Writing in 1855, of the period now known as the Enlightenment, the Scottish Whig Henry Brougham commented that, "the science of chemistry [was] almost entirely … the growth of this remarkable era." One hundred years later, the British historian Herbert Butterfield, renowned for his critique of the Whig interpretation of history, issued a much more negative judgment of the chemistry of the Enlightenment. In his Origins of Modern Science (1949), Butterfield notoriously relegated eighteenth-century chemistry to a kind of limbo, awaiting its "postponed scientific revolution," which arrived only in the last two decades of the century with the work of Antoine-Laurent Lavoisier (1743-94). Enlightenment chemistry had been "immature," hindered by philosophical confusions and the absence of an adequate intellectual framework. The difference of opinion between Brougham and Butterfield has an intriguing connection with their divergent political outlooks. While the Whig writer saw a lengthy period of gradual progress, culminating in Lavoisier’s individual accomplishments, the anti-Whig historian saw the French chemist as the first with true insight into the fundamental ideas of the science, a beacon in an otherwise dark landscape of confusion and error.{1}

The perspectives of whiggism and antiwhiggism have continued to dominate much of the historical writing on the sciences of the eighteenth century, not least chemistry. Whiggish historians have looked to catalog specific and permanent factual discoveries—steadily accumulating positive knowledge—such as findings of new gases, mineral species, and salts. Butterfield’s antiwhiggism reflected the approach of Alexandre Koyré, and before him the tradition of philosophical history derived from Immanuel Kant, which searched for organizing intellectual schemes, worldviews, Weltanschauungen, or paradigms. These have rarely been found before Lavoisier, whose accomplishment has usually been seen as the provision of a previously absent theoretical framework for chemistry. In the twentieth century, various formulations have been given of the essence of Lavoisier’s theoretical achievement, emphasizing different aspects of his work but frequently reiterating his self-representation as one who had broken decisively with previous chemical tradition. Some have identified the key step as a new theory of combustion, in which atmospheric oxygen was accorded its true role, and the fictional principle of combustion, "phlogiston," was discarded. Alternatively, Lavoisier has been hailed for his novel understanding of chemical composition and his pragmatic definition of an element as the product of the best available methods of analysis. Or again, it is the recognition of the gaseous state of matter that has been taken to be the decisive innovation—the realization that substances could be made into gases by the addition of heat, without changing their chemical nature. Or, finally, it has been claimed that the crucial development was the insistence on the conservation of matter as it undergoes chemical change, a law discovered by use of the precision balance and represented formally by chemical equations.

All of these readings of Lavoisier’s accomplishment stress its theoretical character as the laying of an intellectual foundation for subsequent chemical science. Although recent historians have been careful to distance themselves from Butterfield’s pronouncement, assertions that Lavoisier’s work represented the revolutionary first steps in the constitution of chemistry as a true "science" are sometimes still made. The "origin myth" created by Lavoisier himself and his immediate disciples has long retained its hold, with the consequence that the work of his predecessors is relegated to the shadows.{2} The antiwhiggish outlook that identifies Lavoisier’s achievement as a conceptual revolution has made it difficult to discern the lineaments of the chemistry that came before him.

In the essay that follows, I try to avoid the extremes of whiggism and antiwhiggism. I shall not present the chemistry of the Enlightenment simply as a process of accumulating factual data. We shall see that chemical discoveries were not always neutral facts, but could be tokens in disputed theories, crucial to some interpretive schemes and ignored by others. But, we shall also see that chemistry existed as a discipline well before Lavoisier gave it the theoretical framework familiar to modern eyes. Chemistry in the eighteenth century was a body of practical techniques, instruments, and materials, organized in written texts and oral lectures. Students were taught ways of assimilating new information and were given a clear sense of the history of the subject. Chemistry, however, was a discipline without entirely rigid boundaries; it engaged in productive exchanges of concepts and experimental phenomena with neighboring sciences, especially natural philosophy and natural history. Chemists applied various theoretical schemes to the interpretation of such phenomena, sometimes disagreeing about whether they were properly the business of chemistry at all. This diversity assumed critical importance as novel experimental discoveries were produced and their implications for chemical theory explored.

I shall therefore begin with an exploration of the identity of chemistry in the eighteenth century, mentioning how the subject was organized and taught, the social locations in which it was undertaken, and the material, instrumental, and discursive resources that were used to pursue it. Against this background, we can assess the importance of the philosophy of matter, whether mechanistic (invoking specific corpuscular shapes and motions) or Newtonian (invoking specific forces of attraction between particles). These theories of matter can be compared with other available means of conceptually organizing chemical information, such as the schemes for classifying the composition of salts and the popular tables of "affinities" between substances. A survey of chemists’ ways of organizing the properties of the substances they encountered prepares us for the arrival on the scene of novel and anomalous chemical entities around the middle of the eighteenth century. Arguments about how to understand the new gases and "imponderables" (heat, light, and phlogiston) will be seen to have led directly to Lavoisier’s self-proclaimed "revolution." His achievement will thus appear, not as the goal of a teleological progress toward modern science, but as a brilliantly creative response to the need to fulfill chemists’ task of organizing information about substances, their properties and behavior, in the face of unsettling new phenomena. The new system was communicated by Lavoisier and his allies by reworking the traditions of teaching and laboratory practice bequeathed to them by their chemical predecessors. New instruments, new experimental methods, new textbooks, and a new language were the tools of the new chemistry, which, notwithstanding its revolutionary rhetoric, preserved more than a few signs of its historical inheritance.

Lavoisier’s revolution unfolded against the backdrop of a lengthy ancien régime of chemical practice. The beginnings of chemistry as an organized discipline, in textbooks and lectures, have been traced to the end of the sixteenth century. In terms of the overall organization of the contents of the discipline, there was a substantial degree of continuity from this period to the late-eighteenth century. In its instrumental resources, also, chemistry, it has been said, experienced a longue durée of relative stability. Basic laboratory equipment—glassware, crucibles, and furnaces—remained largely unchanged for at least a hundred and fifty years before the middle of the eighteenth century.{3} The advent of novel experimental phenomena concerning heat and gases, and moves toward more precise measurements of chemical quantities, introduced significant changes in laboratory practice in the late eighteenth century. Only then did the ancien régime of chemical practice begin to break down.

Owen Hannaway has persuasively argued that the modern tradition of the chemical textbook was launched by the Alchemia (Frankfurt, 1597) of Andreas Libavius (c.1540-1616). Libavius, a Lutheran schoolmaster, took the ancient textual form of a collection of recipes for chemical preparations and systematically organized them under the headings of the operations involved. He adopted the methods of the Humanist pedagogues, defining the subject matter of the discipline, dividing the definition and defining each part in turn, and so on. Presenting the whole subject in a series of dichotomies, represented in the form of branching tree diagrams, Libavius asserted the autonomy of chemistry as a discipline and its primacy over various practical arts. This kind of pedagogical exposition of chemistry was a way of striking against what Libavius saw as the obscurantism and impious mysticism of the late sixteenth-century followers of the Swiss alchemist and physician Paracelsus.{4}

A proclaimed adherence to systematic method and a declared abhorrence for what was seen as the willful obscurity of alchemical writings remained prevalent aspects of chemical textbooks in the eighteenth century. Other formal features of these texts were also derived in principle from Libavius, albeit subjected to some degree of reorganization as time went on. It was routine to introduce details of preparative procedures with a discussion of the apparatus of the typical laboratory. Chemical operations, such as distillation, sublimation, filtration, and dissolution, would be listed and categorized. The practice of beginning with a definition of chemistry, which would then be unpacked in the course of the exposition, persisted, for example, in the Elementa Chemiae (1732) of Herman Boerhaave (1668-1738). One feature that some eighteenth-century teachers added to the traditional outline of the chemical text was an introductory review of the history of the subject. Boerhaave, at Leiden, and William Cullen (1710-90), at Glasgow and Edinburgh, were among those who did this. The historical introduction, which owed something to other Enlightenment exercises in conjectural history, consolidated the message of the integrity and continuity of the discipline.{5}

In pursuing their careers in universities, Boerhaave and Cullen took advantage of one of the important institutional niches gained by chemistry in the course of its ancien régime. German universities took the lead in establishing positions in chemistry in the early seventeenth century. The academic profile of the subject remained dependent upon the demand for medical education, however. Teachers were frequently unsalaried or underpaid, and relied on collecting fees from medical students, physicians, apothecaries, and anyone else who was interested. While chemistry flourished at Leiden and Edinburgh, due to the highly successful medical schools at those universities, there were many decades when it languished at Oxford and Cambridge.

Universities were not by any means the only places where chemists found employment for their skills in the eighteenth century. In Germany and Scandinavia, there were openings at mining and administrative academies. In Paris, the Académie Royale des Sciences nurtured a distinguished tradition of chemical research by its salaried academicians throughout the century. Lecturers were also appointed at the Jardin du Roi and elsewhere in the French capital, to give courses to members of the public: Diderot and Rousseau were among those who attended lectures at the Jardin. In the second half of the century, the foundation of provincial academies and local learned societies in many European countries provided further opportunities for chemists to lecture and pursue their research. In England, the public scientific lecturer (who might also be an author) became a feature of the expanding commercial market for education and leisure. Peter Shaw (1694-1763) was the first chemist to explore this kind of occupation in the early 1730s; by the 1770s he had several imitators in London and the provinces.{6}

It was in these circumstances of relations with a large and heterogeneous audience in various institutional settings that chemistry acquired the profile of an Enlightenment science. Its perceived utility was the key to this, but utility was understood as more extensive and solid the more securely founded were the scientific or "philosophical" credentials of the discipline. Enlightenment chemists thus further developed Libavius’s claim that chemistry was an autonomous science by virtue of organizing and providing foundations for many of the practical arts. The relationship with medicine was the closest and most venerable, a long-lasting legacy of Paracelsus and his followers, who had pioneered the use of chemically prepared drugs. By the eighteenth century, chemical medicines had an accepted place in the pharmacopoeia, even if their apothecary advocates were still regarded with suspicion by some physicians. For this reason, an up-to-date medical education would include lectures on chemistry. In Germany and Scandinavia, the links between chemistry and the arts of mineralogy and metallurgy were also traceable back to the Renaissance, and were substantially developed during the eighteenth century as new mineral resources were exploited. Entirely new areas of chemical technology were also opened up. In Scotland, Cullen and other chemists participated in local societies devoted to national economic improvement, working on applications of chemistry to dyeing and bleaching, the manufacture of salts, and the use of agricultural fertilizers. Hopes for further progress in these arts, chemists declared, should be invested in the science that was fundamental to them all.{7}

By forging social connections with practitioners and patrons of the arts, and by the concrete work of experimental research, eighteenth-century chemists positioned their science at a pivot-point in the relationship between natural knowledge and power over the material world, which was emerging as a characteristic of the age. It is easier for historians to read the traces of this enterprise in what eighteenth-century chemists said than in what they did. Their writings have survived, but the traces of their actions have been obscured by time. Very little is known about the oral traditions and those of tacit knowledge that sustained the discipline in the course of its longue durée. Textbooks rarely seem to have sufficed to train a chemist, who also had to see and feel to learn—and indeed to smell, taste, and hear, since cultivation of all the senses was understood to be a vital part of the chemist’s formation. The chemist, according to one writer, needed "his thermometer at the tips of his fingers and his clock in his head."{8} Embodied skills like this have left few traces, even fewer than the remains of the material apparatus that displaced some of them in the course of the eighteenth century. By the end of the century, real thermometers and clocks were regular items of laboratory equipment, along with other devices for measurement: barometers, eudiometers, calorimeters, gasometers, and, most important, balances. The end of chemistry’s ancien régime was marked by a shift from reliance on the senses and on informal estimates of quantities to a culture of increasingly precise measurement with a range of refined instrumentation.{9} The subject was reconfigured by disciplinary means that extended well beyond revisions in the textbooks. "Discipline" now acquired a meaning that embraced the training of chemists in the manual skills demanded by the new equipment of the laboratory. Academic chemistry had begun as an outgrowth of Humanist pedagogy; by the beginning of the nineteenth century it was being inculcated through a regimen of laboratory training, like other sciences in the era of the Industrial Revolution.

Butterfield claimed that chemistry had waited until the end of the eighteenth century to achieve a coherent framework of fundamental ideas. Yet an earlier historian, Hélène Metzger, had already demonstrated the importance of philosophies of matter in chemistry from the early seventeenth century. Metzger’s pioneering studies, especially Les doctrines chimiques en France (1923) and Newton, Stahl, Boerhaave et la doctrine chimique (1930), reconstructed the development of the matter theories associated with the mechanical philosophy, with the doctrines of Newton, and with the works of the Halle professor Georg Ernst Stahl (1660-1734). Metzger’s studies continue to command respect, and her lead in the exploration of philosophical theories of matter has been followed by subsequent scholars, but historians have also reconsidered the question of the relationship between matter theory and other domains of chemical thought and practice. The historiography to which Metzger was committed tended to prejudge this issue by assuming the primacy of fundamental philosophical ideas in the development of the sciences, whereas recent research has disclosed a more problematic and ambivalent influence.{10}

The mechanical philosophy of the seventeenth century drew upon the ancient concept of atoms to make several contributions to the theory of chemistry, but these had little impact on practice and remained relatively marginal to the tradition of chemical writing. A mechanistic ontology was applied to account for properties of acids in the Cours de chymie (1675) of Nicholas Lemery (1645-1715). Lemery proposed that the acrid taste and corrosive action of acids could be explained by their sharp-pointed particles, capable of penetrating into the pores of other bodies. But these speculations occupied only a small portion of the "reasonings" he added to descriptions of chemical operations and they did not substantially affect the largely traditional contents of his text. Similarly, Robert Boyle’s attempts to rationalize chemical operations in terms of the shapes and textures of particles of matter were advanced cautiously in the course of some of his experimental essays but were little attended to by other chemical writers. Boyle earned more recognition for his elegant dialogue The Sceptical Chymist (1661), where he argued against the existence of elements or principles that would retain a consistent chemical identity through analysis by fire. These arguments were reiterated by others, including Lemery, and contributed to diminishing the authority of the traditionally identified chemical elements. A contemporary judged that Boyle had "not so much laid a new Foundation of Chemistry, as he has thrown down the Old."{11}

In the early eighteenth century, the mechanical philosophy was succeeded as a source of chemical matter theory by the ideas of Newton. Again, an ambitious program for the reconstruction of chemistry was announced, but it failed to match the pragmatic aims of most chemists. The influence of Newton’s natural philosophy on chemistry was more subtle and indirect than the early "Newtonians" hoped for. The first chemical writers to identify themselves with the Newtonian philosophy were John Keill (1671-1721) and John Freind (1675-1728), both working in Oxford in the first decade of the century. Keill’s 1708 paper in the Philosophical Transactions of the Royal Society followed closely upon Newton’s own remarks about chemical phenomena in the twenty-third "Query" of the Latin translation of Opticks in 1706 (subsequently revised as Query 31 of the 1717 edition of the text). There, Newton had applied to a variety of chemical phenomena the notion of microscopic forces, which would be analogous to the gravitational force but act on a much smaller scale. The notion was especially relevant to displacement reactions, for example when copper was added to a solution of a silver salt in acid, and silver precipitated. Such a reaction was to be explained by saying that the specific force of attraction between copper and the acid is stronger than that between silver and the acid, so that the silver is displaced from combination. Newton noted that the metals could be arranged in a consistent order of their relative attraction for the acid in question, and that similar orders could be constructed for other kinds of reactions. Though the phenomenon was not new to many chemists, Newton’s discussion made it plausible that it could be explained in terms of a microscopic analogue of the force of gravity.{12}

This was the point developed by Keill and Freind. Keill’s paper presented a series of putative axioms for explaining chemical phenomena on the grounds of specific attractions, which would in turn be explained by such factors as the relative density, shapes, and textures of the particles of bodies. Freind’s text, Praelectiones Chymicae (1709), derived from lectures delivered in the basement laboratory of the Ashmolean Museum in Oxford, reproduced these axioms and went on to apply them to generate explanations for various chemical operations. His exposition failed, however, to encompass the familiar chemical attributes of bodies. Attractive powers seemed to bear no relation to the properties that chemists had identified with particular substances. For this reason, Freind’s text was occasionally cited by natural philosophers but rarely by chemists in the following decades. A few writers continued to link the relative attractions or "affinities" among chemical substances to the Newtonian concept of forces, but, as we shall see, the ordering of chemical affinities had an important function in eighteenth-century chemistry quite independently of Newton’s ideas.{13}

As well as discussing attractive forces in his influential Query, Newton also mentioned the possibility of forces of repulsion. He noted how "fermentation"—by which he meant any process that released "air" from solids or liquids—"seems unintelligible, by feigning the particles of Air to be springy and ramous, or rolled up like hoops, or by any other means than by a repulsive power." These remarks assumed considerable importance in the origins of pneumatic chemistry, in which the processes of release of aerial fluids, and the reverse processes of "fixing" air in solids or liquids, were central. Stephen Hales (1677-1761), vicar of Teddington in Middlesex, investigated these phenomena in his Vegetable Staticks (1727), claiming that they demonstrated the existence of repulsive forces between air particles. Hales provided the conceptual vocabulary in which interactions between aerial fluids and solid or liquid substances could be explicated: The repulsive forces responsible for the expansion of air could be overcome by sufficiently strong attraction by particles of more ponderous matter, in which case the air would be "fixed," he claimed. Hales also developed the instrumentation for studying these processes. He used the "pneumatic trough" to collect and measure samples of air given off in chemical reactions, by leading it into vessels filled with water and held upside-down over a water-filled basin. In the light of the subsequent differentiation of aerial fluids into chemically distinct gases, Hales has sometimes been criticized for failing to discriminate between the airs he manipulated; he continued to assume that airy fluids were essentially one kind of entity, albeit sometimes contaminated by mixtures of other substances. He was, in fact, much less interested in the chemical differentiation of airs than in their role in a providential economy of nature sustained by a balance of attractive and repulsive forces. It was in these terms, for example, that he understood the reduction in the volume of air surrounding a burning body as due to the release of sulfurous or acidic particles, which were strongly attractive and so reduced the elasticity of the air they contaminated. In ascribing a crucial providential role to air—"this noble and important element, endued with a most active principle [by] the all-wise Providence of the great Author of nature"—Hales was using it to address the problem of divine action in the world, a theme of central importance in eighteenth-century natural philosophy.{14}

In what became Query 31 of the Opticks, Newton pointed out that chemical substances could be arranged consistently in order of the strength of their attraction for a certain other substance. His idea seemed to have come to fruition in 1718, when Etienne François Geoffroy (1672-1731) presented to the Paris Academy a "Table of the different relationships observed between different substances" [Figure1]. The sixteen columns of the table were headed with symbols for the different acids, alkalis, and metals. Below each were arranged the symbols for those substances that could form combinations with them, in descending order of strength of the combination. Geoffroy carefully avoided use of the terms "attraction," with its specifically Newtonian connotations, and "affinity," which could invoke alchemical notions of occult sympathies. By referring simply to "rapports" (relationships), he tried to maintain neutrality on the theoretical issues that divided Newtonians from the Cartesians who still prevailed in French science—a precaution that did not, in fact, prevent suspicions that Geoffroy was covertly representing the Newtonian outlook.{15}

Geoffroy’s was the first of a large number of such tabulations that appeared in the course of the eighteenth century. Affinity tables, as they came to be called, became larger and more elaborate, summarizing more information about reactions and combinations. In 1775, the Swedish chemist Torbern Bergman (1735-84) presented a table in two parts (for wet and dry reactions), with thirty-four columns and up to twenty-seven substances listed in each column. Some historians have read the prevalence of these tables as an indicator of the influence of the Newtonian philosophy of matter on chemical thinking in the eighteenth century. Accounts of a "Newtonian tradition" of chemistry have referred to the tables as an important thread of that tradition. Others, however, have argued that affinity tables should be understood in relation to their uses in chemists’ research and teaching, reflecting not a specifically Newtonian tradition but intrinsically chemical ways of thinking about such issues as combination and reactions. The latter reading seems more in line with the attitude recommended by Bernard de Fontenelle, perpetual secretary of the Academy, when Geoffroy’s table was first published. Fontenelle wrote: "It is here that sympathies and attractions would become appropriate, if there were such things. However, leaving as unknown that which is unknown, and holding to certain facts, all chemical experiments prove that a particular body has more disposition to unite with one body than with another, and that this disposition has different degrees."{16} Talk of sympathies and attractions was beside the point, Fontenelle suggested. The table should be valued as a means of ordering information about chemical operations, hence easing learning and directing research.

Fontenelle’s remark directs attention away from the possible connections between affinities and the theories of natural philosophy and toward chemical operations themselves and how they were conceived. Ursula Klein has demonstrated that the operations recorded in Geoffroy’s table are all to be found in books of metallurgical and pharmaceutical chemistry in the seventeenth century.{17} Particularly important were the operations by which salts were formed. Frederic L. Holmes has documented the importance of research on analysis and synthesis of the so-called "middle salts"—those formed by combining an acid and an alkali—in the Academy in the two decades before the table appeared. Particularly important contributions to this research were made by Geoffroy himself and by Wilhelm Homberg (1652-1715).{18} In 1702, Homberg had distinguished three classes of middle salts: those formed by acids in combination with, respectively, a fixed alkali, an alkaline earth, and a metal; in addition, he recorded a class of ammoniac salts. Although he did not explicitly reject either the ancient notion of elements or the ontology of corpuscles, Homberg’s understanding of middle salts utilized a more pragmatic and operational concept of chemical compounds and the processes by which they were composed and decomposed. Geoffroy’s table reflected the notion that chemistry was concerned with the combination and separation of chemically identifiable entities by operations that were always in principle reversible. As he put it in a paper of 1704: "What completely assures us that we have succeeded in investigating the composition of bodies is, having reduced mixta into the simplest substances that chemistry can provide, we can recompose them by reuniting these same substances."{19}

Eighteenth-century affinity tables have been discussed by Klein and Holmes as part of a largely autonomous chemical practice, which was substantially independent of the matter theory handed down by natural philosophy. Chemists worked with ideas about composition and chemical processes that were adapted to the materials they dealt with and the operations they carried out. Chemical operations were conceived as essentially reversible combinations and separations of parts that were ascribed stable identities in terms of their chemical properties. Other historians have linked this conceptual outlook specifically with the influence of Stahl. The German chemist provided his contemporaries with a popular vocabulary for labeling chemical substances and changes, which distinguished them from bodies and operations considered from a purely physical point of view. He defined chemistry as concerned specifically with bodies considered as "mixts" or compounds, that is to say from the point of view of their chemical constitution rather than their physical "aggregation." Mechanics was concerned with taking bodies apart into their homogeneous physical components, whereas chemistry, in Stahl’s view, was concerned with a more intimate kind of composition, in which bodies were found to be constituted of heterogeneous substances that did not share the properties of the compound in which they occurred. Stahl’s ontology designated a way of studying the objects in the world that ascribed to chemists distinctive and independent skills of analysis and synthesis.{20}

Stahl’s support for the autonomy of chemistry was one reason for the popularity of his doctrines among European chemists in the mid-eighteenth century. In France, his ideas were introduced in lectures given at the Jardin du Roi between 1742 and 1768 by Guillaume-François Rouelle (1703-70). Rouelle’s influential lectures reiterated the Stahlian insistence on a realm of specifically chemical entities and processes, the domain of an autonomous discipline of chemistry. The same line was taken in the article "Chymie," published by Gabriel François Venel (1723-75) in the Encyclopédie of Diderot and d’Alembert in 1753. Venel urged his readers to reject philosophical hypotheses about the nature of chemical composition and not to assume that the destiny of chemistry lay in its reduction to the principles of natural philosophy. Chemists could take pride, he maintained, in their ability to comprehend chemical processes without resorting to uncertain physical hypotheses. On these grounds, they could assert the authority of their discipline over the practices of the chemical arts.{21}

Along with his often-echoed assertions of the independence of chemistry from physical theory, Stahl also advocated somewhat more debatable doctrines. He identified three different earthy principles in the composition of mixts: a vitrifiable earth, a mercurial or metallic one, and the sulfurous principle or "phlogiston." The last was the principle of flammability, present in all combustible matter, and released as light and heat by burning bodies or by metals in the course of corrosion or calcination. Phlogiston was given a particularly important role in the work of French chemists following a crucial reinterpretation of the doctrine by Rouelle. In his lectures, Rouelle identified the principle not with the earths but with the ancient element "fire," which he characterized as a physical agent (or "instrument") and a chemical element. Fire, in other words, was both a cause of chemical changes and a participant in them, capable of entering into the composition of substances. Rouelle ascribed the same duality of role to the other classical elements, most importantly including air, which, as Hales had shown, was capable of entering into chemical combination. Air could exist free in the atmosphere or fixed in chemical combination, for example in aerated mineral waters. Fire, similarly, could act as a physical instrument, rarefying bodies, or it could enter into chemical combination as phlogiston in metals or combustible matter.{22}

This kind of account was found appealing by many French chemists. It was adopted from Rouelle’s lectures by Venel, for example, in his articles in the Encyclopédie, and by Pierre Joseph Macquer (1718-84), author of the widely read Dictionnaire de chymie (1766). When Lavoisier, who had attended Rouelle’s lectures himself, launched his assault on the phlogiston theory in the 1780s, it was this prevailing version that he attacked. It owed its appeal to the broad sweep of its explanatory capabilities and to its resonance with chemists’ claims to theoretical autonomy. The realization that metallic calcination was the same process as combustion was a dramatic accomplishment, recognized even by whiggish historians as a positive achievement of the phlogiston theory, and it consolidated the claims of chemists to authority over metallurgical practices. As a distinctively chemical entity, phlogiston was a strategic tool in the campaign to establish the boundaries of the discipline and its credentials as an Enlightenment science.

Notwithstanding the efforts of chemists to secure the credentials of their discipline, chemistry did not in fact operate independently from other sciences in the eighteenth century. The study of gases connected the interests of chemists with those of natural philosophers and medical men exploring such issues as respiration and the healthiness of different kinds of air. Phlogiston was given a remarkably broad application in many domains of Enlightenment science. As one of a group of "imponderable" (weightless) fluids, it was invoked in connection with phenomena as diverse as static electricity, nervous impulses, and terrestrial heat. Thus, chemists found themselves sharing a crucial explanatory concept with many other scientific practitioners; far from working in a cultural vacuum, they found they inhabited a climate of ideas dense with distinctive gases and imponderables like phlogiston.

Following the lead of Hales, pneumatic chemistry flourished particularly in Britain. Two fairly distinct paths were explored. The first concerned the role of heat in physical transitions between solid, liquid, and gaseous states. In Scotland, beginning in the 1740s, an attempt was made to connect these phenomena with chemical reactions understood in terms of affinities. In this tradition, phlogiston appeared as a weightless agent of physical and chemical change, rather than as a participant in chemical composition; in fact, it assumed some of the functions of the "ether," a subtle and imponderable fluid freighted with the weight of Newton’s authority by virtue of its mention in the Queries of the Opticks. The second line of research, largely pursued by English natural philosophers, considered the chemical characteristics of different gases, which were initially still regarded as different species of air. In the hands of Joseph Priestley (1733-1804), a number of new gases were produced and distinguished by the degree to which they appeared to contain phlogiston. In the early 1780s, Priestley and others went so far as to identify "inflammable air" with pure phlogiston. For them, phlogiston was not a subtle fluid but a regular factor in chemical composition.

It was Cullen who initiated Scottish research on heat in relation to physical and chemical change. In doing so, he made use of an influential doctrine advanced by Boerhaave, who had taught his students at Leiden that the four ancient elements should be regarded, not as components of matter responsible for its properties, but as "instruments" of all kinds of physical and chemical change. Earth was the "matrix" of certain transformations, water the solvent that permitted others to occur, air was the medium of combustion and respiration, and fire the great instrument of activity in the cosmos.{23} Fire, for Boerhaave, was an imponderable material fluid, capable of passing into or out of normal weighty matter; it was the prime agent of chemical change but it did not itself participate in chemical combination. Cullen identified Boerhaave’s fire with Newton’s ether, characterized as imponderable, expandable, subtle, and repelled by normal matter—properties routinely ascribed to the ether since Newton’s brief description had been elaborated upon in the early 1740s.{24} All substances, Cullen suggested, were composed of normal attractive matter permeated by a cloud of repulsive ether. The relative densities of matter and ether would determine the state of aggregation of the body: If attractions exceeded repulsions, the body would be solid; if they were approximately in balance, it would be liquid; if repulsions overwhelmed attractions, the body would become a vapor. A change of physical state of a body could thus be related to the addition or subtraction of ether, that is, heat or fire.

Cullen extended this perspective to try to relate exchanges of heat to chemical transformations. He explored reactions that involved the production or absorption of heat: the addition of water to dehydrated salts, for example, which releases heat, or the evaporation of volatile liquids, which absorbs it. He was obliged to admit failure, however, to explain chemical reactions in all their qualitative variation. While states of aggregation could be explained in terms of a balance of etherial fluid and ponderable matter, it was not possible to account for the differing strengths of attractions between chemical substances on the same basis. The theory had limited success in the realm of chemical change, but it enjoyed its most triumphant application in the discoveries of specific heat capacities and latent heats by Cullen’s pupil Joseph Black (1728-99). Black showed that different bodies had different capacities to absorb heat to produce a measured change in temperature—a finding that was clearly rooted in the conception of heat as a material fluid absorbed to a greater or lesser degree by normal matter. Similarly, Black’s revelation of the latent heats of fusion and evaporation, which were required to change the state of a body but not revealed by the thermometer, reflected the influence of Cullen’s framing of the problem of heat and aggregation.{25} The significance of this work for chemistry was that it pointed toward an understanding of the gaseous state: A gas came to be seen not as a variety of air but as a state that all bodies could attain, given sufficient heat. Lavoisier, who knew of the work of Black and his Scottish colleague William Irvine, was to turn this insight to telling effect in his research on heat and gases, the first avenue to be explored in his revolutionary remaking of chemical theory.

Black also played a significant part in the second line of development of pneumatic chemistry, investigating the chemical identities of different airs or gases. His Experiments upon Magnesia Alba (1756) scrutinized the air given off by heating "magnesia alba" (magnesium carbonate). Using Hales’s terms, Black labeled this vapor "fixed air" and showed that its release diminished the weight of the salt. Two other findings were crucially important. First, the fixed air proved to have different properties from normal atmospheric air: It turned lime-water milky, and did not support combustion or respiration. Second, after being deprived of its fixed air, the magnesia alba was also found to have lost its alkalinity; apparently the air contributed to its chemical properties when present in the compound. These observations, established by an impressive series of careful experiments, indicated how gases were to assume the status of chemical entities. Black had shown, at least in the case of fixed air, that they could be characterized by tests of their chemical identity and their effects on the properties of the bodies in which they were compounded could be ascertained.{26}

Further exploration of the chemical identities of gases was largely the work of English researchers. In 1766, Henry Cavendish (1731-1810) distinguished Black’s fixed air from the "inflammable air" given off by metals dissolving in acids. Priestley followed by systematically producing and distinguishing numerous new airs, of which he published detailed descriptions in his Experiments and Observations on Different Kinds of Air (3 vols., 1774-7). With quite modest equipment, skillfully used, Priestley was able to identify (among others) "fixed air," "inflammable air," "nitrous air," "marine acid air," "alkaline air," and "phlogisticated air." (To give them their modern names: carbon dioxide, hydrogen, nitric oxide, hydrochloric acid, ammonia, and nitrogen.) To Priestley, the significant difference between them was their degree of phlogistication, i.e., the amount of phlogiston they contained. He developed a diagnostic test for this, the nitrous air test, in which the gas to be tested was mixed with nitrous air and the product showed a diminution in volume as part of it was absorbed by water. The diminution appeared to Priestley to be proportional to the "purity" of the test air; he used the procedure to support his theory that in respiration the purity of air was reduced by discharge of harmful phlogiston from the body into the atmosphere. Priestley’s test became the basis for instruments of various designs, collectively known as "eudiometers," with which investigators toured sites throughout Europe to make assessments of the healthiness of the air. Heavily phlogisticated air, such as that found in marshes or overcrowded urban areas, was regarded as unhealthy. At the other end of the spectrum, Priestley’s most exciting new discovery, prepared by heating the red calx of mercury with a burning glass, was named "dephlogisticated air." This air, which Lavoisier was to construe quite differently as "oxygen," appeared to Priestley as the purest and most suitable air for respiration.{27}

The Swedish chemist Carl Wilhelm Scheele (1742-86), who preceded Priestley in the isolation of this gas, had already interpreted it in the light of the phlogiston theory, as "Feuerluft," the agent responsible for producing fire when united with phlogiston in combustion. The difficulties of assigning priority for the discovery of oxygen in these circumstances have confused—or delighted—historians. It seems sufficient for our purposes to note the significance of the fact that the new gas was understood by Priestley and Scheele in terms of the theory of phlogiston. Gases entered the domain of chemistry, acquiring identities as chemical beings, in connection with the theory by which chemistry had proclaimed its autonomy in the eighteenth century. In the event, however, the phlogistic appropriation of the new gases was a short-lived affair. Lavoisier was to conceptualize their nature in different terms, using resources from the tradition that had linked them to the study of heat. After his work, it turned out that chemistry could survive, and indeed flourish, without phlogiston at all.

Lavoisier’s interest in the nature of gases can be traced to notes he composed in 1766, in which he suggested that air might be a compound of a certain chemical basis with the matter of fire. It has been suggested that his sources included papers by the Berlin academician J. T. Eller and an essay on "Expansibilité," in the Encyclopédie, by the French philosophe A. R. J. Turgot. His model enabled Lavoisier to conceive of different gases as distinct chemical entities, which owed their gaseous form to a temporary combination with the material fluid of fire or heat, which he was to name "caloric."{28} A physical model of the gaseous state appealed to Lavoisier, whose research extended beyond chemistry into many of the other fields of physical science, but it was consistently linked with an emphasis on chemical combination. Lavoisier always viewed caloric as a chemical entity, capable of being exchanged and combined in chemical reactions.

In the early 1770s, Lavoisier put this model to work in connection with the processes of combustion and calcination. He viewed as particularly significant the determination, by Louis Bernard Guyton de Morveau (1737-1816), that metals gained weight as they underwent calcination. Guyton had measured an increase in weight that had been noted on occasion before but not consistently quantified. For Lavoisier, the weight gain was telling evidence that combustion and calcination were processes in which air was fixed by solids, releasing its caloric in the form of light and heat. The flames characteristic of combustion were therefore signs not of phlogiston but of caloric. This new understanding of combustion and calcination had implications for the reverse process, the reduction of calxes to their metallic bases. Lavoisier experimented with the reduction of lead calx (litharge) to the metal by heating with charcoal. Traditionally, the charcoal had been regarded as a source of the phlogiston necessary to form the metal; Lavoisier found it difficult at first to account for its function, since he saw reduction as the release of fixed air from the calx. A critically important step was taken in the wake of Priestley’s visit to Paris in October 1774, when he described to Lavoisier his experiments on the reduction of the red calx of mercury. This reduction was of interest because it could be performed without charcoal, in which case it yielded the fascinating new gas that Priestley was shortly to name dephlogisticated air. Lavoisier repeated these experiments, finding that the air released by the reduction of mercury calx was the only part of the atmosphere absorbed in calcination or combustion. This "purest part of the air" also turned out to be the part consumed in respiration by humans and animals; it was thus also named "eminently respirable air." Finally, this gas was given the name "oxygen" after its role as generator of acids in such processes as the combustion of sulfur and phosphorus.{29}

By 1777, Lavoisier had achieved an understanding of combustion and calcination, and of the role of oxygen in the formation of acids, that enabled him to glimpse a comprehensive alternative to the phlogiston theory. He launched his first outright attack, declaring phlogiston a purely imaginary entity. At first, few chemists were convinced. Some, such as Guyton and Claude Louis Berthollet (1748-1822), accepted that a portion of air was fixed in combustion but continued to believe phlogiston was also released in the process. Lavoisier won them over by his work of the early 1780s, in which he introduced methods of precision measurement to determine quantities of substances in reactions. This new direction to his work followed a period of collaboration with the mathematician and physicist Pierre Simon de Laplace (1749-1827), with whom Lavoisier developed the ice calorimeter in 1782-3 to measure the heat released by processes of combustion and respiration. Precision measurement using the balance was applied particularly to experiments on the analysis and synthesis of water, following Cavendish’s finding that ignition of inflammable air yielded a small quantity of water. In 1785, in Paris, Lavoisier staged public demonstrations of the synthesis of water from oxygen and the gas he named "hydrogen," and its analysis into these constituents. Precise measurements were taken of the weights of reactants and products, to show that quantities were conserved in the course of the reactions. Lavoisier claimed a "demonstrative proof" had been accomplished of the compound nature of water, and the role of oxygen in combustion.{30}

These demonstrations were of considerable importance in securing assent to the new theory among prominent chemists, initially in France and subsequently throughout Europe. Lavoisier had been privately expressing his ambition to make a "revolution" in the science since 1773; ten years later, the chemist Antoine François de Fourcroy (1755-1809) acknowledged his success by using the same term. Lavoisier recruited as allies Fourcroy, Berthollet, and Guyton, who collaborated with him to prepare a new system of chemical nomenclature, published as Méthode de nomenclature chimique (1787).{31} In this system, hydrogen and oxygen appeared as elements, as did the various metals, and simple non-metallic substances like carbon and sulfur. Compounds were to be named to reflect their makeup according to the new chemistry, with different degrees of oxidation indicated by different suffixes, as in "sulfite" and "sulfate." Chemists had long been calling for reform of the language of their science, to eliminate what were recognized as anachronisms and ambiguities. Lavoisier and his allies answered this call in a particular way: Following the lead of the philosopher Etienne Bonnot de Condillac (1715-80), they forged a scientific language designed to reflect nature directly, rather than to follow the conventions among chemists. To speak the new language was henceforth to adopt the new theory. In 1789, the new system was further codified in Lavoisier’s textbook, Traité élémentaire de chimie, which allowed for students to be trained in the new theory and in the use of the apparatus, including the calorimeter and the balance, by which it had been achieved. Again, Lavoisier presented his revolutionary new doctrines in a form adopted from chemical tradition, reworking the genre of the textbook that had been a standard feature of chemical education since Libavius. In a revolutionary gesture of rupture from the past, he dropped the standard historical introduction. Students were now to be taught that chemistry had effectively begun with Lavoisier.{32}

Historians have argued about the balance of the old and the new in Lavoisier’s system. Consideration of the reactions of his contemporaries may help us discern the continuities and the discontinuities between his work and the prior traditions of chemistry. The most striking discontinuities occurred, broadly speaking, in the realm of methods. Lavoisier, in collaboration with Laplace and other physicists, introduced methods of precision measurement that had never previously been put to such telling use in chemistry. The balance, in particular, was Lavoisier’s instrument of choice for accurate measurement, sometimes used in conjunction with other apparatus like the calorimeter or the gasometer, and linked to accountancy procedures for keeping track of the quantities involved in reactions. He had balances of almost unrivaled precision constructed by the best instrument makers in Paris, leading to criticism that the experiments he performed with them could not be readily repeated by others without such resources.{33} Priestley denounced Lavoisier for the expense and exclusivity of his apparatus, connecting his choice of instrumentation with his remaking of the language of the science. Both tactics appeared to Priestley as illegitimate impositions upon the community of chemists—brute displays of power rather than reasonable attempts to persuade. But the steady success of Lavoisier’s attempts to convince chemists encouraged adoption of the apparatus and methods by which conversion was frequently achieved, as, for example, when the experiments of analysis and synthesis of water were repeated in the Netherlands.{34} Thereafter, the new chemistry was taught by textbooks and, increasingly, by laboratory training in use of the new instrumentation. Chemistry was thus prepared to play a central role in the significant growth of laboratory sciences characteristic of the early nineteenth century. Measurements of weights of reactants and quantities of gases, for example, became standard procedure in chemical laboratories; in the hands of John Dalton and others both turned out to be of great theoretical importance in the years after Lavoisier’s revolution.

Although some of the methods of the revolution were borrowed from other physical sciences, it would be wrong to present Lavoisier’s overall accomplishment as anything like a takeover of chemistry by physics. He had almost nothing to say in the Traité about the philosophy of matter, of the Newtonian or any other kind. Instead, his focus was consistently on the kind of issues that had long concerned chemists: the nature of chemical elements and compounds, and the course of chemical reactions. Although he viewed the gaseous state as a physical, rather than chemical, condition, he ascribed a chemical role to caloric, which he continued to list among the elements. One area of traditional chemical theory that Lavoisier did not address was the study of affinities, a lacuna in his system of which he was well aware. His work did however reflect the substantially increased knowledge of chemical composition that eighteenth-century chemists had discovered and represented in affinity tables. Lavoisier was happy to replace his senses with instruments when possible, but he continued to have at his finger-tips and in his head the plentiful information about metals, calxes, acids, alkalis, salts, and the new gases, which his chemical predecessors had gathered.

In some respects, then, Lavoisier’s revolution attested to the disciplinary maturity that chemistry had already attained, particularly in its Enlightenment role as philosophical foundation for many of the practical arts. This accounts for the quite ready acceptance of the new theories in Scandinavia and Germany, the regions where knowledge of chemical composition had made the greatest strides, especially in connection with mineralogy and metallurgy, and where the pragmatic approach to the identification of elements and compounds was already well entrenched.{35} From this point of view, the antiphlogistic account of combustion, calcination, and acidification was of secondary importance, merely displacing a few terms in the previous accounts of these processes. Many German chemists appear to have accepted the new theory quite readily, in a debate largely focused on just these reactions.{36} In other contexts, however, the dislodging of phlogiston was not so easily accomplished; and this is a sign of how chemistry, notwithstanding its maturity, remained a discipline with somewhat permeable boundaries. Scottish chemists and natural philosophers, for example the geologist James Hutton (1726-97), were invoking phlogiston in connection with phenomena such as the earth’s heat; for some of them it was too useful a notion to be given up.{37} In the first decade of the nineteenth century, some chemists in Germany and England resorted to phlogistic explanations for the effects of electric currents passing through solutions of salts. Even Humphry Davy dabbled with the idea of reviving a version of the phlogiston theory to connect chemical phenomena with those of heat, electricity, and light.{38} For all of Lavoisier’s success in answering the questions of chemical composition, his victory over phlogiston deprived chemists of a valuable resource for making interdisciplinary connections of this kind.

Lavoisier’s revolution will probably continue to cast its backward light on the eighteenth century. We can draw from it a salutary lesson on the inadequacy of whiggish views of scientific development. The weight-gain of metals in the course of calcination, for example, was scarcely a significant "fact" until Guyton and Lavoisier gave it a cogent reinterpretation. On the other hand, however, the anti-whiggish belief that Lavoisier created a science de novo by providing a new conceptual scheme must also be abandoned. His new concepts and methods reshaped the discipline, but they did so by exploiting knowledge chemists already had about substances and reactions. His instrumentation and techniques of precision measurement were striking innovations, and Lavoisier very deliberately presented his system as a radically new one, especially in the Traité. But, in its central focus upon chemical composition and processes, the new system fundamentally reaffirmed the autonomy to which chemistry had already laid claim. In this sense, the new chemistry was the fulfillment of the old, not as the outcome of a teleological process, but as a theoretical system created to provide an understanding of what were recognized as intrinsically chemical phenomena. Lavoisier transformed chemistry, but he did so by appropriating and reshaping the traditions consolidated in the course of its ancien régime.