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Abstract

1. Introduction

Primarily, medical science has been hardly separable from natural science. Early modern European medicine was no exception. Isaac Newton’s Principia Mathematica published in 1687 fascinated those who appreciated its mathematical and mechanical precision. Among them were “Newtonian physicians” who applied Newton’s theories in the realm of physiology and medicine. Particularly in Britain, a group of Newtonian physicians formed a medical mainstream from the late seventeenth to early eighteenth centuries.

Two major scholars on the British Newtonian physicians – Theodore M. Brown and Anita Guerrini - started their story with Archibald Pitcairne. Pitcairne, born in Edinburgh in 1652, obtained his M.D. from the University of Rheims in 1680. Returning to Edinburgh, he became one of the twenty-one original fellows of the Royal College of Physicians of Edinburgh established in 1681, and was appointed professor of medicine at the University of Edinburgh in 1685. Pitcairne, through his friend, mathematician David Gregory, encountered Newton’s theory in Principia. In 1691 Pitcairne was invited to the prestigious chair of the practice of medicine at the University of Leyden. On his way to Leyden, he visited Newton at Cambridge and strengthened his resolve to formulate a new physiology based on Newton’s ideas. His inaugural lecture in 1692 was a declaration for mechanical medicine. Pitcairne stayed at Leyden for only about a year before returning to Edinburgh, but his teaching there had a lasting impact. A number of students, most notably Richard Mead, George Cheyne, George Hepburn, and William Cockburn became firm defenders of Pitcairne’s theory. These students, except for Mead, who would practice medicine in London, followed their professor to Edinburgh and formed an influential circle. Pitcairne’s friend Gregory became Savilian Professor of astronomy at Oxford and, like Pitcairne, he spread Newtonian ideas to his students. One of them was the Edinburgh-born mathematician and natural philosopher John Keill. His younger brother, James Keill, joined John at Oxford, after studying medicine at Leyden. When James studied at Leyden, Pitcairne had already left for Edinburgh, but James developed his medical theory along the lines of Pitcairne’s and joined the Pitcairnian-Newtonian school of medicine (Brown, 1981: 211-217; Guerrini, 1983: 13-23, 33-43).

Pitcairne found his scientific model for medicine in mathematical astronomy revealed by Newton’s Principia (Brown, 1981; Guerrini, 1983). Astronomers observed the motion of celestial bodies and explained it mathematically. In imitation of astronomers, physicians needed to “weigh and consider the Powers of Medicines and Diseases as far as they are discoverable by their Operations, and to reduce them to Laws” (Pitcairne, 1727: 10). With this methodology, Pitcairne established a system of hydraulic iatromechanism in 1690s. In this system, an animal body was viewed as a machine which consisted of “small Pipes” and “fluids that supply and nourish those Pipes” (Pitcairne, 1727: 115). Life “consists in the Circulation of the Blood, produced by the Motion of the Heart and Arteries” (Pitcairne, 1727: 99). The school of hydraulic iatromechanism developed with Pitcairne’s followers.

However, according to Brown, this movement lost its momentum when Newton died in 1727, because “with Newton’s death, the clear if unconscious and unarticulated pressure to do Newtonian science was removed” (Brown, 1981: 343). The death of James Keill in 1719, who was an outstanding theorist, was also critical (Brown, 1987: 644). In terms of medical theory, Pitcairn’s theory was gradually displaced by Herman Boerhaave’s in 1720s. Boerhaavean physicians focused on the solid parts rather than the fluids of the body, and the solid parts (bodily fibres) were more difficult to be the subject of mathematical analysis than the fluids at the time. As the moment of the Newtonian medicine’s decline, Guerrini mentioned an earlier point. She wrote “the Newtonian circle began to disperse after 1708” when Keill published his last major work Account of Animal Secretion, and subsequently the group “produced little more work on matter theory and physiology.” This downturn was, according to Guerrini, due to the group’s intellectual weakness and financial prosperity. After the Newtonian physiologists, thanks to Newton’s influence, attained profitable status like university professors or successful medical practitioners, their intellectual activity waned considerably (Guerrini, 1983: 208-210).

As seen above, the research on “the British school of Newtonian-style hydraulic iatromechanism” (Brown, 1981: 630) has been centred around those who were connected with Newton or Pitcairne. And the existent studies noted that Newtonian interpretation in physiology culminated in James Keill and then its energy dispersed (Brown, 1981: 342-343; Guerrini, 1983: 208-210; Schofield, 1969: 53, 57). However, this paper raises a question here. Is this a full story? Of course, the past cannot be perfectly narrated, but important developments might be omitted in the story of British Newtonian iatromechanism, because the charm and appeal of Newton’s scientific theory must have spread beyond the confines of the direct social network. A physician of Newton’s day, without a personal acquaintance with Newton or someone in his network, could read and appreciate Newton’s theory and utilise it to develop his own medical ideas, just as Pitcairne and his disciples did. This being considered, the range of Newtonian iatromechanists could be widened and the picture of the British Newtonian medicine could extend and become more detailed by researching into the contemporary medical authors who have not been seriously considered before. A case study showed that the Newtonian approach to medicine, for its intellectual attractiveness, was adopted in Spain and its colony New Granada in the eighteenth century (Molina-Batancur, 2010). This paper suggests that in Britain Thomas Morgan was one of such independent Newtonian iatromechanists.

Today Morgan is known more as a deist than a medical writer. However, scholarly interest in Morgan even as a deist has been minimal while more famous deists like John Toland and Matthew Tindal have received increased attention. Berg, calling Morgan “a forgotten Christian deist,” examined his theology and gave him his due in intellectual history. Berg also found that Morgan was a notable medical writer. He published two significant medical books – Philosophical Principles of Medicine (1725) and The Mechanical Practice of Physick (1735). These books were frequently quoted in favour by several contemporary medical writers and Morgan was recognized as an outstanding physician. For example, Charles Perry, putting Morgan in the list of leading medical authors (also including Mead, Cheyne, Pitcairne, and Keill), wrote that Morgan was exceptional in “explaining the true, genuine Natures and Essences of Diseases” (Perry, 1741: v, xiv). Cotton Mather, New England minister and scientist known as a proponent of smallpox inoculation and also as “the first significant figure in American medicine” referred to Morgan as follows.

How much would the Art of Medicine be improved, if our Physicians more generally had the mathematical skill of a Dr Mead or a Dr Morgan, and would go his Way to work, mathematically, and by the Laws of Matter and Motion, to find out the Cause and Cure of Diseases (Beall & Shryock, 1954: 194).

Due to such recognition, Morgan’s medical works, particularly Philosophical Principles, had widespread distribution. According to Berg’s research, the book is “found in many eighteenth-century libraries and booksellers’ catalogs in Britain and abroad” and “a copy was found in the library of Isaac Newton” (Berg, 2021: 65).

However, Morgan’s medical works seems to have been largely forgotten, as observed by Berg. Most medical histories do not mention him. A renowned twentieth-century British physician, Henry Cohen, in his presidential address to the Royal Society of Medicine in 1953, introduced Morgan’s Philosophical Principles as a representative Newtonian iatromechanical work and also as “an interesting but neglected book” (Cohen, 1955: 159). Berg reminded the importance of Morgan as a medical author but he did not go further, because his main concern was Morgan’s deism. However, another scholar who studied Morgan as a deist explored more than his deism.

Jeffrey R. Wigelsworth, in his interesting study on English deists, included Morgan among the representative deists. Wigelsworth’s main argument was that, although English deists had been considered as attempting to subvert the established social order supported by the Anglican Church and Newtonian philosophy, they “endorsed the institutions that characterised the establishment: the monarchy, the Church of England, and Newtonian natural philosophy” and “wished to participate in the very institutions that they supposedly sought to destroy.” Wigelsworth examined not only how deists addressed theological and political issues in the late seventeenth and early eighteenth centuries, but also how they interpreted Newtonian science, which fascinated the age. In the section discussing Morgan’s works, the focus was on general topics, such as his Newtonian matter theory, rather than on Morgan’s particular interest, namely medical science. Though Wigelsworth’s main concern was different from that of this paper, both show that Newtonian science was far from the exclusive domain of those who defended the established church. It has been maintained that Newtonianism gelled with Anglicanism (Jacob, 1976) or Tory ideology (Guerrini, 1986; Friesen, 2003; Gino, 2023). However, Wigelsworth revealed that deists accepted and utilised Newtonianism for their own purposes, and this paper supports his argument by looking into Morgan’s Newtonian medicine (Wigelsworth, 2009).

From the perspective of medical history, Morgan’s medical theory could be viewed in the context of transition from iatromechanism to vitalism. Robert E. Schofield maintained that the transition started around 1740 and progressed during the latter half of the eighteenth century. Mechanistic explanation about the phenomena in living bodies gave way to one based on “vital principles.” The British physician William Heberden’s remark, posthumously published in 1802, embodied the change: “to living bodies belong many additional powers, the operation of which can never be accounted for by the laws of lifeless matter.” To explain living things, “fluid spirits, the aether, electricity, fire, and irritable or sensitive fibers” were increasingly emphasised over mechanical concepts such as matter, motion, and forces (Schofield, 1969: 191-192, 209-210). Sergio Moravia noted that irritability or sensitivity of organic fibres was considered as the principle of life by the eighteenth century vitalists such as Albrecht von Haller and Théophile de Bordeu (Moravia, 1978).

Hisao Ishizuka suggested that beneath the transition from machine to life, there was a strong element of continuity - namely, the concept of elasticity. He reinterpreted early modern iatromechanists, emphasising that they were not only interested in circulating fluids and vascular pipes within the body, but also appreciated the role of the elastic muscular fibres. Morgan and other iatromechanists, such as George Cheyne, James Keill, and Richard Mead, proved to be solidists who believed that solid parts of the body played a crucial role in sustaining life. In their view, according to Ishizuka, any vital motions could not properly function without the elasticity of the animal fibre. For example, blood circulated not only by the force of the heart but also by the elasticity of the fibrous vessels, which endeavoured to restore their former state when the blood pushed against the vascular walls. Ishizuka suggested that the concept of elasticity served as a mediator between iatromechanism and vitalism. Vitalists emphasised sensibility or irritability as properties unique to living organisms, considering elasticity as a mere mechanical property. However, Ishizuka argued that despite the strategic distinctions made by vitalists, some sort of continuity existed between ‘mechanical’ elasticity and ‘vital’ sensibility or irritability (Ishizuka, 2006). Certainly, Ishizuka discussed part of Morgan’s (and other iatromechanists’) medical thought, but significant aspects remain to be explored. This paper will show that Morgan’s medical theory can also be seen as transitional to vitalism in another way.

That said, the main aim of this paper is to identify the key characteristics of Morgan’s medical ideas that have not been systematically analysed in the existing scholarship. Current medical histories rarely mention Morgan, and even when they do, he is viewed as a second-class Newtonian writer. For example, Schofield called Morgan’s Philosophical Principles “a journeyman’s version of Pitcairne or James Keill” i.e. a work which fell short of the masters’ works (Schofield, 1969: 129). Another scholar Guerrini’s comment is interesting and includes more detail. Guerrini saw Morgan’s theory as “a rather unorthodox variety of Newtonian medicine.” Morgan was a Newtonian in that “his text presented a familiar hydraulic model of the body” but he was unorthodox because he “interpreted Newtonian physics as a Manichean dichotomy of expansive and contractive forces exemplified by fire and air.” For Morgan, “light was elemental fire” and “the original expansive force” (Guerrini, 1996: 305-6). In fact, this expansive force was new to the Newtonian medicine and might be considered “unorthodox” but alternatively it could be more attractive and persuasive, as the present article suggests.

This study, as a whole, aims to show that heat was a central element in Morgan’s physiological understanding, as a form of Newtonian medicine. After briefly examining his life and writings in Section 2, Morgan’s physiology, with a focus on circulation and secretion, will be described in Section 3. For the Pitcairnian-Newtonian physicians, the proper circulation and secretion of blood and other body fluids were crucial to sustaining life, so they took great pains to explain them and to provide remedies when irregularities arose. This section, laying the groundwork for the subsequent parts, shows that Morgan’s explanation was largely related to the effects of heat within his original framework. Section 4 deals with Morgan’s analysis and remedy for fever which was considered as a distinct disease at the time. In fact, fever included many different diseases by today’s standards, and its treatment was actively debated among contemporary physicians. Particularly in Morgan’s medicine, in which heat was important, fever provides a relevant case, because it was understood as a failure to control body heat. Finally, Section 5 examines how digestion works in health and disease in his physiological system. It will show that heat played a pivotal role in Morgan’s digestion theory and that digestive dysfunction was caused by excessive or insufficient heat.

2. Morgan’s Life and Medical Writings

The time and place of Morgan’s birth and his early life – or his life as a whole - are vaguely documented. It is conjectured that he was born in Wales or in Bridgwater, Somerset around 1671/72. In early life, Morgan was a poor boy in a farmer’s house near Bridgwater. His genius was recognised, and a local minister supported Morgan to study at an academy in Bridgwater in 1690s. The academy was established around 1690 for guiding the youth intended for the ministry among Dissenters. He was ordained as a Presbyterian minister in 1716. Later that year Morgan started his ministry at Marlborough in Wiltshire, east of Somerset. However, in 1724, he was dismissed from his employment for heresy, mostly conjectured as Arianism. At the time of his dismissal, he had already developed a different interest. Although unknown when he started studying medicine, he ultimately gained a doctorate in medicine from Glasgow University in 1723 or 1724. He published his first medical book in 1725. From 1727 onwards Morgan practiced medicine in Bristol, but he lived in London at least from 1735 continuing as a medical practitioner. Additionally, he became popular as a deist after publishing The Moral Philosopher in 1737. Due to this book to which he later added two additional volumes, he gained notoriety as a leading deist until his death in 1743 and beyond (Berg, 2021: 21-39).

It would be useful to understand Morgan’s medical thought if we could trace how Morgan learned medical science, particularly Newtonian one, but unfortunately the relevant materials are scanty. From 1724 onwards, the title pages of Morgan’s publications bear the abbreviation M.D. after his name, but his 1723 pamphlet does not, so he earned a doctor of medicine degree in 1723 or 1724. Morgan gained it at the University of Glasgow, according to the register of the Marischal College at the University of Aberdeen. This register shows that on 26 June 1738 Thomas Morgan, M.D. gave testimony in Aberdeen at the ceremony of graduation as doctor of medicine of John Cunningham of Falmouth in Cornwall (Anderson, 1898: 113). Morgan’s link with Glasgow is also confirmed by the fact that he signed in 1726 a testimonial to John Stirling, the principal of Glasgow University (Berg, 2013: 557). However, more detailed information on the process of Morgan’s medical education has not been found. We may surmise that Morgan started studying medicine in Glasgow around 1720 when Newtonian medicine was influential in Edinburgh due to the activities of Pitcairne’s disciples. Dingwall wrote in his book on the history of Scottish medicine that in the late seventeenth and eighteenth century the contribution of leading physicians was applied collectively to the teaching of medicine in Edinburgh, Glasgow, and elsewhere in Scotland (Dingwall, 2003: 120-121). It means that Morgan was in a position to learn the Newtonian medicine developed in Edinburgh.

The sources to trace his intellectual network are also limited. His personal correspondence is not extant. The above-mentioned registers of University of Aberdeen show that Morgan gave testimony for John Cunningham with two other medical doctors. One is John Allen, M.D. of Aberdeen, medical practitioner in Bridgwater in Somerset. Because Morgan spent his early years in Bridgwater, he and Allen might have known each other from their youth, but there is no more data about their relationship. The other is Andrew Hooke, M.D. of Glasgow. It seems that he and Morgan were like-minded from the years they studied at Glasgow. Hooke in his book commented favourably on Morgan’s book Philosophical Principles: “a Book that can never be too much studied by the young Physician” (Hooke, 1734: 41). Hooke’s book shows that he and Morgan shared some common understandings about physiology. There is no evidence that Morgan had a personal acquaintance with more renowned medical or scientific figures. He sought to gain recognition from important medical figures of the day, but probably not successfully. He dedicated Philosophical Principles to Hans Sloane, President of the Royal College of Physicians in London and Mechanical Practice to Richard Mead, physician to King George II, but there is no evidence that both dedicatees reacted (Morgan, 1725: v; 1735: v; Berg, 2021: 31, 66).

Though it is hard to closely trace Morgan’s medical education and professional network, his medical writings show that he was well versed in contemporary medicine. Morgan’s first medical book Philosophical Principles published in 1725 contained main themes discussed in this paper - his principles of physiology based on heat, circulation, and secretion; theory of fever and digestion. These themes consisted of the core part of Morgan’s medicine. Some other medical topics not deeply analysed in this article included the muscle and its motion, the effects of opium and other soporiferous drugs, and the mechanical explanation about emotions such as love and grief - hopefully, these could be the topics of future research.¹⁾ The second edition of Philosophical Principles was published in 1730 with a minor addition. Morgan added some explanation about secretion and muscular motion between pages 188 and 201, though the nearly 500 remaining pages remained unchanged. Due in part to the added pages, but also to the clearness of printing, mostly the second edition is utilised by this research.

Morgan’s second medical book The Mechanical Practice of Physick was published in 1735. It complemented the first book rather than advancing new principles. As can be inferred by its title, the second book focused on the medical ‘practice’ based on the mechanical principles presented by the first book. Already in the preface of the first edition of Philosophical Principles Morgan expressed his hope for “an opportunity some time or other to publish something more particular upon the Practice, as grounded upon the Principles here laid down” (Morgan, 1725: xxxvi). In Mechanical Practice, Morgan gave a further discussion on the digestive process and the cure of fever. Mechanical effects of mercury, health problems of women in pregnancy and childbirth, and the cure of children’s diseases were also discussed. These practical aspects are, of course, worth noting, but this paper, as an initial attempt to look into Morgan’s medicine, put more weight on his underlying theories, so Mechanical Practice is used as an auxiliary source.

The last book which should be mentioned is Physico-Theology published in 1741. In it, Morgan dealt with the topics of both natural science and theology. He used his theory of matter to support his argument that God governed the world not arbitrarily but by uniform and established laws. Though not a medical book, Physico-Theology is consulted to find Morgan’s view on the nature of light, because his concept of light was related to fire and heat which played an important role in his physiology.

As mentioned above, this paper does not aim to discuss the entirety of Morgan’s medical works; rather, it seeks to contribute to a better understanding of Newtonian medicine by examining key aspects of his medical theory. His Philosophical Principles and related works provide sufficient material for this task. The selected materials show that Morgan shared certain similarities with other Newtonian medical theorists on the one hand, and that he also pursued his own approach to improving Newtonian explanations of animal bodies on the other.

3. Morgan’s “Animal Oeconomy”: Heat, Circulation, and Secretion²⁾

In the preface of Philosophical Principles, Morgan expressed his mechanical view of an animal body. It was a machine, but a very complicated machine of Nature endowed with life. Life, in medical point of view, “consists in the Circulation of the Blood; and Death is nothing else but a Cessation of that Motion.” Therefore, health lay in the orderly circulation of blood and “the consequent Derivation of the several Liquors secreted by the Glands, in their natural and due Proportions.” Any irregularity of body fluids, whether excess or deficiency, would cause diseases (Morgan, 1730: viii). This was a basic physiological system shared by Newtonian physicians at the time. And its root could be traced back to Archibald Pitcairne. In the following paragraphs, the development made by Pitcairne and James Keill is briefly sketched first, then the model of Morgan follows.

While staying in Leyden Pitcairne delivered lectures and some lectures on important topics were published as dissertations. One of them was titled “On the circulation of the Blood through the minutest vessels of the body” delivered in January 1693. In it, based on William Harvey’s theory of blood circulation, Pitcairne discussed the secretion of body fluids, which was considered a process of great importance for animal life. He rebutted what he saw as wrong ideas: chemical and Cartesian theories. According to the chemical physicians such as Jan Baptist Van Helmont and Thomas Willis, different glands had their own stagnating “fermenting Liquors” by whose power blood changed into fluids of different natures. Pitcairne asserted that this theory was incompatible with the circulation of blood, because the fermenting fluid could not stagnate but would be washed away by the circulating blood. In Cartesian theory, blood was composed of particles of different shapes, and glands functioned as sieves which only allowed some particles in blood for proper secretion. The particles could pass through a given gland when their geometrical shape corresponded to the shape of the holes on the glands. This explanation is flawed, Pitcairne argued, because several different shapes could fit through a specific pore. For example, a cone could pass through a hole intended for a sphere. Moreover, the theory did not consider the orientation in which the particles might face the pores – a cube-shaped particle could pass through a corresponding size of square hole only if precisely oriented (Pitcairne, 1727: 39-53; Guerrini, 1983: 69-71; Gino, 2023: 218-221).

Denying the Cartesian explanation, Pitcairne maintained that glandular pores were all circular and different only in diameter (large or small) and he distinguished between “thinner” and “grosser” fluids. The thinner fluids could pass through small secreting glands, which prevented the grosser fluids from passing through. When the grosser fluids were secreted through large glands, they were secreted with the thinner at the same time, so the grosser fluids were dilated by the thinner ones. According to Pitcairne, sweat was a thinner fluid and urine and saliva were grosser. However, he did not attempt to further differentiate the types of body fluids or to explain how they were secreted (Pitcairne, 1727: 56-63).

It was James Keill who developed a more detailed theory of secretion along the lines of Newtonian iatromechanism. Keill introduced the forces of attraction between small particles within animal bodies to explain the secretion of different body fluids. Keill published Account of Animal Secretion in 1708 – “the high point of Newtonian interpretations in physiology in this period” according to Guerrini (Guerrini, 1983: Abstract). Its enlarged version appeared in 1717. In the essay, Keill stressed an attractive power which “only exerts itself in Particles that are at a small distance from one another” i.e. cohesion. Similar to gravity, the existence of this short-range attractive force was demonstrated by Newton. As gravity explained the motions of celestial bodies, for Keill, cohesion “seems to be the only Principle from which there can be a satisfactory solution given of the Phenomena produc’d by the Minima Nature.” In other words, “the whole Animal Oeconomy” depended upon this attraction (Keill, 1717: 101-102).

Keill used this attraction to explain the secretion of body fluids from blood. He noted that blood, just as most other fluids, included attracting particles. Animal blood, when drawn out of the body, divided into two different parts – the red globules soon coagulated, but the serum remained fluidic. Keill explained this by arguing that the particles of the globules were more strongly attracted to one another than to the serum. Besides globules, microscopic examination revealed that the serum contained “a great number of Corpuscles of various Figures and Magnitudes” and the different corpuscles were endued with “different Degrees of an attractive force” (Keill, 1717: 95-97). Various body fluids were secerned from blood by different glands and Keill attributed the variety of fluids – for example, sweat, saliva, and bile - to different particles with different attractive powers. According to him, the particles in the blood returning through the veins attracted one another and formed globules too big for secretion. Keill argued, following Pitcairne, that the role of the lungs was to comminute the large globules in the blood into minute particles “by the Force of Respiration” i.e. by air pressure (Keill, 1717: 117; Pitcairne, 1727: 102-103). In the course of circulation, the various particles reunited, according to their different attractive powers. The rapid stream of blood pushed by the heart disturbed the cohesion of particles, so “the Particles which unite first… must be such as have the strongest attractive Force; and such as have the least, unite last; and all the intermediate ones according to their several Natures.” In other words, when the blood was flowing the fastest, the particles with the strongest attractive force cohered, and as the blood became distant from the heart and the speed of its flow decreased, the particles with less strong attractive force began cohering. The velocity of blood also affected the viscidity of the fluids. The particles united in the slower stream made the fluid containing them more viscid (Keill, 1717: 129-130).

As a result of the process above, for Keill, blood in the artery contained different particles with different size, and the sorts of contained particles varied according to the distance from the heart. When secretion was made through the gland, the size of its orifices decided which particles could be secreted. Particles with a diameter lager than the size of the orifice could not enter. In this way, many different kinds of body fluids could be secreted from blood (Keill, 1717: 198-199). At Keill’s time, “it is not easie to determine, how many different sorts of Particles are in the Blood” but “a few different sorts of particles variously combined, will produce great Variety of Fluids.” Keill showed only five sorts of particles could made twenty-six different combinations (Keill, 1717: 162).

Morgan agreed with Keill that the force of cohesion was crucial in physiology, but he added a concept of the expansive force. This expansive force was related to Morgan’s understanding of light, fire, and air. His concept of light was influenced by contemporary Newtonian trends (Wigelsworth, 2009: 159-160). In Physico-Theology Morgan started his discussion of light with the praise for Newton’s achievement - “several great Discoveries concerning the sensible Properties of this visive element, with regard to the different Refrangibility of its Rays, and the different Sensations from thence arising.” However, Morgan believed Newton “has left enough for the farther Disquisition and Investigation of future Ages.” Morgan noted the materiality of light which was confirmed by experiments and observations. Just like other matters, the rays of light moved through a space and could be intercepted by an opaque object. The rays could be contracted into a less space or diffused into a larger space, through a convex or concave lens, while the quantity of light stayed the same. The experiment with a convex lens led Morgan to articulate another point independent from Newton’s view. The action of concentrated light “produces all the Effects of the most intense Fire… even to the Dissolution and Evaporation of Gold.” Thus, it is clear that “Fire and Light are essentially and substantially the same, and that Fire is nothing else, but condensed Light” (Morgan, 1735: 29-31).

Morgan also expanded on Newton’s idea about light and ethereal substance. Newton in the ‘General Scholium’ of the second edition of Principia (1713) mentioned “a very subtle spirit pervading gross bodies and lying hidden in them; by its force and actions, the particles of bodies attract one another at very small distances and cohere… and light is emitted, reflected, refracted, inflected, and heats bodies; and all sensation is excited” (Newton, 1999: 943-944). Morgan subscribed to Henry Pemberton’s book for popularisation of Newtonian science. In it, Pemberton again communicated that Newton had not discovered yet the cause of the power by which light and bodies interact, but he had “in general hinted at his opinion… that probably it is owing to some very subtle and elastic substance diffused through the universe, in which such vibrations may be excited by the rays of light.” This subtle substance did not give any sensible resistance to bodies in motion, so the planets moved free from resistance in space (Pemberton, 1728: 376-377).

Morgan argued that the subtle substance was elementary light. It was not the light in the general sense - that is, visible light. Elementary light was the medium of light. “This element appears to be universal, and to be equally diffused thro’ the whole Universe.” As a result, “wherever there are any luminous Bodies, tho’ at immense Distances, such as the fix’d Stars, this Element is moved and actuated by them, so as to communicate the Motion and make such Bodies sensible to us” (Morgan, 1741: 39-41). The reason Morgan identified the subtle substance with elementary light was that it was light that could exist without resisting or being resisted by other moving bodies. Experiment showed that light moving in straight line was not affected by lateral pressure. People standing in a circle were able to see each other in the same moment “by Rays which must cross and intersect each other in all possible Angles, and this without interrupting, or in the least disturbing or confounding the Action of Light.” Light was the only material which gave no resistance, so the subtle substance diffused universally was the extremely minute element of light (Morgan, 1741: 49-53).

Important for Morgan’s physiology is that elementary light was ubiquitous, and light and fire were essentially the same. Heat was defined by Morgan as action of fire. Heated air was expanded and could function as expansive power. Air heated by sunlight was expanded just as “a small Portion of Air… by Heat or the Action of Fire, have its expansive or centrifugal Force increased to prodigious degree.” Air containing elementary light (fire) was breathed in by animals, so “the expansive Force of heated Air, or the mutual conjunct Action of Fire and Air” operated in the opposite way to the cohesive power in animal bodies. This expansive force “separates and dissolves the Parts of solid Bodys; and intimately mixes and blends together the Parts of Fluids” (Morgan, 1730: 90-93).

As far as blood was concerned, red globules had a greater importance for Morgan than for Keill. Keill wrote the globules were comminuted in the lungs, did not exist in the artery, and were reshaped in the vein where the speed of blood flow was slow. However, Morgan thought the globules remained through the circulation. The globules were heavier and denser than the serum and “endued with a stronger corpuscular Attraction.” Also, they were “capable of receiving a greater Degree of Heat, and of retaining the Heat it once acquir’d longer than the Serum” (Morgan, 1730: 112). These globules had a role of keeping the natural heat of body and a regular pulse (Morgan, 1730: 331). Morgan’s colleague Hooke accepted the idea that the globules retained heat and were related to body temperature (Hooke, 1734: 32-35).

With regard to secretion, Morgan concurred, to some degree, with Keill when he discussed the secretion of different body fluids. Different secretions mainly depended on “the different Diameters or Orifices of the secretory Ducts” and on “the different Velocity of the Blood at the several secretory Ducts, as the Arterys are more or less complicated and divided before the Secretion is made” (Morgan, 1730: 169). The quantity of secretion was proportional to ‘the Momentum Motus of the Blood, or as the Quantity of Blood carry’d to the Orifices of these secretory Ducts, in a given Time” and “vary according to the different degrees of Visicity or Fluxility of the Blood; for where the Serum itself is very crass and viscid, it will not pass so freely thro’ the smaller Orifices of the glandular secretorys” (Morgan, 1730: 188-189). According to Morgan, unlike Keill, the globules existed in the arterial blood; therefore, he added that secretion was made only from serum, not from red globules, because secretory glands were too small to admit the globules (Morgan, 1730: 114). The globules were also taken into account when Morgan discussed the purpose of secretion, as described below.

He differentiated two sorts of secretory glands. One is what he called “expurgatory glands” which cast secreted fluids out of the body to prevent the excess of serum in the blood - for example, urine and sweat. The other is “circulatory or lymphatic glands.” These glands secreted useful fluids for the body such as bile and synovial fluid, and more watery part (lymph) unused for the intended fluids were sent back to the blood in the vein (Morgan, 1730: 162-164). The explanation given so far could be accepted by the contemporary physicians in general. However, Morgan added his original interpretation about the drain and return of serum, or lymph. He said that the phenomenon of the large and continual drain of serum from the arteries, which is continually returned to the blood in the veins could not be overlooked by all anatomists, but little has been said about its reason. Morgan found the reason in red globules which he thought retained “the main Heat, Fervor, Efflatus, and Activity of the Blood.” The serum near the globules was attracted and heated by them. Contrarily, the serum passing through the glands was not affected by the globules. This serum was “for some time drawn off and separated from the Globules and hotter Serum” to be “cool’d and condens’d” and returned to the veins to cool and dilute the blood. By this means, according to Morgan, the natural heat of the blood was moderated and suppressed which otherwise would “become unsufferable, and inconsistent with the Animal Life (Morgan, 1730: 179-180).

Before finishing this section, one more thing is worth considering - the function of the lungs and air. It was obvious to contemporaries that the role of lungs exchanging air was indispensable for life, but how breathing sustained life was still unclear and remained a matter of debate. Morgan agreed with Keill and Pitcairne in that the lungs helped the circulation of blood, but he differed on the mechanism. As seen above, Keill and Pitcairne argued that the lungs comminuted the large globules in the blood by air pressure, but Morgan, without mentioning the comminuting role, said the lungs using air pressure propelled blood like the heart. Air “rush’d into the bronchial Vesiculae, so as to distend and fill them up.” These distended vesicles compressed the blood vessels of the lungs to push forward blood (Morgan, 1730: 147-148).

From a different point of view, the Irish physician Bryan Robinson proposed a chemical explanation of the role of air. He maintained “the Life of Animals is preserved by acid Parts of the Air” which “dissolve or attenuate the Blood, and preserve its Heat: and by both these, keep up the Motion of the Heart” (Roos, 2017: 141; Robinson, 1732: 187). However, Morgan, viewing animal life as analogous to the combustion of fire, rebutted the claim that “Acids will check and extinguish Fire sooner than common Water, and any Acid Vapour in the Air is more suffocating and destructive than any common watery Vapour or Fume.” Morgan also argued that acids could not generate heat by mixing with blood, because blood was a neutral substance. A continual supply of fresh air was necessary to preserve the life of animals, but it was not acid parts of air that sustained life. For Morgan, air supplied “a continual Supply of a proper exhaling Medium to receive and carry off those copious discharges of humid effluvia or moist Vapour, which all living Creatures, and all combustible Matter under the Action of Fire, are incessantly emitting and throwing out.” Without the exhaling medium “the Life of Animals, and the Action of Fire, must soon be suffocated and extinguish’d” (Morgan, 1735: 75-77). Roos, who studied Robinson, commented that “indirectly, Morgan was observing the effects of increased concentrations of carbon dioxide, as did Robinson recognize there was an element in the air responsible for respiration” - that is, oxygen. The controversy between Morgan and Robinson was noted by the Philosophical Society of Edinburgh. The Edinburgh society’s Medical Essays and Observations published in 1738, while discussing some parts of Morgan’s Mechanical Practice, introduced Morgan’s position attacking Robinson’s (Anon., 1738: 472-473).

As seen above, Morgan thought that air sustained life in another way as well. For him, the air entering an animal body from the atmosphere contained elementary fire, so “Air join’d with elementary Fire, is absolutely necessary to keep up the Fluidity and natural Heat of the Blood.” Thus, the air inhaled was vital; however, because the air circulating with blood was continually heated by the red globules, it had to be exhaled while fresh air was supplied to the blood. Therefore, the lungs, by breathing, played an important role in maintaining the temperature of blood and body (Morgan, 1730: 147-148).³⁾

Morgan noted that the air with elementary fire was “retain’d in the internal Constitution of Bodys.” Breathing in air and using it for vital purposes, including preserving body heat before breathing out, was an ability only living animals had. In this regard, it may be said that Morgan’s physiology included some vitalistic aspects. This point will be discussed again in the conclusion of this article (Morgan, 1730: 104-105).

Morgan believed that the expanding forces caused by heated air as well as the attractive forces of small particles operated in the animal body. In his physiology, the animal life lay in the circulation of the blood and other body fluids. The air, containing elementary fire and breathed in through the lungs, mixed with the blood, and its expansive power prevented the body fluids from coagulating and stagnating. The red globules in blood absorbed and retained heat to maintain the body dynamics. In the course of the circulation of blood, the serum and air in it were heated by the red globules. To avoid over-heating, some amount of the serum was secreted from the artery to cool down before returning to the vein, and heated air was cast out to be replaced by fresh air. Failure to sustain appropriate levels of heat would lead to diseases. An example in point - fever - will be discussed in the next section.

4. Fever

At Morgan’s time, fever was generally looked upon as a distinct disease. By the nineteenth century, fever included “heightened sensory perception, often a feeling of burning heat and sometimes subsequent chills, disorientation and even delirium.” Fever indicated many diseases - such as pneumonia, malaria, and influenza - and was considered a brutal condition that threatened disability and even death (Hamlin, 2014: viii, 6). In his History of Scottish Medicine Comrie, introducing Pitcairne’s dissertation on the “Cure of Fever,” mentioned that at the time “fevers formed two-thirds of all diseases” (Comrie, 1932: 274). The Newtonian hydraulic school of Pitcairne advanced a new theory of fever, although its therapies remained largely traditional. There was a trigger for the development of the new theory: in 1691 an Edinburgh physician Andrew Brown published Vindicatory Schedule, Concerning the New Cure of Fevers advocating a new therapy for fevers. In response, Pitcairne and his disciples defended old therapies by developing a novel theory (Cunningham, 1981). Because Morgan broadly belonged to the Newtonian school regarding the nature of fever, the controversy between Brown and the Edinburgh Newtonian physicians is discussed as background, before examining Morgan’s comprehension of fever.

Before Brown published his therapy, the way physicians in Edinburgh treated fever in general, or ‘continued fevers’ can be summarised as follows. The cause of the disease was considered to be a disturbance of the blood caused by external morbid matter. The initial treatment procedure was to bleed the patient (bloodletting or phlebotomy). It reduced the quantity of deteriorated blood and slowed down the pulse which had been elevated due to fever. The next step was vomiting, because some consumed matter was likely to be flowed into the blood and sustain the fever. Then, the physician administered a diaphoretic that would remove the morbid matter through sweating (Cunningham, 1981: 75-76).

Brown’s therapeutic procedure differed significantly. In essence it consisted of bleeding, purging, and paregoric. The fact that both approaches began with bleeding signifies only a minor area of agreement between them. Defenders of the traditional method saw sweating as the central element, but the main part of Brown’s was purging. Brown’s therapy was based on the teachings of the English physician Thomas Sydenham. The pillar of Sydenham’s method was the experiential basis: the cure was discovered by facing the disease through skilled trial and error. For Sydenham, the physiological and pathological theory was unimportant. However, Brown was not satisfied with Sydenham’s experience-based approach and sought to support his mentor’s method with a theoretical explanation. Brown argued, in essence, fevers continued because undigested particles kept coming into the blood from the intestines. They were undigested because digestion was weakened by the fever. To cut off the supply from the intestines, purging was to be used (Cunningham, 1981: 76-81).

Brown challenged the practice of the members of Royal College of Physicians in Edinburgh which was established to defend and promote proper medical practice in the city. The College’s legitimacy being attacked, some measures was to be taken. Within the College, the prosecution of unlicensed practitioners was the responsibility of the two Censors. In 1690-91 Pitcairne was one of these Censors. However, Brown, having a medical doctorate, could not be prosecuted. A legal measure being unavailable, the College had to meet his challenge by undermining the validity of his medical practice and theory. So, a group of physicians, led by Pitcairne, published pamphlets attacking Brown’s position, and Brown and his supporters responded in a protracted pamphlet war that lasted until 1702 (Cunningham, 1981: 84-85).

Pitcairne entered the war under his own name, publishing Dissertione de Curatione Febrium in 1694. In it, he defended the old method of sweating with his hydraulic physiology, arguing that purging was ineffective. He observed that in fever there was no perspiration, and the other evacuations were diminished; thus, fevers resulted from the suppression of evacuations. The cure of fevers was achieved when the evacuations are released. He did not mean to talk about bleeding and vomiting, as both could be used if physicians found them necessary. The main question for Pitcairne was “what Kind of Secretion or Evacuation must be us’d in Fevers, if any is to be us’d?” To answer, he drew on the figures of the Italian physician Santorio Santorio who measured the quantities of the different evacuations in health and found that perspiration was the largest bodily secretion. Therefore, fevers would be alleviated by the restoration of perspiration. According to the figures, excretion “thro’ the Pores of the Skin” was ten times as great as “Excretion by Stool,” thus, perspiration was ten times as effective as purging (Cunningham, 1981: 92-93; Pitcairne, 1727: 193, 200-201).

Two of Pitcairne’s disciples joined the ranks of participants in the controversy. William Cockburn, in his book about sailors’ diseases, supported sweating over purging, but his argument was not much different from Pitcairne’s (Guerrini, 1983:103). More noteworthy is George Cheyne’s A New Theory of Continu’d Fevers published in 1701. Pitcairne arranged the publication of this book (Guerrini, 1983: 112-113, 117) and James Keill, enumerating the contemporary achievements of his fellow Newtonian physicians, praised Cheyne for having explained the subject of fever well (Keill, 1708: iv). Thus, Cheyne’s book can be seen as broadly representing the view of the Pitcairn circle.

Cheyne argued that the heat was caused by the friction of the particles of the blood. In the course of circulation, the blood particles clashed with the vascular walls and also with each other, and these clashes generated heat (Guerrini, 1983: 86).⁴⁾ Therefore, fever - an abnormal increase of heat - would be caused by the increased friction. Based on this assumption about body heat, Cheyne argued that “the most effectual Cause of continual Fevers, is an Obstruction of the Glands.” The obstruction of some glands decreased the number of glands through which body fluids could flow. This would augment the quantity of the blood and other fluids in the passable canals (Cheyne, 1701: 12-13). The increased quantity of fluid meant the increased number of particles in it, so “there must be a greater Motion than ordinary, and consequently a greater Heat” (Cheyne, 1701: 17).

What caused the obstruction of the glands? Cheyne observed that people with a cold developed a fever. Particularly, sudden cold weather after a hot period could cause many cases of such a fever. According to him, the excessive heat dilated the glands - “all the cutaneous Glands, the Glands of the Trachea, Bronchi, Oesophagus, Stomach, and of the Intestines.” Subsequent cold weather would “contract their orifices and congeal their flowing Liquors” and consequently the obstruction of the glands would occur. Another reason for the obstruction was “Surfeiting or Drunkenness; which is still to be suspected as a considerable part of the Cause of Fever in adult Persons in great Cities” (Cheyne, 1701: 25-30).⁵⁾

How could fever be cured? Cheyne’s strategy was not different from Pitcairne’s. The increased amount of body fluid in the passable canals needed to be reduced. Cheyne provided more detailed explanation about the therapeutic procedure. He listed four proper remedies of fevers: (1) bleeding, (2) vomiting, (3) purging, and (4) sweating. He also mentioned blistering, but he said that blistering and outward applications were only useful to remove the incidental effects, not the cause of fevers (Morgan had a different view on blistering, as will be seen below). Initial bleeding, by reducing the amount of circulating blood, increased the velocity of the blood, so the blood stream could more easily resolve the obstruction. Bleeding also prepared the patient for the next step, vomiting. It removed the obstructions of the stomach and intestines and also eliminated the obstructions in many other canals by its violent shaking over the whole body. The violent concussion of the body posed a danger of bursting narrow and weak canals, such as the capillary vessels, but this was prevented by the preceding bloodletting. Cheyne emphasised that vomiting was a safe measure. Emetics were less dangerous than many of the medications for internal use, because their effects were confined to the stomach and intestines, where the canals were strong and wide. Other medicines that flowed into the circulating blood carried greater danger because of their stimulating effects on the blood and nerves (Cheyne, 1701: 57-75). This general caution regarding medicines - quite unusual for a professional physician - paved the way for his discussion of purging.

Many contemporary readers must have been surprised by Cheyne’s inclusion of purging among his recommended remedies, because it was the primary therapy favoured by his antagonist, Andrew Brown. It turned out that Cheyne was employing a tactical feint. Purging was similar to vomiting in that they took away the obstruction of the stomach and the intestines. Though a gentle purge had the same effect as vomiting, a more violent purge, that is, Brown’s method “has so much Danger, and so many Inconveniences in it, as render it as unsafe.” A strong purgative entered the mass of the blood through the lacteals, and produced many unnatural fermentations, separating the natural cohesions of body fluids and promoting the unnatural ones. It was possible that a strong purgative could help removing the obstructions of glands, by stimulating nerves and canals, but “these unnatural Fermentations and Changes of the Cohesion of the Fluids, instead of promoting the Cure, often increases the Cause of Fevers, to wit, the Obstruction of the Arteries which constitute the Glands.” Cheyne’s description of, and warning about, the dangerous side effects of purging must have satisfied the members of his circle. About the last step of sweating, Cheyne referred readers to Pitcairne’s Dissertione de Curatione Febrium as “all that can be pertinently said on this head; is so learn’dly and accurately already handled” in the treatise. He added this “most Universal, and surest step” needed to be used after bleeding and vomiting. When he summarised the therapeutic procedure, purging was not even mentioned. As such, Cheyne managed to defend the Newtonian theory of treating fevers while rebutting Brown’s challenge (Cheyne, 1701: 75-86, 90).

Although Morgan shared some Newtonian hydraulic assumptions with the Pitcairne circle, he developed his own system of animal oeconomy, as seen in the previous section. Moreover, since he was not involved in the Edinburgh controversy over the treatment of fevers, writing some twenty years later, he formulated an independent theory and practice regarding the disease. And his discussion was generally more detailed. Morgan had a different concept of body heat. For him, air containing elementary fire, not the friction of the particles of the blood, was the cause of heat in the animal body; and accordingly, his explanation of fever differed.

Morgan agreed with Cheyne that the obstruction of the glands caused fever but differed on how it led to fever. For Morgan, one of the purposes the serum in blood was secreted through lymphatic glands was to cool down blood. The secreted serum was, as shown in the previous section, cooled down because it was separated from the hot red globules. The cooled serum returned to the vein to reduce the heat of blood in general. So, when the glands were obstructed, the blood heat increased excessively, and the red globules absorbed more serum with stronger expansive power, which was reinforced by heat. Additionally, because the glands were blocked, a larger amount of blood circulated through the blood vessels - the point said by Cheyne as well. The result was “a preternatural Distension of the small capillary Arterys… with Heat or inflammatory Pain” (Morgan, 1730: 213-214). Like Cheyne, Morgan saw the obstruction of stomach or intestines as a common cause of this kind of fever (Morgan, 1730: 221).

Morgan, as Cheyne did, discussed fever, which was caused by a cold, but again somewhat differently. While Cheyne paid attention to the compression of the canal, Morgan focused on the influence of cold air on blood. As blood cooled, the expansive power of red globules was diminished, so serum was more easily separated from the globules. As a result, the larger amount of serum was secreted through the glands and “there must ensue a Surcharge of the secreted Matter upon the glandular Lymphaticks and Aqueducts, occasioning a painful Distension and Tumefaction of the Glands thus affected” (Morgan, 1730: 271-272).

Morgan talked about yet another kind of fever which was not discussed by Cheyne - fever occasioned not by the excess but by the diminution of the circulating fluid. He said that this could be caused by excessive sweating, diarrhoea, over-fasting, loss of blood, or any too large discharge of the body fluids. When the body fluids were evacuated, the globules remained and only the serum part was removed. In this case, fever occurred because the ratio of the red globules, which contained more heat, increased compared to the relatively cool serum (Morgan, 1730: 242, 247-248).

About the cure of fever, Morgan emphasised the way performed by Nature and argued that it should be followed by human art. At the early stage of disease, the blood was gradually heated up and the glands were being obstructed. In case the stomach or intestines - the place of “the first Principles and original Stamina of the Disease” - were obstructed, their muscles, being urged by the irritation of the indigested and heated matter, were excited to strong contractions, by which the obstructing matter would be thrown out by natural vomiting or purging. If a physician had a chance to care, his role should be helping Nature by taking necessary measures to facilitate vomiting or purging (Morgan, 1730: 221-223).

At the second stage where fever was at its peak and the lymphatic glands were obstructed, Nature changed her way. She took care to shut up all the outlets of the serum, so that expurgatory secretions like sweating decreased. By this means, the serum of the blood increased “so as to be capable by its natural Weight and Impetus to make its way again into the obstructed Lymphaticks.” This being done successfully, the blood would be cooled and diluted, the expurgatory glands would be open again in order to let out the morbid serum accumulated during the progress of the fever (Morgan, 1730: 218). Though Cheyne too mentioned this healing mechanism of nature in a passing remark, he did not recommend it for the physician to follow (Cheyne, 1701: 15-16). However, Morgan advised the reader to take the way of Nature. The physician needed to increase the amount of serum in blood to collect enough body fluids to break through the obstruction. This would be “best done by convenient Drinks in sufficient Quantitys” (Morgan, 1730: 226). At this stage, to evacuate body fluids by vomiting or purging only hindered the work of Nature who endeavoured to increase them.

Morgan was aware that his remedy was slightly different from other physicians. It could be argued, Morgan wrote, his advice was “contrary to the approv’d establish’d Practice of very good and well-experienc’d Physicians” who “very often draw off and cause a considerable Evacuation of the Serum, both by Blistering and Sudorificks as well as Catharticks.” Morgan acknowledged such a practice often led to successful results, but he maintained that its intention was “not to draw off the Serum from the Blood, or to lessen the Quantity of Serum in the Mass of Blood,” because the physicians who used the method “if they act rationally and know what they are doing, are always careful to enjoin free and plentiful drinking” of cooling and diluting liquor to increase body fluids (Morgan, 1730: 232-233).

According to Morgan, the reason of evacuating body fluids when fever was in full swing was to reduce the excessive concentration of fluids. Where the circulation was hindered, the glandular canals were “distended with a hot, viscid, acrid Serum” and the muscular part affected would be “attended with hot and burning, or with sharp, pungent, and acute Pains.” To prevent the particular part from serious injuries, a well-managed evacuation was necessary, and Morgan recommended blistering. By this way, the heated and stagnated serum would be drawn off and the stimulus and irritation on the affected part would be removed. He went on to talk about another benefit of blistering “not so commonly observ’d.” As seen above, Morgan believed that the heated air in the blood had an expansive force. This expansive force gave added pressure on glandular canals in case of fever. By blistering, the physician evacuated not only the hot serum but also performed “a considerable Drain of the heated, rarefy’d, expanded Air contain’d in the Blood, and imprison’d or shut up in the internal, tumefy’d, inflam’d Part.” In this way, the careful fluid and air evacuation by blistering could help to relieve fever (Morgan, 1730: 234-236).

Morgan’s recommendation of blistering showed his characteristic emphasis on the role and effects of heated air in the animal body. Whereas Cheyne thought that blistering played only an incidental role in the treatment of fever, Morgan believed that, when necessary, it was an important measure for relieving fever by evacuating the heated air as well as hot body fluids. Morgan discussed this method again in his Mechanical Practice of Physick (Morgan, 1735: 173-176). Medical Essays published by the Society of Edinburgh introduced Morgan’s use of blistering along with several of his other “uncommon Methods of Cure in different Diseases.” This introduction appeared under the head of “An Account of the most remarkable Improvements and Discoveries in physick, made or proposed since the Beginning of the Year 1734.” This part introduced not only Morgan’s suggestion but also observations made by other contemporary physicians, without criticising them. It is clear that Morgan’s new approach, together with those of other physicians, was the subject of serious consideration among contemporary medical academics (Anon., 1738: 454, 497).

In Morgan’s physiology, keeping the proper degree of body heat was crucial for health. To do so, the circulation of body fluids through many pipes needed to be properly maintained. The trouble with the circulation could lead to the disease of fever. The failure of the secretion of serum caused by the obstructed gland would increase body heat, because the serum, secreted through the gland and returning to the vein, cooled down the blood which was continually heated by the red globules. The heated air contained in the blood, with its expanding force, worsened the symptom of fever. For recovery, the obstruction of the gland needed to be broken through by the force of ample amounts of circulating serum, which was increased by abundant drinking of cooling liquor. Additionally, to prevent the over-heating of the obstructed part, if necessary, the fluid and air should be carefully evacuated from the area by blistering.

5. Digestion

In early modern Europe, different theories about how food became assimilated into the animal body were competing. By the mid-seventeenth century, chemical interpretations had become influential. According to Clericuzio, Ortus Medicinae (1648) of the Flemish physician Jean Baptiste van Helmont marked a turning point in the development of the chemical theory of digestion. For van Helmont, digestion was caused by a chemical reaction produced by an acid ferment. A ferment in the stomach changed ingested food into an “acid chyme.” In the intestine, a neutralising ferment transformed the acid chyme into an “alkaline chyme” to be absorbed by veins. Franciscus Sylvius, professor of medicine at Leyden was also an advocate of iatrochemistry. In Praxeos Medicae Idea Nova (1671) he disagreed with van Helmont over the preparation for the absorption of “acid chyme” by the lacteals of the intestine. Sylvius argued that the process depended on bile and pancreatic juice, not on a neutralising ferment in the intestine. However, an alternative approach also existed. In De Motu Animalium (1685) the Italian physiologist Giovanni Borelli, who influenced Pitcairne, maintained that stomach digestion was a process of crushing and grinding (Clericuzio, 2012: 332-333; Bloch, 1987: 1224).

Pitcairne and other iatromechanists believed that digestion was a mechanical process. Pitcairne rejected the argument of contemporary iatrochemists that some sort of acidic digestive fluid secreted from the stomach was instrumental in dissolution of food. If that was the case, he said, the digestive fluid would dissolve not only food in the stomach but also the stomach itself. For Pitcairne, the stomach contributed nothing but its muscular motion to the digestive procedure. By its motion the parts of the food were separated from one another and dissolved into a fluid (Pitcairne, 1727: 113-114). Keill agreed with Pitcairne that the physical motion of the stomach was the primary digestive force, but he thought that saliva and juice from the stomach played a role. However, Kell considered them simple liquids not very different from drinking beverages, and their role was mechanical, not chemical. According to him, the function of “the Saliva, the Succus of the Glands in the Stomach, and the Liquors we drink” was “to soften the Aliments, as they are fluids, which easily enter the Pores of most Bodies, and swelling them, break their most intimate Cohesions.” This softening function of the liquids made the work of stomachic muscles easier (Keill, 1703: 26-27).

Morgan, as well, denied the existence of an acid menstruum in the stomach. He mentioned the function of the stomach’s muscular motion for digestion. Also, he, like Keill, provided a mechanical role of moistening and softening for the digestive fluid. However, for Morgan, the main agent of digestion was heat. This section deals with his theory of digestive process and maldigestion.

Morgan started with the idea that all solid bodies, including food, contained the natural sources of dissolution - elementary fire and air. They had, he thought, a potential expansive force to disunite closely cohering bodies. What was needed for the exertion of the force was “nothing but a sufficient degree of Heat, which is the Action of the forgoing expansive Elements [elementary fire and air] already in motion” (Morgan, 1730: 323-324). According to Morgan, during the time of digestion, the upper and lower orifices of the stomach were closed, so the moistened food was shut up and dissolved in a digestive natural heat. In addition, the orifices opened at certain intervals to let in fresh air and let out vapid fumes. By this process in the stomach, “the whole Mass will be reduc’d to a sort of thin frothy Gelly, somewhat like a very soft, tender, and slightly cohering Curd” (Morgan, 1730: 324-325; 1735: 28-29).

The muscular action of the stomach pushed the digested matter into the duodenum and small intestine where it met the bile and pancreatic juice. According to Morgan, the proper action of these digestive fluids was precipitation. The harder parts which could not be perfectly liquified were “attracted and precipitated by the bile, and thereby separated from the more fluid Parts.” And the liquid parts were “reduc’d to a thin, fluxile, white and oily Liquor, like Milk, and then receives the name of Chyle.” The chyle was received into the lacteals where it joined the lymph from the lymphatic glands for more attenuation and dilution before it passed into the blood, providing the necessary nutrition. The bile had another role which was attenuating the feculence separated from the chyle and therefore preventing the excrements from hardening. At the same time, the bile, by gently stimulating the intestines, helped the bowel movements (Morgan, 1730: 325-327; 1735: 29-30).

We can see that in Morgan’s digestion theory, the heat in the stomach played a crucial role. Consequently, if something went wrong in digestion, it was largely because the heat was excessive or deficient. The excessive heat would make the dissolving, or digestive power of the expansive elements too intense while the deficiency would make it too weak.

When the excessive heat brought too strong dissolving power, the digestion would be “hot and flatulent; that is, the concocted Matter, instead of retaining its natural and due Consistence, will run into a frothy, hot, and windy Flatus.” This “too highly rarefy’d Concoction would continually replenish the Blood with an [sic] hot, rarefy’d, and viscid Serum,” resulting the consequences of heat and viscidity throughout the whole body - the inordinate inflation of the stomach and intestines during digestion with hot flushing and shortness of breath, restlessness at night, periodical burning heats, and cold sweat. Morgan said that this kind of maldigestion was generally due to intemperance and want of exercise. Overeating and binge drinking would increase the amount of the elementary fire and air in the stomach, leading to a high degree of heat. The only remedy was “obstinate Temperance and proper Exercise.” Those who use this treatment would need no other medicines (Morgan, 1730: 328-329).

In case the digestive heat was insufficient, leading to a weak dissolving ability, there followed “a cold and hard Digestion.” In it, only the watery parts of food were digested while the rich oils and volatile salts, which cohered more strongly with the solid parts of food, were not separated for nutrition. “These volatile oily Salts,” which were supposed to be separated in a proper digestion, were precipitated together with the faeces and excreted. Morgan wrote that the volatile oily salts were supposed to pass into the blood and be attracted into the red globules. When this supply was not sufficient, the serum part of the blood increased while the proportion of the globules dropped. Consequently, the pulse became weak and low, the natural body heat sank, the muscles grew weak, and “the Person is dull, listless, and averse to Motion.” Also, the bile could not be properly drawn off from the blood due to the want of the volatile oily salts in the blood. Low-quality bile could not perform its role of stimulating the intestines, resulting “an obstinate Costiveness and Obstruction of the Bowels” with “a continual Nausea, loss of Appetite, and inclination to vomit at everything” (Morgan, 1730: 330-331). As causes of this sort of maldigestion, Morgan mentioned unwholesome diet, bad air, and excessive grief (Morgan, 1730: 336). For recovery, anything which would help to raise and strengthen the body heat was recommended - for example, a glass of wine or two, smooth stale beer, or any such warming liquor, fish and flesh broths, and riding on horseback (Morgan, 1730: 333; 1735: 31-32).

Morgan, like other Newtonian physicians, rejected chemical explanations of digestion and suggested a mechanical one. However, he characteristically emphasised the role of heat in the digestive process while others focused on the muscular motion. In Morgan’s view, the heat in the stomach worked on the elementary fire and air contained in food, and the heated air expanded and dissolve the food, separating nutritious parts from excrements. Excessive or deficient heat caused maldigestion.

6. Conclusion

Judging by what Thomas Morgan wrote about medicine, he was a serious Newtonian iatromechanist who was as knowledgeable as the Pitcairneled Newtonian physicians. While he shared some common grounds with them, he developed the Newtonian medicine in his original way. Morgan’s view was characterised by his emphasis on heat as an important factor in physiology. For him, heated air mixed in the blood acted as an expansive force to counterbalance the attractive forces between small particles in the animal body. Fever occurred when the heated air in the blood was not properly evacuated or when the warmed body fluids were not cooled down. Morgan also believed that the main agent of digestion was the heat operating in the stomach. His heat-based interpretation was novel but fitted into the existing understanding of physical matters including the animal body, and it provided explanation about some physiological phenomena which were not fully explained before. The explanatory power of Morgan’s medical writing was, the present paper suggests, probably the reason of its good reputation among the contemporaries which we saw in the introduction.

Morgan’s medical writings received a degree of attention from the medical world of the day. The German scientific journal Acta Eruditorum, in its February 1728 volume, contained a review in Latin of Philosophical Principle of Medicine. The review summarised its main contents, including those discussed in this paper - the attractive force between particles, the expansive force operated by elementary fire and air, the function of red globules and serum in the blood, the manner in which secretions were produced, the nature and cure of fevers, and stomachic digestion driven by heat (Anon., 1728: 53-63). Many parts of Mechanical Practice of Medicine were introduced in Medical Essays and Observations of the Philosophical Society of Edinburgh. For example, as we saw in Section 3, Medical Essays, in discussing the controversy between Morgan and Robinson, presented Morgan’s view that body heat, as the operation of elementary fire, was comparable to combustion (Anon., 1738: 472-473). Andrew Hooke, who probably met Morgan in Glasgow, seemed to accept Morgan’s views on many aspects of physiology, including digestion. In his Essay on Physick, he wrote that ingested food was “dissolved by the Digestive Heat and Action of the Stomach” (Hooke, 1734: 46).

Charles Perry and Cotton Mather, mentioned in the introduction, who favorably assessed Morgan’s medical works, paid particular attention to his explanations of the causes of diseases and their cures in accordance with those explanations. Although not directly stated, such an evaluation was likely related to Morgan’s theory of secretion and fever, which this paper discusses in detail. For Newtonian physicians, secretion was a key to animal life and health. Most diseases, including fever, were explained with reference to abnormal secretion. As mentioned earlier, fevers accounted for a considerable proportion of all diseases, and their proper management was one of the most debated topics in early eighteenth-century medicine (Jenner, 1996: 317).

John Hancocke, a London clergyman and popular medical writer, used Morgan’s ideas to support his argument. He recommended drinking cold water as a remedy for fevers and many other diseases. Though he differed from Morgan in detail, he agreed that the obstruction of lymphatic glands caused fevers and that drinking “plenty of cooling and diluting Liquors, and especially Water” would relieve them. “To make it better understood” Hancocke cited Morgan’s view that serum, after secreting from blood, returned “into the Blood-Veins, to cool and dilute the Blood.” Another disease, epilepsy, according to Hancocke, might be caused by the expansion of the nerves. This could be caused by air, “the most Elastic Body we know.” And “some Philosophers, particularly Dr. Morgan says, there is Fire and Air in all Bodies” and if a great quantity of air in the nerves was expanded by heat, “it may perhaps be more apt to be expanded and give some trouble to the Nerves and Muscles of the Body.” Supplying water would ease the nerves by lowering heat and reducing the elasticity of the air (Jenner, 1996: 313-320; Hancocke, 1726: 109-110, 182-3).

Thomas Knight, a member of the Royal College of Physicians, also explained the way lithontriptics worked based on Morgan’s view of elementary fire and air. Lithontriptics, which were injected into the bladder, were composed of “Quick-lime and alkaline Salt, each a Creature of the Fire by Calcination” and, as a result, “the material Particles of Fire” were lodged in them. These particles of fire in lithontriptics dissolved bladder-stones, by “acting upon the unelastic Air” inside the stones and by rarefying the air “so as to burst its Confines and Strong Holds.” The theoretical base of this effect came from Morgan’s theory (Munc, 1878; Knight, 1749: 130-139). Knight worte:

His [Morgan’s] Principle is, That the expansive Force of Air, rarify’d by the Action of Fire, or the mutual Action of Fire and Air, is the universal Cause of Fermentation and Dissolution… When it [fire] has Activity enough to get from the Interstices into the Moleculae of some Fluids or Solids, and finds a hidden Fire, this secret Fire, joining with the external, acquires an extreme Violence (Knight, 1749: 134-135).

Such approval and application of Morgan’s ideas for further development reflected their influence and utility within the medical community of the eighteenth century.

It has been generally said that the mid-eighteenth century was a period of transition in British physiology from mechanism to vitalism (Brown, 1974). Given that Morgan was active as a medical writer towards the mid-eighteenth century, an attempt to gauge his position in this respect may be of some value. It seems hardly deniable that the influence of Newtonian iatromechanists declined and gave way to the view that physiological phenomena could be properly explained only by “vital principles,” rather than by the mechanical properties shared by dead matter. However, rather than seeing it simply as an abrupt shift, some continuity between mechanism and vitalism has also been sought. We already saw one example in the introduction: Ishizuka maintained that the concept of elasticity, which was valued by the iatromechanists, served as the mediator between mechanism and vitalism. Ishizuka included Morgan among those iatromechanists who paid notable attention to the elasticity of muscular fibres. The final suggestion of this paper is that his physiology had a further transitional dimension. However, one more example of the transition from another figure is worth noting before Morgan’s case is examined.

Theodore M. Brown detected an undercurrent in Keill’s application of attractive forces to selective purgation by drugs, arguing that it provided a back door to vitalism. Sudorifics increased perspiration but did not increase urine. Likewise, mercurial medicine stimulated only salivation, not affecting the discharge of other fluids. The reason for this selective purgation, which was unaddressed by Pitcairne, was explained by attraction. According to Keill, a medicine was more strongly attracted to certain particles than to others; in this way, different medicines affected different body fluids. Only living animals could be selectively purged as such. Inanimate objects could not exhibit similar reactions. In this way, Brown argued, Keill was adopting “specific and unique properties of biological organisms, which could be described but not easily explained” (Brown, 1981: 346-349).

Morgan also posited that living bodies had unique characteristics. He saw the animal body as a machine, but a living machine. In the preface of Philosophical Principles of Medicine, he wrote that “the animal Automaton is contriv’d and adjusted upon Principle of Self-preservation.” Ordinary machines, such as clocks, had no power to restore themselves, but in the animal machine, “an irregular or diseased Motion in one part, necessarily excites such Motions in other parts, as tend to rectify the first Disorder and set all right in a natural way” (Morgan, 1730: viii-ix). He did not mention body heat regarding this self-preserving principle, but heat must have contributed to the unique power, considering the crucial role of heat in Morgan’s physiology. When Morgan introduced the expansive force of heated air in the animal body which counteracted the attractive force, he remarked that “the elementary Fire and Air” was “retain’d in the internal Constitution of Bodys” (Morgan, 1730: 104-105). Clearly, the ability to absorb, retain, and emit elementary fire and air belonged solely to the living body. In this regard, part of Morgan’s heat-based physiology may, to some degree, be considered to lie on the threshold between mechanism and vitalism.

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