Читать книгу The Fontana History of Chemistry - William Brock J. - Страница 27

Lavoisier was now in a position to bring about a revolution in chemistry by ridding it of phlogiston and by introducing a new theory of composition. His first move in this direction was made in 1785 in an essay attacking the concept of phlogiston. Since all chemical phenomena were explicable without its aid, it seemed highly improbable that the substance existed. He concluded:

All these reflections confirm what I have advanced, what I set out to prove [in 1773] and what I am going to repeat again. Chemists have made phlogiston a vague principle, which is not strictly defined and which consequently fits all the explanations demanded of it. Sometimes it has weight, sometimes it has not; sometimes it is free fire, sometimes it is fire combined with an earth; sometimes it passes through the pores of vessels, sometimes they are impenetrable to it. It explains at once causticity and non-causticity, transparency and opacity, colour and the absence of colours. It is a veritable Proteus that changes its form every instant!

By collaborating with younger assistants, whom he gradually converted to his way of interpreting combustion, acidity, respiration and other chemical phenomena, and by twice-weekly soirées at his home for visiting scientists where demonstrations and discussions could be held, Lavoisier gradually won over a devoted group of anti-phlogistonists. Finding that editorial control of the monthly Journal de physique had been seized by a phlogistonist, Lavoisier and his young disciple, Pierre Adet (1763–1834), founded their own journal, the Annales de Chimie in April 1789. The editorial board soon included most converts to the new system: Guyton, Berthollet, Fourcroy, G. Monge, A. Seguin and N. L. Vauquelin. This is still a leading chemical periodical. While Director of the Academy of Sciences from 1785, Lavoisier was also able to alter its structure so that the chemistry section consisted only of anti-phlogistonists.

It is significant that Lavoisier’s new theory was one of acidity as much as combustion. Stahlian chemists had not foreseen that there were many types of ‘airs’ or gases, but, as Priestley’s career shows, they actually had little difficulty in conceptualizing them within a phlogistic framework. The appearance of gases also led to a modification in the phlogistic theory of acidity. According to Stahl, vitriolic acid (sulphuric acid) was the universal acid – ‘universal’ in the sense of being the acid principle present in all substances that displayed acidic properties. However, with the discovery of fixed air, several chemists, led by Bergman in Sweden, had decided that this, not vitriol, was the true universal acid. Such a view was argued vociferously by the Italian, Marsilio Landriani, during the 1770s and 1780s. Landriani claimed to have found evidence that fixed air was a component of all three mineral acids as well as the growing number of vegetable acids such as formic, acetic, tartaric and saccharic acids. It was really this theory of acidity that Lavoisier had to challenge in the 1780s.

Lavoisier’s method was to challenge the theory as displayed in the French translation undertaken by his wife of Richard Kirwan’s Essay on Phlogiston and the Constitution of Acids. He was able to convince Kirwan that the acidity of fixed air was sufficiently explained by the fact that it contained oxygen. The irony here was that Lavoisier’s new theory retained in effect the Stahlian notion of a universal acid principle in the form of oxygen. In practice, the explanation of properties by principles was not to last much longer after the advent of Dalton’s atomism and the evidence that not all acids contained oxygen.

The demonstration by Hales that fixed air formed part of the composition of many solids and liquids had also given rise to speculations that this air was vital to vegetable and animal metabolisms. For example, in 1764, an Irish physician, David Macbride, concluded that ‘this air, extensively united with every part of our body’, served to prevent putrefaction, a prime example of which was the disease called scurvy. The recognized value of fresh vegetables in inhibiting scurvy, he suggested, was due to their fermentative powers. The fixed air that they produced during digestion served to prevent putrefaction inside the body.

It was this suggestion that inspired Priestley to investigate the effects of airs on living organisms – a programme of research that was to form the basis of Davy’s earliest research some time later. Initially, in 1772, Priestley concluded that fixed air was fatal to vegetable life, but this was probably due to the fact that he used impure carbon dioxide from a brewery, or that he was using it in excess. Others, including Priestley’s Mancunian friend, Thomas Henry, found the opposite, that flowers thrived in fixed air. It was while repeating these findings that Priestley discovered that, in the presence of sunlight (but not otherwise), plants growing in water, such as sprigs of mint, gave off dephlogisticated air. This had already been anticipated in 1779 by Jan Ingenhousz (1730–99) who, together with Jean Senebier (1742–1809) in Geneva, laid the foundations of a theory of photosynthesis in plants.

Three particularly important converts to the new chemistry were Guyton (whose work had earlier catalysed Lavoisier’s interest in combustion), Claude-Louis Berthollet (1748–1822) and Antoine Fourcroy (1755–1809). Berthollet’s conversion to Lavoisier’s views seems to have arisen because of his own perturbation at the weight changes involved in calcination, to which Guyton had drawn attention. In his Observations sur l’air (1776), Berthollet explained acidity and weight changes in combustion by means of fixed air, and otherwise incorporated Lavoisier’s work on oxygen into the phlogiston framework. It was the analysis of water, together with increasing personal contact with Lavoisier in the Academy, where they found themselves drawing up joint referees’ reports, that converted Berthollet to Lavoisier’s position by 1785. In fact, Berthollet always had certain reservations. In particular, he never accepted the oxygen theory of acidity, and his investigation of chlorine (first prepared by Scheele in 1774 and assumed by Lavoisier to be oxygenated muriatic acid) seemed to confirm his doubts. In later life he also firmly rejected the notion that chemical properties could be explained in terms of property-bearing principles.

Fourcroy was Lavoisier’s principal interpreter to the younger generation. His ten-volume Système des connaissances chimiques (1800) codified and organized chemistry for the next fifty years around the concepts of elements, acids, bases and salts. Fourcroy saw this structure not only as ‘consolidating the pneumatic doctrine’ but as affording ‘incalculable advantage(s)’ for learning and understanding chemistry (see Table 3.1).

While still a phlogistonist, Guyton was much exercised by the inconsistent nomenclature of chemists and pharmacists. Unlike botany and zoology, whose terminology had been revised and made more precise earlier in the century by the

TABLE 3.1 The contents of Fourcroy’s Système des connaissances chimiques (1800) arranged by classes of substances.

Class	Volume
1. Undecomposed or simple bodies	1
2. Burned bodies; oxides or acids	2
3. Salifiable bases; earths and alkalis	2
4. Salts	3, 4
5. Metals and metal salts containing an excess of acid (although belonging to class 1, these were dealt with separately because of their number and importance)	5, 6
6. Minerals	6
7. Vegetable compounds	7, 8
8. Animal compounds	9, 10

Swede, Karl Linnaeus, chemical language remained crude and confusing. In 1782 Guyton made a series of proposals for the systematization of chemical language.

Alchemical and chemical texts written before the end of the eighteenth century can be difficult to read because of the absence of any common chemical language. Greek, Hebrew, Arabic and Latin words are found, there was widespread use of analogy in naming chemicals or in referring to chemical processes, and the same substance might receive a different name according to the place from which it was derived (for example, Aquila coelestis for ammonia; ‘father and mother’ for sulphur and mercury; ‘gestation’ as a metaphor for reaction; ‘butter of antimony’ for deliquescent antimony chloride; and ‘Spanish green’ for copper acetate). Names might also be based upon smell, taste, consistency, crystalline form, colour, properties or uses. Although several of these names have lingered on as ‘trivial’ names (which have even had to be reintroduced in organic chemistry in the twentieth century because systematic names are too long to speak), Lavoisier and his colleagues in 1797 decided to systematize nomenclature by basing it solely upon what was known of a substance’s composition. Since the theory of composition chosen was the oxygen system, Lavoisier’s suggestions were initially resisted by phlogistonists; adoption of the new nomenclature involved a commitment to the new chemistry.

Following the inspiration of Linnaeus, Guyton suggested in 1782 that chemical language should be based upon three principles: substances should have one fixed name; names ought to reflect composition when known (and if unknown, they should be non-committal); and names should generally be chosen from Greek and Latin roots and be euphonious with the French language. In 1787, Guyton, together with Lavoisier, Berthollet and Fourcroy, published the 300-page Méthode de nomenclature chimique, which appeared in English and German translations a year later. One-third of this book consisted of a dictionary, which enabled the reader to identify the new name of a substance from its older one. For example, ‘oil of vitriol’ became ‘sulphuric acid’ and its salts ‘sulphates’ instead of ‘vitriols’; ‘flowers of zinc’ became ‘zinc oxide’.

Perhaps the most significant assumption in the nomenclature was that substances that could not be decomposed were simple (i.e. elements), and that their names should form the basis of the entire nomenclature. Thus the elements oxygen and sulphur would combine to form either sulphurous or sulphuric acids depending on the quantity of oxygen combined. These acids when combined with metallic oxides would form the two groups of salts, sulphites and sulphates. In the case of what later became called hydrochloric acid, Lavoisier assumed that he was dealing with an oxide of an unknown element, murium. Because of some confusion over the differences between hypochlorous and hydrochloric acids, in Lavoisier’s nomenclature hydrochloric acid became muriatic acid and the future chlorine was ‘oxygenated muriatic acid’. The issue of whether the latter contained oxygen at all was to be the subject of fierce debate between Davy, Gay-Lussac and Berzelius during the three decades following Lavoisier’s death.

The French system also included suggestions by Hassenfratz and Adet for ways in which chemicals could be symbolized by geometrical patterns: elements were straight lines at various inclinations, metals were circles, alkalis were triangles. However, such symbols were inconvenient for printers and never became widely established; a more convenient system was to be devised by Berzelius a quarter of a century later.

During the eighteenth century some chemists had turned their minds to quantification and the possible role of mathematics in chemistry. On the whole, most chemists agreed with Macquer that chemistry was insufficiently advanced to be treated mathematically. Although he believed, correctly as it turned out, that the weight of bodies bore some relationship to chemical properties and reactions, the emphasis on affinity suggested that the project was hopeless. Nevertheless, Lavoisier, inspired by the writings of the philosopher, Condillac, believed fervently that algebra was the language to which scientific statements should aspire⁷:

We think only through the medium of words. Languages are true analytical methods. Algebra, which is adapted to its purpose in every species of expression, in the most simple, most exact, and best manner possible, is at the same time a language and an analytical method. The art of reasoning is nothing more than a language well arranged.

In a paper on the composition of water published in 1785, Lavoisier stressed that his work was based upon repeated measuring and weighing experiments ‘without which neither physics nor chemistry can any longer admit anything whatever’. Again, in another essay analysing the way metals dissolve in acids, Lavoisier used the Hassenfratz – Adet symbols:

In order to show at a glance the results of what happens in the solution of metals, I have constituted formulae of a kind that could at first be taken for algebraic formulae, but which do not have the same object and which do not derive from the same principles; we are still very far from being able to obtain mathematical precision in chemistry and therefore I beg you to consider the formulae that I am going to give you only as simple annotations, the object of which is to ease the workings of the mind.

The important point here was that Lavoisier used symbols to denote both constitution and quantity. Although he did not use an equals sign, he had effectively hit upon the idea of a chemical equation. As we shall see, once Berzelius’ symbols became firmly established in the 1830s, chemists began almost immediately to use equations to represent chemical reactions.

While producing the Méthode de nomenclature chimique with Lavoisier and the others, Guyton was converted to the new chemistry. Because the new language was also the vehicle of anti-phlogiston chemistry, it aroused much opposition. Nevertheless, through translation, it rapidly became and still remains the international language of chemistry.

TABLE 3.2 Lavoisier’s ‘elements’ or ‘simple substances’.

Lavoisier’s final piece of propaganda for the new chemistry was a textbook published in 1789 called Traité élémentaire de chimie (An Elementary Treatise on Chemistry). Together with Fourcroy’s larger text (published in 1801), this became a model for chemical instruction for several decades. In it Lavoisier defined the chemical element pragmatically and operationally as any substance that could not be analysed by chemical means. Such a definition was already a commonplace in mineralogical chemistry and metallurgy, where the analytical definition of simple substances had become the basis of mineralogical classification in the hands of J. H. Pott, A. F. Cronstedt and T. Bergman. It was for this reason that Lavoisier’s list of 33 basic substances bore some resemblance to the headings of the columns in traditional affinity tables. Lavoisier’s list included substances such as barytes, magnesia and silica, which later proved to be compound bodies.

After discussing the oxygen theory in part I of the Traité, he discussed their preparation and properties, their oxides and then their salts formed from acidic and basic oxides in part II. Caloric disengaged from oxygen explained the heat and light of combustion. It has been said that the elements formed the bricks while his new views on calcination and combustion formed the blueprint. The Traité itself formed a dualistic compositional edifice. Whenever an acidic earth and metal oxide (or earth) combined, they produced a salt, the oxygen they shared constituting a bond of union between them. As was appropriate for an elementary text, part III, a good third of the book, was devoted to chemical instrumentation and to the art of practical chemistry.

Lavoisier’s table of elements did not include the alkalis, soda and potash, even though these had not been decomposed. Why were they excluded from his pragmatic definition of simple substances? Two reasons have been suggested. In the first place, he was prepared to violate his criterion because of the chemical analogy between these two alkalis and ‘ammonia’, which Berthollet had decomposed into azote (nitrogen) and hydrogen in 1785. Lavoisier was so confident that soda and potash would be similarly decomposed into nitrogen and other unknown principles, that he withheld them from the table of simple substances. On the other hand, although confident that muriatic acid was also compound, because the evidence was not so strong as for the alkalis, he included it in the list of elements. While we may admire Lavoisier’s prescience – Davy was to decompose soda and potash in 1808 – this was a disturbing violation of his own pragmatism. What guarantee did the chemist have that any of Lavoisier’s simple substances were really simple? As we shall see, Lavoisier’s operational approach caused a century of uncertainty and helped to revive the fortunes of the ancient idea of primary matter.

A second explanation is more subtle. Lavoisier’s simple substances were arranged into four groups (see table 3.2). Three of the groups contained the six non-metals and seventeen metals then known, both of which were readily oxidizable and acidifiable, together with the group of five simple ‘earths’. The remaining group was light, caloric, oxygen, azote (nitrogen) and hydrogen. At first glance these elements appear to have nothing in common, but the heading Lavoisier gave them, ‘simple substances belonging to all the kingdoms of nature, which may be considered the elements of bodies’, provides the clue. Lavoisier probably saw these five elements as ‘principles’ that conveyed fundamental generic properties. Light was evidently a fundamental principle of vegetable chemistry; caloric was a principle of heat and expansibility; oxygen was the principle of acidity; hydrogen was the principle of water that played a fundamental role in all three kingdoms of Nature; and nitrogen was a principle of alkalinity. If the 1789 list of elements is compared with a preliminary list he published in 1787, it is found that azote was moved from its original position among the non-metals. It is not unlikely that this change was connected with the decomposition of ammonia and Lavoisier’s decision that soda and potash were compounds of ‘alcaligne’, a nitrogenous principle of alkalinity.

If this interpretation is correct, it illustrates again the role of continuity in Lavoisier’s revolutionary chemistry. Although we cannot now know if this was the position Lavoisier held – a position that was in any case subject to refutation and modification within a few years – it is intriguing to notice that organic chemists (beginning with Liebig) came to see certain elements, namely hydrogen, oxygen, carbon and nitrogen, as the ‘universal’ or ‘typical’ elements of mineral, animal and vegetable chemistry. It was on the basis of this that Gerhardt and Hofmann were to build a ‘type theory’ or organic classification and from which Mendeleev was to learn to classify a greatly extended list of elements in 1869.

By the mid 1790s the anti-phlogistonian camp had triumphed and only a few prominent chemists, such as Joseph Priestley, continued as significant critics. Unfortunately, by then the French Revolution had put paid to the possibility that Lavoisier would apply his insights to fresh fields of chemistry.