In the end, however, Lavoisier would become a chemist. He would specialize in chemical analysis: separating elements from mixtures and measuring out the elements of compounds were activities well suited to his analytical mind. It was in Rouelle's chemistry courses that he was going to acquire the techniques necessary for his studies of mineralogy and hydrology. 

Chemistry in the Mid-Eighteenth Century

Around 1750, chemistry was seeking to gain its independence from the four disciplines which had engendered it: industry, the natural sciences, alchemy and medicine.
Industry - building construction, metallurgy, the fabrication of glass and textiles, processing of leather, production of gunpowder and saltpeter, food preservation - was making governments aware of the possibilities for applied chemistry. Concurrently, the natural sciences were undergoing rapid development and alchemy was contributing to methods of distillation and sublimation for preparing drugs. Distillation soon shed its mystery, becoming a simple method of analysis. But it was especially medicine which, since the sixteenth century, had been promoting the development of chemistry through pharmacy. Men such as the Swiss Leonhard Thurneysser (1530-1595), the Italian Angelo Sala (1576-1637), the Germans Johann Glauber (1604-1670), Otto Tachenius (1630-1700), Johann Kunckel (1630-703) and Johann Becher (1635-1682) created medical chemistry. in 1597, Andreas Libavius published in Frankfort Alchymia recognita, emendata et aucta, cum dogmatibus et experimentis nonnullis, cum commentario medico-physico-chymicothe first collection of chemical texts. The title clearly indicates its links with medicine and pharmacy. In 1609, Johannes Hartmann gave the first courses in chemistry at the University of Marburg.

In France, Bernard Palissy (1520-1590), who can be considered the first professor of chemistry, had denounced the teachings of certain professors, whom he considered charlatans. He advocated returning to the observations of nature. It was at Sedan, Montpellier and Paris that the links between chemistry and pharmacy were forged. In 1610, Jean Béguin (1605- ? ) published in Paris the Tyrocinium chymicum e naturae fonte et manuali experiencia depromptum, the prototype of pharmaceutical texts which would have more than fifty editions. In 1635, Jean Riolan (1580-1657), eminent member of the Faculty of Medicine in Paris, created the Jardin du Roy. Initially destined uniquely for growing medicinal plants, it soon became an important center for the teaching of chemistry. The first "demonstrator," Guillaume Davisson (1593-1669) was hired in 1648. He was succeeded by Nicolas Lefèvre (1615-1669) and then by Christopher Glaser (1628-1672). In 1666, Colbert created the Academy of Sciences which among its twenty-one members included two chemists: Claude Bourdelin (1621-1699), a pharmacist, and Samuel Cottereau du Clos ( ? - 1715), physician to the King. In 1691, Wilhelm Homberg (1652-1715) joined them and gave the first modern definition of mineral salts: "Acids combine with fixed salts to compose bivalent salts according to the nature of the acids that have been used. For example, spirits of nitre added to tartar salt produces true common salt, spirits of vitriol combined with tartar salt produces true vitriol. Both are bivalent salts, that is partially fixed, partially volatile, because the two salts that compose them are and remain fixed and volatile, respectively." (W. Homberg, quoted by F. L. Holmes in "Cum grano salis," Cahiers de Science et Vie, no. 14, Paris, 1993, p. 81.) The successors, Moïse Charas (1619-1698), G.F. Boulduc (1675-1742) and Nicolas Lémery (1645-1715), abandoned the old technique of analyzing salts by heating them dry and developed the more rigorous method of analysis in solution. Thus they could define precisely a growing number of acids and bases. In 1737, Henri Louis Duhamel du Monceau (1700-1782) isolated two types of fixed alkalis: one drawn from plant ashes (potassium hydroxide or potash), the other derived from sea water (soda). In Prussia, Frederick I reorganized the Berlin Academy of Sciences in 1744. Johann Pott (1692-1777) published there his celebrated Lithéogéognosie ou examen chymique des pierres et des terres en général. Andreas Marggraf (1709-1780) produced phosphorus from the phosphates found in urine, discovered magnesia and manganese, studies phosphoric acid and platinum and extracted zinc from its minerals. "Around 1750 then, " writes F.L. Holmes, "chemists began to dare to venture beyond the limits of what was known, discovering new acids and new bases, combining them to form new salts or, conversely, extracting known salts from acids and bases not yet identified." ("Lavoisier," Cahiers de Science et Vie, no. 14, Paris 1993, p. 82.) In France, Louis Lémery (1677-1743), Etienne François Geoffroy (1672-1731) and Pierre Joseph Macquer (1718-1784) were congratulating themselves on this rapid progress. And yet chemistry was still far from being an independent discipline. Bartolomeo Beccaria (1716-1781), holder of the first chair in chemistry at Bologna n 1737, also taught medicine and pharmacy. Georg Ernst Stahl (1660-1734) was a professor of medicine at Halle, as was his student, Frederick Hoffman (1660-1743). Joseph Black (1728-1799) wrote his famous paper, Experiments on Magnesis Allba, with the goal of providing a treatment for kidney stones. Louis de La Planche, Antoine Baumé (1728-1804) and Gabriel Francis Venel (1723-1775) taught as much medicine and pharmacy as chemistry. Along with his course in chemistry, Rouelle gave lectures in pharmacy in which he specified, "There should be a distinction between the pharmaceutical production process and chemistry. Without the latter, the former makes only chance combinations and mixtures which, far from reaching the desired end, are often very harmful. It is chemistry that lays the foundations for all good pharmacy. It is from the exact knowledge of analysis that principles are deduced." (G.F. Rouelle, Cours de Pharmacie, manuscript in 1 vol. in 4°, p. 4.) But the old concepts continued to reign.


  Aristotle's Four Elements

Rouelle, Lavoisier's professor, was still defining the constituent elements of matter as Aristotle had: "We call principles or elements simple, homogeneous, indivisible, immutable and insensible bodies, more or less mobile according to their different configurations, stature and mass, and which are differentiated by their volume and particular figure. It is impossible to detect them in isolation, separated from other elements, unless they come together in a very large numerical quantity. Their particular figure is also unknown and it would be quite ridiculous to pretend to determine it, as several physicists have done. What can be ascertained is that they exist in very small numbers and yet their different combinations suffice to form all the bodies found in Nature. We acknowledge four principles or elements: phlogiston or fire, earth, water and air." (G.F. Rouelle, Cours de Chymie, pp. 27-28.)

Meanwhile, Macquer wrote in 1756, in Eléments de chymie théorique:: " The object and principal goal of chemistry is to separate the different substances composing a body, examine each one individually, determine their properties and analogies, decompose them still another time if that is possible; compare and combine them with other substances, reunite them and put them back together so as to cause the reappearance of the first mixture with all its properties; or by differently combined mixtures, produce anew composed bodies for which not even Nature has given us a model . But this analysis and decomposition of bodies is limited: we can pursue it only up to a certain point, beyond which all our efforts are useless. Regardless of how we proceed, we are always stopped by substances that are stable, that we cannot decompose and which serve as barriers to our progress. It is these substances that we must, I believe, call principles or elements. At least, they are truly so for us. Such substances are principally earth and water, air and fire. For although there is reason to believe that these substances are not the essential parts of matter, are not its simplest elements - since experience has taught us that it is impossible to recognize by our senses the principles of which they are themselves composed -, I believe that it is more reasonable to stop there, and to consider them as simple, homogeneous bodies and the principles of other bodies." (P. Macquer, Elements de chymie théorique, Paris, Jean-Thomas Hérissant, 1756, pp. 1-2.)

Lavoisier was soon to expose the archaic character of these ideas: "A very remarkable thing," he wrote, "is that while teaching the doctrine of the four elements, no chemist has been led by the force of evidence to acknowledge a larger number. (...) All that can be said on the number and nature of elements is limited, I believe, to purely metaphysical discussions: the problems which one proposes to resolve are unspecified and susceptible to an infinity of solutions. But it is most likely that none in particular agrees with Nature." (Lavoisier, Traité élémentaire de chimie, Paris, Cuchet, vol. I, 1789, pp. xvi-xvii.)
  The Theory of Phlogiston

This theory, formulated by Georg Ernst Stahl, had been accepted by all chemists. Its object was to explain the combustion of bodies and the calcination of metals. The effect of these phenomena, according to Stahl, was to release phlogiston, the inflammable and subtle principle contained in these materials. The loss of phlogiston transformed metals into calx, or metallic oxides with very different physical properties (brilliance, ductility and malleability). But it is in the form of oxides that metallurgists receive metallic minerals from mines. To obtain the original metals, they believed they had to restore the missing phlogiston to the oxides, and thus conducted an operation that was the opposite of calcination, that is, a reduction in the presence of charcoal. Stahl's theory had the advantage of explaining not only the phenomena of combustion, calcination, the reduction of metals, and the solution of metals by acids but even that of the respiration of human beings. But its major flaw was to be purely qualitative and not quantitative. If calcination consisted of releasing the phlogiston contained in a metal, one should be able to observe a decrease in the weight of the product obtained. But the products of the calcination of metals are heavier than the original metals. Stahl recognized the contradiction, but made no attempt to explain it. Louis Bernard Guyton de Morveau (1737-1816) suggested that the phlogiston released from the calcined metal was replaced by air, heavier than it, and that, therefore, phlogiston was endowed with a negative weight. Lavoisier, who believed in the mathematical virtue of the scale, accepted neither the theory nor the hypothesis: his faith in the law of the conservation of matter forbade him to do so.

  Transmutations

In 1771, it was still believed possible to transmute one element into another. Johann Gottschalk Wallerius (1709-1785) was one of the proponents of the theory of the transmutation of metals. In his lectures, Rouelle still reserved a place for ideas coming from traditional alchemy: " Ordinary chemists doubt the truth of the principles of this science, but they cannot be judges in a matter entirely unknown to them. (...) Although I do not wish to cast doubt on the testimony of great men who affirm that they have seen transmutations, I would like to see for myself before shedding my remaining reservations. However, I would not advise anyone to attempt such expensive undertakings given the uncertainty of the outcome, unless he has a reliable guide to lead him in an operation which is preserved only by tradition." (G.F. Rouelle, quoted by M. Daumas, Lavoisier, Paris, Gallimard, 1941, pp. 27-28.)

A Swiss scientist, Bengt Ferner, attributed the steady lowering of the levels of the oceans to a transformation of water into earth. This hypothesis was accepted by a number of chemists who, after a prolonged boiling and evaporation of water, found an earthy residue at the bottom of the recipient. According to Stahl himself, "Water, through a great number of repeated distillations can be carried to such a degree of refinement that it can penetrate glass ." (G.E. Stahl, quoted by Lavoisier, "Does the Purest Water Contain Soil and Can This Water be Changed into Soil?" , Introduction aux Observations sur la physique, Paris, Le Jay et Barrois, 1777, vol. 1, p. 79.) But Boerhaave in his Eléments de chymie, Duhamel du Monceau in his Physique des arbres and Le Roy in a paper read at the Academy contested the possibility that water could change into earth.

In one of his first works, Lavoisier attacked this myth of the transmutation of water into earth. He boiled water for a hundred days in a "pelican", a glass recipient whose shape resembles the bird's form, and demonstrated that the residue obtained was not due to a transmutation of water, but rather to the dissolving of the pelican's inner surface in the water. He carried out this demonstration by applying for the first time what would become the basis of his scientific method: the weighing of the elements of reaction thanks to the use of precise scales.

  Lavoisier's Method

In the article, Chymie, in the Encyclopédie, Venel prophecied: "It is clear that the revolution which would place chemistry in the rank it merits - which would at least place it alongside experimental physics - can be carried out only by a clever, enthusiastic, and bold chemist who, finding himself in a favorable position and skillfully profiting from a few fortunate circumstances, can attract the attention of scientists, first by a noisy ostentation and a determined, assertive tone, and then by reason, if his first method has stirred up prejudice." The ambitious Lavoisier was determined to be that chemist. (Encyclopédie ou dictionnaires raisonné des sciences, des arts et des métiers..., de Diderot-d'Alembert, Paris, 1753, vol. III, p. 409. Gabriel François Venel, a doctor from Montpellier and Inspector General of Mineral Waters, was along with his friend Pierre Bayen, a pharmacist and chemist, responsible for analyzing all the mineral waters in France. "He is better known for what he has promised to the sciences than for what he as actually done for them, " was Fourcroy's severe comment in the Dictionnaire de Chimie, Encyclopédie méthodique, vol. III, p. 262.)

In 1768, the death of Théodore Baron liberated a place for a chemist at the Royal Academy of Sciences. Lavoisier had been on the list of candidates for two years. Supported by his father's friends, Maraldi and Duhamel du Monceau as well as by Bernard de Jussieu, Macquer and Joseph Jérome Le François de Lalande (1732-1807), he was elected. On Wednesday June 1, 1768 he sat for the first time at the Academy. He would remain there for twenty-five years, taking on increasing responsibilities. His first papers were reports on analysis: studies of gypsum, the diamond, meteorites, charcoal, lead and mineral waters. He perfected a new model of the hydrometer which he used to measure the density of mineral waters. His work was always on a high level, but contained nothing revolutionary.

One has the impression that he had not yet found a research subject worthy of his talents. But, on the other hand, he had already defined his working method, based on three principles: 1- Every chemical reaction is an equation; this equality is of a quantitative nature and is verified by weighing the bodies before the reaction and the new compositions at its conclusion. 2- The validity of a chemical analysis must be confirmed by a synthesis reconstituting exactly the original body from the elements defined by the analysis. 3- The principle of the conservation of matter is a mathematical law of general value and not just a simple philosophical concept, and it is applicable to all the sciences. In chemistry, it is verified by the systematic use of the scale.

Although the paternity of the law of the conservation of matter is generally attributed to Lavoisier, it was known well long before him. It goes back to the ancients Greeks. Anaxagoras expressed it this way in 450 B.C.: " Nothing is born or perishes, but already existing things combine, then separate anew." (Quoted by R. Taton, Histoire générale des sciences, Paris, P.U.F., 1957, vol. 1, p. 217.)

In 1630, in his Essais sur la recherche de la cause pour laquelle l'étain et le plomb augmentent de poids quand on les calcine (Essays on the Search for the Reason that the Weights of Tin and Lead Increase When They are Calcined), Jean Rey (1583-1645), doctor from the Périgord and correspondent of Père Mersenne, wrote, "The heaviness is so closely linked to the basic matter of the elements that, when changing from one to the other, they always keep the same weight." (Jean Rey, Essais sur la recherche de la cause pour laquelle l'étain et le plomb augmentent de poids quand on les calcine , new edition based on the original and supplemented by manuscripts from the Bibliothèque du Roi and the Minimes de Paris, with notes by M. Gobet, Paris, Ruault, 1777, p. 21.)

In 1678, the Abbé Edme Mariotte (1620-1684) wrote in his Essai de logique: "It is a maxim or natural rule that nature makes nothing from nothing and that matter is never lost." In 1704, Isaac Newton (1642-1727), borrowing the atomists' argument on the eternal similarity of the material world, wrote in Optics that it seemed most likely that in the beginning matter had been formed into solid, massive hard, impenetrable and mobile particles of such size and shape and in such numbers and proportions as to make them most suited to the ends for which they were intended. For this very reason Newton believed primitive particles to be solid and incomparably harder than any of the "porous" bodies which they composed. He considered them to be so hard that they could never worn down or broken up, since nothing could, in the ordinary course of nature, divide into several parts what had originally been made whole "by the will of God himself." As long as the particles remain whole, he continued, they could make up over the centuries bodies of the same nature and texture. But if they were broken down, the nature of the things that had depended on these particles such as they had been, would inevitably change . The nature of water and earth composed of altered particles and fragments of these particles could not be of the same as that of water and earth which had been composed in the beginning with the whole particles. Consequently, he argued, so that nature can endure, the alteration of bodies must consist only of separations, new combinations and movements of these solid bodies, but in places where these particles are joined together and touch only slightly. (Newton, quoted by H. Metzger, in Newton,Stahl, Boerhaave et la doctrine chimique ,Paris, Félix Alcan, 1930, p. 30.) And in 1764, Dr. Chardenon asserted more simply: "It is a generally accepted principle that the absolute weight of a body can be increased only by adding new matter. The law of opposites thus indicates that they can become lighter only by the removal of these same parts." (Chardenon, "Mémoire sur l'augmentation de poids des métaux calcinés," in Memoires de l'Académie de Dijon, 1796, p. 314.)

Although Lavoisier is not the author of the law, nor did he ever seek to demonstrate its exactitude, it was for him a true paradigm which entirely defined his scientific method: everything can be measured, hence calculated and, just as in a balance sheet, the total of the outflow must always equal that of the inflow. It was with this goal in mind that he commissioned the costly scales fabricated by the best craftsmen of the day, Mégnié and Fortin. His expression of the law in the Traité élémentaire de chemie is interesting because it includes three notions that were essential for him: the experimental method, the method of equation, and that of analysis and synthesis. He wrote in the chapter on the fermentation of wine: "Nothing is created, neither in the operations of the art, nor in those of nature, and one can assume in principle that in every operation there is an equal quantity of matter before and after the operation, the qualtity and the quantity of the principles are the same, and that there are only modifications. The entire art of carrying out experiments in chemistry is based on this principle. For all of them one is obliged to assume a true equality of equation between the principles of the bodies being examined and those that are drawn from them through analysis. Thus since the must of grapes produces carbonic acid gas and alcohol, I can say that grape must = carbonic acid + alcohol. (...) The effects of the fermentation of wine are thus reduced to separating into two portions the sugar which is an oxide; to oxygenating one at the expense of the other to form carbonic acid; and to deoxygenating the other in favor of the first to form a combustible substance which is alcohol. And all this occurs in such a way that if it were possible to recombine these two substances, alcohol and carbonic acid, one would reform sugar." (Lavoisier, Traité élémentaire de chimie, Paris, Cuchet, 1793, vol. 1, pp. 140-41 and 150.)

  The Chemistry of Gases

In 1772, a new field of investigation, the chemistry of gases, was opened up to Lavoisier. Succeeding Robert Boyle (1627-1691), John Mayow (1645-1679) and Stephen Hales (1677-1761), the British chemists Joseph Black (1728-1799), Joseph Priestley (1733-1804) and Henry Cavendish (1731-1810) founded pneumatic chemistry during the 1760s. The French did not see the importance of the chemical role of gases and still considered atmospheric air as an inert gas, "a simple receptacle of exhalations."

In September , Jean Charles Philibert Trudaine de Montigny (1733-1777) a high official in t he Ministry of Finances and a colleague of Lavoisier at the Academy of Sciences, asked him to verify information provided by his spy in London, Joâo Jacinto de Magalhaens, according to which it was possible to treat scurvy by administering to patients a preparation of fixed air (CO2 or carbon dioxide) in the form of water impregnated with the gas. This, at least, is what the chemist Joseph Priestley had reported at the Royal Society.

Fixed air, this newly discovered gas which the British said was "fixed" in certain organic compounds, posed an enigma for Lavoisier. Was it atmospheric air itself which became fixed, Turgot (1727-1781) wrote to Condorcet (1743-1794), or was it only a part of atmospheric air? And could this fixation of air be the cause of the increase in weight observed when, by simple heating, a metal was transformed into its oxide? This operation, called calcination by analogy with the method used to transform chalk into lime, made it possible to transform numerous metals into oxides. But these oxides - it had beeen known for a long time - weighed more than the metals that produced them.

Thus the Mémoires de l'Académie des Sciences had taken note in 1667, the year the Academy was created, of the Expériences de l'augmentation du poids de certaines matières par la calcination ((Experiments Concerning the Increase in Weight of Certain Materials Through Calcination). "It would be quite natural to believe," the author of the report wrote, "that a body cannot become heavier unless it joins itself to some perceptible matter. But M. du Clos has shown the Academy that a pound of régule of antimony, so finely ground that it was reduced to impalpable dust, having been exposed to the focus of a burning glass, and reduced to ashes after one hour, became heavier by one-tenth, although during all the time it had burned, it was giving off a quite thick white smoke. While this material was on fire, its surface became covered with a great many small whitish filaments. The fire from the charcoal had the same effect as that from the sun. When the experiment was repeated, it was found that the finer the antimony powder, the more quickly it heated and the greater its increase in weight. It was also found that sulphurated minerals, such as tin and lead, acquired this increase in weight when they were calcined. (...) M. du Clos conjectured that the air that is incessantly drawn to places where there is fire, deposits on these burning materials full of sulphur from the earth more volatile sulphurated particles which join with them, become fixed there and form the filaments of which we have spoken and which apparently account completely for the increase in weight."

The only way of finding out was to repeat the experiments done by the other authors. Lavoisier chose phosphorus, that astonishing body which calcines easily while producing phosphoric acid. Having placed in an open flask a half gram of phosphorus, he weighed both and placed the flash under a bell jar. With the aid of the burning glass he had used to calcine diamonds, he ignited the phosphorus, which burned in the flask producing phosphoric acid. At the end of the process, the air in the bell jar had diminished by 0.3 liters whereas the weight of the flask had increased by 0.3 grams. It was clear that the air in the bell jar had been fixed by the phosphorus and that this fixation explained the increase in weight.

The same was true with sulfur, which burned under the bell jar, transforming itself into sulfuric acid. The weight of the whole - bell jar and sulfur - was the same both before and after the experiment; that of the bell jar had not changed, only the weight of the sulfur had increased. Therefore this increase had necessarily been made at the expense of the air contained under the bell jar. The calcination of lead and tin gave the same results.

Here then was a revolutionary discovery, which was hardly compatible with the official theory formulated by Stahl. If the simple heating of a metal caused it to lose its phlogiston, it should at the same time decrease its weight. But just the contrary occurred: its weight increased. Stahl's assertion thus had to be false: combustion did not consist of a release of phlogiston but in an acquisistion of air and was accompanied by an increase in weight.

In November, Lavoisier was ready to establish the chemical role of air and to question openly the existence of phlogiston. The research program he defined on this theme at the beginning of 1773 was to be followed scrupulously for the next twenty years. It would be punctuated by true discoveries, borrowings from English authors and especially brilliant conceptual syntheses which provided the bases for modern chemistry.

  The Notion of Element

Was it - as Turgot argued - the atmospheric air as a whole which was acquired during the calcination of metals and came to be called fixed air, or only a part of that air? And would that part not be precisely the gas that is released by the calcination of metallic oxides? In March 1775, Lavoisier carried out his famous experiment of twelve days and twelve night on red oxide of mercury which is also known as mercurius calcinatus. This oxide obtained by prolonged heating of mercury at 350 degrees has the remarkable property of spontaneously reducing itself when it is heated to more than 400 degrees. Determined to weigh not only the solids and liquids, but also the gaseous products of the reaction, Lavoisier collected these gases thanks to the pneumatic trough invented by Hales and perfected by Cavendish and Priestley.

The liberated fixed gas that Lavoisier studied had very remarkable qualities: it activated combustions and sustained animal respiration: it was oxygen. Priestley called it dephlogisticated air, but Lavoisier, already a physiologist, preferred called it air vital. The theory of acids ensued quite naturally from these first discoveries: since by burning sulphur in the fire of oxygen, sulphuric acid is obtained and by burning phosphorus, phosphoric acid is obtained, the denomination "oxygen" (that which engenders acids) `was surely suitable for this new gas. "The definition of the composition of air ensues from this program of research: atmospheric air is not an element, that is, a simple body, but a mixture of several gases. Approximately a quarter of atmospheric air is composed by dephologisticated or eminently breathable air (oxygen) and three-quarters, of noxious and harmful air (nitrogen)." (Lavoisier, Oeuvres, vol. II, p. 143.)

In the quarrels over precedence concerning the discovery of oxygen, it is thus possible to define Lavoisier's original contribution: if the Swede Scheele was the first to isolate it , it was Priestley who defined its properties and Lavoisier who identified it as an element. Whether he had wished it or not, Lavoisier had been led to the methodical study of Aristotle's four elements: could they really be considered as elements, elementary constituents of matter? It was no longer true for earth, air and fire. What about water?

  The Analysis and Synthesis of Water

In 1783, seeking to identify the product obtained from the combustion of hydrogen in the presence of oxygen, Lavoisier obtained water - but only after Cavendish and Priestley. Thus water as well was not a simple element: it was composed of hydrogen and oxygen. But Lavoisier still had to demonstrate this in an irrefutable way. Thus in February 1785, he achieved in a single experiment the analysis and then, synthesis, of water. His demonstration before a large audience lasted three days. With the help of J.-B. Meunier de La Place (1754-1793), he passed water vapor over incandescent iron, which decomposed it into hydrogen and oxygen. The two gases were collected in a separate gasometer constructed by Mégnié and were then mixed in a glass balloon and ignited by an electric spark. Water was reconstitued.

  The Contents of the Chemical Revolution

- The Notion of Element. Lavoisier challenged Aristotle's conception of the four elements - earth, water, air and fire - and redefined the notion of element. He showed that atmospheric air is a mixture of oxygen and nitrogen and that water is a compound body, formed from oxygen and hydrogen. He brought to light the role of oxygen in combustions, calcinations, oxidations and the formation of acids. In asserting with Laplace that the amount of heat discharged during a reaction is equal to the amount of heat absorbed during the opposite reaction, he formulated the first principles of thermochemistry.

- The Rejection of Phlogiston. By attacking the "sublime theory" of Stahl in 1785, Lavoisier in his Réflexions sur le phlogistique, crowned the scientific revolution that was underway. But his rights of ownership of this revolution had their limits. For the discovery of oxygen, Scheele, the Swede, and Priestley, the Englishman, had preceded him. The determination of the composition of water was implicit in the results obtained by Cavendish, that of heat specific to those of Crawford and Black. Moreover, the way he envisaged different degrees of oxygenation is scarcely compatible with the modern concept of oxydo-reduction and the exchange of electrons which accompanies it: the gain in electrons during the reductions goes more in the direction of the acquisition of phlogiston, imagined by Stahl.

-The New Method of Nomenclature. In giving chemistry its first general laws, he made it a science; by imposing on it the use of the scale and the exact weighing of bodies before and after every reaction, he invented an experimental method. But his most important contribution was no doubt the modern language he gave it by codifying, along with Guyton de Morveau, the new method of chemical nomenclature. He did more than simply give it a new language: "He formalized this language by taking advantage of the quasi-algebraic character and universality of the great linguistic functions, according to an open combinatoire, conforming to the capacity possessed by chemistry for the rational creation of constantly new material spaces." (J.P. Malrieu, L'Actualité chimique, 1987, XI.)

In this case, Lavoisier was relying on the Abbé Condillac (1715-1780), whom he quotes in the introduction to the Traité élémentaire de chimie: "Languages are true analytical methods; algebra, the means of expression which is the simplest, most exact and best adapted to its object, is both a language and an analytical method. In short, the art of reasoning can be reduced to a well constructed language."

Some people reproached Lavoisier for having replaced Stahl's phlogiston theory by its opposite - absorption of oxygen instead of the discharge of phlogiston during combustion - and for this reason called the new chemistry "anti-phlogiston." Others criticized his choice of the new dominations or inisisted that the theory of acids was too rigid, since some, such as hydrochloric acid, do not contain oxygen. Lavoisier is also sometimes reproached for having engaged chemistry and physics in an impasse by replacing phlogiston by the hypothetical caloric or matière de la chaleur , an imponderable material substance. According to Lavoisier, this matter, combined with the oxygen principle to form oxygen gas. Between heat-substance and heat-movement, the two conceptions coexisting in his times, he made the wrong choice. (Maurice Pasdeloup, "Lavoisier géant de la science, nain et victime de la politique," in Fréquence Chimie, November 1994, p. 26.)

As for chemical affinities, he justified himself by the evasive reply that he had not studied them. "M. Geoffroy, M. Gellert, M. Bergman, M. Scheele, M. de Morveau, M. Kirwan and many others have already collected a multitude of particular facts which are only waiting for the places to be assigned to them. But the principal data are still lacking, or at least those we possess do not yet have enough precision or certainty to become the fundamental base on which such an important part of chemistry must stand." (Lavoisier, Traité élémentaire de chimie, vol. I, p. xiv.)

It is a good idea to let him speak for himself, since he made the point of specifying in 1792, at a time when a certain pride on his part was no longer out of place, what he felt to be his personal contribution: "One will not deny me, I trust, all the theory of oxidation and combustion; the analysis and decomposition of air by metals and combustible bodies; the theory of acidification; more precise knowledge on the nature of a great number of acids, notably vegetable ones; the first ideas on the compositon of vegetable and animal substances, and the theory of respiration to which Seguin also contributed." (Mémoires de Chimie, vol II, p. 87.)

In making this dry enumeration with the succinctness of a self-assured man, he well knew that he had had to do much more than simply reject phlogiston in order to attract the attention of the scientific community. His hard work, his experimental method, his fortune and costly instruments, his influence at the Academy of Sciences, his membership in "enlightened circles," his wife's talent for public relations, the Monday dinners at the Arsenal and the political role of Paris and France in Europe did the rest.

Lavoisier opened the way for organic chemistry by inventing the method of analyzing organic bodies by combustion. It was, however, this opening, associated with his concerns for public health, which was going to lead him towards medicine, physiology and biology.