Marat's Science

3: Marat's Investigations of Heat



The earliest natural philosophers identified fire as a central concern. In the sixth century bce, Heraclitus of Ephesus concluded that fire was the underlying unifying principle of all of nature:

All things are an equal exchange for fire and fire for all things, as goods are for gold and gold for goods.[1]

Empedocles’ four-element theory, which included fire as one of the four fundamental materials of nature, was taken up by Aristotle and dominated natural philosophy for more than two millennia. Although an alternative concept of element championed especially by Robert Boyle began to challenge this notion in the seventeenth century, the four-element theory retained a strong position among natural philosophers even at the end of the eighteenth century. Lacépède, for example, took the four-element theory for granted and Lamarck wielded it vigorously against Lavoisier and his new chemistry.[2]

It is understandable, then, that from earliest times “What is fire?” would have presented itself as an obvious focus of scientific research. Some historians of science have been unable to resist the temptation to call it a “burning question.”[3]

The difficulties presented by the question can be appreciated by temporarily stepping out of the eighteenth-century context and considering it in the framework of present knowledge. What answer would be given to a modern high-school science student who, contemplating the dancing flame of a candle, asks “What is fire?”

Philosopher of science Gaston Bachelard writes that when he asked “educated persons and even eminent scientists” this question, he generally received “vague or tautological answers which unconsciously repeat the most ancient and fanciful philosophical theories.”[4] Nor, generally speaking, could he find better answers in published writings:

In the course of time the chapters on fire in chemistry textbooks have become shorter and shorter. There are, indeed, a good many modern books on chemistry in which it is impossible to find any mention of flame or fire. Fire is no longer a reality for science.[5]

The course of scientific development has treated Marat’s other main concerns differently. A high-school student asking “What is light?” or “What is electricity?” would likely elicit confident, definitive answers that both teacher and student would find satisfying. The question of what fire is, however, seems to have simply faded away.[6]

For Marat and his contemporaries this was still a central issue of physics. The obvious connection between flame and hotness was taken to indicate that fire is in some sense a pure expression of heat, and therefore that the appearance of flame is a special case of the general phenomenon of heat, or at least that the two phenomena are intimately related. Marat was far from alone in assuming that heat and fire are “two effects of the same cause.”[7] To inquire into the nature of fire, then, also meant investigating the nature of heat.

There were two broad generalizations current with regard to the nature of heat. The first held that heat is a material substance that, when added to another material substance, made the latter hot. The difference between a cold piece of iron and a hot piece of iron, according to this view, was the absence or presence of a substance that might be called either “heat matter” or “fire matter.” The fact that hot and cold objects put in contact with each other soon reach a state of temperature equilibrium suggested that heat must be a fluid that flows into and out of objects. A great body of observational and experimental data seemed to confirm this common-sense conception.

The alternative generalization was that heat is not a substance at all, but a pure product of matter in motion. This idea has ancient roots and was given renewed currency in the seventeenth century by Francis Bacon in his De forma calidi (1620).

Bacon observed that the production of heat always seemed to require imparting motion to a substance, whether by the friction of rubbing, the blows of a blacksmith’s hammer on iron, or the striking of flint on metal. Other seventeenth-century natural philosophers, including Newton and Leibniz, developed Bacon’s concept and attributed heat to the vibratory motions of an object’s constituent particles. A hot substance, according to this hypothesis of “intestine motion,” would be one whose particles are in a state of relatively rapid motion.

Although this idea enjoyed a great deal of support among seventeenth-century natural philosophers, by the last decades of the eighteenth century the material theory of heat had become dominant. After all, when a hot object and a cold object are gently placed in contact with each other, heat apparently flows from one to the other even though no obvious motion is imparted.

The notion of “heat matter” or “fire matter” was in harmony with the generally mechanical outlook of many eighteenth-century natural philosophers. Fundamental explanations of such physical phenomena as heat, light, electricity, magnetism, gravity, sound, and even mental processes frequently depended upon postulating the existence of a “subtle fluid” that could mechanically account for observed properties. The fact, for example, that objects tend to increase in volume when heated, or liquefied, or evaporated, was explained by the mechanical action of particles of the heat fluid insinuating themselves into the spaces between the object’s particles and pushing them apart.

The important discoveries of Joseph Black with regard to heat, especially latent heat, were apparently made about 1760 but were not published in his name until 1803, after his death.[8] In spite of the publication delay, Black’s ideas have frequently been credited with influencing an extremely important technological development that occurred before they were put into print: James Watt’s improved steam engine.[9] In any event, Black’s findings and concepts with regard to heat were propagated, albeit imperfectly, in the works of other authors.[10] Black’s discoveries seemed to favor the matter-of-fire theory over the intestine motion theory. Of the latter, Black declared:

I myself cannot form a conception of this internal tremor which has any tendency to explain even the more simple effects of heat.[11]

That the heat fluid was subtle to the point, perhaps, of being imponderable, or weightless, resulted from the observational fact that objects do not increase in weight, or increase very little, when heated. Whether heat is absolutely weightless or not was a matter of controversy, as will be seen below. Another adjective frequently used to describe subtle fluids was “elastic”; the concept of elasticity will also be discussed below.

Both the subtle fluid and intestine motion concepts, of course, were hypothetical. Their existence was inferred from their “consequences”; no one could credibly claim to have observed either directly. Although the authority of Newton’s hypotheses non fingo was often invoked to discourage this sort of speculation about the hidden causes of phenomena, Newton himself provided the best-known models for speculative subtle fluid theories. Newton’s influence on his eighteenth-century successors was complex; it can be roughly described as two somewhat contradictory influences, one stemming from his Principia Mathematica and the other from his Opticks.[12]

The Principia was the primary source of Newton’s immense prestige among scientists. By uniting the principles of planetary motion and terrestrial gravity, and by giving precise mathematical expression to their common attractive force, Newton was thought to have established a model for all subsequent scientific investigation. Following Newton, all natural phenomena might be reducible in principle to mathematically describable matter in motion. Those who were left unsatisfied with abstract mathematical formulae and who continued to demand to know the cause of gravitational attraction were told that such questions were inherently hypothetical and therefore unscientific. “What is gravity?”—like “What is fire?”—was turned into a nonquestion for science.

The Opticks, however, revealed another side of Isaac Newton. Rather than depending upon deduction from quantifiable data, Newton’s optical investigations were primarily empirical and qualitative. It was the Newton of the Opticks that provided the primary inspiration for experimental physicists such as Benjamin Franklin and Marat, whose investigations were also qualitative rather than quantitative. “The Opticks,” a leading historian of science points out,

is the only one of Newton’s two major books on physical science to have been read by every Newtonian scientist. . . . The Opticks is the more important of the pair for our understanding of Newtonian natural philosophy in the eighteenth century.[13]

Most significant was the lack of hesitation to frame hypotheses that Newton exhibited in his Opticks. In a series of queries appended to that work, he developed speculative hypotheses regarding subtle elastic fluids or “aethers” as putative first causes of light, gravity, and other natural phenomena. The Newtonian legacy was therefore more than a little ambivalent:

The scientists of the eighteenth century often took a curious position with respect to hypotheses in the Newtonian philosophy. For instance, they would attack Cartesian philosophy as resting on hypotheses and quote Newton’s phrase about Hypotheses non fingo. Then, without further ado, they would present Newton’s own hypotheses as if they derived them from phenomena and were accepted principles of natural philosophy.[14]

Belief in hypothetical fluids, then, and in a heat fluid in particular, was not a source of disagreement between elite academicians and independent researchers like Marat. It was part of the common ground upon which they all stood.

The Problem of Combustion

The investigation of heat was not the only approach taken in the quest to understand the nature of fire. Rather than contemplating the continuity of increasingly hot objects with burning objects, some natural philosophers chose to consider the discontinuity represented by the act of combustion. The observation that some materials burst into flame when heated while others do so less readily or not at all led to the concept of a material basis of combustion. Substances that share the property of being burnable, it was reasoned, must have a specific component in common, a principle of combustion. This idea was championed in the seventeenth century by Johann Joachim Becher and was most fully elaborated by Georg Ernst Stahl, who in a 1703 publication named the principle of combustion phlogiston.[15] Phlogiston was envisioned as the “food of fire” that was consumed in the burning process. It was not thought to be destroyed, however, but transferred in a chemical reaction from combination with one substance to combination with another.

The phlogiston theory developed largely out of attempts to understand the economically critical industrial process in which iron was produced from iron ore by heating it over a charcoal fire. This process could be convincingly explained as a transfer of phlogiston out of the burning wood, leaving an ash, into chemical combination with the ore, producing the metal.

The phlogiston theory provided a satisfying explanation of combustion and remained dominant until near the end of the eighteenth century when a rival theory proposed by Lavoisier challenged it. Lavoisier’s explanation of combustion held that burning objects did not give up a substance named phlogiston, but instead gained a substance—a sort of antiphlogiston—that happened to be a component of ordinary air. He named this substance oxygen. In the earlier theory, the air had played the role of a neutral but necessary medium of transmission for phlogiston.

Although Lavoisier began to work out the new theory during the years 1772–1775, it only began to mount a serious challenge to the old paradigm after 1785, when such members of the French scientific elite as Berthollet, Fourcroy, Laplace, and Monge declared their allegiance to the new chemistry.

It had not completely conquered the field even at the end of the eighteenth century, however. Important pockets of resistance remained in France and abroad. The most important defender of phlogiston theory, Joseph Priestley, continued to oppose Lavoisier’s innovation to the end; his last scientific work, published in 1800, was entitled Doctrine of Phlogiston Established. In France the banner of phlogiston was held aloft by Jean Claude Delamétherie. Although Delamétherie  was not a member of the Academy of Sciences, as editor of the Journal de Physique he was an important scientific opinion-maker. Lamarck’s massive Réfutation de la théorie pneumatique, published in 1796, was directly aimed against the new chemistry, but it rejected phlogiston as well and tried to stake out a third alternative.

While Lavoisier was investigating combustion, he was also studying the related subject of heat. In 1782 and 1783 he collaborated with Laplace on a series of classic experiments utilizing a device Laplace had invented to measure heat, the ice calorimeter. Their resulting monograph, Mémoire sur la chaleur, suggested that the intestine motion and matter-of-heat doctrines were fundamentally equivalent. Lavoisier, nevertheless, preferred to consider heat as a material substance that flows back and forth among other substances. In the famous work that codified the chemical revolution, along with the new terms “oxygen,” “hydrogen,” and others, Lavoisier gave the name “caloric” to the matter of heat. Caloric was the second entry in Lavoisier’s “Table of Simple Substances”; only light was higher on the list.[16]

Lavoisier had earlier concluded that all aeriform fluids are composite substances containing caloric. The papers that he read before the Academy of Sciences could contain hypotheses as speculative as any of Marat’s theoretical musings. In one, for example, he explained that the matter-of-heat and matter-of-light produced by ordinary combustion, as well as the matter-of-electricity produced by another kind of combustion, are all normally components of the air. “It is astonishing,” he said, “to see how much this new theory adds to the explanation of a very great number of phenomena.”[17]

Marat’s Recherches physiques sur le feu

The preceding comments give a general idea of the state of knowledge and some of the debated issues concerning heat, combustion, and fire when Marat’s Recherches physiques sur le feu (Research on the Physics of Fire) appeared in 1780. Marat sought to contribute to the discussions by reporting and interpreting a large number of experiments he had performed with the aim of developing a general theory of the subject.

Recherches physiques sur le feu was published in Paris with the approval of the official censors. There are 166 numbered experiments described in the book; the descriptions are careful enough in most cases to permit replication. Many of them required the use of sophisticated apparatus that Marat described in detail; M. Sikes, optician to the King at the Palais Royal, was identified as the master craftsman who had constructed the equipment according to Marat’s specifications. The number of experiments may seem inflated by the fact that separate numbers were sometimes assigned to similar experiments in which the conditions were only minimally varied. On the other hand, some of the individually numbered experiments represented a dozen or more separate trials, and Marat reports that many were repeated numerous times.

Marat evidently put a great deal of thought and work into trying to unravel the mysteries of fire and heat. Although he was not always successful in preventing extraneous factors from influencing the outcome of his investigations, his carefulness as an experimenter appears to have been beyond reasonable reproach, at least by contemporary standards.

In the introduction, he stated that his book aimed to challenge “the opinion of the ages” with regard to fire:

It was long ago believed, and is still believed today, that fire is a material element, dispersed throughout all material bodies and remaining hidden as long as it is not manifested by movement.[18]

Marat treated the notion of elemental fire as the virtually unanimous opinion of physicists and cited Hermann Boerhaave, “one of the most famous partisans of this system,” as a specific example of someone identified with the doctrine he intended to argue against. “For the physicists,” Marat said, “fire is a material destined to produce, by its very presence, light and heat.” Since all material bodies contain it, why are not all bodies hot and luminous? “Because, they respond, this element is included with other basic elements in a mixture wherein it remains hidden until it manifests itself by means of movement.”[19]

Marat contended that this doctrine leads to absurdities. “What?” he exclaimed, “Imperceptible atoms of fire are maintained for thousands of centuries in the midst of the least combustible materials?” If the sparks produced by striking two stones together is thought to represent the release of elemental fire, how is it, he asks, that this element can “lose all of the properties that it is known to have and then regain them by means of a simple collision?”[20]

Marat took strong issue with those who claimed that phlogiston, the matter of combustion, is equivalent to the matter of fire:

Not content with holding that fire is material, they claim to be able to show it in the decomposition of mixtures. Open their books and you will see that this matter is the phlogistic part of bodies.[21]

Marat was by no means an opponent of phlogiston theory. In a footnote he disputed “a famous author” who “has strongly cried out against the nomenclature of chemistry.” Marat agreed with this author that many terms used by chemists were empty of meaning, but disagreed that phlogiston was one of them. “By phlogiston, one understands the inflammable principle of bodies. This principle surely exists in nature.”[22]

Marat conceded that the idea of fire being nothing more than phlogiston seems reasonable at first glance. Since “materials impregnated with phlogiston” burst vividly into flame, it is tempting to conclude “that they are almost completely made up of fire.” The “true inflammable principle,” on the other hand, “however pure it may be,” is entirely unlike fire in that it is “never luminous, and is always found to be at the ambient temperature.”[23]

Further insight can be gained by examining the concentrated acids, such as marine acid, vitriolic acid, and nitrous acid, “those liquors that the chemists regard as the mixtures most impregnated with pure fire.” The action of nitrous acid, for example, which holds phlogiston in solution, radically differs from that of fire: “Instead of making animal and vegetable material pass into the carbonous state in extracting their phlogiston, as fire does, it dissolves them entirely.” On the basis of such considerations, Marat concluded that “phlogiston is absolutely distinct from fire.”[24]

Having demonstrated to his satisfaction that “fire is not material,” Marat believed he had destroyed the standard paradigm and was ready to put forward his own theory as an alternative.[25] Although he appeared to be preparing the reader for a conception not depending upon a postulated material substance, that was not the case. Fire is not a material fluid per se, he said, but a modification of a material fluid. He explained this by means of an analogy: Colors are not themselves distinctly material entities; they are modifications of a material entity (i.e., light). In fact, Marat said, he hoped his theory would do for the doctrine of fire what Newton’s had done for the doctrine of colors.

Although fire is not material, then, there exists a material substratum without which fire would not exist. Marat identified it as a subtle fluid and designated it by the name “igneous fluid.” However full of igneous fluid bodies may be, in the absense of combustion they “are always at the temperature of the medium that surrounds them.” From this fact he concluded that “it is therefore the movement of this fluid, and not its presence, that produces heat and fire.”[26]

By way of elaboration he added: “Heat and fire are two effects of the same cause that differ between themselves as more or less.”[27] If heat is caused by a moderate motion of igneous fluid, fire is the result of a qualitative increase in that motion. Marat had, in a sense, combined the “matter of heat” and “intestine motion” doctrines into a synthesis wherein the essential motion is not of the particles of the heated object itself, but rather of the corpuscles of a special material fluid.

Apart from whatever intrinsic scientific value this conception may have had, was it original? Marat claimed that it represented a genuine theoretical innovation, and there is no evidence that he or his readers believed otherwise. None of the extensive critical commentaries by Lamarck, Marivetz, Carra, or others accused him of simply rehashing an old doctrine.[28] The general notion, however, was not new. A decade or two earlier, Joseph Black had reviewed all of the various theories of heat known to him:

The greater number of French and German philosophers, and Dr. Boerhaave, have supposed that the motion in which heat consists is not a tremor, or vibration of the particles of the hot body itself, but of the particles of a subtile, highly elastic, and penetrating fluid matter, which is contained in the pores of hot bodies, or interposed among their particles.[29]

It would seem, then, that the essence of Marat’s synthesis had earlier been considered by a perceptive English observer to be representative of continental physical thought in general. Was Marat ignorant of the literature on the subject, or was he falsely trying to ignore it? Neither of these alternatives is likely; he knew the subject well and he was writing for a sophisticated audience that he could not have hoped to so easily dupe. Perhaps the answer lies in differing emphases of interpretation on the part of Black and the authors whose work he was characterizing; perhaps the latter had not attributed as much importance to the motion of a special fluid’s particles as Black thought they had.

The Solar Microscope

Be that as it may, Marat stressed the originality of his contribution to heat theory. His primary innovation, however, was a methodological one, based on turning a little-known piece of apparatus to new use. Although the truth of his theory could be “deduced from the necessity of the facts,” reliance on reason alone was unnecessary since it could now “be demonstrated to the eye itself.”[30] The igneous fluid could be made visible by means of a solar microscope that Marat had devised.

The solar microscope was an instrument used in a darkroom. A narrow beam of sunlight was introduced into the darkroom through a tube fitted into a hole in the shutters; one or more lenses were employed to focus the incoming light and to magnify images of shadows cast on a screen. The baron de Marivetz included detailed illustrations of the apparatus in his Physique du monde.[31] Here is how Marat described his initial efforts: “Trying one day to examine the flame of a candle in the darkroom, I put it at the focus of the lens, but no image was produced.”[32] The incoming sunlight had been too strong; he decreased the amount of sunlight entering the room and found that “all matter less subtle than that of light can therefore intercept the light” and produce a shadow on the screen. “How surprised I was,” he wrote,

at seeing the image of the candle’s flame in the form of a whitish cylinder, bordered by a white halo and crowned with a tuft of swirling jets that were less white. Recovering from my astonishment, I judged that this image was that of the igneous fluid, not of the flame. To assure myself of that, I substituted a piece of red-hot iron for the candle.[33]

Seeing a similar image, he continued to examine a number of other substances by the same method.

Having become adept at rendering the igneous fluid visible, I undertook the reexamination of all the phenomena of fire in the darkroom; I performed a large number of experiments, and the truths that I had previously deduced from the necessity of facts were almost all confirmed.[34]


Images produced by Marat’s helioscope reproduced from his Découvertes sur le feu.

Photo courtesy of Division of Rare and Manuscript Collections, Carl A. Kroch Library, Cornell University

Marat’s use of the solar microscope in the darkroom would be the central axis of all his future research in physics, and he believed that further developing the technique would represent a major gain for science.

This method of observing is absolutely new and I strongly recommend that physicists give it a try. To take it into certain branches of physics would be, I think, to open a source of new knowledge. I myself have already applied it to electricity, air, and light, and will soon communicate my discoveries to the public.[35]

This commitment was fulfilled with the publication of his Découvertes sur la lumière and his Recherches physiques sur l’électricité, which will be discussed in Chapters 4 and 5, respectively.

Observation of the candle flame, described above, was experiment number one; number two was the substitution of the red-hot iron for the candle. Before pursuing Marat’s investigation of the igneous fluid, however, let us consider the question of his originality with regard to the solar microscope. Few innovations are absolutely unprecedented and this was no exception.

Joseph Priestley’s 1772 history of optics, which was almost surely known to Marat, stated that “in 1738 or 1739 M. Lieberkuhn” invented a solar microscope. In 1739 Lieberkuhn was in England where “he showed an apparatus of his own making . . . to several gentlemen of the Royal Society, as well as to some opticians.” One of the latter, a Mr. Cuff of Fleet Street, “took great pains to improve” the instrument.

The solar microscope, as made by Mr. Cuff, was composed of a tube, a looking glass, a convex lens, and a Wilson’s microscope. The tube is of brass, near two inches in diameter, fixed in a circular collar of mahogany; which, turning round at pleasure, in a square frame, may be easily adjusted to a hole in the shutter of the window, in such a manner, that no light can pass into the room, but through that tube. Fastened to the frame by hinges, on the side that goes without the window, is a looking glass, which, by means of a jointed brass wire, coming through the frame, may be either moved vertically or horizontally, to throw the sun’s rays through the brass tube into the darkened room. The end of the brass tube, without the shutter, has a convex lens, to collect the rays, and bring them to a focus; and on the end within the room Wilson’s pocket microscope is screwed, with the object to be examined applied to it, in a slider. The sun’s rays being directed by the looking glass through the tube upon the object, the image or picture of the object is thrown distinctly and beautifully upon a screen of white paper, and may be magnified beyond the imagination of those who have not seen it.[36]

Priestley also mentions a later variation on this instrument that was described by a Mr. Brander in a 1769 pamphlet on “eines Sonnen Microscops.” In this version, the solar microscope “has been introduced into the small, and portable, as well as the large Camera Obscura.”[37]

Marat did not mention these predecessors, although he certainly had knowledge of them. While Marat did not refer directly to Priestley’s history of optics, it would seem unlikely that he had not been familiar with it. In his later work on electricity, Marat repeatedly cited Priestley’s History and Present State of Electricity as an authoritative source.

In any event, Marat did not claim to have invented the solar microscope, but to have devised an improved kind that he called a helioscope, and to have extended its use into new fields of observation:

I did nothing more than use, in a certain manner, an instrument that had for a century been in the hands of all those who occupy themselves with physics.[38]

Marat drafted a codicil to his will in order to prevent members of the Academy of Sciences, and particularly a “M. Bettancourt,” from stealing the credit for “my little helioscope,” which was “capable of producing the solar image in all its whiteness.” Bettancourt, he said, is “a man above all others in knowing how to appropriate the results of others’ work for his own use.” In the event of Marat’s death, the executor of the will was instructed to turn the design over to the Academy of Sciences. This would indicate that his hostility was not directed toward the Academy as an institution, but toward certain of its members.[39]

Marat and the Academy: Round One

Marat had requested, through the good offices of his patron the comte de Maillebois, that the Academy of Sciences evaluate his experimental work.[40] The Academy appointed a commission to do so, and its report, dated April 17, 1779, stated that Marat’s experiments

were carried out by a new and ingenious means that opens up a great field of new research in physics. This means is the solar microscope. Until now this instrument has not been used except for small objects. [Marat, however,] has used it to show, in a perceptible manner, various emanations that without this means could not be seen at all, or at least not so clearly and distinctly.[41]

Accompanying the commissioners to Marat’s laboratory to observe his experiments was one of the most renowned of contemporary scientists, Benjamin Franklin. Franklin injected a note of humor into the proceedings by volunteering his own bald head as one of the series of objects to be placed at the focus of the solar microscope. It, too, gave off the emanations whose shadows Marat interpreted as visible evidence of igneous fluid.

This first official encounter between Marat and the Parisian academicians apparently ended in mutual respect. The commissioners seemed genuinely impressed with what they believed to be a new and valuable experimental technique.

The commissioners would not go so far as to affirm that the shadowy emanations they had seen projected on the screen were indeed produced by igneous fluid, as Marat claimed. The wording of their report suggests that they differed among themselves on that point; to declare positively that they had witnessed igneous fluid “would get us involved in overly lengthy discussions.” They did, however, state for the record that all of the experiments they had seen were consistent with the conclusion that some kind of fluid was seen emanating from heated bodies, and that this fluid was carrying heat with it. The report ended with what can only be described as a glowing tribute to Marat’s set of “new, precise, and well-executed experiments, ingeniously and appropriately designed,” which, they repeated, “open up a vast field of research for physicists.”[42]

Marat could hardly have desired a more supportive statement. He published it in its entirety at the front of one of his short works, with the claim that it constituted the Academy’s stamp of approval on the contents of the book.[43] Unfortunately for Marat, he had neglected to take the commissioners’ apparently innocuous qualification into account. Lavoisier noticed Marat’s claim and demanded that the Academy publicly repudiate it, which is what occurred. Marat’s relationship with the Academy rapidly deteriorated.

Condorcet would later refer to this episode as “the Marat incident,” and would use it to illustrate the Academy’s role as “barrier against charlatanism of all kinds.” The Academy, he said in retrospect, had been “too soft” in its dealings with Marat; it should not have “accepted as new experiments which were already known and which were novel only through the jargon the author had given them.”[44] Although not all of the academicians were opposed to Marat, the opposition of Condorcet, Lavoisier, and Laplace was sufficient to turn the institution as a whole against him. The response to Marat’s next approach to the Academy, which had to do with his optical experiments, will be discussed in Chapter 4.

That Marat’s experimental techniques, at least, were considered to be of genuine value by many academicians was confirmed in 1784 by the report of a commission headed by Franklin that investigated Mesmer’s animal magnetism. While not mentioning Marat by name, the report cited the results of observations with a solar microscope as evidence against the existence of animal magnetism.[45]

One academician who considered Marat’s theories worth discussing was Lamarck, who later wrote a lengthy critical appreciation (26 printed pages) of Marat’s researches into the nature of fire.[46] This “Exposition of several very interesting experiments by means of which the matter of fire (in expansion) is rendered visible” was published in 1794, after Marat had become famous as a revolutionary martyr. Apparently wanting to keep the focus on the scientific issues at hand, Lamarck consistently referred to him only as “the author,” identifying him in a footnote as “the patriot Marat.”[47] Lamarck writes:

Several interesting experiments that the author had occasion to perform by the ingenious means that he devised certainly revealed some things to him that other physicists knew nothing about.[48]

Lamarck carefully described and interpreted Marat’s investigations, lauding his “beautiful experiments” and his “striking proofs.” He concluded by counterposing Marat’s work to that of “pneumatic chemists” such as Lavoisier and Fourcroy:

I recommend, moreover, that the reader consult this interesting work, and above all to repeat the fascinating experiments that its ingenious author has performed. These experiments are very important for the true theory of fire, which the pneumatic chemists have assuredly not foreseen at all.[49]

Lamarck’s comments testify eloquently to the seriousness with which Marat’s endeavors were treated in important scientific circles. When this was published, Lamarck was already prominent for his botanical studies and had just begun his pioneering studies of invertebrate anatomy. His accomplishments had earned him entry into the Academy of Sciences in 1779, but his natural philosophy was not in harmony with that of its dominant faction, as his polemics against pneumatic chemistry attest. Lamarck’s account of Marat’s work was not entirely uncritical; his criticisms will be discussed below.

Another important contemporary scientist and member of the Academy of Sciences, the comte de Lacépède, also expressed admiration for Marat’s experimental innovation. He wrote that molecules of fire

have been able to intercept rays of light and to project a shadow onto a surface; Mr. Marat has found an ingenious means to augment this shadow and render it more perceptible.[50]

He went on to argue, as Marat had, that the heat fluid and the light fluid are distinct entities,

and the series of experiments that Mr. Marat has made public prove it even better than all my reasoning.[51]

Jean Louis Carra’s popular Nouveaux Principes de physique (New Principles of Physics), in a section entitled “Theory of Fire and Heat,” summarized the views of Newton, Musschenbroek, Boerhaave, the Cartesians, and Marat. He then declared:

I therefore adopt, in relation to my own principles, the opinion of Boerhaave, that of the Cartesians, and that of M. Marat.[52]

It is evident that Marat’s contributions to physics, and especially his solar microscope technique, attracted some attention in contemporary scientific circles. While never a central focus of scientific concern, his work was certainly noticed, acknowledged, reviewed, commented upon, and discussed.

Investigating the Igneous Fluid

Let us now return to Recherches physiques sur le feu to find out what Marat was able to learn about the igneous fluid. First of all, he anticipated a possible criticism of his interpretation of what the solar microscope reveals:

It will undoubtedly be objected that the image on the screen may be the result of some lightweight vapor escaping from these bodies.[53]

He disposed of this objection by experiments showing that the same shadows are produced by hot objects in the evacuated chamber of an air pump.

Marat admitted a limitation of the technique and expressed the desire for more powerful microscopes in the future:

The fluid appears here only in its collectivity; perhaps dioptrics will some day be capable of allowing us to distinguish its individual atoms.[54]

Although Marat acknowledged being unable to see these atoms of the igneous fluid, he “deduced from facts” that they “must be of an astonishing smallness and spherical in shape,” and therefore called them little globes, or “globules.”[55]

Another property of the igneous fluid is “diaphaneity”; it never forms an opaque medium, as his experiments numbered 10 through 12 demonstrate. In fact, it seems more than diaphanous; it could even be said to be lucid, since on the screen it shows up brighter than its surroundings.

A most significant property of the igneous fluid is one that saved Marat from a philosophical dilemma:

Like all other bodies, this fluid has weight, for heated metals lose this weight when cooled.[56]

Watching, in the darkroom experiments, the prodigious quantity of igneous fluid that escapes from incandescent bodies “causes one to cease being astonished” at their weight loss. Experiment 27 consisted of heating a 16-ounce ball of fine silver to cherry-red, whereupon it registered a five-and-a-half grain weight gain. Experiment 28 similarly compared the weight of a piece of copper before and after being heated white-hot; again a weight differential is reported.

Repeated four consecutive times, these experiments always gave the same results, from which it follows that the same mass of the same metal never requires other than the same quantity of igneous fluid to redden it to the same point.[57]

Marat, of course, was not the first to perform such experiments designed to ascertain whether or not heat has weight; Boerhaave, Buffon, and many others had long disputed the meaning of contradictory findings. Some investigators who found no weight differential between a cold object and the same object after it had been heated argued that this supported the intestine motion doctrine of heat. Others, likewise failing to detect a weight differential, refused to give up the concept of a material fluid, but declared it “imponderable”; i.e., weightless by definition. Critics of the latter claim objected that weight is the sine qua non of materiality; a weightless material substance is a contradiction in terms.

Complicating this discussion even further was the attempt to explain why a metal apparently gains weight when it loses phlogiston: Phlogiston must possess negative weight (or an antigravitational property, which is the same thing). To those who proclaimed this notion absurd, its defenders replied that if the electric and magnetic forces can attract and repel, why, in principle, could gravitation not work both ways?

Marat’s doctrine of heat, however, had no such difficulties, thanks to his conviction that he had measured the igneous fluid’s weight. Thus there was no contradiction with his fundamentally Newtonian conception of matter: “Certainly all matter is extended, is divisible, has weight, is impenetrable, etc.”[58]

“From my experiments,” he continued, “it is certain that the igneous fluid weighs much more than the air it displaces.” This fact was obscured by the igneous fluid’s great subtlety. However,

the igneous fluid, far from being the lightest of bodies, as one might think, is very heavy, considering its subtlety.[59]

“In spite of its weight,” Marat said, “it is extremely mobile. And to this astonishing mobility is added a great expansive force,” as can easily be seen by means of the solar microscope.[60]

“This fluid is compressible, but only when it is in action.”[61] A footnote warned against confusing compressibility with elasticity. The concept of elasticity, it should be noted, created difficulties for the internal consistency of Marat’s natural philosophy. In his later book on electricity he seemed to accept the standard Newtonian conception of elastic fluids as composed of particles endowed with a repulsive force. According to that standard conception, an elastic fluid—of which air is the most familiar example—expands without limit and resists compression because its constituent particles mutually repel each other. At the same time, however, Marat argued that no forces of repulsion exist in nature, that all apparent repulsions can in fact be explained in terms of attraction. This would seem to reflect a metaphysical preference on Marat’s part, but he believed he could substantiate it experimentally, as will be seen in his work on electricity, where repulsion presented itself as an unavoidable issue.[62]

Returning to the physical qualities of the igneous fluid: “It equals all others in the solidity of its globules. This solidity is extreme.”[63] Igneous globules can pulverize diamonds; “thus are they as unalterable as the elements themselves.”[64] Although denying fire the status of element, it would seem that Marat has here given his igneous fluid the essential characteristic of an element.

Finally, the igneous fluid has specific affinities. It has more affinity for water than for air, for example, as is shown by boiling water, wherein the matter of heat hesitates to make the leap from water to air. Although Marat nowhere in this work mentioned Joseph Black’s explanation of latent heat, this reference to the igneous fluid’s hesitation reveals that he was at least familiar with the phenomenon, if not with its implications for a theory of heat. A work on latent heat submitted to the Academy of Rouen under the pseudonym “chevalier de Soycourt” has been attributed to Marat, but the evidence of his authorship is not conclusive.[65]

Heat and Light

The igneous fluid’s most pronounced affinity, according to Marat, is for light; i.e., the luminous fluid, which, it will be recalled, Lavoisier placed at the top of his revised list of elements. The affinity between light and heat is suggested by the fact that “light always accompanies a vivid heat.”[66] This had led some physicists to jump to the false conclusion that igneous fluid and the luminous fluid are one and the same. That cannot be true, however, because the converse is not true: “Heat does not always accompany a vivid light.” Glowworms, for example, can be “as luminous as white-hot iron, while remaining at the temperature of their surroundings.”[67] Furthermore, “heat penetrates all bodies, but not all bodies are permeable to light; that can only come from the difference in the fluids that do the permeating.”[68]

These are but two of several plausible reasons Marat cites to refute the claim that the two fluids are identical. The clincher, however, is that in the darkroom the igneous fluid throws a shadow but the luminous fluid does not. “It is therefore proven that light and heat are not the same principle.”[69]

Light is not the only thing, Marat said, that other physicists have confused with the igneous fluid; some have also asserted that heat matter is identical to the electric fluid. There are, he admitted, many points of analogy between the two, but there are also a number of crucial distinctions. Igneous matter “always distributes itself equally in the entire substance of bodies it permeates”; electrical matter “often accumulates on the surface of the same whole.”[70] Heat, moreover, penetrates a large body slowly, while electricity can do it with astonishing speed. Heat escapes from bodies over their entire surfaces, while electricity often escapes from a single point.

Again, their difference “can be seen by simple inspection when they are compared in the darkroom. Both are transparent, but electric fluid is much less so than igneous fluid.”[71] Experiment 53 demonstrated this contention by comparing the shadow-image of a candle with that of the discharge from a Leyden jar.

Experiment 56 demonstrated that an electric discharge pushes and disrupts the shadow of igneous fluid emanating from a candle flame. Electric fluid—insofar as it represents a positive charge—repulses the igneous fluid. This seems to contradict Marat’s later explicit disavowal of repulsive forces in nature, but perhaps he simply neglected to specify the distinction between apparent repulsion (which is an illusion) and real repulsion (which does not exist). Or it may be that he had not yet fully worked out this point in his own mind. In any event, this particular “repulsion” could have been attributed to mere mechanical interaction, with igneous fluid globules simply bouncing off of electric fluid particles.

A number of other distinctions between igneous and electric fluids were given, leading to a general conclusion:

Since the igneous material differs so essentially from the electric material and from the luminous material, the only substances with which it can be confused, it constitutes therefore a separate fluid.[72]

All that has been stated above, Marat said, goes to show “that heat, fire, and flame are produced by a fluid in motion. . . . But what does this movement consist of?”[73]

After considering various means of producing heat, including friction, percussion, and the mixture of certain chemicals, Marat explained heat transmission in classical mechanical terms. Globules of igneous fluid apparently act like tiny spherical balls bumping into each other and rebounding according to the laws of Newtonian mechanics. “However hard one tries,” he said, “one cannot conceive of intestine motion as other than the result of rectilinear movement broken up by some obstacle.”[74]

What is the primary source of this motion? Where does it originate? “If it is asked what is the force that causes these molecules to interpenetrate, I will answer: the principle of attraction.” Marat, unlike some of his contemporaries, was willing to accept attraction as a fundamental fact of nature requiring no further explanation: “This principle seems to me to be a property inherent in matter.”[75]

This is a classical Newtonian stance, of course, in opposition to Cartesian efforts to explain gravitation by means of yet another subtle fluid. The position Marat defended was more characteristic of Newton’s eighteenth-century followers than of Newton himself, who took various positions at various times. His well-known hypotheses non fingo represented a purely agnostic stance. In an exchange of correspondence with the churchman Richard Bentley he warned Bentley against ascribing to him the position that attraction is inherent in matter.[76] But in the Opticks Newton seemed to accept precisely that position, while at the same time speculating on a “universal aether” as a possible cause of gravity.

Marat argued that attributing attraction to a universal fluid simply “transposes the difficulty from the effect to the cause.” In other words, if the motion of igneous globules is explained as resulting from the motion of particles of another fluid, what is the cause of this latter motion? Marat was not the only natural philosopher to recognize that this mode of explanation leads to an infinite regression of causes and effects. The only escape is to call a halt at some point and accept some basic force—some “action at a distance”—as inherent in natural law and thereby beyond further explanation.[77]

One of the greatest difficulties facing heat theorists was to explain how the sun’s heat is transmitted. Some previous thinkers, Marat said, believed that the sun was the only source of the heat that is felt when one is exposed to the sun’s rays. Others, citing the fact that even in the tropics the tops of mountains are bitter cold, claimed that the heat emanates from the earth.

“To settle our ideas on this matter, let’s consult nature.”[78] If the sun’s rays are focused with a strong magnifying lens, all combustibles at its focus catch fire. “On icy mountaintops and on burning plains these experiments always produce the same results. It would seem beyond doubt, then, that the principle of fire is in the sun’s rays.”[79]

Marat says, however, that the solar rays themselves have no heat. They agitate the igneous fluid contained in bodies and set it in motion. “The sun’s rays, then, are the agent, not the principle, of heat.”[80] If this be doubted, experiment 73 should remove all skepticism: The sun’s rays are focused with a burning mirror and directed to the focus of the solar microscope. No evidence of igneous fluid is seen until an object is placed at the focus and begins to heat up.

We conclude that the solar rays are nothing other than the matter of light itself, impelled in a straight line by the action of the sun, and if they produce heat it is only by exciting in bodies the intestine movement of the igneous fluid.[81]

Lamarck’s Critique

As mentioned earlier, Lamarck applauded Marat’s ocular demonstration of the igneous fluid, although he preferred to call this fluid fire, “the name by which it is most generally known.”[82] While generally supportive, Lamarck was by no means uncritical of Marat’s theory. Responding to Marat’s notion of igneous globules colliding and rebounding in accord with Newtonian mechanics, Lamarck said: “As far as I am concerned, this fine piece of reasoning is not as simple to follow as its author thought it was.”[83] How can it be, he asked,

that the motion communicated to these globules by collisions is not diminished proportionately as it is transmitted, as it must be?[84]

Fire does not behave according to this rule. “The smallest quantity of fire,” Lamarck observed,

such as the spark produced by striking a piece of flint, can produce an immense conflagration, burn a fleet of ships, destroy a large city, etc., etc. . . . What, then! Does this small spark, which in fewer than four seconds can produce the complete combustion of the most considerable powder magazine, all by itself communicate motion to the enormous quantity of igneous fluid that in an instant manifests an inconceivable force?[85]

Lamarck concluded that mere motion of the igneous fluid is an insufficient explanation.

Whether Marat had heard this objection directly from Lamarck or not is unknown, but he was certainly familiar with the argument because he gave an explicit answer to it.[86] “Someone will object,” Marat wrote,

that motion diminishes proportionately as it is transmitted. Fire, to the contrary, increases: A spark can become a conflagration.[87]

This is a point well taken, he acknowledged, but does the difficulty go away by considering fire to be a species of matter?

How could a particle of this matter so quickly transform an enormous volume of heterogeneous substances into a substance identical to itself? Isn’t this difficulty insurmountable?[88]

The objection to the motion hypothesis, however, is easier to resolve:

It is true that motion generally diminishes proportionately as it is transmitted, but only when the thing that is moving receives but a single impulsion and when the objects involved are inelastic. In the opposite case, rather than diminishing, motion increases, and that is what happens with the igneous globules.[89]

Marat proposed a chain-reaction scenario, where each globule is involved in multiple collisions and “each new impulsion adds to the preceding ones, resulting in an augmentation of speed.”[90] He compared the spread of fire to the way a storm develops: Powerful winds that at first can create only the lightest ripples on the surface of the sea end up shaking the foundations of the earth. Although Marat was able to solve this problem to his own satisfaction within the framework of his own system, there is every reason to believe that his answer would not have satisfied Lamarck.

The question “What is fire?” was implicitly posed by the title of Marat’s book. The long series of experiments was prologue to an attempt at a direct answer: “Fire consists of a violent intestine motion of igneous globules.”[91] Fire, furthermore, “only occurs in combustibles by virtue of a specific attraction between phlogiston and the igneous fluid.”[92] The form of the flame is determined by air pressure, which always makes it take a vertical direction and the shape of an elongated cone.

In closing, Marat summarized the many manifestations of heat and fire that are accounted for by the intestine motion of igneous fluid.

This principle has been established in an uncontestable manner, all carefully supported by a knowledge of physical phenomena, and the explanation of the phenomena are derived only from the laws of rational mechanics. Thus in comparing this doctrine to that of the so-called elementary fire, it will be found to clarify all the phenomena of which the other cannot make sense.[93]


[1] See The Presocratic Philosophers, edited by G. S. Kirk, J. E. Raven, and M. Schofield, 197–8.

[2] Lacépède, Essai sur l’électricité naturelle et artificielle, vol. I, 18; Lamarck, Recherches sur les causes des principaux faits physiques, vol. I, 316.

[3] W. P. D. Wightman, The Growth of Scientific Ideas, 176–96.

[4] Gaston Bachelard, The Psychoanalysis of Fire, 2–3.

[5] Ibid., 2; emphasis in original.

[6] In the nineteenth century, Michael Faraday considered “What is fire?” an appropriate question to stimulate the interest of young people in science; the topic became a staple of his Juvenile, or “Christmas” lectures. Faraday, Chemical History of a Candle.

[7] Marat, Recherches physiques sur le feu, 17.

[8] Joseph Black, Lectures on the Elements of Chemistry.

[9] Watt, however, insisted that Black’s ideas “certainly did not directly point out the improvements I have made upon the steam engine.” Eric Robinson and Douglas McKie, eds., Partners in Science: Letters of James Watt and Joseph Black, 416.

[10] For example, Adair Crawford, Experiments and Observations on Animal Heat and the Inflammation of Combustible Bodies, and J. H. de Magellan, Collection de différens traités sur des instruments d’astronomie, physique.

[11] Black, Lectures on the Elements of Chemistry. Quoted by Duane Roller in “The Early Development of the Concepts of Temperature and Heat” (Harvard Case Studies in Experimental Science, vol. I, 152.

[12] See especially I. B. Cohen, Franklin and Newton, chapters 6 and 7; and Henry Guerlac, Newton on the Continent.

[13] I. B. Cohen, Franklin and Newton, 143.

[14] Ibid., 145.

[15] Stahl first stated his phlogiston theory in a lengthy commentary on Becher’s Physica subterranea (1663), which Stahl republished in 1703.

[16] Antoine Laurent Lavoisier, Elements of Chemistry, 175.

[17] Lavoisier, “Mémoire sur quelques fluides qu’on peut obtenir dans l’état aériforme, à un degré de chaleur peu supérieur à la température moyenne de la terre,” 334–43.

[18] Marat, Recherches physiques sur le feu, 1.

[19] Ibid., 3.

[20] Ibid., 4.

[21] Ibid., 8; emphasis in original.

[22] Ibid., 8. Who was the “famous author” Marat referred to? Gillispie (Science and Polity, 311) believes it was “certainly” Lavoisier; Mandelbaum (Rebel as Savant, 160) assumes it was Buffon. It could have been either.

[23] Marat, Recherches physiques sur le feu, 8–11.

[24] Ibid., 12–14.

[25] Ibid., 14.

[26] Ibid., 17.

[27] Ibid.

[28] Jean Baptiste Lamarck, Réfutation de la théorie pneumatique; Étienne Claude Marivetz and Louis Jacques Goussier, Physique du monde, Jean Louis Carra, Dissertation élémentaire sur la nature de la lumière, du feu at de l’électricité; Jean Baptiste Louis Romé de l’Isle, L’Action du feu central; Baltazar Georges Sage, Institutions de physique; Jan Heindrich Van Swinden, Recueil de mémoires sur l’analogie de l’électricité et du magnétisme; Johann Wolfgang von Goethe, Zur Farbenlehre; Bernard Germain de Lacépède, Essai sur l’électricité naturelle et artificielle.

[29] Black, Lectures on the Elements of Chemistry, 31.

[30] Marat, Recherches physiques sur le feu, 18.

[31] Marivetz, Physique du monde, “Supplément,” Plate XV, illustrations 115 through 120; the notes to these illustrations are on pages 10 through 15.

[32] Marat, Recherches physiques sur le feu, 18.

[33] Ibid., 18–19.

[34] Ibid., 19.

[35] Ibid., 19–20.

[36] Joseph Priestley, History and Present State of Discoveries Relating to Vision, Light, and Colours, 741–2. Priestley gives the source as “Phil. Trans. ab. Vol. 3, p. 127.”

[37] Ibid., 801. Priestley cites Brander’s pamphlet as Kurtze Beschribung, &c: eines Sonnen Microscops, Ausburg, 1769. Another account of the same device “may also be seen in Martin’s Gentlemen’s Philosophy, Vol. 2, p. 249.”

[38] Letter to Roume de Saint-Laurent (20 November 1783), Correspondance de Marat, 39.

[39] The codicil to Marat’s will is a signed, undated, four-page document that is part of the Olin Library’s collection of manuscripts (Cornell University). “Bettancourt” was perhaps the Spanish engineer Agustin Bethencourt y Molina (1758–1824), alias Betancourt, but he was not elected a corresponding member of the Academy until 1809. See also Correspondance de Marat, 96–7, where Marat seems to refer to the same man as “M. de Bellancourt.”

[40] Yves Marie Desmarets, comte de Maillebois (1715–91), was an honorary member of the Academy of Sciences from 1749.

[41] Académie des Sciences, “Rapport de la commission nommée par l’Académie des Sciences, pour l’examen des découvertes de Marat” (Registres de l’Académie des Sciences [1779], 97–100).

[42] Ibid.

[43] Marat, Découvertes de M. Marat sur le feu, l’électricité, et la lumière.

[44] Quoted by Hahn, Anatomy of a Scientific Institution, 158. The source is given as: Bibliothèque de l’Institute, MS. 876, fols. 95–6.

[45] Report of Dr. Benjamin Franklin and other Commissioners Charged by the King of France, with the Examination of the Animal Magnetism as Now Practised at Paris (London, 1785), 31. The report was signed by Lavoisier and Bailly, among others.

[46] Lamarck, Recherches sur les causes des principaux faits physiques, vol. I, 343–68. This critique was not based on Marat’s Recherches physiques sur le feu but on his earlier synopsis, Découvertes de M. Marat sur le feu, l’électricité, et la lumière.

[47] Ibid., vol. I, 344.

[48] Ibid., vol. I, 352.

[49] Ibid., vol. I, 367–8.

[50] Lacépède, Essai sur l’électricité naturelle et artificielle, vol. I, 39.

[51] Ibid., 40.

[52] J. L. Carra, Nouveaux Principes de physique, vol. IV, 8.

[53] Marat, Recherches physiques sur le feu, 21.

[54] Ibid., 23.

[55] Ibid., 39.

[56] Ibid., 27.

[57] Ibid., 31.

[58] Ibid., 42.

[59] Ibid., 34.

[60] Ibid., 34–5.

[61] Ibid., 37.

[62] See chapter 5, “The Illusion of Repulsion,” on this website.

[63] Marat, Recherches physiques sur le feu, 39.

[64] Ibid., 39.

[65] Claudius Roux, in “Documents bio-biblio-iconographiques sur Marat” (Albums du Crocodile, Jan.–fév. 1954), hypothesized that Marat authored the “Soycourt Mémoire.” Subsequent authors have often accepted the attribution as fact, but it remains unproven.

[66] Marat, Recherches physiques sur le feu, 43.

[67] Ibid., 43.

[68] Ibid., 43.

[69] Ibid., 46.

[70] Ibid., 48–9.

[71] Ibid., 53.

[72] Ibid., 56.

[73] Ibid., 56–7.

[74] Ibid., 59.

[75] Ibid., 60.

[76] Letter from Newton to Bentley, 17 January 1693, in Richard Bentley, Works, vol. III, 210.

[77] Some modern cosmologists have postulated an exchange of “gravitons” between other elementary particles as a means of pushing the explanation of gravity to a deeper level. Should the existence of gravitons be confirmed, of course, the question of the cause of their motion would be posed. And so it goes.

[78] Marat, Recherches physiques sur le feu, 72.

[79] Ibid., 72–3.

[80] Ibid., 75; emphasis added.

[81] Ibid., 78.

[82] Lamarck, Recherches sur les causes des principaux faits physiques, vol. I, 349.

[83] Ibid., 361.

[84] Ibid., 362.

[85] Ibid., 362–3.

[86] These writings do not constitute a direct exchange between Marat and Lamarck. The chronology is as follows: Marat’s original publication, Découvertes sur le feu, l’électricité, et la lumière, which Lamarck was discussing, appeared in 1779. Lamarck’s discussion was published in 1794 after Marat’s death. But Marat had responded to the identical objection in Recherches physiques sur le feu, published in 1780.

[87] Marat, Recherches physiques sur le feu, 84.

[88] Ibid., 85.

[89] Ibid.

[90] Ibid.

[91] Ibid., 182.

[92] Ibid., 183.

[93] Ibid., 193–4.