Читать книгу The Fontana History of Chemistry - William Brock J. - Страница 22

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’.