скачать книгу бесплатно
Newton was also the inspiration behind the experimentally deft attempts made by Stephen Hales (1677–1761) to discover the mechanism of plant growth through an investigation of the movement of sap. It was while making these investigations in the 1720s that he discovered that plants and minerals contained, or held within their pores, large quantities of air. In his Vegetable Staticks published in 1727, Hales devoted over a third of the book to a demonstration of this finding, which he proved by heating solids and liquids in a gun barrel and collecting the ejected air over water in a vessel suspended from a beam. This discovery that air could be ‘fixed’ was the beginning of pneumatic chemistry, and a key factor in the eighteenth-century ‘chemical revolution’.
THE PHLOGISTONISTS (#ulink_e226d10a-93ab-517c-bd43-ca59d84cef38)
By rejecting the claim that the ultimate elements could ever be identified by fire analysis alone, and by arguing that whatever was released by fire were not elements but classes of substances, Boyle failed to be helpful to the practical chemist. The result was that practical chemists went back to the elements. But with one difference. They now began to separate physical from chemical theories of matter and to accept that, to all intents and purposes, substances that could not be further refined by fire or some other method of analytical separation were effectively chemical elements. This did not preclude the possibility that these ‘elements’ were composed from smaller physical units of matter, but this was a possibility that the investigative chemists could ignore. Such a pragmatic attitude was to reach its final form in Lavoisier’s definition of the element in 1789. We find a good example of this attitude in the theory of elements advocated by Georg Stahl (1660–1734), which is customarily referred to as the phlogiston theory. This in turn had been developed from the writings of Becher.
The severe economic problems of the several small and scattered states and principalities that made up the Holy Roman Empire had encouraged rulers to surround themselves with advisors and experts. As we have seen, this was one of the reasons why alchemists were often to be found at European courts, as were ‘projectors’ and inventors of various kinds. With the growth of government and civil service, the Germanies developed a tradition of cameralism (economics), which strove to make their countries self-sufficient through the strict control of the domestic economy and the efficient exploitation of raw materials and industry. It was the problems connected with mining and with glass, textile, ceramic, beer and wine manufacturing that encouraged the German states to take chemistry seriously. By the beginning of the eighteenth century, chemistry was to be found in many German universities in both the contexts of medicine and cameralism.
Johann Becher (1635–82?) was an early cameralist. With the backing of the Austrian emperor, Leopold I, he founded a technical school in Vienna in 1676 for the encouragement of trade and manufacture. Some years later he moved to the Netherlands to try to launch a scheme for recovering gold from silver by means of sea sand, and he is reputed to have died in London after investigating Cornish mining techniques. Becher wrote of himself in his most important book, Physica Subterranea (1667), that he was
(#litres_trial_promo):
… One to whom neither a gorgious home, nor security of occupation, nor fame, nor health appeals to me; for me rather my chemicals amid the smoke, soot and flame of coals blown by bellows. Stronger than Hercules, I work forever in an Augean stable, blind almost from the furnace glare, my breathing (sic) affected by the vapour of mercury. I am another Mithridates saturated with poison. Deprived of the esteem and company of others, a beggar in things material, in things of the mind I am Croesus. Yet among all these evils I seem to live so happily that I would die rather than change places with a Persian king.
Despite its title ‘Subterranean physics’, Becher’s treatise was concerned with the age-old problem of the chemical growth of economically important minerals. A deeply religious work, it was vitalistic and Paracelsian in tone. For Becher, Nature, created by God the chemist, was a perpetual cycle of change and exchange, to which the mercantile economy was an analogy. He could not agree with Helmont’s reduction of the elements to water, claiming that this was a misreading of Genesis; for the Bible had said nothing about the creation of minerals. Since these had clearly developed after the organic world, he supposed that they had been generated from earth and water. Although he rejected Paracelsus’ tria prima, he argued that there were three forms of earth, which, for our convenience, can be symbolized as El, E2 and E3:
terra fluida (E1), or mercurious earth, which contributed fluidity, subtility, volatility and metallicity to substances;
terra pinguis (E2), or fatty earth (the ancient unctuous moisture of the alchemists), which produced oily, sulphureous and combustible properties; and
terra lapidea (E3) or vitreous earth, which was the principle of fusibility.
Air was not a part of mineral creation. Becher implied that the terra pinguis was an essential feature of combustibility, but, unlike Stahl later, he did not notice its participation in reversible reactions. He treated fire solely as an instrument, or agent, of change. Minerals grew from seeds of earth and water in varying proportions under the guidance of a formative principle. Because he had a unified view of Nature, he also referred at length to the more complex compositions of the vegetable and animal kingdoms, where both fire and air were incorporated. However, in re-editing the Physica in 1703, Stahl concentrated solely on the mineral theory.
Stahl, a Professor of Medicine at the newly opened University of Halle, was a Lutheran pietist and a vitalist who kept his chemistry separate from his medicine and vehemently denounced the claims of iatrochemistry. Like Becher, he worked in the cameralist tradition, his first publication, the Zymotechnia Fundamentalis (1697), being concerned with the preparation of fermented beers, wines and bread. It was to help improve the smelting of ores that he first turned to Becher’s treatise.
Like Boyle and Newton, he believed that matter was composed of particles arranged hierarchically in groups or clumps to form ‘mixts’ or compounds. There were four basic types of corpuscle, Becher’s three ‘earths’ and water. In 1718 Stahl redesignated Becher’s terra pinguis (E2) as ‘phlogiston’. If we symbolize water by W, then the four elements, whose existence we can only deduce from experiment, combine together by affinity or the cohesion of water to form secondary (chemical) principles. These substances, like gold and silver and many calces (earths) are extremely stable and cannot be simplified. They are in practice the simplest entities with which the chemist can work, and were to become the elements of modern chemistry. Further combinations among these secondary principles produced mixts such as the metals and salts:
E1 + E2 + E3 + W → secondary principles (e.g. gold) → mixts (e.g. metals) → higher mixts, etc. (e.g. salts)
Moreover, following Boyle, the ultimate four elements are not necessarily omnipresent; but for the secondary principles and mixts to be visible, the particles of the elements and secondary principles have to aggregate among themselves. Echoing Helmont, Stahl believed that ‘gas’ was a release of water vapour from a decomposing mixt.
Stahl, who appears to have had a good working knowledge of the practice of metallurgy, saw an analogy between organic combustion and the calcination of metals. Whereas contemporary metallurgists used charcoal in smelting to provide heat and to ‘protect’ the metal from burning, Stahl supposed that all flammable bodies contained the second earth, phlogiston, which was ejected and lost to the atmosphere during combustion:
In the particular case of metals, X is the calx (oxide).
Stahl was astute enough to see that the reaction was reversed when a calx was heated with charcoal, and interpreted this as due to the transfer of fresh phlogiston from the charcoal:
X + phlogiston → metal [reduction]
Another brilliant explanation was the combustion of sulphur, and its recovery (synthesis) after treatment with salt of tartar (potassium carbonate):
burn
sulphur → universal acid + phlogiston
universal acid + salt of tartar → vitriolated tartar
vitriolated tartar + charcoal → sulphur
This cyclic transaction confirmed Stahl’s belief that sulphur was a mixt containing phlogiston and the principle of acidity, which, following Becher, he called the ‘universal acid’ since he assumed that it was present in all acids. The universal acid itself was a mixt composed from the vitriolic earth and water.
Such transfers as occurred with metals, sulphur and acids were not possible with organic substances, that is, with materials extracted from animals and vegetables, and this made the study of mineral, or inorganic, chemistry all the more interesting. A metal could be made to undergo a series of chemical transformations and be restored completely weight for weight; but an organic material such as a potato would be totally destroyed by chemical manipulation and no amount of added charcoal would ever restore it. Stahl, still unaware of the significance of air in chemical change, had drawn a definite line between inorganic and organic chemistry. In the case of the latter, it appeared that an appeal to the supernumerary properties of a vital soul or organizing principle was still necessary. This was not needed in mineral chemistry, and Stahl rejected Becher’s belief that minerals grew beneath the ground.
Stahl’s phlogistic principle readily explained the known facts of combustion. Combustion obviously ceased because a limited amount of air could only absorb a limited amount of phlogiston. When the air became saturated, or ‘phlogisticated air’, combustion ceased. Equally, combustion might cease simply because substances only contained a limited amount of phlogiston. Obviously, however, phlogiston could not remain permanently in the atmosphere otherwise respiration and combustion would be impossible. Unlike Becher, Stahl assumed that phlogiston was absorbed by plants (as Helmont’s willow tree experiment, and the properties of wood charcoal, demonstrated), which were then eaten by animals. There was a phlogiston cycle in Nature and phlogiston was the link between the three kingdoms of Nature. It was this cycle that was transformed into photosynthesis at the end of the eighteenth century.
To the modern mind the principal snag, indeed absurdity, of the phlogiston theory is that metals and other combustibles gain in weight when burned in air. But according to the phlogiston theory something is lost. Why, then, was there not a corresponding reduction in weight? Stahl himself noticed without comment that, in the reduction of lead oxide (i.e. during the addition of phlogiston), the lead formed weighed a sixth less than the original calx. Possibly this was an exception to the rule, for if Stahl’s paradigm was organic distillation, organic substances do appear to lose weight when they are burned and if the gaseous products of combustion are ignored.
In any case, Stahl’s phlogiston was a principle of far more than mere combustion; it did duty to explain acidity and alkalinity, the colours and odours of plants, and chemical reactivity and composition. Weight change was a physical phenomenon and, while it might be indicative of chemical change, it clearly did not assume a fundamental role in Stahl’s conception of chemistry. Finally, we should note that eighteenth-century chemists were by no means unanimous that metals increased in weight during calcination. Improvements in heating technology had actually made it more difficult to demonstrate. Because experiments were frequently made with powerful burning lenses, which produced temperatures well in excess of the sublimation or vaporization points of oxides, we can well understand why chemists frequently reported losses in weight.
In reality, what seems to us today to be an acute problem with the credibility of the phlogiston theory only became problematical when the gaseous state of matter began to be explored in the 1760s. It was then that phlogiston began to take on bizarre and inconsistent guises: as an incorporeal, etherial fire; as a substance with negative weight; as the lightest known substance, which buoyed up heavier substances; or as one of the newly discovered factitious airs, inflammable air (hydrogen). Boyle’s sceptical and investigative tradition then came into its own again when Lavoisier dismissed Stahl’s theory of composition, and phlogiston in particular, as a ‘veritable Proteus’.
CONCLUSION (#ulink_20351629-b471-5aad-8d29-83e0ee00309e)
It is clear that the kind of chemistry inherited from the seventeenth century was changed in at least six ways by the chemists of Lavoisier’s generation: air had to be adopted as a chemically interactive species; the elemental status of air had to be abolished and exchanged for the concept of the gaseous state; the balance had to be used to take account of gases; the weight increases of substances burned in air had to be experimentally established; a working, practical definition of elements had to be established; and a revised theory of composition had to be adopted, together with a more satisfactory and less-confusing terminology and nomenclature that reflected compositional ideas. The thrust of these revisions was accomplished by Lavoisier and has usually been referred to as the chemical revolution. Does this mean, therefore, that we have to accept that there was no mood for change in the seventeenth century comparable to the revolutionary accomplishments of astronomers, physicists, anatomists and physiologists?
Seventeenth-century chemical practice encompassed four distinctive fields of endeavour. Alchemy, though intellectually moribund, still attracted attention both as a religious exercise and because, in principle, it would have given support to the new corpuscular philosophy. Practical alchemists even at this late stage of its development could still stumble upon important empirical discoveries. In 1675, for example, Hennig Brand, while exploring the golden colour of urine, caused excitement with his discovery of phosphorus. Among medically oriented chemists, iatrochemistry had received its impetus from the writings of Paracelsus, Helmont and the exponents of the acid-alkali theory. The iatrochemists were an important group because they considered their calling worth teaching. In France, in particular, chemistry came to acquire a public following that was reflected in the production of large numbers of textbooks and instruction manuals. The iatrochemists thereby helped to establish chemistry’s respectability and ensured that it would become an important part of the medical and pharmaceutical curriculum. In effect, they began the first phase of the long chemical revolution. A third chemical constituency was that of the chemical technologists, who, in a small but significant way, continued to provide data from their observations and experiments, and who encouraged the cameralistic interest in chemistry.
Finally, there was the critical, but experimentally fruitful, work of Boyle, who did not hesitate to draw upon the work of the other three fields as evidence for the mechanical-corpuscular philosophy. In his hands chemistry became a respectable science. The ‘occult’ forms and qualities of Aristotle were replaced by geometrical arrangements and (in the hands of Newton) forces of attraction and repulsion; the secrecy of the alchemists and that of the technologists was abandoned, and an attempt was made to reform the chaotic and imprecise language of chemistry. While none of these reforms resulted in chemistry as we know it, it would be churlish to deny that chemistry changed during the seventeenth century and shared in the momentum of the general Scientific Revolution.
Nevertheless, the pragmatic element remained undefined and the subject remained the two-dimensional study of solids and liquids and ignored the gaseous state until the time of Hales. Until the role of gases was established and understood, there was a technical frontier that hindered further innovation. That was why late-eighteenth-century chemical progress has always seemed so much more impressive and why, fairly or unfairly, Lavoisier’s synthesis of constitutional ideas and experiment appears as impressive as the work of Newton in physics the century before.
3 Elements of Chemistry (#ulink_495f604c-ca00-58ed-b349-9fa8c18e8669)
Doubtless a vigorous error vigorously pursued has kept the embryos of truth a-breathing: the quest for gold being at the same time a questioning of substances, the body of chemistry is prepared for its soul, and Lavoisier is born.
(GEORGE ELIOT, Middlemarch, 1872)
‘Chemistry is a French science; it was founded by Lavoisier of immortal fame.’ So wrote Adolph Wurtz in the historical ‘Discours préliminaire’ of his Dictionnaire de chimie pure et appliquée (1869). Needless to say, at a time of intense European nationalism and rivalry, in science as much as in politics, such a claim proved instantly controversial. In fact, as early as 1794, Georg Lichtenberg (1742–99) had argued that the anti-phlogistic chemistry was bringing nothing new to Germany. ‘France’, he claimed, ‘is not the country from which we Germans are accustomed to expect lasting scientific principles.’ As far as Lichtenberg was concerned, whatever might be of value in Lavoisier’s new system of chemistry was really of German origin. Thorpe’s riposte to Wurtz seventy years later was that ‘chemistry is an English science, its founder was Cavendish of immortal memory’ – thus invoking an earlier controversy over which European nation’s chemists had first synthesized water. Raoul Jagnaux’s Histoire de chimie (1896) presented the history of chemistry almost entirely as a French affair, with Lavoisier, once again, as its founder. This led twentieth-century German historians to write histories that emphasized that the origins of modern chemistry lay in the chemical contributions of Stahl and, before him, of Paracelsus.
Today we can smile at such nationalistic obsessions and agree that, even though Lavoisier could never have achieved what he did without the prior and contemporary investigations and interpretations of British, Scandinavian and German chemists and pharmacists, there is an essential grain of truth in Wurtz’s statement. For Lavoisier restructured chemistry from fundamental principles, provided it with a new language and fresh goals. To put this another way, a modern chemist, on looking at a chemical treatise published before Lavoisier’s time, would find it largely incomprehensible; but everything written by Lavoisier himself, or composed a few years after his death, would cause a modern reader little difficulty. Lavoisier modernized chemistry, and the benchmark of this was the publication of his Traité élémentaire de chimie in 1789. On the other hand, historians have come to recognize the continuities between Lavoisier’s work and that of his predecessors. Lavoisier’s deliberate decision to break with the past and to put chemistry on a new footing inevitably meant that he was cavalier with history and that he paid scant attention to his predecessors – thus indirectly providing a source of his own mythology as the father of chemistry.
A SCIENTIFIC CIVIL SERVANT (#ulink_546de43c-9773-5eb3-ac16-c868204a5181)
Antoine-Laurent Lavoisier was born in Paris on 27 August 1743, the son of a lawyer who held the important position of solicitor to the Parisian Parlement, the chief court of France. His wealthy mother, who also came from a legal family, died when Lavoisier was only five. Not surprisingly, therefore, Lavoisier’s education was geared to his expected entry into the legal profession. This meant that he attended, as a day pupil, the best school in Paris, the Collège des Quatres Nations, which was known popularly as the Collège Mazarin. The building still survives and now houses the Institut de France, of which the French Academy of Sciences is a part. The Collège Mazarin was renowned for the excellence of both its classical and scientific teaching. Lavoisier spent nine years at the Collège, graduating with a baccalaureate in law in 1763. This legal training was to help him greatly in the daily pursuit of his career and can be discerned in the precision of his scientific arguments; but his spare time was always to be devoted entirely to scientific pursuits.
One of the close friends of the Lavoisier family was a cantankerous bachelor geologist named Jean-Étienne Guettard (1715–86). Aware of young Lavoisier’s scientific bent, Guettard advised him, while still at the Collège Mazarin, to join a popular chemistry course being given by Guillaume-François Rouelle (1703–70) in the lecture rooms of the Jardin du Roi. Rouelle was following in the tradition established in the seventeenth century of giving public lectures in chemistry aimed at students of pharmacy and medicine. Among his innovations was a new theory of salts, which abandoned both the Paracelsian view that they were variations of a salt principle, and Stahl’s view that they were combinations of water and one or more earths. Instead, Rouelle classified salts according to their crystalline shapes and according to the acids and bases from which they were prepared. Rouelle was also responsible for propagating the phlogiston theory among French chemists by incorporating it into his broader view, adopted from Boerhaave and Stahl, that the four traditional elements could function both as chemical elements and as physical instruments. Thus, fire or phlogiston served a double function as a component of matter and as an instrument capable of altering the physical states of matter. This was different from Stahl, who allowed air and fire only instrumental functions. Air, water and earth could similarly serve as instruments of pressure and solution, and for the construction of vessels, as well as entering into the composition of substances. Rouelle, therefore, accepted Hales’ proof that air could act chemically; like the other three elements, it could exist either ‘fixed’ or ‘free’.
Rouelle’s pupil, G. F. Venel, was one of the few French chemists to pursue Hales’ work before the 1760s. He argued that natural mineral waters were chemical combinations of water and air, and that seltzer water could be reproduced by dissolving soda (sodium carbonate) and hydrochloric acid in water. He also advocated that the reactions of air had to be subsumed ‘under the laws of affinity’. In this way, air came to occupy one of the columns of the many dozens of different affinity tables that were published during the middle of the eighteenth century.
Lavoisier’s earliest knowledge of contemporary ideas concerning the elements, acidity, air and combustion was probably derived from Rouelle’s lectures, which he attended in 1762, as well as from Macquer’s Élémens de chymie théorique (1749) and Venel’s article on ‘chemistry’ in the third volume of the great French Encyclopédie (1753). Between them, Rouelle, Macquer and Venel turned their backs on Boyle’s seventeenth-century physical programme of attempting to reduce chemistry to ‘local motion, rest, bigness, shape, order, situation and contexture of material substances’. Instead, inspired by Newton, they intended to fuse the corpuscular tradition with the more pragmatic chemical explanations of Stahl. They also introduced Lavoisier to the quantitative analysis of minerals.
During the 1750s and 1760s the French government became aware that industry was ‘pushed much further in England than it is in France’. Wondering whether Britain’s increasing wealth and prosperity from trade and manufacture came because ‘the English are not hindered by regulations and inspections’, the French commissioned a series of reports on their country’s industries and natural resources. This interest had several effects: there was a sudden wave of translations of, chiefly, German and Scandinavian technical works on mining, metallurgy and mineral analysis; with these works, part and parcel, came an awareness of the phlogistic theory of chemical composition; moreover, chemists who had trained in pharmacy and medicine, like Macquer, began to find their services in demand for the solution of industrial problems. Guettard had long cherished an ambition to map the whole of France’s mineral possessions and geological formations, and the government readily gave approval in 1763. Needing an assistant who could identify minerals, Guettard persuaded Lavoisier to join him on his geological survey, which lasted until 1766.
In their travels through the French countryside, Lavoisier paid particular attention to water supplies and to their chemical contents. One mineral that particularly interested him was gypsum, popularly known as ‘plaster of Paris’ because it was used for plastering the walls of Parisian houses. Why, Lavoisier wondered, did the gypsum have to be heated before it could be applied as a plaster? Since water could be driven from the plaster by further heating, it seemed that the water could be ‘fixed’ into the composition of this and other minerals – a phenomenon that Rouelle had already termed ‘water of crystallization’. He then showed that it was the loss of some of the fixed water that explained the transformation of gypsum into plaster by heating. Lavoisier was to find the idea of ‘fixation’ significant.
Although Guettard’s geological map of France was never published and Lavoisier’s geological work remained largely unknown to his contemporaries, the work on gypsum was presented to the Academy of Sciences in February 1765, when Lavoisier was twenty-two. With a clear, ambitious eye on being elected to the Academy, the year before he had entered the Academy’s competition for an economical way of lighting Parisian streets. (This was some forty years before coal gas began to be used for this purpose.) Although his involved, meticulous study of the illuminating powers of candles and oil and pieces of lighting apparatus did not win him first prize when the adjudication was made in 1766, his report was judged the best theoretical treatment. King Louis XV ordered that the young man should be given a special medal.
Thus by 1766, this ambitious man had succeeded in bringing his name before the small world of Parisian intellectuals. In the same year, two years before he reached his legal majority of 25, Lavoisier’s father made a large inheritance over to him. To further his complete financial independence, in 1768 Lavoisier purchased a share in the Ferme Générale, a private finance company that the government employed to collect taxes on tobacco, salt and imported goods in exchange for paying the State a fixed sum of money each year. Members received a salary plus expenses, together with a ten per cent interest on the sum they had invested in the company. Such a tax system was clearly open to abuse; consequently, the fermiers were universally disliked and were to reap the dire consequences of their membership of the company during the French Revolution. All the evidence suggests that Lavoisier’s motives in joining the company were purely financial and that, as political events moved later, he strove actively to rid the system of corruption and fraud. Unfortunately, Lavoisier’s later suggestion that the fermiers should beat the smugglers by building a wall around Paris for customs surveillance was to lead to hostility towards him, as may be gathered from the punning aphorism ‘Le mur murent Paris fait Paris murmurant’ (The wall enclosing Paris made Paris mutter).
In 1771, at the age of twenty-eight, Lavoisier further cemented his membership of the Ferme Générale by marrying the fourteen-year-old daughter of a fellow member of the company, Marie-Anne Pierrette Paultze (1758–1836). Despite their difference of age and their childlessness, their marriage was an extremely happy one. Marie-Anne became her husband’s secretary and personal assistant. She learned English (which Lavoisier never learned to read) and translated papers by Priestley and Cavendish for him, as well as an Essay on Phlogiston by the Irish chemist, Richard Kirwan. The latter was then subjected to a critical anti-phlogistic commentary by Lavoisier and his friends, which actually led to Kirwan’s conversion. She also took lessons from the great artist, Louis David, in order to be able to engrave the extensive illustrations of chemical apparatus that appeared in Lavoisier’s Elements. David, in turn, portrayed the Lavoisiers together.
Madame Lavoisier was also hostess at weekly gatherings of Lavoisier’s scientific friends – a role she continued after his execution. It was through such continuing social activities in her widowhood that she met the American physicist, Benjamin Thompson (1753–1814), better known as Count Rumford, whose experiments on the heat produced during the boring of cannon had led him to question the validity of Lavoisier’s caloric theory of heat. After rejecting the suits of Charles Blagden and Pierre du Pont (whose son, Irénée, was to found the huge American chemical company), widow Lavoisier married Rumford in 1805; but they soon proved incompatible and quickly separated. Madame Lavoisier is a good example of how, before the time when they enjoyed opportunities to engage in higher education and in independent scientific research, women played a discrete, but essential, role in the development of science. At a time when the well-off could afford domestic servants, wives and sisters had abundant leisure to help their scientifically inclined fathers, husbands and brothers in their researches.
As a rich and talented man, Lavoisier was an obvious candidate for election to the prestigious Academy of Sciences. Unlike the Royal Society, whose Fellows have always been non-salaried, the French Academy of Sciences was composed of eighteen working ‘academicians’ or pensionnaires. As civil servants, they were paid by the French government (until 1793, by the Crown) to advise the State and to report on any official questions put to them as a body. There were also a dozen honorary members drawn from the nobility and clergy, a dozen working, but unpaid, ‘associates’ (associée) and, to complete the pecking order, a further dozen unpaid assistants (élèves or adjoints). The Academy also made room for its retired pensioners and for foreign honorary associates.
Because of its tight restriction on the number of salaried members, and of members generally, election to the Academy was a prestigious event in the career of a French scientist. This accolade was in contrast to Britain’s Royal Society, which allowed relatively easy access to its fellowship by those with wealth or social status as well as those with scientific talent; consequently, its fellowship lacked prestige. Indeed, until its election procedures were reformed in 1847, fellowship of the Royal Society was not necessarily the mark of scientific distinction that it is today.
The three working grades of the Académie, together with its aristocratic honorary membership, clearly reflected the rigid hierarchical structure of eighteenth-century French society. In practice, the pensioners were allocated between the six sciences of mathematics, astronomy, mechanics, chemistry, botany and anatomy (or medicine). Biology and physics were added under Lavoisier’s directorship of the Académie in 1785. Like the Nobel prizes today, such a distribution frequently prevented the election of a deserving candidate because the most appropriate scientific section was full. There was also a tendency to elect or to promote on grounds of seniority rather than merit. Because membership was restricted, vacancies often led to intense lobbying for positions, factionalism, ill-feeling and sometimes (as with Lavoisier’s election as an associé in 1772) to the bending of rules. The repeated failure of the revolutionary, Jean-Paul Marat, who fancied himself an expert chemist, to gain admission in the 1780s, led him and others to oppose the Academy. Its close association with Royal patronage and its reflection of the ‘corrupt’ hierarchical structure of the ancien régime in any case made it inevitable that it would be suppressed by the revolutionary government in August 1793.
Although, as was to be expected for one so brash and young, Lavoisier failed on his first attempt to join the Academy in 1766, by a modest bending of the rules to create an extra vacancy for him, he was successfully admitted to the lowest rank of assistant chemist in 1768. His chief sponsor described him as ‘a young man of excellent repute, high intellect and clear mind whose considerable fortune permits him to devote himself wholly to science’. Any fears that his membership of the tax company would interfere with his role as academician were probably repressed by the thought that he would be able to entertain on a lavish scale!
Much of Lavoisier’s fortune was probably spent on the best scientific apparatus that money could buy. Some of his apparatus was unique and so complex that his followers were forced to simplify his experimental procedures and demonstrations in order to verify their validity. It should not be thought from this that Lavoisier threw money away on instruments unnecessarily. For example, when measuring the quantity of oxygen liberated from lead calx in 1774, he found that traditional glass retorts were unusable because the lead attacked the glass; clay retorts gave similarly erroneous readings because of their porosity; hence for precise volumetric measurements Lavoisier was forced to design and have made an airtight iron retort. Expense was justified, then, because of the new standard of precision that Lavoisier demanded in chemistry. In the Traité he recognized that economies and simplifications would be possible, ‘but this ought by no means to be attempted at the expense of application, or much less of accuracy’.
Lavoisier was to be a loyal servant of the Academy, by helping to prepare its official reports on a whole range of subjects including – to select from one biographer’s pagelong list – the water supply of Paris, prisons, hypnotism, food adulteration, the Montgolfier hydrogen balloon, bleaching, ceramics, the manufacture of gunpowder, the storage of fresh water on ships, dyeing, inks, the rusting of iron, the manufacture of glass and the respiration of insects. It has been pointed out that, without an ethic of service, such as was entailed in a centralized Royalist state, a privileged citizen such as Lavoisier would have had no incentive to involve himself in such a ‘dirty’ subject as chemistry.
THE CHEMISTRY OF AIR (#ulink_9f1111d5-5cfe-5154-8478-ca4f35d81fed)
The problem of the Parisian water supply came to Lavoisier’s attention during the year of his election to the Academy when the purity of water brought to Paris by an open canal was questioned. The test for the potability of water involved evaporating it to dryness in order to determine its solid content. But the use of this technique reminded academicians, including Lavoisier, of the long tradition in the history of chemistry that water could be transmuted into earth. Obviously, if this were the case, the determination of the solid content ‘dissolved’ in water would reveal nothing about its purity.
As we have seen, the transmutation of water into earth had been a basic principle of Aristotle’s theory of the four elements, and a crucial, experimental, factor in van Helmont’s decision that water was the unique element and basis of all things. Although by the 1760s most chemists could no longer credit that such an apparently simple pure substance as water could be transmuted into an incredibly large number of complicated solid materials, it was seriously argued by a German chemist, Johann Eller, in 1746 that water could be changed into both earth and air by the action of fire or phlogiston. For Eller this was evidence that there were only two elements, fire and water. The active element of fire acted on passive water to produce all other substances.
It seems clear from the design of Lavoisier’s experiment on the distillaton of water, which he began in October 1768, that he suspected that the earth described in Eller’s experiment (which he probably read about in Venel’s article on ‘water’ in the fifth volume of the Encyclopédie in 1755) was really derived from the glass of the apparatus by a leaching effect. By weighing the apparatus before and afterwards, and also weighing the water before and after heating continuously for three months, Lavoisier showed that the weight of ‘earth’ formed was more or less equal to the weight loss of the apparatus. Intriguingly, Lavoisier did not clinch his quantitative argument by analysing the materials in the sediment and showing that they were identical to those in glass. Moreover, since the correlation of weights was not exact, some room for doubt remained until two decades later when Lavoisier showed that water was composed of hydrogen and oxygen.
Enough had been done, however, to convince Lavoisier that Eller’s contention that water could be transmuted into earth was nonsense. This was reported to the Academy in 1770. He also surmised, under the influence of Venel’s views on the chemical dissolution of air in liquids and solids, that there was a more plausible explanation of water’s apparent change into vapour or air when heated – namely, that heat, when combined with water and other fluids, might expand their parts into an aerial condition. Conversely, when air was stripped of its heat it lost its voluminous free aerial state and collapsed into, or was ‘fixed’ into, a solid or liquid condition, just as Stephen Hales had found in the 1720s when analysing the air content of minerals and vegetables.
Lavoisier recorded these ideas in an unpublished essay on the nature of air in 1772. Here was the basis for a theory of gases – though at this juncture Lavoisier knew nothing at all of the work of Priestley and others on pneumatic chemistry. He was also, not surprisingly, still interpreting his model of the gaseous state in terms of phlogiston. When air was fixed
(#litres_trial_promo):
… there had to be a simultaneous release of phlogiston or the matter of fire; likewise when we want to release fixed air, we can succeed only by providing the quantity of fire matter, of phlogiston, necessary for the existence of the gaseous state [l’état de fluide en vapeurs].
Lavoisier was now clear that there were three distinct states of matter
(#litres_trial_promo):
All bodies in nature present themselves to us in three different states. Some are solid like stones, earth, salts, and metals. Others are fluid like water, mercury, spirits of wine; and others finally are in a third state which I shall call the state of expansion or of vapours, such as water when one heats it above the boiling point. The same body can pass successively through each of these states, and in order to make this phenomenon occur it is necessary only to combine it with a greater or lesser quantity of the matter of fire.
Moreover, it followed from the fact that metals disengaged ‘air’ when they were calcined, that metals contained fixed air:
Apparatus for the preparation, collection and study of gases was a necessary factor in the chemical revolution. It was not until 1727 that Stephen Hales hit upon a way to isolate the ‘air’ produced from a heated solid. In order to estimate as accurately as possible the amount of ‘air’ produced and to remove any impurities from it, Hales ‘washed’ his airs by passing them through water before collecting them in a suspended vessel by the downward displacement of water.
Hales, like John Mayow in the seventeenth century, still thought in terms of a unique air element, but Joseph Black’s demonstration that ‘fixed air’ (carbon dioxide) was different from ordinary air encouraged Henry Cavendish, Joseph Priestley and others to develop Hales’ apparatus to study different varieties of air – or gases, as Lavoisier was to call them. An incentive here was the invention of soda water by Priestley, which encouraged interest in the potentially health-giving properties of artificial mineral waters generally. In 1765, while investigating spa waters, the English doctor, William Brownrigg, invented a simple shelf with a central hole to support a receiving flask or gas holder. This creation of the ‘pneumatic trough’ enabled gas samples to be transferred from one container to another and for gases to join solids and liquids on the chemical balance sheet.
Joseph Priestley (1733–1804) is surely one of the most engaging figures in the history of science. The son of a Yorkshire Congregational weaver and cloth-dresser, Priestley was trained for the Nonconformist ministry at a Dissenting academy in Daventry. Like most Nonconformist academies of the period, this taught a wider curriculum than the universities that included the sciences. After serving a string of ministries, where his theological views became increasingly Unitarian, and a teaching post at the famous Warrington Academy, in 1773 Priestley became the librarian and household tutor of William Petty, the second Earl of Shelburne who, while Secretary of State in Chatham’s cabinet, had opposed George III’s aggressive policy towards American colonists. Already the author of innumerable educational works, in 1767 Priestley had published a History and Present State of Electricity, which launched him upon a part-time career in science. While minister of a Presbyterian congregation at Leeds, and living next door to a brewery, Priestley had begun investigating the preparation and properties of airs. Under Shelburne’s patronage, Priestley had the necessary leisure to prepare some five volumes containing detailed accounts of these experiments on airs, as well as a number of theological works. There was a connection here in that Priestley was attempting to explore the relationship between matter and spirit.
In 1780, retaining a life annuity from Shelburne, Priestley returned to the ministry at Birmingham’s New Meeting. Here he found convivial philosophical and scientific company in the Lunar Society composed of rising industrialists and intellectuals such as Mathew Boulton, James Watt, Josiah Wedgwood and Erasmus Darwin. Although its members were united in their support for the American War of Independence and for the initial stages of the French Revolution, it was Priestley the preacher-orator who was publicly identified with radical criticism of English politics and the discrimination against Dissenters. In 1791 a ‘Church and King’ mob destroyed Priestley’s home and chapel, forcing him to flee to London. Although he was eventually compensated for the loss of his property, in 1794 he decided to emigrate and to join two of his sons in America.
Here he was warmly welcomed in Philadelphia, where he was offered the Chair of Chemistry at the University of Pennsylvania. Instead, Priestley moved to Northumberland, in rural Pennsylvania, where he hoped to found an academy for the sons and daughters of political refugees who would join him there. It did not work out and Priestley spent his declining years cut off by distance from European, and even American, intelligence, and fighting a rearguard action against Lavoisier’s chemistry in his fascinating Considerations on the Doctrine of Phlogiston (1796). Although outmanoeuvred by Lavoisier, Priestley lived on in two ways. His young executor, Thomas Cooper (1759–1839), a fellow refugee from English politics, acquired sufficient up-to-date knowledge of chemistry from studying in Priestley’s library and laboratory to become one of America’s leading chemical educators. A century after Priestley’s discovery of oxygen, in August 1874, a national meeting of chemists, gathered at his home in Northumberland (now a Priestley Museum), decided to create the American Chemical Society.
It was Cavendish who began the collection of water-soluble gases over mercury, but Priestley who brought their study and manipulation to perfection. Curiously, believing that chemistry, like physics, required expensive and complicated instruments, Lavoisier only rarely used the pneumatic trough; instead, he developed an expensive and sophisticated gasometer. A good third of Lavoisier’s Elements of Chemistry was devoted to chemical apparatus. Until the appearance of Michael Faraday’s Chemical Manipulation in 1827, Lavoisier’s descriptions remained the bible of instrumentation and chemical manipulative techniques.
In the spring of 1772, Lavoisier read an essay on phlogiston by a Dijon lawyer and part-time chemist, Louis-Bernard Guyton de Morveau (hereafter Guyton) (1737–1816). In a brilliantly designed experimental investigation, Guyton showed that all his tested metals increased in weight when they were roasted in air; and since he still believed that their combustibility was caused by a loss of phlogiston, he saved the phenomena by supposing that phlogiston was so light a substance that it ‘buoyed’ up the bodies that contained it. Its loss during decomposition therefore caused an increase of weight. Most academicians, including Lavoisier, thought Guyton’s explanation absurd. Following his previous reflections on the role of air, Lavoisier speculated immediately that a more likely explanation was that, somehow, air was being ‘fixed’ during the combustion and that this air was the cause of the increase in weight. It followed that ‘fixed air’ should be released when calces were decomposed – just as Hales’ earlier experiments in Vegetable Staticks had suggested.
One final Encyclopédie article seems to have influenced Lavoisier decisively at this juncture. This was an essay on ‘expansibility’ published in the sixth volume in 1756 by another pupil of Rouelle’s, the philosopher and civil servant, Jacques Turgot. Like Lavoisier, Turgot combined a career of public service with spare-time research in chemistry. But he never published his reflections (or if he did so, he did it anonymously), and we only know of his interesting thoughts from his private correspondence. Turgot arrived independently at the same solution as Lavoisier, namely that Guyton’s experiments could be explained as due to the fixing of air. He had actually learned of Guyton’s work before Lavoisier in August 1771. In a private letter to Condorcet, Turgot noted
(#litres_trial_promo):
The air, a ponderable substance which constantly enters into the state of a vapour or expansive fluid according to the degree of heat contained, but which is also capable of uniting with all the other principles of bodies and forming in that state part of the constitution of different compounds … this air combines or separates in different chemical reactions because of a greater or lesser affinity that it has for the principles to which it was attached or with those that one presents to it.
Given that Lavoisier was party to the same intellectual influences as Turgot, it was not surprising that they should have reached the same conclusions. Whether Lavoisier was aware or not of Turgot’s thoughts, he took pains constantly to preserve priority of the idea that it was air that was fixed in calcination, rather than liberated, as he had first thought earlier in 1772. If air was an expanded fluid combined with phlogiston, as Turgot’s Encyclopédie article had suggested, then the phlogiston released during combustion (the process of ‘fixing air’) would explain the heat and light generated during the reaction. It followed that heat and light came from the air, not the metal as the Stahlians had always maintained:
Lavoisier was able to verify this in October 1772 by using a large burning lens belonging to the Academy. When litharge (an oxide of lead) was roasted with charcoal, an enormous volume of ‘air’ was, indeed, liberated. In order to investigate this phenomenon more closely, and in order to ensure priority after finding that sulphur and phosphorus also gained in weight when burned in air, Lavoisier deposited a sealed account of his findings in the archives of the Academy, which he allowed to be opened in May 1773
(#litres_trial_promo):
What is observed in the combustion of sulphur and phosphorus, may take place also with all bodies which acquire weight by combustion and calcination, and I am persuaded that the augmentation of the metallic calces is owing to the same cause. Experiment has completely confirmed my conjectures: I have carried out a reduction of litharge in a closed vessel, with the apparatus of Hales, and I have observed that there is disengaged at the moment of passage from the calx to the metal, a considerable quantity of air, and that this air forms a volume a thousand times as great as the quantity of litharge employed. This discovery seems to me one of the most interesting that has been made since Stahl and as it is difficult in conversation with friends not to drop a hint of something that would set them on the right track, I thought I ought to make the present deposition into the hand of the Secretary of the Academy until I make my experiments public.
In committing himself to the hypothesis that ordinary air was responsible for combustion and for the increased weight of burning bodies, Lavoisier demonstrated that he was ignorant of most contemporary chemical work on the many different kinds of airs that can be produced in chemical reactions. In Scotland, a decade earlier in 1756, Joseph Black had succeeded in demonstrating that what we call ‘carbonates’ (e.g. magnesium carbonate) contained a fixed air (carbon dioxide) that was fundamentally different in its properties from ordinary atmospheric air. Unlike ordinary air, for example, it turned lime water milky and it would not support combustion. Black’s work did not achieve much publicity or publication in France until March 1773. A few years later, Henry Cavendish studied the properties of a light inflammable air (hydrogen), which he prepared by adding dilute sulphuric acid to iron. These experiments were to stimulate the astonishing industry of Priestley who, between 1770 and 1800, prepared and differentiated some twenty new ‘airs’. These included (in our terminology) the oxides of sulphur and nitrogen, carbon monoxide, hydrogen chloride and oxygen. The fact that most of these were ‘acid’ airs was to be, for Lavoisier, an intriguing phenomenon.
Hence, although largely unknown to Lavoisier in 1772, there was already considerable evidence that atmospheric air was a complex body and that it would be by no means sufficient to claim that air alone was responsible for combustion. Lavoisier seems to have been aware of his chemical ignorance. He wrote in his laboratory notebook on 20 February 1773:
I have felt bound to look upon all that has been done before me as merely suggestive. I have proposed to repeat it all with new safeguards, in order to link our knowledge of the air that goes into combination or is liberated from substances, with other acquired knowledge, and to form a theory.
And, with the firm and confident intention of bringing about, in his own prescient words, ‘a revolution of physics and chemistry’, he spent the whole of 1773 studying the history of chemistry – reading everything that chemists had ever said about air or airs since the seventeenth century and repeating their experiments ‘with new safeguards’. His results were summarized in Opuscules physiques et chimiques published in January 1774.
Ironically, far from clarifying his ideas, his new-found familiarity with the work of pneumatic chemists now led him to suppose that carbon dioxide, ‘fixed air’, in the atmosphere was responsible for the burning of metals and the increase of their weight. This was not unreasonable, and the explanation for Lavoisier’s misconception will be clear. Most calces (that is, oxides) can only be reduced to the metal by burning them with the reducing agent, charcoal (C), when the gas carbon dioxide is produced:
calx + C → metal + fixed air
It was easy to suppose, therefore, that the same fixed air was responsible for combustion:
metal + fixed air → calx
As he noted plaintively in a notebook
(#litres_trial_promo):
I have sometimes created an objection against my own system of metallic reduction which consists of the following: lime [CaO] according to me is a calcareous earth deprived of air; the metallic calces, on the contrary, are metals saturated with air. However, both produce a similar effect on alkalies, they render them caustic.
Obviously, Lavoisier needed to distinguish between air and fixed air, carbon dioxide. It should be noted how this reasoning was based upon the complementarity of analysis and synthesis. If two simple substances could be combined together to form a compound, then, in principle, it ought to be possible to decompose the compound back into the same components. Lavoisier was to find a perfect example of this in the red calx of mercury, a substance that caused him to revise his original hypothesis significantly.
Two things caused Lavoisier to change his mind. First, his attention was drawn by Pierre Bayen, a Parisian pharmacist, to the fact that, when heated, the calx of mercury (HgO), a remedy used in the treatment of venereal disease, decomposed directly into the metal mercury without the addition of charcoal. No fixed air was evolved. As Bayen pointed out, this observation made it difficult to see how the phlogiston theory could be right. Here was a calx regenerating the metal without the aid of phlogiston in the form of charcoal! Secondly, the mercury calx had also come to the attention of Priestley because of a contemporary uncertainty whether the red calx produced by heating nitrated mercury was the same as that produced when mercury was heated in air. In August 1774 he heated the calx in an enclosed vessel and collected a new ‘dephlogisticated air’, which he found, after some months of confusing it with nitrous oxide, supported combustion far better than ordinary air did. Unknown to Priestley the Swedish apothecary, Scheele, had already isolated what he called ‘fire air’ from a variety of oxides and carbonates in the years 1771–2. But Scheele, working in isolation even in Sweden, did not help to shape Lavoisier’s views in the same way that Bayen and Priestley did. These experiments were reported directly to Lavoisier by Priestley when he was on a visit to Paris during October 1774, but he also published an account of the new air at the end of the same year.
Bayen’s and Priestley’s observations, together with his own experiments with mercuric oxide, caused Lavoisier to revise his hypothesis of 1774. In April 1775, Lavoisier read a paper to the Academy of Sciences ‘on the principle which combines with metals during calcination and increases their weight’ in which, still more confused, he identified the principle of combustion with ‘pure air’ and not any particular constituent of the air. This new hypothesis, which was published in May, was seen by Priestley. The latter, realizing that Lavoisier had not quite grasped that the ‘dephlogisticated air’ generated from the calx of mercury was a constituent part of ordinary air, gently put him right in another book he published at the end of 1775. This, together with further experiments of his own, finally led Lavoisier to the oxygen theory of combustion. In revising the so-called ‘Easter Memoir’ for publication in 1778, and in an essay published the year before, he wrote as follows:
The principle which unites with metals during calcination, which increases their weight and which is a constituent part of the calx is: nothing else than the healthiest and purest part of air, which after entering into combination with a metal, [can be] set free again; and emerge in an eminently respirable condition, more suited than atmospheric air to support ignition and combustion.
Because this ‘eminently respirable air’ burned carbon to form the weak acid, carbon dioxide, while non-metals generally formed acidic oxides, Lavoisier called the new substance oxygen, meaning ‘acid former’
