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.
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