed, which intangle the subtil particles, and detain them in their embraces, whilst they endeavour to fly away….”⁵⁹ Fermentation is a qualitative change, which is the outcome of “an intestine motion of particles, or the principles of every body, either tending to the perfection of the same body, or because of its change into another.”⁶⁰ He identified the fermentative agent with spirits, “substances highly subtil, and aetherial particles of a more divine breathing, which our parent nature hath hid in this sublunary world, as it were the instruments of life and soul , of motion and sense ….”⁶¹ Because of their affinity with the corpuscles of sulfur, spirits produce with them a sweet, stable and lasting compound, which is the main component of both vital and animal spirit s and the agent of fermentation.⁶² Like van Helmont’s Archeus, Willis’ ferments are located in the main organs performing vital functions. Vital spirits originate from a small particle of spirit, which is activated in the heart by a ferment keeping blood in constant fermentation: “so by the fermentation, or accension which the blood suffers in the bosom of the heart, very many particles of spirit, salt and sulphur endeavour to break forth from its loosened frame: by which being much rarified, and like water boyling over a fire, the moved and boyling blood is carried through the vessels, not without great tumult and turgescency.”⁶³ Willis adopted the iatrochemical outlook by claiming that an acid digestive ferment was found in the stomach. Through the action of the ferment, food is broken into small particles and chyle is fermented, acquiring a whitish colour, “by the reason that sulphureous particles are dissolved with the saline and mixed with the acid ferment.”⁶⁴

Willis explained fevers on the basis of the chemical composition and motion of blood. Blood is constant fermentation , which “depends on both on the heterogeneity of the parts of the blood itself, and on the various ferments, which are breathed into the mass of the blood from the bowels.”⁶⁵ Willis’ main argument was that fevers resulted from an abnormal fermentation in the blood, causing it to overheat. Abnormal fermentation might come from an alteration of the chemical composition of blood or by inordinate motions of its particles.⁶⁶ Hence, he advocated chemical therapies to heal distempers of blood, such as by introducing chemicals that can trigger fermentation. He maintained that, as ferments are required to make blood, “when they are wanting by nature, they are with good success supplied by art: for fixed salts, alkali salts, extracts, digestives, openers, and especially chalybeat remedies, help for this reason , that, as it were by a certain ferment, they restore anew the weak, or almost extinct ebullition, or boyling of blood.”⁶⁷ It is apparent that Willis’ inquiry into ferments and fermentation was not confined to medical theory. As attested by his Oxford Casebook, Willis had recourse to fermentation in his medical practice as early as 1650. The case of Robert Wylde, a Royalist of Worcester reported in the entry bearing the date of September 10, 1650, provides evidence of Willis’ chemically oriented approach to pathology. He wrote that in his patient “the blood mass emerges in an impure state and arrives at the kidneys full of tartarean feculencies. When this stone-bearing stuff reaches there… while the serum is being strained from the blood (or rather precipitated from a ferment) that tartarean and thicker matter, in the face of its Gorgon-like ferment, is straightway congealed into sand or little stones.”⁶⁸

Like Willis, Glisson and Charleton bracketed ferments and fermentation with spirits and investigated the chemical composition of blood, a process producing the exaltation and spiritualization of bodies. In his Anatomia hepatis (1654) Glisson explained animal heat as the outcome of fermentation in blood. Such a process results from the struggle of spirits trying to disentangle from coarser parts.⁶⁹ For Charleton, fermentation plays a central part in nature and in animal economy. He claims that ingested food undergoes fermentation so that the particles of spirits therein contained are activated and get into a state of volatility. A follower of Gassendi, Charleton states that blood, which is rich in vital spirits, is the product of chemical and mechanical processes.⁷⁰

Like the iatrochemists, Boyle put special emphasis on fermentation as the key to understanding digestion, claiming that digestion and the generation of stones within the body are chemical processes brought about by active principles i.e., ferments. He spelled out his view in The usefulnesse of natural philosophy:

… and it seems a mistake to imagine (how many soever do so) that heat must needs be the efficient of all the changes the matter of our aliments may happen to undergoe in a humane body: where there are streiners, and solvents, and new mixtions, and perhaps ferments, and diverse other powerfull agents, which by successively working upon the assumed matter, may so fashion and qualifie it, as, in some cases, to bring the more disposed part of it to be not unlike even fossile salts or other mineral substances.⁷¹

For Boyle, the assimilation of ingested food was the outcome of a fermentative process that in turn triggered a series of chemical reactions. Fermentation produces the breaking down of food and the assimilation of the nutrients into blood:

For in fermentation , the sulphurous (as Chymists call them) active and the spirituous parts of vegetables, are much better loosened, and more intirely separated from the grosser and clogging parts, in most mixts, then they are by the vulgar ways of distillation, wherein the concrete is not opened by previous fermentation. And these nobler parts being incorporated with our aliments, are with them received freely, and without resistance carryed into the mass of blood, and therewith, by circulation, conveyed to the whole body where their operation is requisite.⁷²

4 Extraordinary Cures, Ferments and Effluvia

Ferments, fermentation and Helmontian medicine played a central part in the controversies over the extraordinary cures performed in England in 1665–1666 by the Irish healer Valentine Greatrakes, also known as “the Stroker.” Boyle, like other natural philosophers, took an active part in the investigations of Greatrakes’ supposed miraculous healing powers. Boyle’s interest in Greatrakes’ cures was not confined to the medical aspects of the case, as he dealt with the Stroker’s supposed miraculous gift and his piety – a matter of contention among natural philosophers and divines. Boyle was a spectator of at least 60 performances by Greatrakes, who made his cures in Lady Ranelagh’s house, where he also cured Boyle’s brother-in-law. Boyle himself stroked a patient with the inside of Greatrakes’ glove.⁷³ Furthermore, Boyle compiled a questionnaire aimed at acquiring information on the Stroker’s cures and collected information on the Irish healer in his Diaries of 1666. In 1665 John Beale (a former member of the Hartlib Circle), sent Boyle via Oldenburg an early account of Greatrakes’ cures, which he reported as being successful most times in treating “pox, scorbutus and withered limbs,” yet he failed to cure the blind and to raise the dead.⁷⁴ Then Boyle asked the chemist Daniel Coxe to investigate the case. Coxe sent Boyle a detailed account of Greatrakes’ religious beliefs and curative powers in a letter of 5 March 1666. He reassured Boyle about the Irish healer’s piety and ruled out supernatural explanations, describing Greatrakes’ gift of healing as “merely complexional.”⁷⁵ Coxe went on by comparing Greatrakes’ to Butler’s cures as related by van Helmont. He added that active particles transpiring from Greatrakes’ body had the power to “mitigate the furious Archeus” of the patient. On 9 March 1666 Boyle wrote a long letter to Henry Stubbe dealing with the Irish healer’s powers and with a number of medical issues, notably fermentation and its role in diseases.⁷⁶ Stubbe was actively involved in the study of Greatrakes’ cures. In the Miraculous conformist, dedicated to Willis and published in 1666 in the form of a letter to Boyle, Stubbe explained Greatrakes’ cures along Helmontian lines, i.e., as an effect of fermentation. Yet, in the dedicatory epistle he added, “the gift of healing was bestowed on [Greatrakes] since the Restoration of His Sacred Majesty and the Restitution of the Doctrine and Discipline of the English Church.” For Stubbe, Valentine Greatrakes’ healing power was God’s gift in the sense that he was given a peculiar temperament. He wrote that “his body being composed of some particular ferments, the effluvia whereof, being introduced sometimes by a light, sometimes by a violent Friction, should restore the temperaments of the debilitated parts, re-invigorate the bloud, and dissipate all heterogeneous ferments out of the Bodies of the diseased, by the eye s, nose, mouth, hand and feet.”⁷⁷ In his letter to Stubbe, Boyle endorsed the naturalistic explanation and recognized the role of ferments as specific agents of disease in the extraordinary cures performed by the Irish healer: “I doe not see why it may not be possible for the sanative, and perhaps Anodine steames of his Body to be of such a Texture that they may both reinvigorate the spirits & by appropriated Qualitys, oppose & subdue the morbifick Matter or Ferment.”⁷⁸ In the same year (1666) John Beale wrote to Boyle saying that the study of fermentation disclosed most arcana of nature: “All motions in nature seem to follow the course of some Ferment.”⁷⁹ Despite his commitment to explore fermentation as a cause of pathological conditions, Boyle was not so confident that all diseases originated from specific ferments.⁸⁰ In his view, fermentation was but one of the causes of abnormal heat of the blood, therefore suggesting that different causes could be discovered by means of more accurate chemical investigations.⁸¹

5 Respiration, Fermentation and Aerial Nitre

In the 1660s Boyle and the Oxonians took issue with van Helmont’s (and Willis’) ferments and spirits, and set out to investigate by means of experiments the chemical reactions occurring in the human body, notably the role of air in respiration and in blood heating. Their physiological research gave increasing importance to nitre and its role in respiration and animal heat. Several chemical philosophers, notably Michael Sendivogius, saw nitre as the vital substance (of celestial origin) contained in the air. Bathurst and his fellow physiologists in Oxford rejected the old view that the use of respiration was refrigerating blood and carrying off fumes, and maintained that the use of particles of aerial nitre was to transform blood. As documented by Frank, aerial nitre became the focus of Oxford physiological investigations for nearly two decades.⁸² Yet, fermentation did not disappear from their medical agenda, it was rather reinterpreted as a series of chemical reactions. According to Bathurst, spiritus nitrosus i.e., a volatile salt contained in air, was necessary to the life of plants and animals – not for the refrigeration of blood, but because it was the food of vital and animal spirit s . Bathurst describes the way corpuscles of aerial nitre penetrate blood as a chemical process, similar to distillation.⁸³ Bathurst’s lectures spurred John Mayow’s research, which saw the light first in Tractatus duo (1668) and then in the Tractatus Quinque (1674). Mayow maintained that aerial nitre is necessary to both combustion and respiration and explored the composition of this salt. Nitre, Mayow stated, is composed of three parts: an extremely fiery and volatile acid salt (the aerial part that could be identified as nitric acid), an alkaline salt (potassium carbonate), and a saline volatile salt (ammonium carbonate). He reached the conclusion that part of the air is necessary for combustion, and that this part is present in nitre. The volatile component of nitre provides “food for fire,” and also passes into the blood of animals by respiration. This brought about Mayow’s rejection of spirits as factotum and their replacement with nitro-aerial spirit, again a sort of universal explanans.⁸⁴ On the grounds of his well-known experiments, he concluded that nitro-aerial particles contained in air mixing with the saline-sulphurous parts of blood brought about fermentation.⁸⁵

In 1668 Willis issued Pathologiae Cerebri, adopting the aerial nitre theory. He explained the origin of muscular motion by means of aerial nitre, and maintained that muscular motion was produced by an explosion caused by the encounter of the spirito-saline particles of animal spirit s (coming from the nerves) and the nitro-aerial ones contained in the blood.⁸⁶ In De Sanguinis Incalescentia (1670) Willis developed the notion of nitre and dropped the view of fermentation he had maintained in the De Fermentatione. In 1670 Willis denied that fermentation could produce heat in liquids. Having stated that fermentation was not the cause of the warming of blood, Willis suggested instead that heat was generated by the reaction of particles of nitre coming from air with those of sulphur contained in the blood.⁸⁷ Willis’ main concern was to establish the chemical reactions occurring in blood and for this reason he wholeheartedly adopted the aerial niter as an explanans.

As attested by his well-known essay on nitre to be found in his 1661 Certain physiological essays, Boyle took special interest in spirit of nitre. He investigated its role in respiration too. He took active part in the Oxford physiologists’ research on respiration, yet he did not subscribe to Mayow’s view of nitre. For Boyle, the chemical properties of the aerial nitre were not understood properly, therefore he refrained from adopting the theory of aerial nitre as the vital component of air. In General History of Air (published posthumously in 1692), containing notes that he had been collecting for more than 20 years, Boyle criticized the theory of spirit of nitre as the vital part of the air. For Boyle, spirit of nitre was an “exceedingly corrosive” substance, which could scarcely be conceived as “refreshing to the nature of animals.” In addition, he conceived of spirit of nitre as an acid spirit, whereas he classified spirit of blood as an alkaline volatile salt. These two substances, he concluded, were of “opposing natures.”⁸⁸

Throughout his career, Boyle kept investigating the composition and the properties of blood by means of a series of chemical experiments, as shown by his Memoirs for the History of Human Blood, Especially the Spirit of that Liquor (published in 1684, but largely written in the late 1660s). The encouragement to write a natural history of blood came from Locke, to whom Boyle addressed the Preface to the work. At the outset of his work on blood, Boyle adopted the Paracelsian stance, stressing that chemistry disclosed invisible, hidden active substances contained in blood. He insisted on the importance of chemical investigation of blood, saying that most of the extant studies on blood “consisted much more of observations than experiments; being suggested far more by the phenomena that nature her self has afforded physicians, than by trials industriously made, to find what she will not, unsolicited by art, discover.”⁸⁹ Boyle’s main goal in this work was to establish by chemical experiments the composition and properties of blood. As part of his chemical exploration of blood composition, he thoroughly investigated the chemical change air produced in blood.

Boyle dealt with blood fermentation as the cause of fevers in Specifick medicines, published in 1685, but conceivably written in 1680. He explicitly referred to the research on fermentation carried out by Willis and other physiologists: “many modern physitians, especially since the learned Dr Willis’s notions came to be in request, have looked upon fevers and agues to consist in, or be produced by vitious fermentation of the blood.”⁹⁰ He argued that blood fermentation might be altered in different ways, bringing about various distempers:

as there is oftentimes a vitious fermentation of the blood, so there may be sometimes a want of fermentation, or a certain sluggishness, upon whose account, either the brisk intestine agitation, that it ought to have as a warm fluid of such a nature as ‘tis wont to be in sound persons, or a due quickness of circulation through the heart is wanting: to which sluggish state of the blood, if it be obstinate and lasting, several distempers are wont to be consequent.⁹¹

Boyle was too careful an experimental philosopher to commit himself to the view that fermentation accounted for all kinds of diseases. He maintained that a number of diseases could not by explained as the outcome of imperfect fermentation occurring in blood. In addition, he expressed doubts about the analogies between blood fermentation and the fermentation of other liquors such as wine and vinegar, an analogy that was widely employed by seventeenth-century iatrochemists. Boyle’s aim was to investigate the specific chemical substances responsible for blood fermentation.⁹²

6 Epilogue

Thanks to the Paracelsians’ and van Helmont’s contributions, the study of fermentation flourished in Europe in the second half of the seventeenth century. In England, Boyle and the Oxford physiologists gave a substantial impetus to research on fermentation and to its uses in medicine. They reinterpreted the Paracelsian and Helmontian ferments as units of matter and tried to establish the chemical rationale for the fermentation. In addition, they experimentally explored the composition of blood and the chemical reactions responsible for its fermentation.

A major impulse to explore ferments and fermentation came from the Great Plague. A number of physicians endorsed Helmontian doctrines, notably the view that pestilential seeds and ferments played a central part in the origin of plague. Two champions of the Helmontian revival of the mid-1660s, George Thomson and William Simpson stood out for their insistence upon the importance of fermentation as a key to the understanding of physiology and pathology.⁹³ In a work containing a head-on attack on Galenism and the medical establishment, Thomson extolled van Helmont because he disclosed “the seminal virtues and properties of things… bringing to light the doctrine of fermentation, the original and activity of spirits.”⁹⁴ Thomson’s works on plague are a synthesis of empirical observations and Helmontian notions such as Archeus, spirits and ferments. This is apparent in his understanding of plague symptoms: “when the natural ferment of the stomack in the Pest is so far lost, that instead of white a black juice is engendered, it is a certain sign of the abolition of spirit and consequently of approaching death.”⁹⁵ In 1670 Thomson published a tract on blood, where he opposed bloodletting and insisted on the spiritual nature of active principle, stressing that ferments are the source of life as they are responsible for digestion and the origin of vital spirit in blood.⁹⁶

Like Thomson, Simpson was engaged in providing chemical foundation to medicine. In Zenexton anti-pestilentiale he argued that: “Ferments [are] … certain powers in Nature whereby all things are put into a way of change… Ferments are Parents of transmutation out of one form into another, or from one degree to another, whereby things are brought to their highest energy.”⁹⁷ Simpson continued his research on fermentation and in Zymologia physica (1675) identified the particles of acid and those of sulphur as the agents of any fermentative process.⁹⁸

Approaches to the study of fermentation diverged both in methods and aims. Whereas the Helmontians explained it as the outcome of ferments, i.e, semi-material principles, Boyle and the Oxford physiologists focused on the chemicals responsible for the fermentative process. There is little doubt that fermentation occupied a central part in English science and it comes as no surprise that Newton paid special attention to it. In Hypothesis explaining the properties of light, sent to Oldenburg on 7 December 1675, Newton maintained that “The whole frame of nature may be nothing but aether condensed by a fermental principle.” ⁹⁹ Evidence of his continuing research on fermentation is the famous “Query” 31 of 1717 Opticks, where Newton deals with active principles. He states that forces acting at very small distance are “…the cause of fermentation, by which the heart and blood of animals are kept in perpetual motion and heat.” According to Newton, air “abounds with acid vapours fit to promote fermentation, as appears by the rusting of iron and copper in it, the kindling of fire by blowing, and the beating of heart by respiration.”¹⁰⁰