Читать книгу The Fontana History of Chemistry - William Brock J. - Страница 35

Dalton presented his theory within the context of ideas concerning heat at a time when the chemical world had become excited by the news of galvanic or current electricity. In 1800 the Italian physicist, Alessandro Volta (1774–1827), described his ‘voltaic pile’ or battery in a paper published by the Royal Society. This simple machine made from a ‘pile’ or ‘battery’ of alternating zinc and silver discs gave chemists a powerful new analytical tool. As Davy said later, its use caused great excitement and it acted as ‘an alarm-bell to experimenters in every part of Europe’. Almost immediately it was found that the battery would decompose water into its elements. While there was nothing extraordinary about this further confirmation of Lavoisier’s chemistry, the puzzling fact was that hydrogen and oxygen were ejected from the water at different poles – the hydrogen at what Volta designated as the negative pole, and the oxygen at the positive pole. Two chemists who particularly concerned themselves with this galvanic phenomenon (the term ‘electrolysis’ was not coined by Faraday until 1832) were Davy and Berzelius.

Humphry Davy (1778–1829) was born at Penzance in Cornwall and educated locally. Intending to qualify as a doctor, he was apprenticed to a surgeon in 1795 and began to read Lavoisier’s Elements of Chemistry in French in his spare time. Though ignorant and completely self-taught, like Priestley before him, Davy began to repeat, correct and devise new experiments. Apart from this growing interest in chemistry, he wrote poetry (for this was the era of Romanticism when young men poured forth their individual feelings in verse), he admired the rich Cornish scenery and he fished. Through a friendship with Gregory Watt, the tubercular son of James Watt, Davy came to the attention of Thomas Beddoes, a pupil of Joseph Black and a former lecturer in chemistry at the University of Oxford, who had resigned from ‘that place’ because of his support for the French Revolution and his suspiciously radical politics. In 1798 Beddoes, convinced that the many gases that Priestley had discovered might prove beneficial in the treatment of tuberculosis (TB) and other urban diseases, founded a subscription-based Pneumatic Institute in Bristol. He persuaded Davy, whom he recognized as a man of talent, to join him as a research assistant. Davy probably still expected to qualify as a doctor, perhaps by saving sufficient money to enter Edinburgh University as a result of this experience. In the event, he became a chemist.

Davy’s risky and foolhardy experiments at Bristol, in which he narrowly escaped suffocation on several occasions, brought him fame and notoriety in 1800 when he published his results in Researches, Chemical and Philosophical; Chiefly Concerning Nitrous Oxide … and its Respiration. None of his inhalations demonstrated chemotherapeutic benefits – though his results with nitrous oxide (laughing gas) were to be the cause of regular student ‘saturnalia’ in chemical laboratories throughout the nineteenth century. Not until 1846 was the gas used as an anaesthetic. This inhalation research, and some further essays published in 1799, which included an attack on Lavoisier’s notion of caloric and the substitution of light for caloric in gaseous oxygen (phosoxygen), brought Davy’s name to the attention of another patron, Benjamin Thompson, who had also denied that heat was an imponderable fluid.

Count Rumford, as he is better known, had founded the Royal Institution in London in 1799 as a venue for publicizing ways in which science could help to improve the quality of life of the deserving poor and for the rising middle classes. By 1801 Rumford needed a new Professor of Chemistry. Davy’s appointment coincided with the wave of contemporary interest in electrolytic phenomena and, although he lectured, dazzlingly, on many other subjects at the Royal Institution, it was his research on electrochemistry that captured the public’s imagination and ensured the middle-class success of the Institution.

By building bigger and more powerful batteries, and by using fused electrolytes rather than electrolytes in solution, Davy confirmed Lavoisier’s hunch that soda and potash were not elementary by isolating sodium and potassium in 1807. In the next few years he demonstrated that Lavoisier’s alkaline earths were also compounds and prepared calcium, strontium and barium electrolytically. Later still, Davy argued convincingly against the view that muriatic acid contained oxygen, and for the opinion that oxymuriatic acid, which he renamed chlorine, was an undecompounded elementary body – a point supported by his isolation of its conjoiner, iodine, in 1813.

This succession of corrections to Lavoisier’s chemistry has led some historians to feel that Davy set out systematically to destroy French chemistry. Indeed, by 1815 he had critically and effectively questioned most of the assumptions of the antiphlogistic chemistry – that acidity was due to oxygen, that properties were due to ‘principles’ rather than arrangement, that heat was an imponderable fluid rather than a motion of particles, and that Lavoisier’s elements were truly elementary. Although Davy was often bold in his speculations and use of analogical reasoning, in stripping Lavoisier’s system to its empirical essentials he did not replace it with any grand system of his own, except to suggest that chemical affinity was, in the final analysis, an electrical phenomenon.

In the early 1800s there were two different opinions on the cause of electrolysis. According to the ‘contact theory’ advocated by Volta, electricity arose from the mere contact of different metals; an imposed liquid merely acted as a conductor. Since this theory did not easily account for the fact that the conducting liquid was always decomposed, the alternative ‘chemical theory’ argued that it was the chemical decomposition that produced the electric current. Davy found fault with both theories and as so often in the history of science, he drew a compromise: the contact theory explained the ‘power of action’ of, say, zinc becoming positively charged when placed in contact with copper; this power then disturbed the chemical equilibrium of substances dissolved in water, leading to a ‘permanent action’ of the voltaic pile. As to the cause of the initial ‘power of action’, Davy was in no doubt that it was chemical affinity⁵:

Is not what has been called chemical affinity merely the union or coalescence of particles in naturally opposite states. And are not chemical attractions of particles and electrical attractions of masses owing to one property and governed by one simple law?

If Davy was the first chemist to link chemical reactivity with electrolytic phenomena, it was the Swede, Berzelius, who created an electrical theory of chemistry. Davy had concluded from his long and accurate work on electrolysis that, in general, combustible bodies and bases tended to be released at the negative pole, while oxygen and oxidized bodies were evolved at the positive pole⁶:

It will be a general expression of the facts in common philosophical language, to say, that hydrogen, the alkaline substances, the metals, and certain metallic oxides, are attracted by negatively metallic surfaces [i.e. electrodes]; and repelled by positively electrified metallic surfaces; and contrariwise, that oxygen and acid substances are attracted by positively electrified metallic surfaces, and repelled by negatively electrified metallic surfaces; and these attractive and repulsive forces are sufficiently energetic to destroy or suspend the usual operation of elective affinity.

Berzelius, with his patron-collaborator, William Hisinger, had reached the same conclusion independently in 1804, but only developed the important and influential electrochemical theory, which was to leave a permanent mark on chemistry, in 1810 after he had learned of Dalton’s atomic theory. Jöns Jacob Berzelius (1779–1848), after being brought up by his stepfather, studied medicine at the University of Uppsala. Here he read Fourcroy’s Philosophie chimique (1792) and became convinced of Lavoisier’s new system. A competent reader and writer of English, French and German, and alert to the latest developments outside Sweden, his graduation thesis in 1802 was on the medical applications of galvanism. This brought him to the attention of Hisinger, a wealthy mine owner, who invited Berzelius to use the facilities of his home laboratory in Stockholm. Together they not only drew important conclusions about electrolysis, but discovered a new element, ‘ceria’, in 1803, which later turned out to be the parent of several ‘rare-earth’ elements (see chapter 9).

By 1807 Berzelius had become independent of Hisinger’s patronage when he was elected to a Chair of Chemistry and Pharmacy at the Carolian Medico-Chirurgical Institute in Stockholm. His light lecturing duties allowed him plenty of time to research in the Institute’s modest laboratory. Elected a member of the Swedish Academy of Sciences in 1808, in 1818 he became one of its joint secretaries. The appointment included a grace-and-favour house in which he built a simple laboratory adjacent to the kitchen. Here he took occasional pupils, such as Mitscherlich and Wöhler.

Berzelius first learned of Dalton when planning his own influential textbook, Larbok i kemien, the first volume of which was published in 1808. Somehow Berzelius had acquired a copy of Richter’s writings on stoichiometry (he remarked on how unusual this was) and so learned of the law of reciprocal proportions and of the idea of equivalents. He saw immediately how useful these generalizations were for analytical chemistry. An avid follower of British chemical investigations, Berzelius learned of Dalton’s theory when he read a reference to it in Wollaston’s report on multiple proportions in Nicholson’s Journal. Because of the European wars, which made scientific communication difficult, he was unable to obtain a copy of Dalton’s New System (from Dalton himself) until 1812. Nevertheless, just from Wollaston’s brief account he saw immediately that a corpuscular interpretation of these analytical regularities was ‘the greatest step which chemistry had made towards its completion as a science’.

His own analytical results more than confirmed that, whenever substances combined together in different proportions, they were always, as Dalton had already concluded, in the proportions A + B, A + 2B, 2A + 3B, A + 4B, etc. Berzelius reconciled this regularity with Berthollet’s views on the influence of mass in chemical reactions. He agreed that Berthollet was right in supposing that substances could combine together in varying proportions; but these proportions were never continuously variable, as Berthollet had argued against Proust, but fixed according to Dalton’s corpuscular ratios.

Berzelius’ teaching duties included the training of pharmacists. He was, therefore, conscious of the fact that the Swedish Pharmacopoeia had not been revised since the days of phlogiston chemistry and that by 1810 its language had become embarrassingly out of date. In 1811, in an attempt to persuade the government to make a sensible decision on its pharmaceutical nomenclature, Berzelius devised a new Latin classification of substances, which exploited the electrochemical phenomena that he and Davy had studied, and firmly founded the organization of ponderable matter on the dualistic system that lay at the basis of Lavoisier’s antiphlogistic nomenclature.

Ponderable bodies were divided into two classes, ‘electropositive’ and ‘electronegative’ according to whether during electrolysis they were deposited or evolved around the positive or negative pole. Since these definitions reversed

FIGURE 4.1 Berzelius’ classification of substances. (Based on C. A. Russell, Annals of Science, 19 (1963): 124.)

the convention that Davy had already introduced, Berzelius was soon obliged to conform to the definition that electropositive substances were attracted to the negative pole. It was because of the theoretical implications of galvanic language that Faraday, in 1832, introduced the valueneutral nomenclature of electrodes, cathodes, anodes and so on. Berzelius’ electropositive and electronegative substances then became anions and cations respectively.

Oxygen, according to Berzelius, was unique in its extreme electronegativity. Other, less electronegative substances, like sulphur, could be positive towards oxygen and negative towards metals. On combination, a small residual contact charge was left, which allowed further combination to occur to form salts and complex salts. Thus, electropositive metals might form electropositive (basic) oxides (as electrolysis demonstrated), which would combine with electronegative acidic oxides to form neutral salts. The latter, however, might still have a residual charge that allowed them to hydrate and to form complex salts:

The scheme allowed the elements to be arranged in an electrochemical series from oxygen to potassium, based upon the electrolytic behaviour of elements and their oxides. Because salts were defined as combinations of oxides, Berzelius had to insist for a long time that chlorine and iodine were oxides of unknown elements, and that ammonia was similarly an oxide of ‘ammonia’. It was not until the 1820s that Berzelius finally capitulated and agreed that chlorine, iodine and bromine (which he placed in the special category of forming electronegative ‘haloid’ salts) were elements and that ammonia was a compound of nitrogen and hydrogen only.

It was this electrochemical system which was to have far-reaching analogical implications for the classification of organic substances. It also allowed Berzelius in 1813 to introduce a rational symbolism based upon the Latin names of the elements. Compounds were denoted by a plus sign between the constituents, as in copper oxide, Cu + O, the electropositive element being written first. Later, Berzelius dispensed with the plus sign and set the two elements side by side as in algebra. Different numbers of elements were then indicated by superscripts, e.g. S²O^3, a molecule of ‘hyposulphuric acid’. These joined symbols, which were criticized initially for being potentially confusing with algebraic symbolism, only began to be used in the 1830s. It was Liebig who, in 1834, introduced the subscript convention we still use today, though French chemists went on using superscripts well into the twentieth century. Because of the importance of oxygen in Berzelius’ system, he abbreviated it to a dot over its electropositive congener, i.e. Cu = Cu + O. In 1827 he extended this to sulphur, which was indicated by a comma, i.e. copper sulphide, Cú.

In a further ‘simplification’, which in practice wrought havoc in the classification of organic compounds and in communication between chemists, Berzelius in 1827 introduced ‘barred’ or underlined symbols to indicate two atoms of an element. (Since the bar was one-third up the stem of the symbol it involved printers making a special type, thereby losing one advantage over Dalton’s symbols; hence the use of underlined symbols in some texts.) The symbols for water and potash alum thus became, respectively:

Although Berzelius introduced symbols as a memory aid to chemical proportions, they were initially adopted by few chemists. Berzelius himself virtually ignored his own suggestions until 1827, when he published the organic chemistry section of his textbook, which appeared in German and French translations soon afterwards. Indeed, the development of organic chemistry was undoubtedly the key factor into pushing chemists into symbolic representations. Following the determination of a group of younger British chemists to introduce Continental organic research into Britain, Edward Turner employed Berzelius’ symbols in the fourth edition of his Elements of Chemistry in 1834. From then on, together with chemical equations, whose use in Britain was pioneered by Thomas Graham, symbols became an indispensable part of chemical communication.

TABLE 4.2 The development of the chemical equation.

As we have seen, Dalton angrily rejected Berzelius’ symbols mainly on the grounds that they did not indicate structure but were merely synoptic. Nor was he at all pleased with the way Berzelius had taken over his creation and transformed it electrochemically. On his part, Berzelius, after struggling for years to obtain a copy of Dalton’s New System, expressed deep disappointment with the book when he eventually read it in 1812⁷:

I have been able to skim through the book in haste, but I will not conceal that I was surprised to see how the author has disappointed my hopes. Incorrect even in the mathematical part (e.g. in determining the maximum density of water), in the chemical part he allows himself lapses from the truth at which we have the right to be astonished.

Berzelius’ extensive account of his interpretation of Dalton’s theory was published in English in Thomas Thomson’s monthly Annals of Philosophy in 1813. These articles were criticized by Dalton on at least five grounds. Whereas Dalton could see no good reason geometrically why atoms had to be spherical or all the same size, these were cardinal assumptions of Berzelius, who put them to good use in 1819 when he explained the isomorphism of crystals that Mitscherlich had discovered when studying with him in Stockholm. (Isomorphism refers to the fact that a family of salts containing different metals tend to have similar or identical crystal shapes.) Again, unlike Dalton, Berzelius refused to allow combinations of the type 2A + 2B or 2A + 3B on the grounds that, logically, nothing would prevent such ‘atoms’ from being divided. Dalton disagreed, since self-repulsions could be appealed to. Only after a lifetime’s analysis, in 1831, did Berzelius accept that occasionally two atoms of an element could combine with two or more other atoms. Before then this had led Berzelius to assume that all metallic oxides had the form MO. In the cases of the alkali metals and of silver, which are actually M₂