Berthollet, Claude Louis

(b. Talloire, near Annecy, Savoy, 9 December 1748; d. Arcueil, France, 6 November 1822), chemistry

Berthollet came from a French family that had emigrated to Savoy during the previous century and had become members of the noblesse de robe. The family was, however, in straitened circumstances when Claude Louis was born. He first studied at the collèges in Annecy and Chambéry, and later qualified as a physician at the University of Turin in 1768. After this he settled in the Piedmont for four years before moving to Paris in 1772, where he studied chemistry under Macquer and Bucquet while continuing to study medicine. As a Savoyard he could introduce himself to his near-compatriot Tronchin, from Geneva, an associate of the Académie des Sciences, propagator of vaccination in France, and the chief personal physician to the regent, the duke of Orléans. Upon Tronchin’s recommendation the duke bad Berthollet appointed private physician to Mme. de Montesson, and allowed him to carry out research in the private laboratory installed by the regent and his son in the Palais Royal. Here Berthollet repeated the experiments on elastic fluids of Lavoisier, Priestley, and Scheele, and met Lavoisier. He qualified as a doctor of medicine at the University of Paris in 1778 and married Marguerite Baur in the same year.

Between 1778 and 1780 Berthollet presented seventeen memoirs to the Academy; these led to his election as a member on 15 April 1780, on the death of Bucquet (Fourcroy opposed him). In 1784, on Macquer’s death, he was appointed inspector of dye works and director of Manufacture Nationale des Gobelins. He subsequently collaborated with Lavoisier, Fourcroy, and Guyton de Morveau in the publication of Méthode de nomenclature chimique (1787), incorporating the principles of the new chemistry of Lavoisier.

Berthollet flourished under four different political regimes. In 1792 he was appointed a member of the commission for the reform of the monetary system, and in 1793 the Committee of Public Safety made him an important member of the scientific commission concerned with war production, particularly that of munitions. He was appointed to the commission on agriculture and arts on 22 September 1794 and was made a professor at the École Normale. Berthollet was also charged, with his lifelong friends Monge and Guyton de Morveau, with the organization of the École Polytechnique, where for a time he taught animal chemistry. In 1795 he was one of the first members elected to the Institut France, de which replaced the suppressed Academy in 1793.

With the fall of Robespierre and the revolutionaries, Berthollet’s star shone even more brightly under Napoleon, who showed a deep admiration and affection for the chemist. In 1796 Napoleon appointed Berthollet and Monge to accompany the commission that was to bring back the great works of Italian art to France. In the execution of this assignment Berthollet developed some of the earliest chemical methods for the restoration of paintings. Two years later, Berthollet and Monge accompanied Napoleon as scientific members of his expedition to Egypt, where they stayed for two years and established an Institute modeled on that of Paris. In 1804 Napoleon made Berthollet a count, senator for Montpellier, administrator of the mint, and grand officer of the Légion d’Honneur. After this, Berthollet led a semi-retired existence in the Paris suburb of Arcueil. In 1807 he and Laplace founded the Société d’Arcueil, which met regularly to discuss scientific problems, and published three volumes of its proceedings. In 1811 Berthollet’s son, Amédée committed suicide when his business (manufacturing sodium carbonate according to a new method developed by his father) failed.

In tracing the development of Berthollet’s scientific work, it must be emphasized that, for all his original contributions, he was essentially part of the continuous historical tradition of chemistry. Unlike his senior contemporary Lavoisier, Berthollet wanted to improve rather than to revolutionize the basis of the science. Whereas Lavoisier had tried to found a new chemistry deriving from the analysis of its most fundamental principles, Berthollet wanted to reinvigorate the traditional science by synthesizing ideas derived from various sources. He was trained as a physician, as was common for chemists from Paracelsus to Boerhaave and Black; in addition, like them, he not only sought an adequate theoretical explanation of chemical phenomena but also strove to find an immediate application for his ideas. His enthusiastic espousal of the ideals of the French Encyclopedists reinforced this longing to put science at the service of man’s practical needs.

From 1778 to 1783 Berthollet sent a large number of memoirs to the Academy, all of them admitting the essential correctness of the phlogiston theory. The main characteristic of his mature work is foreshadowed in these early contributions: the quest for a synthesis that would lead to a more adequate understanding of chemical phenomena, by fusing the divergent principles of Stahl’s and Lavoisier’s systems while incorporating the eighteenth-century tradition of the chemistry of affinities. His preoccupation with practical applications was revealed in such investigations as the attempt to revive asphyxiated animals with the help of “dephlogisticated air.” He also studied the properties of soap, which he explained by assuming different distributions of the respective affinities of its components during their interaction with a solvent, and developed original procedures for making new types of “metallic” soap that would have been useful for medicinal purposes.

Berthollet’s earlier investigations were largely concerned with Lavoisier’s major preoccupation, the study of gases. In the beginning he opposed Lavoisier’s ideas while defending the phlogiston theory. It might help to dispel the popular caricature of the opponents of Lavoisier if we recall some of Berthollet’s arguments in this debate.

Tradition. Stahl’s doctrine, according to Berthollet, had been so successful in accounting for chemical phenomena that there appeared to be no adequate reason for rejecting it. Berthollet argued that the study of air and other elastic fluids (or gases) had shown the need for modifying this doctrine in some details, with which many of the other chemists agreed, but he did not think that it followed from this that the traditional basis of chemistry was to be swept aside. Lavoisier alone appeared radical, setting out systematically to disprove the existence of phlogiston by a series of experiments, the precision of which Berthollet admired. Berthollet held that Lavoisier’s positive contributions could be synthesized with a revised phlogiston theory, while the negative conclusions as to the existence of phlogiston could only be harmful to chemistry, the unity of which would be destroyed by such iconoclasm.

Weight. While later historians have made the whole debate on phlogiston revolve about Lavoisier’s so-called crucial experiments showing the increase in weight of substances during combustion, it can be seen by studying Berthollet’s earlier work that it was possible to have accepted this part of Lavoisier’s work without rejecting Stahl. In a series of experiments on the conversion of sulfur, phosphorus, and arsenic to the corresponding “acids” (oxides), Berthollet confirmed Lavoisier’s conclusion that there was an increase in weight in all three cases because “vital air” was added. He also agreed that, notwithstanding earlier investigations in which a diminution in the quantity of the surrounding air had been observed, Lavoisier was the first to have indicated that there was an increase in weight when metals, sulfur, and phosphorus were calcinated. So far from seeing any inconsistency between these ponderable considerations and the phlogiston theory, Berthollet, although a supporter of Stahl, felt justified in criticizing Lavoisier for not having been sufficiently vigorous in insisting on his conclusions (a deficiency attributed, paradoxically enough, to Lavoisier’s rejection of the phlogiston theory). While it would be tedious to go into the explanations offered by Berthollet in accordance with Stahl’s doctrines, it is historically interesting to list some alleged inconsistencies on Lavoisier’s part when dealing with quantitative relations.

(a) Lavoisier had maintained that the causticity of metals was due to their combination with “vital air.” To Berthollet this was doubly suspect. First, oxygen combined with all the metals in almost the same proportion to form calces, yet the latter varied greatly in their causticity, contrary to what should have happened according to Lavoisier’s doctrine. Second, the red precipitate of mercury was a highly caustic calx. This implied that it contained a large amount of oxygen, if causticity were due to the presence of this substance—but in fact the red oxide of mercury contained only a tiny proportion of oxygen, not more than one part to every ten or twelve parts of the metal.

(b) When hydrogen and carbon were separately burned, Lavoisier’s theory led to inconsistencies with regard to the quantity of oxygen required in each case. For hydrogen, he had to say that the heat generated was due only fractionally to the consumption of the oxygen present—about 1/21 of the total—while the rest was due to hydrogen itself. For the burning of carbon, he attributed all the heat to the oxygen consumed. Berthollet rightly questioned the discrepancy in the quantitative factors involved in the explanation of the same type of chemical phenomenon, combustion.

Accompanying effects. The rejection of phlogiston had led Lavoisier to postulate an alternative set of explanations for the physical phenomena accompanying combustion: production of a flame (heat and light), and the change of physical state from solid or liquid to vapor. This he achieved by postulating a generic principle called caloric, which was also brought in to explain the existence of substances that were permanently in the gaseous state. Berthollet pointed out that not only was it impossible to clarify, even in principle, the exact relationship of heat to light in Lavoisier’s system (unlike Stahl’s, in which all such phenomena were explained uniformly by the presence of phlogiston), but also that empirical evidence refuted Lavoisier’s ideas about such physical effects. Thus carbon, sulfur, and the metals detonated strongly, producing a flame, when distilled with niter. For Lavoisier substances in the solid state, such as all the reactants in these cases, were deprived of caloric and therefore should not have been able to produce a flame giving out heat and light. He had explicitly stated that only substances in the gaseous or liquid state contained sufficient caloric to produce these effects.

Lavoisier had asserted that the physical effects accompanying combustion required a diminution in the volume of the elastic fluids or air present, since caloric was required for the production of the effects. On the other hand, Berthollet pointed out that in cases such as the detonation of a mixture of carbon and gunpowder, an elastic fluid possessing a greater volume than the reactants was actually given out. Consequently this experiment should have been accompanied by the production of cold, since caloric was being absorbed and not given out, contrary to the experimental evidence, which showed that heat and light were produced.

Berthollet had here seized upon the real weakness of Lavoisier’s explanation of combustion: that this was not only a chemical phenomenon in which something (oxygen) was absorbed, but also a physical phenomenon in which something (energy or, rather, enthalpy, heat minus entropy) was released. This inquiry was not undertaken, however, until the notion of energy was clarified in the third and fourth decades of the nineteenth century.

The foregoing controversy ushered in Berthollet’s preoccupation in his later scientific career, the desire to reformulate the basic principles of chemistry by synthesizing the traditional views with the important new discoveries of Lavoisier, a task that was to take him the better part of twenty years. In the intervening period he rejected the phlogiston theory and embraced Lavoisier’s doctrine.

In 1785, after he had set out to test its correctness by performing a large number of experiments on “dephlogisticated marine acid” (chlorine), Berthollet explicitly stated for the first time that “this principle, which Stahl had ingeniously imagined in order to explain a large number of phenomena, and by means of which a genuine relationship could be established between them, namely phlogiston, having sufficed for the needs of chemistry during a long period,” had at last become a useless hypothesis. Ironically, the test case chosen was more correctly explained by the followers of Stahl than by Lavoisier. As the name suggested, Stahl correctly thought of chlorine as marine (hydrochloric) acid that had lost phlogiston (often identified with hydrogen), while Lavoisier assumed that he was dealing with oxygenated marine acid (oxygen plus hydrochloric acid).

Berthollet’s reasons for agreeing with Lavoisier were summarized in the statement that in the preparation of chlorine, using a mixture of manganese dioxide and hydrochloric acid, the “vital air” of the former could be shown to have combined with the latter, thereby proving that oxygenated marine acid had been formed during the reaction. The actual details of Berthollet’s reasoning were rather obscure. He cited the formation of fixed air and common salt (by boiling oxygenated marine acid with soda) as an important detail in this context, without making it clear why and how it was relevant. Three other experiments were also given in support of the Lavoisienne view. All of them, however, appeared to be curiously vitiated by circular reasoning, in that Berthollet assumed that the phenomena observed were best explained by supposing that oxygen was combined with marine acid in chlorine. These experiments were (α) the dissolution of metals in a solution of oxygenated marine acid when no gas was given off, because Berthollet thought that the metals combined directly with the oxygen of the acid and therefore did not decompose the water, which otherwise would have given off hydrogen: (b) the conversion of hydrogen sulfide gas into vitriolic acid when it was passed through the solution of chlorine, which he explained by assuming that the oxygen of the latter combined with the gas; and (c) the transformation of mercury into a corrosive sublimate which contained a large proportion of “vital air” and marine acid, the two supposed ingredients of the solution of chlorine in which the metal was dissolved.

Two points are worth observing here. First, Berthollet was already preoccupied with finding an explanation of phenomena, interpreted on Lavoisienne principles, that would finally be understood through a complete theory of affinity. Thus, he mentioned that it was not a simple elective affinity between the two that caused marine acid to combine with the oxygen of manganese dioxide, but that a more complex distribution of affinities was required to account for the production of oxygenated marine acid. He suggested that marine acid had only a feeble affinity for oxygen and believed that its combination with the “vital air” of the calx was to be explained by a change in the state of manganese, which was easily dissolved by the marine acid, this dissolution being accompanied by an expulsion of oxygen with which the metal was originally combined. In this process a more concentrated form of oxygen, which lost a large part of its caloric when separated from the manganese, was obtained. The more concentrated form of the oxygen helped it to combine with marine acid despite their weak mutual affinities. Berthollet later expanded this explanation into a complete system of chemistry.

Second, Berthollet’s change of allegiance from Stahl to Lavoisier was directed more by practical expediency than by any apparent superiority in either the logic or the adequacy of the new system. Lavoisier’s ideas enabled Bertholletto give a much simpler explanation of the phenomena, although one that was obviously quite inadequate on many counts. Not only were Berthollet’s original objections to Stahl’s system not answered, but many new ones also arose in the application of Lavoisier’s principles.

A glaring instance was Lavoisier’s explanation of the properties of acids as deriving from the presence of oxygen. Berthollet had already shown, in 1778, that hydrogen sulfide, while possessing the characteristics of a feeble acid, did not contain oxygen. He was to reconfirm this discovery nearly twenty years later. But Berthollet’s most important contribution in this domain was his analysis of prussic acid (1787), which he correctly showed to be composed of hydrogen, carbon, and nitrogen. Although he did not succeed in determining the relative proportions of its components, he was convinced that it contained no oxygen. Lavoisier himself accepted these results but avoided their theoretical implications by suggesting that prussic acid was perhaps not an acid after all. Several years later Berthollet followed up this investigation with those of some other acids—hydrochloric, uric, boric, and fluoric—all of which contained no oxygen, according to him.

Although Berthollet never rejected Lavoisier’s ideas after 1785, he gave indications of how he was eventually going to suggest an alternative explanation that would avoid such difficulties as that of the composition of acids. Lavoisier had explained the properties of substances by referring to the elements of which they were composed; Berthollet proposed to derive them from the relations between their constituents. He did not define acidity in isolation, depending upon the presence of oxygen, but by the interaction between the components of one substance in the presence of another; the former was an acid if it was neutralized by a base, and if together the two gave rise to a series of salts.

A valuable consequence of Berthollet’s adherence to Lavoisier’s system was the determination of the composition of ammonia, for which he gave the earliest accurate analysis (1785). He had tried in 1778 to explain the origin of this alkali, which had been formed in various experiments concerned with distilling alcohol over ammonium carbonate and other “fixed” alkalies. Lavoisier, who had given an account of this work to the Academy, had enjoined Berthollet not to publish it because it contained serious errors. This advice was accepted, although Berthollet intended to repeat the analysis of ammonia, which he carried out soon after his conversion to Lavoisier’s ideas.

Berthollet also acknowledged his debt to Priestley (who had decomposed ammonia in 1775–1777) in the method of the analysis: the passage of an electric current through ammonia. The resulting mixture of gases was exploded with oxygen and its composition was analyzed. From the results obtained it was shown that ammonia was composed of 2.9 volumes of inflammablegas (hydrogen) to 1.1 volumes of moffette (nitrogen).

These investigations into pneumatic chemistry were prompted not only by Berthollet’s interest in the theoretical issues raised by the differences between the systems of Lavoisier and Stahl, but also by his interest in the practical implications of these new discoveries. An illustration of this was his study of chlorine, which immediately led to two separate applications. The first of these stemmed from his preparation of potassium chlorate by saturating a concentrated solution of caustic potash with chlorine. A mixture of the chlorate and carbon exploded energetically, leading Berthollet to try to replace niter with potassium chlorate to obtain a more powerful kind of gunpowder. A public experiment was carried out with this new type of gunpowder in 1788, with unexpected results—the director of the plant and four other people were killed on the spot. It appears that this innovation was later effectively used in military operations.

Berthollet was more successful in his other attempt to put the properties of chlorine to practical use. Having observed that chlorine had bleaching properties, he wanted to find a simple technique for introducing it as a bleaching agent for textiles. This was partly prompted by his affiliations with Gobelins, but chiefly (as he often insisted) by the humanitarian ideals inculcated in him by the Encyclopedists. The traditional methods of bleaching, which involved soaking cloth in whey and spreading it in a sunny field, was, he said, wasteful, since it prevented large tracts of land from being tilled. It was a measure of his humanitarian impulse that, unlike such contemporaries as Watt, who amassed large fortunes from their industrial inventions, Berthollet published his technique for the bleaching of textiles by chlorine without bothering to patent it. It is some reflection on the morality of pioneers in the same field that they invited Berthollet to demonstrate the application of his method—which he did gratis—and then tried to patent his discovery of the bleaching liquid, calling it lye de Javelle.

Berthollet’s method of chlorine bleaching consisted of pouring sulfuric acid on a mixture of six ounces of manganese monoxide and sixteen ounces of salt. This mixture was heated by immersion in boiling water and a bleaching solution was obtained by collecting the chlorine in water (100 quarts for every pound of salt). The cloth to be bleached was first soaked in diluted caustic potash, then washed and eventually immersed in the bleaching solution for three to four hours; the operation was repeated several times. Finally the bleached cloth was washed with soft soap and rinsed in diluted sulfuric acid.

Another of Berthallet’s important contributions to the textile industry was the treatise in which he endeavored to place the ancient craft of dyeing on a Scientific basis by a systematic discussion of its procedures, coupled with an attempt to find an adequate set of theoretical principles to explain the chemical actions involved. His explanation was that, depending on the variable physical conditions of temperature, quantity of solvent employed, and so forth, when a cloth was dyed the reciprocal affinities of the particles of the dye, the mordants, and the cloth itself were responsible for the kind and quality of dyeing. The colors produced were due to the oxidation of the mordant by the atmosphere.

During his stay in Egypt, Berthollet noticed the apparently inexhaustible source of sodium carbonate constituted by Lake Natron, at the threshold of the desert. He sought an explanation for this natural phenomenon in terms of a chemical theory of affinity that had been maturing in his mind over the years. The ground surface of Egypt, he reflected, was covered with a layer of ordinary salt, while the neighboring mountains of Libya were formed of limestone. If these substances reacted with each other, a double decomposition would have occurred, forming sodium carbonate and-calcium chloride. But limestone did not ordinarily react with salt—an explanation in terms of double decomposition for the formation of sodium carbonate in Egypt would therefore have been acceptable only if some special circumstances obtained in this case. Berthollet pointed out that the physical conditions in the area were sufficiently unusual to warrant this assumption. Two factors probably intervened: the high temperatures prevailing in the region and the relatively large quantities of limestone present. When a salt solution filtered slowly through the pores of the limestone, the relatively weak affinities between these two substances were enhanced by the combined effects of the temperature and the enormous mass of limestone. This led to decomposition of the salt, assuring a constant production of sodium carbonate and calcium chloride through double decomposition, as a result of the redistribution of the affinities between the original reactants.

These observations lent a renewed interest to Berthollet’s earlier suggestion that such physical conditions as temperature, relative concentration, and quantities of reactants affected the nature and direction of affinities in a chemical reaction. Berthollet read a memoir on the general theory of affinities while he was still in Egypt. This was the starting point of his complete new system of chemistry, first briefly sketched in Recherches sur les lois de l’affinité (1801) and later developed into the comprehensive, two volume Essai de statique chimique. Here he attempted to provide a proper basis for chemistry, so that its experimental results could be viewed in the light of theoretical first principles. Berthollet developed a theory and a model adequate for the understanding and the interpretation of the rapidly growing body of chemical knowledge in his time. He was aware that the positive work of constructing a new theory had yet to be performed after the shock of Lavoisier’s criticism of the old chemistry.

In his attempt to provide chemistry with an adequate theoretical foundation, Berthollet recognized the importance of the theory of affinity. He pointed out that for lack of a proper critical appraisal of the principles involved, his predecessors’ ideas on affinity, as expressed in the construction of “tables of affinity” through the eighteenth century, were perhaps some what crude and immature. The main objection to these tables was that they assumed affinity to be a general force, unaffected by the experimental conditions and always constant. For example, it had been supposed that if two acids, A and B, were considered, the table of affinities could show at a glance which of the two had a stronger affinity for a base, Z. This led to the view that the acid with the stronger affinity could always replace the other acid in a compound with the base, no matter what the experimental conditions might be. Thus if, according to a table of affinities, A had a stronger affinity for Z than B did, then it would always be the case that on adding acid A to a compound of B and Z, all the B would be replaced in the compound by A, giving the substitution product AZ. With the introduction of rudimentary quantitative methods in the latter half of the eighteenth century, this view had been extended to justify the doctrine that all substances combined in constant proportions: two substances always combined in the same proportion because of their fixed affinity for each other.

Before the time of Berthollet, it had already been maintained by various chemists that this view oversimplified the nature of chemical combinations. Lavoisier had shown, for example, that the nature of chemical combination varied with the temperature. Consequently, he pointed out, a separate table of affinity should be constructed for each degree of temperature. Bergman actually constructed tables showing how affinity varied with temperature. Inspite of a few efforts along these lines, affinity remained in some sense an “absolute” that could be determined once and for all from a given table, irrespective of any variation of the conditions under which a reaction took place.

Berthollet undertook a thorough examination of the notion of affinity as his predecessors had employed it. His main contribution to its development was the proof that affinity was a relative concept which varied with the physical conditions accompanying an experiment: quantity, temperature, solubility, pressure, and physical state (solid, liquid, or gas) determined the relative force with which one substance attracted another. Berthollet then tried to prove that the proportions in which two substances combined also varied according to the conditions. This led to his famous controversy with J. L. Proust.

According to the Essai, there were two main types of forces in nature: gravitation, which accounted for astronomical phenomena, and chemical affinity. It was quite possible that they had a common origin, but they were best treated separately, so as not to lose sight of the very important differences between chemical affinity and astronomical attraction. For, unlike affinity, astronomical attraction operated at such enormous distances that its action could always be considered to be uniform. The shapes, sizes, and specific properties of the molecules composing a particular substance determined the way in which chemical affinity was defined in any given case, so that the exercise of this force was not uniform in all cases. Besides, the results produced were quite different in the two cases. The end product of chemical affinity was always a combination of the substances concerned, whereas no such phenomenon could be associated with astronomical attraction.

To elucidate the nature of chemical combination and affinity, Berthollet employed an explanatory model. This was a mixture of the two main types of methods later utilized by chemists, the atomic and the planetary, although he was not very consistent in the use of either model. Thus, he envisaged substances as composed of minute particles or molecules, which roughly corresponded to atoms. But he also asserted that the proportions in which two substances combined varied continuously, thereby making it impossible to think of these molecules as possessing the most characteristic property of atoms, that of indivisibility. Berthollet’s molecules were supposed to be endowed with mutual attraction. The interplay between the total number of forces in a given substance was supposed to lead to the production of a stable system. This was analogous to a planetary system, where the sum of the forces between the heavenly bodies results in a state of equilibrium. The analogy did not extend further than this: there was no central nucleus or sun around which the molecules of the substances revolved as in a planetary system. It is not clear whether the model was a dynamic one.

Berthollet Pointed out that a chemical substance represented a state of equilibrium between the forces of its component molecules. Likewise, all chemical combinations were caused by an interplay of the forces of molecules composing the reactants. The manner in which the different forces influenced a chemical reaction required that they be distinguished from each other. The former theories of affinity had failed to take these differences into account because of the belief in a uniform force of affinity. The factors that had to be considered in evaluating these forces, according to Berthollet, were the following:

Chemical affinity. The attraction that different substances had for each other, or that the molecules of the same substance exercised upon one another, ultimately depended on their chemical natures, which determined their chemical affinities for each other. The affinity of one substance for another had an upper, but generally no lower, limit. The proportions in which two substances combined could vary from the smallest part to the maximum. The variation normally was continuous and differed very slightly from one compound to another. When the maximum reciprocal affinities between two substances had been satisfied, they were said to be “saturated” with respect to each other.

Quantity. Given a certain force of attraction between two substances, it was natural to suppose that the larger the quantities used, the greater the force deployed. Since a substance was thought of as an aggregate of minute particles or molecules, it was only natural to suppose that each of these would bear a determinate portion of the total force. In any reaction, therefore, affinity was not a constant force that could be determined once and for all: it would vary according to the quantities of the substances. The nature of the combination resulting from the interaction of two or more substances was not a simple function of their respective affinities, considered independently of their masses: in fact, it was not possible to give any precise meaning to the idea of chemical affinity unless it was associated with that of the masses of the reactants. If a salt was formed by the combination of an acid, A, and a base, Z, then it could not be determined in advance whether another acid, B, would displace A in the given salt. For example, A might have a greater affinity for Z than B had for Z in a given reaction. Nevertheless, if a sufficient quantity of B was added to AZ, a point would be reached at which the joint action of the quantity of B and its (relatively weaker) affinity for Z would start to counteract the affinity of A for the base: part of AZ would be converted into BZ.

The forces responsible for chemical combinations depended on the relative attraction or affinity of one substance for another, according to its chemical nature, as well as on the number of its reacting molecules, measured by its quantity. Instead of considering affinity in isolation, as in the tables of affinity. Berthollet proposed to use a more complex concept combining the idea of affinity with that of the mass of a reacting substance.

The relative affinities of different substances could be measured by comparing the quantities of each that would saturate a given amount of the same substance, provided the physical conditions were constant. The idea of saturation was extended by Berthollet to the maximum quantity of any given substance that would combine with another under given conditions, rather than limiting it to the neutralization of acids by bases and of bases by acids.

Berthollet combined the concept of relative affinity with that of the mass of reactants in a chemical combination. This gave the total force with which a given quantity of a substance reacted with another. Instead of taking the quantity of the affinity by itself, Berthollet suggested the use of a concept such as “effective mass” or “chemical mass” of a substance in given reaction. He added that the use of such terms as “mass” and “affinity” which implies that they have clear meanings, should be abandoned if chemistry did not want to be stunted for lack of properly analyzed theoretical foundations. Instead of saying, for instance, that sulfuric acid had a greater affinity for caustic soda than acetic acid did, it was necessary to take into account how much of either would combine with the alkali under given conditions. ‘this would indicate the total attraction exerted by either acid, under similar conditions, upon the alkali: this idea is properly expressed by the complex concept of chemical mass, representing the product of the power of saturation and its mass. Although this was one of the most important ideas introduced into chemistry by Berthollet, its importance was generally overlooked until the second half of the nineteenth century, when there was a revival of interest in the nature of chemical equilibrium and the physical conditions that affect that equilibrium.

Distance (cohesion and elasticity). Besides affinity and quantity, Berthollet pointed out that a chemical reaction was strongly influenced by the physical conditions under which substances were made to react. This was one of his most original contributions: he was probably the first chemist to undertake an exhaustive examination of these conditions. In order to grasp his position, we have to understand the details of the model he constructed to represent the course of a reaction. Although his model was not stated very explicitly, the following appears to be the most coherent interpretation of his ideas on the subject.

It was generally observed that substances increased in volume when they were heated, and contracted when cooled. From this Berthollet inferred a particulate structure of matter; substances were composed of small, discrete particles, invisible to the naked eye, and located at definite distances from each other. The application of heat increased the distances between these particles. If substances had been compact masses or undivided wholes, rather than aggregates of discrete particles, it would have been difficult to account for the expansion of bodies when they were heated and their contraction when cooled. This was particularly true when one attempted to explain how an apparently compact mass, such as a solid, could contract when cooled. The different states of matter were thus explicable by relative increases or decreases in the distances between particles as a substance passed from one state to another.

Berthollet used this model to elucidate the relationship between the physical states of matter and the phenomena of chemical combination. This made him the first chemist to attempt a detailed explanation of chemical reactions in quasi-mechanical terms. He asserted that the affinity between two particles attracted to each other was influenced by their distances. The closer together they were, the more strongly they were attracted. It followed that the following minimum conditions had to be fulfilled before any two substances combined: The chemical nature of each substance had to be such that its molecules attracted, and were attracted by, those of the other. Also, the attraction of the molecules of substance A for substance B had to be greater than the reciprocal attraction of the molecules of A for each other. Conversely, the attraction that the molecules of A exercised upon those of B, combined with the attraction of B for A, had to be powerful enough to over come the reciprocal affinity between the molecules of B. The closer together the molecules of a given substance, the stronger their reciprocal affinity and, consequently, the more difficult would it be for them to combine with any other substance. In more concrete terms, it was often difficult to make two substances combine in the solid state, although they might combine readily in the liquid state. As solids, their molecules were so closely packed together that the reciprocal affinity was too great to be overcome by the attraction exerted on them by the molecules of another substance. In the liquid state, however, the distances between the molecules of a substance were much greater. There was a corresponding decrease in their reciprocal attractions, which rendered them more susceptible to the affinity exerted by the molecules of another substance. For the same reason, chemical combinations took place more easily between substances in the gaseous state than in either the solid or the liquid state: in a gas the distances between the molecules were, relatively speaking, the greatest, so that their reciprocal attractions were reduced to a minimum. For instance, when steam was passed over iron filings, an oxide of iron was formed more readily than when the metal was immersed in water. The reaction took place with far greater facility when the iron had been powdered into small filings than when a block of the metal was used.

Apart from this effect on the physical states, the relative distances between the molecules of reacting substances explained the role of such factors as temperature and pressure. The most important factor influencing chemical combination was heat. The reason for this was to be sought in the expansive power of caloric, the active principle underlying the effects of heat. Caloric caused substances to expand, thereby increasing the distances between their molecules. The reciprocal attraction between the molecules of any given substance consequently decreased. If a substance in the solid state was heated, it passed, as a general rule, through the liquid to the gaseous state. When the reciprocal affinities between the molecules of the same substance decreased, the molecules were more easily attracted to those of another substance, with which they could then combine. For the same reason, the variation of pressure was important in studying the reactions between gases: distances were inversely proportional to the pressure.

A problem that arose in Berthollet’s model, with the important role he assigned to the variation of distance between molecules when they reacted with a given substance, was the explanation of the nature of the force responsible for the increase and the decrease of distances between molecules. So far as the more compact states of matter were concerned, the closeness of the molecules could be attributed to the force of reciprocal attraction. Thus, crystallization was explained as a consequence of the tendency to attain a maximum effect by the symmetrical arrangement of molecules: distances were thus reduced to a minimum, with a corresponding increase in reciprocal affinities. The tendency to cohere as closely as possible was the result of mutual attraction between the molecules, and crystallization was a secondary effect derived from this primary attraction. In fact, the Essai distinguished between the different kinds of effects due to affinity. One of them accounted for chemical combination of two or more different substances. The other expressed itself as the reciprocal affinity between the molecules of a given substance: its intensity was measured by the state of cohesion. Obviously the two effects could work in opposite directions during a chemical reaction. Berthollet may therefore be considered to be the originator of crystal chemistry: unlike his contemporary Haüy, he did not have to assume that the internal symmetry of the crystal (i.e., the symmetry around a particular atom or molecule) had to be the same as its external (or macroscopic) symmetry.

Thanks to this model and the accompanying analysis of chemical reactions, especially combination, the analyzed idea of affinity had been replaced by a group of concepts. This had an important influence upon early nineteenth-century chemistry. On the negative side, chemists dispensed with the use of tables of affinity as guiding hypotheses after the publication of the Essai Nobody could accept such a simplified account of chemical phenomena after Berthollet’s criticisms. On the positive side, there were the continued efforts of chemists to provide theories accompanied by adequate models showing the internal workings and structure of chemical substances. This was nowhere more evident than in the development of chemical kinetics and thermochemistry in the late 1850’s, with the accompanying clarifications of the notion of mass action and the mechanism of chemical equilibria.

Berthollet’s compatriot J. L. Proust had asserted in 1799 that all combinations occurred indefinite proportions; in the formation of any chemical compound the same elements were always combined in the same proportions by weight. Berthollet interpreted this to be yet another version of the doctrine of elective affinities and challenged Proust’s notions.

From Berthollet’s concept of chemical mass it followed that the proportions in which one substance combined with another increased directly with its chemical mass, the “active” quantity in any given reaction. There were a maximum and a minimum proportion in which one substance would combine with another. Between these two limits, which one might call the “threshold” and “saturation” points, the substances would combine in any proportions, depending on their respective quantities, the difference in the proportions being continuous between one extreme and the other. From this it obviously followed that the proportions in which substances combined were not fixed, at least within limits. This was not borne out, however, by the facts in all cases-a point that Berthollet had to concede to Proust. There was at least an appearance that substances combined, in some cases, in definite proportions that could not be made to vary indefinitely. For instance, oxygen and hydrogen did not combine in varying degrees, but in the same proportions, to form water; likewise, ammonia was always formed by the same proportions of nitrogen and hydrogen, as Berthollet himself had been the first to demonstrate. Berthollet did not contest such evidence, although he continued to affirm that in the majority of cases, combinations occurred in conformity with his theory of variability of proportions. For him the problem was to reconcile these two apparently conflicting views of chemical combination. He attempted to do this in two different ways.

On the one hand, Berthollet admitted the existence of fixed proportions. Some substances did combine in only one fixed proportion, but this could be explained quite satisfactorily in accordance with his principles. The reason was to be found in the special conditions under which some combinations took place, so that it was not possible for the substances involved to follow the general law of variability of proportions within limits. On the other hand, he maintained that it was only the poverty or superficiality of experimental observations that had led chemists to attribute a fixity of proportions in combinations where, in fact, there was none to be discovered. A case such as the fixed proportions in which hydrogen and oxygen combined to form water vapor was explicable in terms of the physical state of the resulting combination. The reaction was accompanied by a strong condensation: in fact, with the application of slight pressure the resulting product was converted into the liquid state instead of being gaseous; such a conversion was accompanied by a thousand fold (or more) condensation in the volume of the product. As a result of the condensation, the molecules of water vapor were packed together more closely than the molecules of hydrogen and oxygen surrounding them. The reciprocal affinity between the compound molecules of water vapor was considerably greater, due to their closeness, than that between the individual hydrogen and oxygen molecules Surrounding them. Berthollet assumed that it was a special characteristic of such molecules that their combination would be accompanied by condensation only when they came together in a particular proportion, such as 2: 1 for hydrogen and oxygen. When they were mixed in proportions other than this, they did not form stable compounds for two reasons. First, their distances might be too great for the mutual affinity to be effective. Second, even if they did combine in small quantities, the surrounding molecules, by their physical impact or their opposing affinities (e. g., of cohesion), succeeded in dissociating whatever compound molecules might be produced. This explained why oxygen and hydrogen combined in a fixed proportion to produce water. When they were present in the ratio of 1:2, their molecules were so combined that the particles of thee product were closer together, and hence relatively isolated, due to condensation.

The arguments advanced by Berthollet and Proust in their controversy (1801–1807)were both empirical and theoretical. The empirical objections revolved about the ability to distinguish between a genuine chemical compound and a mere mixture. Here Proust’s intuition was more often correct than Berthollet’s. The theoretical difficulties showed Berthollet at an advantage because, unlike Proust, he had worked out a complete set of principles in terms of which he tried to account for all the known chemical phenomena. While Berthollet agreed that there was clearly a difference between such substances as glass and the metallic oxides, he could discover no criterion by which any definite distinction could be established between them. On Berthollet’s model it was quite possible to account for the existence of the substances in which the constituents were always combined in the same proportions, but no difference of principle was involved in distinguishing substances held together by weaker affinities (their combination being unaccompanied by condensation). Proust’s reply was a circular one. He was supposed to state a principle by which substances that he considered to be of fixed composition could be distinguished from those that he acknowledged to be of variable composition. His reply took the form that compounds were substances whose constituents always combined in fixed proportions, whereas solutions or mixtures were substances having variable constitutions.

BIBLIOGRAPHY

I. Original Works. Most of Berthollet’s papers were published in Mémoires de l’Académie/Institut or, after 1789, in Annales de chimie. He also wrote three books: Éléments de l’art de la teinture, 2 vols. (I, 1791; 2nd ed., 1804); Recherchees sur les lois de l’affinité (1801); and Essai de statique chimique, 2 vols. (1803). Also see Mémoires de physique et de chimie de la Société d’Arcueil, 3 vols. (1807–1817),

II. Secondary Literature. No comprehensive study of Berthollet’s life and works has been published, but see E. F. Jomard, Notice sur la vie et les ouvrages de C. L. Berthollet (Annecy, 1844); and S. C. Kapoor, “Berthollet, Proust, and Proportions,” in Chymia, 10 (1965), 53–110. Also of value is M. P. Crosland, The Society’ of Arcueil. A View of French Science at the Time of Napoleon I (Cambridge, Mass., 1967).

Satish C. Kapoor

BERTHOLLET, CLAUDE LOUIS

Talloires, near Annecy, Savoy, 9 December 1748; d. Arcueil, France, 6 November 1822)

pure and applied chemistry. For the original article on Berthollet see DSB, vol. 2.

Berthollet was one of the leading French chemists in the late eighteenth century. He became a close associate of Antoine Lavoisier. As the founder of the Society of Arcueil in the opening years of the nineteenth century he began a further career, not only with his own ideas on chemical reactions, but also as the patron and friend of a group of

talented young scientists. Some areas of Berthollet’s life and work previously unexamined are: (1) his association with Lavoisier; (2) his activities during the French Revolution; (3) his founding of the Society of Arcueil and his friendship with Pierre-Simon Laplace; and (4) his influence.

Life. Berthollet was broad in build, amiable, and even homely in disposition, a character quite different from most of his scientific colleagues. He was the first major convert to Lavoisier’s oxygen theory (1785) despite justifiable disagreement on the idea of oxygen as the principle of acidity. Only slightly junior, Berthollet worked closely together with Lavoisier. His importance in the Lavoisier school is suggested by the inclusion of his name on the title page of the collaborative Méthode de nomenclature chimique (Paris, 1787), despite the fact that successive chapters of the book appear only under the names of his colleagues Lavoisier, Louis-Bernard Guyton de Morveau, and Antoine Fourcroy. Again when the key chemical journal the Annales de chimie began publication in 1789 Berthollet’s name appeared on the title page as a founding member of the editorial board. He was active in soliciting contributions to the journal. When finally it had reached nearly its hundredth volume in 1815 it was he who organized a new series with his protégés Louis-Joseph GayLussac and François Arago as editors.

All “the big four” (authors of the Méthode) were appointed to different major positions of responsibility during the momentous revolutionary period, with the tragic death of one (Lavoisier, 1794) and Berthollet as the last survivor (d. 1822). In 1791 Berthollet was appointed successively as commissioner of the mint and member of the Bureau de Consultation des Arts et Métiers, concerned with rewarding artisans for their inventions. He joined the Bureau of Weights and Measures in 1793 at a time when several members had been arbitrarily excluded as politically unreliable. (Berthollet prudently avoided all political association.) Also in 1793 he became a member of the Commission des Arts, where his duties included making an inventory of the contents of the laboratory of the unfortunate Lavoisier. In 1794 he was appointed to the Commission for Agriculture.

Yet all these posts were arguably less important than those involving the direct application of chemistry to the war effort. In September 1793 he and two others were ordered by the minister for war to write a booklet to explain the manufacture of iron and steel. As the discoverer of potassium chlorate and its dangerous explosive properties (1787), probably his main contribution to the war effort was in the manufacture of gunpowder and the extraction of its main constituent saltpeter (potassium nitrate). For example, in December 1793 he was put in charge of a new refinery of saltpeter. In 1794, together with Guyton and Fourcroy, he taught a crash course to prospective gunners on the manufacture of gunpowder.

A more constructive period followed after the fall of Maximilien Robespierre (July 1794), when many educational plans were made. November 1794 saw the foundation of the École Normale, where Berthollet was appointed as professor of chemistry. Yet the students were an astonishingly mixed crowd and Berthollet was a poor teacher. He had a little more success as professor at the École Polytechnique (September 1795), where its students had been selected by examination.

Yet Berthollet’s role as professor was to be interrupted in May 1796, when he was sent to Italy as a member of a commission to confiscate art treasures in the territory newly conquered by General Napoléon Bonaparte. Contact with the future emperor was to affect Berthollet’s future. The chemist was chosen again in May 1798 to accompany Bonaparte to Egypt. Bonaparte returned suddenly to Paris in November 1799, leaving the army behind but taking with him Berthollet and his friend Gaspard Monge.

The turn of the next century was to find Berthollet at Arcueil, then a village a few miles south of Paris. He bought a country house, built a chemistry laboratory, and invited Gay-Lussac, a promising young graduate from the École Polytechnique, to join him as an assistant. This was to lead to the foundation of the private Society of Arcueil that announced its existence to the world by the publication of its first volume of Mémoires in 1807. Berthollet had recently been joined by the distinguished mathematician Laplace, who had bought a neighboring house at Arcueil, a summer retreat away from the noise and pollution of Paris. They assembled a small group of outstanding young men of science whom they advised and encouraged, even helping several to become members of the first class of the institute, which now replaced the former Royal Academy of Sciences.

In 1803 Bonaparte had made Berthollet a senator, a position which carried a large income. Yet the chemist did not handle his fortune well and by 1807 was in some difficulty. Laplace, also a friend of the head of state, wrote to Napoléon, mentioning the plight of his friend. The emperor immediately authorized payment of a substantial sum for the man he once called “his chemist.”

In 1783 Berthollet had met Charles Blagden, very soon to be appointed secretary to the Royal Society of London, then on one of his many visits to Paris. They soon became great friends, reinforced by their common early medical careers, and they kept up a correspondence for the next forty years. The letters were of great importance in conveying scientific news between the two capitals. In 1784 Blagden was appointed as one of the official correspondents of the Academy of Sciences. Berthollet was also to correspond with many other foreign men of science, including Martin van Marum, Joseph Proust, and Jöns Jakob Berzelius.

Work. The close association of Berthollet with Laplace in the Arcueil period was based on more than personal friendship. They shared a common interest in the legacy of Isaac Newton’s theory of gravitational attraction. This had played a prominent part in Laplace’s previous astronomical work, but in the early 1800s he became interested in attraction on the microscale, that is, the short-range attraction between particles of matter. Having explained satisfactorily capillary attraction, he directed his ambition towards a theory of “the identity of the attractive forces governing capillarity with those responsible for [chemical] affinities” (see also Crosland, 2006). He tried unsuccessfully for some time but eventually realized that his equations would have to take into consideration the size and the shape of the ultimate particles—an impossible task.

Of course chemical affinity was one of Berthollet’s great interests since his visit to Egypt, where he had discovered that large masses of reactants could overwhelm normal affinities. Thus he had set out to study what he called “chemical statics,” as in the title of his book. He even introduced the term chemical mass (1803, vol. 1, p. 16).

Although John Dalton published his atomic theory only in 1808, it had been described in Thomas Thomson’s textbook of 1807. Thomson sent Berthollet a copy and Berthollet had it translated into French (9 volumes, 1809). Berthollet wrote a long introduction and, although he was critical, describing Dalton’s atoms as “an ingenious hypothesis,” it was often through this book that, in time of war, Dalton’s theory was known on the continent of Europe.

The defeat of Napoléon marked the effective end of the Society of Arcueil but Berthollet decided to work on a second edition of his Essai, taking into account recent research. In particular he was able to comment on the work of Dalton, William Wollaston, and Berzelius. He was still critical of Proust’s theory of definite proportions despite growing evidence in its favor but he allowed for its possibility in certain cases. The new edition of the Essai was never published but the manuscript has recently been found and analyzed. Although the Society of Arcueil was no longer in existence, Berthollet managed in 1817 to bring out a final volume of its Mémoires, consisting of papers by some of its former members.

Influence. The person on whom Berthollet had the greatest immediate influence was Gay-Lussac who, for example, suspected that “oxymuriatic acid” (chlorine) was a simple substance but said that “it appeared so extraordinary that

M. Berthollet prevailed upon us to state it with the greatest reserve” (1814, p. 97). It was Berthollet who had discovered the bleaching action of “oxymuriatic acid,” which was simply and logically explained as oxidation due to its oxygen content. Thus the French chemists lost any claim to the discovery of the elementary nature of chlorine, which went to Humphry Davy. Even after Berthollet’s death Gay-Lussac admitted that he still felt the influence of his mentor in the interpretation of chemical phenomena. In 1823 Jean-Baptiste Dumas, the young aspiring chemist from Geneva, had hoped to study under Berthollet, whose works he had studied, but when he arrived in Paris he found that the great chemist had died in the previous year. After this Berthollet’s work seems to have been largely forgotten until the time of Cato Maximilian Guldberg and Peter Waage with their law of mass action (1867).

They referred to Berthollet’s work but he had not gone far enough with the question of mass for them. When Wilhelm Ostwald in 1896 republished Berthollet’s Essai in his series of science classics, he remarked that the book was often praised but seldom read.

SUPPLEMENTARY BIBLIOGRAPHY

WORK BY BERTHOLLET

Essai de statique chimique. 2 vols. Paris, 1803.

OTHER SOURCES

Crosland, Maurice. Introduction to reprint of Essai de statique chimique [1803], by Claude Louis Berthollet. 2 vols. New York: Johnson Reprint Corp., 1952.

———.The Society of Arcueil: A View of French Science at theTime of Napoleon I. London: Heineman, 1967.

———. “A Science Empire in Napoleonic France.” History ofScience 44 (2006): 29–48.

Cuvier, Georges. “Eloge historique de M. le comte Berthollet.” In Recueil des éloges historiques, vol. 3. Paris, 1827.

Gay-Lussac, Joseph-Louis. Mémoire sur l’iode.” Annales deChimie (31 July 1814): 5–160.

Grapí, Pere. “The Marginalization of Berthollet’s Chemical Affinities in the French Textbook Tradition at the Beginning of the Nineteenth Century.” Annals of Science58 (2001): 111–135.

Laplace, Pierre-Simon. Mécanique céleste, vol. 4, Supplément à la théorie de l’action capillaire. Paris, 1807.

Sadoun-Goupil, Michelle. Le chimiste Claude-Louis Berthollet,

1748–1822: Sa vie, son œuvre. Paris: J. Vrin, 1977. This includes correspondence and a full bibliography of Berthollet’s publications.

———, ed. Revue de l’Essai de statique chimique, by Claude Louis Berthollet. Paris: École Polytechnique, 1980.

———.Du flou au clair? Histoire de l’affinité chimique. Paris: Editions du Comité des Travaux historiques et scientifiques, 1991.

Thomson, Thomas. Système de chimie. Translated by Jean Riffault. 9 vols. Paris, 1809. With an introduction by Berthollet.

Maurice Crosland