The Fontana History of Chemistry

Newton’s ether, the active principle of chemical change, was exploited by large numbers of eighteenth-century chemists, including the important Dutch teacher, Hermann Boerhaave (1668–1738). The latter’s Elementa Chemiae (1732), which appeared in English in 1741, assimilated ether to fire. Fire, said Boerhaave, consisted of subtle, immutable bodies that were capable of insinuating themselves into the pores of bodies; it was ‘the great changer of all things in the universe, while itself remaining unchanged’. Like his German contemporary Georg Stahl, whose work he ignored, Boerhaave treated fire, together with the other three Aristotelian elements, as one of the four ‘physical instruments’ available to chemists. Because of the connections that were established between the Scottish universities and the University of Leiden, where Boerhaave taught, Boerhaave came to have considerable influence on the teaching of chemistry to medical students in Scotland by William Cullen (1710–90) and his pupil, Joseph Black (1728–99).

Cullen, for example, explained chemical attraction as due to the self-repulsive character of the particles of etherial fire and to the relative densities of ether within two attracting bodies compared with the density of ether in the external environment. The solid and liquid states similarly depended upon the relative quantities of ether and ordinary matter within a substance – a model that was to have important consequences for the conceptualization of gases. This identification of ether and fire, or heat, stimulated Cullen’s pupil, Joseph Black, to the study of calorimetry, the establishment of the concepts of specific heat capacity and latent heat, and the exploration of the qualitative difference between air and a ‘fixed air’ (carbon dioxide), whose presence in magnesia alba (basic magnesium carbonate) he had deduced in 1766.

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

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 was4:

… 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

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

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

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’.