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… the purest air, eminently respirable air, is the principle constituting acidity; this principle is common to all acids.
The etymology, for those who no longer read Greek, is still obvious in the German word for oxygen, Sauerstoff. By this Lavoisier did not mean that all substances containing oxygen were acids, otherwise he would have been hard pressed to explain the basic reactions of metallic oxides. Oxygen was only a potentially acidifying principle; for its actualization, a non-metal had also to be present. Although soon destined to be overthrown as a model of acidity, this was the first chemical theory of acidity; it suggested a general way of preparing acids (by the oxidation of non-metals with nitric acid) and, in terms of ‘degrees of oxidation’, it provided for the time a very reasonable explanation of the different reactivities of acids.
By 1779 half of Lavoisier’s revolution was over. Oxygen gas was a ponderable element containing heat (or caloric, as Lavoisier called it to avoid the word phlogiston), which kept it in a gaseous state. On reacting with metals and non-metals, the heat was released and the oxygen element affixed to the substance, causing it to increase in weight. Metals formed basic oxides, non-metals formed acids (acid anhydrides). In respiration, oxygen burned the carbon in foodstuffs to form the carbon dioxide exhaled in breath, while the heat released was the source of an animal’s internal warmth. (Lavoisier and the mathematician, Pierre Simon Laplace, demonstrated this quantitatively with a guinea pig in 1783 – the origin of the expression ‘to be a guinea pig’.) Respiration was a slow form of combustion. The non-respirable part of air, mofette or azote, later called nitrogen, was exhaled unaltered.
At first glance, in this new theory, phlogiston seems to be transferred from a combustible, such as a metal, to oxygen gas. In reality, although Lavoisier waited some years before articulating the new theory in detail, there were major differences between caloric and phlogiston. Caloric was absorbed or emitted during most chemical reactions, not just those of oxidation and reduction; like Boerhaave’s etherial ‘fiery vigour’, it was present in all substances, whereas phlogiston was usually supposed absent from incombustibles; when added to a substance, caloric caused expansion or a change of state from solid to liquid, or liquid to gas; above all, caloric could be measured thermometrically, whereas phlogiston could not.
Nevertheless, Lavoisier did not challenge the old theory until 1785.
The principal reason why Lavoisier was unable to suggest in 1777 that chemists would be better off by abandoning the theory of phlogiston was that only this theory could explain why an inflammable air (in fact hydrogen) was evolved when a metal was treated with an acid, but no air was evolved when the basic oxide of the same metal was used. If the metal contained phlogiston, the explanation, as Cavendish suggested, was simple:
Lavoisier’s gas theory gave no hint why these two reactions behaved differently. Similarly, his belief that all non-metals burned to form an acid oxide appeared to be weakened by the case of hydrogen, which seemed to produce no identifiable product. If this seems odd, it must be borne in mind that moisture is so ubiquitous in chemical reactions that it must have been easy to ignore and overlook its presence.
It was Priestley who first noticed the presence of water when air and ‘inflammable air’ (hydrogen) were sparked together by means of an electrostatic machine. He described this observation to Cavendish in 1781, who repeated the experiment and reported it to the Royal Society in 1784:
By the experiments … it appeared that when inflammable air and common air are exploded in a proper proportion, almost all of the inflammable air, and near one-fifth of the common air, lose their elasticity and are condensed into dew. It appears that this dew is plain water.
Cavendish told Priestley verbally about his findings. Priestley then told his Birmingham friend James Watt, the instrument maker, who independently of Cavendish arrived at the conclusion that water must be a compound body of ‘pure air and phlogiston’. Watt made no statement to this effect until after Lavoisier announced his own experiments and conclusions, which themselves were triggered by references to Cavendish’s experiments that were made by Cavendish’s secretary, Charles Blagden, during a visit to Paris in 1783. Watt then claimed priority, but found himself forestalled by the prior appearance of Cavendish’s paper.
Much ink and rhetoric was to be spilled over rival claims – Cavendish or Watt in England, or Lavoisier in France. In fact, it was only Lavoisier who interpreted water as a compound of hydrogen and oxygen; Watt agreed, albeit within the conceptual framework of the phlogiston theory, while Cavendish instead viewed water as the product of the elimination of phlogiston from hydrogen and oxygen:
In other words, for Cavendish this was not a synthesis of water at all; instead, as a phlogistonist, he preferred to see inflammable air as water saturated with phlogiston and oxygen as water deprived of this substance. When placed together the product was water, which remained for him a simple substance. As we shall see, it was this same experiment of Cavendish’s that led him to record that nitrous acid was also produced – owing to the combination of oxygen with nitrogen – but that a small bubble of uncondensed air remained (chapter 9).
For Lavoisier, however, Cavendish’s work was evidence that water was not an element. Assisted by the mathematical physicist, Simon Laplace (1749–1827), he quickly showed that water could be synthesized by burning inflammable air and oxygen together in a closed vessel; and with the help of another assistant, Jean-Baptiste Meusnier, he showed that steam could be decomposed by passing it over red-hot iron. Priestley was never convinced by this analysis, arguing that the hydrogen could have come from the iron, not the water. The matter was settled (though never for Priestley) in 1789 when two Dutch chemists, Adriaan van Troostwijk (1752–1837) and Jan Deiman (1743–1808), synthesized water from its elements with an electric spark. The same electric machine could be used to decompose water into its constituents. Once current electricity became available with the voltaic cell in 1800, this same experiment was to usher in the age of electrochemistry. Given Lavoisier’s commitment to oxygen as an acid former, it is not surprising that he should have been so quick off the mark if Cavendish’s work provided him with an essential clue; in fact Lavoisier’s notebooks show that after 1781 he had repeatedly burned hydrogen in search of an acidic product.
Whatever the merits of the claim that Lavoisier was the first to grasp that water was a compound of hydrogen (meaning ‘water producer’) and oxygen, the important point was that he could now explain why metals dissolved in acids to produce hydrogen. This, he asserted, came not from the metal (as the phlogistonists claimed, some even identifying phlogiston with inflammable air), but from the water in which the acid oxide was dissolved:
Although it was left to Davy and others to develop the point, the understanding of water also helped lead to a hydrogen theory of acidity.
THE CHEMICAL REVOLUTION (#ulink_f2609218-441b-5206-9331-f5f941916e5a)
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.
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 aspire
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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.
THE AFTERMATH (#ulink_4e76879e-ea98-5f21-bf06-5d697f42b7d1)
Although opposition to Lavoisier’s chemistry remained strong in Germany for a decade or more, largely for patriotic reasons, and although Cavendish and Priestley never converted, the speed of its uptake is impressive. Much depended, of course, on key teachers. In Germany, Sigismund Hermstadt (1760–1832) translated the Traité in 1792, and in the same year Christoph Girtanner (1760–1800) published a survey of Lavoisier’s chemistry. At Edinburgh the French-born Joseph Black, who had always taught that phlogiston was a principle of levity, lectured on the new chemistry while not necessarily committing himself to it until 1790. His successor, Thomas Charles Hope (1766–1844), ensured that large audiences of medical students learned the new theory after 1787. Scottish opposition seems to have been largely confined to geology, where James Hutton found phlogiston more accommodating to his theory that it was solar light and his need for a plutonic ignitor in the absence of oxygen deep inside the earth; and animal physiology, where, despite Lavoisier’s view of animal heat as the natural exothermic product of burning food inside the body, Adair Crawford developed a complex mechanism involving air, heat, blood, phlogiston and the specific heat capacity of blood.
Despite Lavoisier’s continued research after 1789 – for example, he began some promising work on the analysis of organic substances – he found his official activities as an academician and fermier taking up more and more of his time as the Revolution, which broke out in that year, created more and more technical and administrative problems.
When Lavoisier was born, France was still a monarchy and power lay firmly in the hands of the Crown and aristocracy together with the Roman Catholic church. These two powerful and sometimes corrupt groups, or Estates, which were virtually exempt from taxation, were the landlords of the majority Third Estate of peasant farmers, merchants, teachers and bankers from whom France’s wealth was derived. Agricultural depression, a rise in population and a succession of expensive wars (including France’s intervention in the American War of Independence in 1778) led France towards bankruptcy in the 1780s. The only solution to this seemed to be to introduce a more equitable system of taxation, which, in turn, involved the reformation of political structure, including the reduction of King Louis XVI’s despotic powers.
On 14 July 1789 revolution broke out with the storming of the Bastille prison in Paris. In fear of their lives, Crown and aristocracy renounced their privileges, while a National Assembly composed of the Third Estate drew up the Declaration of the Rights of Man. National unity was short-lived, however, as the more radical Jacobins manoeuvred for political power and the downfall of the monarchy. War with Austria and Prussia was to prove the excuse for the King’s execution on 21 January 1793. In the period of terror and anarchy that followed, Lavoisier was to lose his life. For, despite his undoubted support for the initial phase of the Revolution and his hard work within the Academy in improving the quality of gunpowder or in devising the metric system in 1790, his services to France and his international reputation were, in the words of one historian, ‘as dust in the balance when weighed against his profession as a Fermier-Géneral’. On 24 November 1793 Lavoisier and his fellow shareholders (including his father-in-law) were arrested and charged, ludicrously, with having mixed water and other ‘harmful’ ingredients in tobacco, charging excessive rates of interest and withholding money owed to the Treasury.
Although later investigations by historians have revealed the worthlessness of these charges, they were more than sufficient in the aptly named ‘Age of Terror’ to ensure the death penalty. Even so, there is some evidence that Lavoisier, alone of the fermiers, might have escaped but for the evidence that he corresponded with France’s political enemies abroad. The fact that his correspondence was scientific did not, in the eyes of his enemies, rule out the possibility that Lavoisier was engaged in counter-revolutionary activities with overseas friends.
Lavoisier was guillotined on 8 May 1794. The mathematician Lagrange commented, ‘It required only a moment to sever his head, and probably one hundred years will not suffice to produce another like it.’ Following the centenary of the French Revolution in the 1890s, a public statue was erected to commemorate Lavoisier. Some years later it was discovered that the sculptor had copied the face of the philosopher, Condorcet, the Secretary of the Academy of Sciences during Lavoisier’s last years. Lack of money prevented alterations being made and, in any case, the French argued pragmatically that all men in wigs looked alike anyway. The statue was melted down during the Second World War and has never been replaced. Lavoisier’s real memorial is chemistry itself.
CONCLUSION (#ulink_e38c81d3-e072-549b-b330-d02817b8e00f)
A rational reconstruction of what seem to have been the essential features of the ‘chemical revolution’ would draw attention to six necessary and sufficient conditions. First, it was necessary to accept that the element, air, did participate in chemical reactions. This was first firmly established by Hales in 1727 and accepted in France by Rouelle and Venel. Although Hales tried to explain the fixation of air by solids by appealing to the attractions and repulsions of Newtonian particle theory, there was no satisfactory explanation for its change of state. Secondly, it was necessary to abandon the belief that air was elementary. This was essentially the contribution of the British school of pneumatic chemists. Beginning in 1754 with Black, who showed that the ‘fixed air’ released from magnesia alba had different properties from ordinary air, and continuing through Rutherford, Cavendish and Priestley, it was found possible to prepare and study some twenty or more ‘factitious airs’ that were different from ordinary air in properties and density. Their preparation and study were made possible by the development of apparatus by Hales for washing air, the pneumatic trough, thus extending the traditional ‘alchemical’ apparatus of furnaces and still-heads that had hitherto largely sufficed in chemical investigations. Whether factitious airs were merely modifications of air depending upon the amounts of phlogiston they contained, or distinct chemical species in an aerial condition, or the expanded particles of solid and liquid substances, was decided by Lavoisier’s development of a model of the gaseous state.
The concept of a gas was a necessary third condition for the reconstruction of chemistry. By imaging the aerial state as due to the expansion of solids and liquids by heat, or caloric, Lavoisier brought chemistry closer to physics and made possible the later adoption of the kinetic theory of heat and the development of chemical thermodynamics. The balance pan had always been the principal tool of assayers and pharmacists, while the conservation of mass and matter had always been implicit in chemists’ rejection of alchemical transmutation and their commitment to chemistry as the art of analysis and synthesis. With the conceptualization of a whole new dimension of gaseous-state chemistry, however, it was necessary that chemical analysis and book-keeping should always account for the aerial state. Here was a fourth necessary condition that raised problems for phlogistonists when Guyton demonstrated conclusively in 1771 that metals increased in weight when they were calcined in air. Many historians, like Henry Guerlac, saw this as the ‘crucial’ condition for effecting a chemical revolution and the event that set Lavoisier on his path to glory.
Largely for pedagogic reasons, generations of historians, chemistry teachers and philosophers of science have interpreted the chemical revolution as hinging upon rival interpretations of combustion – phlogiston theory versus oxygen theory. More recently, those historians who have seen Lavoisier’s chemistry as literally an anti-phlogistic chemistry have had a wider agenda than combustion in mind. In particular, it now seems clear that the interpretations of acidity was a major issue for Lavoisier and the phlogistonists. Indeed, it could be argued that, once Lavoisier had the concept of a gas, it was the issue of acidity, not combustion, that led him to oxygen – as its very name implies. The transformation of ideas of acidity, therefore, formed a fifth factor in the production of a new chemistry.
Finally, and not least, the sixth necessary condition was a new theory of chemical composition and organization of matter in which acids and bases were composed from oxygen and elements operationally defined as the substances that chemists had not succeeded in analysing into simpler bodies. Oxygen formed the glue or bond of dualistic union between acid and base to form salts, which then compounded in unknown ways to form minerals. To make this more articulate and to avoid confusion with the unnecessary thought patterns of phlogiston chemistry, a new language was required – one that reflected composition and instantly told a reader what a substance was compounded from. After 1787 chemists, in effect, spoke French, and this underlined the new chemistry as a French achievement.
Although he pretended at the beginning of the Traité that it had been his intent to reform the language of chemistry that had forced the reform of chemistry itself, it was clearly because he had done the latter that a new language of composition was needed. As historians have stressed, the new nomenclature was Lavoisier’s theoretical system. He justified its adoption in terms of Condillac’s empirical philosophy that a well constructed language based upon precise observation and rationally constructed in the algebraic way of equal balances of known and unknown would serve as a tool of analysis and synthesis.
Observation itself involved chemical apparatus – not merely the balance, but an array of eudiometers, gasometers, combustion globes and ice calorimeters, which would enable precise quantitative data to be assembled. In this way chemical science would approach the model of the experimental physicists that Lavoisier clearly admired and with whose advocates he frequently collaborated.
This last point has led some historians to question whether Lavoisier was a chemist at all and whether the chemical revolution was instead the result of a brief and useful invasion of chemistry by French physicists. Others, while admitting the influence of experimental physics on Lavoisier’s approach, continue to stress Lavoisier’s participation in a long French tradition of investigative analysis of acids and salts to which he added a gaseous dimension. Even Lavoisier’s choice of apparatus, though imbued with a care and precision lacking in his predecessors’ work, was hallmarked by the investigative procedures of a long line of analytical and pharmaceutical chemistry. All historians agree, however, that until about 1772, when events triggered a definite programme of pneumatic and acid research in his mind, Lavoisier’s research was pretty random and dull, as if he were casting around for a subject (‘une belle carrière d’expériences à faire’) that would make him famous. Seizing the opportunity, the right moment, is often the mark of greatness in science. Priestley and Scheele believed that science progressed through the immediate communication of raw discoveries and ‘ingenious simplicity’. Lavoisier’s way, to Priestley’s annoyance, was to work within a system and to theorize in a new language that legislated phlogiston out of existence.
Like Darwin’s Origin of Species, Lavoisier’s Traité was a hastily written abstract or prolegomena to a much larger work he intended to write that would have included a discussion of affinity, and animal and vegetable chemistry. Like Darwin’s book, it was all the more readable and influential for being short and introductory. If more information was required, Fourcroy’s encyclopedic text and its many English and German imitations soon provided reference and instruction. But this was not the end of the chemical revolution. To complete it, Lavoisier’s elements had to be reunited with the older corpuscular traditions of Boyle and Newton. This was to be the contribution of John Dalton.
4 A New System of Chemical Philosophy (#ulink_a55c4187-4324-550a-b45b-590a1a56345b)
Atoms are round bits of wood invented by Mr Dalton.
(H. E. ROSCOE, 1887)
Before Dalton came on the scene, chemistry can hardly be described as an exact science. A wealth of empirical facts had been established and many theories had been erected that bound them together, not the least impressive of which were Lavoisier’s new dualistic views of chemical composition and his explanations of combustion and acidity. Most of eighteenth-century chemical activity had been qualitative. Despite the Newtonian dream of quantifying the forces of attraction between chemical substances and the compilation of elaborate tables of chemical affinity, no powerful quantitative generalizations had emerged. Although these empirically derived affinity relations often allowed the course of a particular chemical reaction to be predicted, it was not possible to say, or to calculate, how much of each ingredient was needed to perform a reaction successfully and most economically. Dalton’s chemical atomic theory, and the laws of chemical combination that were explained by it, were to make such calculations and estimates possible – to the benefit of efficient analysis, synthesis and chemical manufacture.
As a consequence of the power of the corpuscular philosophy, by the end of the seventeenth century it had become a regulative principle, or self-evident truth, that all matter was ultimately composed of microscopic ‘solid, hard, impenetrable, moveable’ particles. As we saw in the second chapter, however, such ultimate descriptions of Nature were of little use to practical chemists, who preferred to adopt a number of empirically derived elementary substances as the basic ‘stuffs’ of chemical investigation. Lavoisier’s famous definition of the element in 1789 made it clear that speculations concerning the ultimate particles or atoms of matter were a waste of time; chemistry was to be based on experimental knowledge
(#litres_trial_promo):
All that can be said upon the number and nature of elements [i.e. in an Aristotelian or Paracelsian sense] is, in my opinion, confined to discussions entirely of a metaphysical nature. It is an unsolvable problem capable of an infinity of solutions none of which probably accord with Nature. I shall be content, therefore, in saying that if by the term elements we mean to express those simple and indivisible atoms of which matter is composed, it seems extremely probable we know nothing at all about them; however, if instead we apply the term elements or principles of bodies, to express our idea of the last point which analysis is capable of reaching, we must admit as elements, all the substances into which we are capable, by any means, to reduce bodies during decomposition. Not that we can be certain that these substances we consider as simple may not be compounded of two, or even a greater number of principles; but, since these principles cannot be separated, or rather since we have not hitherto discovered the means of separating them, they act with regard to us as simple substances, and we ought never to suppose them compounded until experiment and observation has proved them to be so.
For the same reason, although Dalton believed in physical atoms, most of his interpreters were content with a theory of chemical atoms – the ‘minima’ of the experimentally defined elements. Whether these chemical atoms were themselves composed from homogeneous or heterogeneous physical atoms was to go beyond the evidence of pure stoichiometry.
Stoichiometry was a subject invented by the German chemist Jeremias Richter (1762–1807), who had studied mathematics with the great philosopher, Immanuel Kant, at the University of Königsberg, and for whom he wrote a doctoral thesis on the use of mathematics in chemistry. This was, in practice, nothing grander than an account of the determination of specific gravities, from which Richter calculated the supposed weights of phlogiston in substances. Just as Kepler had searched for mathematical relations and harmony in astronomical data gathered by Tycho Brahe, so Richter spent his spare time as a chemical analyst in the Berlin porcelain works searching for arithmetical relations in chemistry. As Partington noted sardonically, Richter spent his entire life finding ‘regularities among the combining proportions where nature had not provided any’.
The exception was his discovery in 1792, while investigating double decompositions, that, because neutral products were formed, the reactants must ‘have amongst themselves a certain fixed ratio of mass’.
If, e.g., the components of two neutral compounds are A – a, a and B – b, b, then the mass ratios of the new neutral compounds produced by double decomposition are unchangeably A – a:b and B – b:a.
This law of neutrality was a special case of what came to be known as the law of reciprocal proportions. Richter referred to the study of these ratios as ‘stoichiometry’ and went on to examine how a fixed weight of an acid was neutralized by different weights of various bases. This investigation led him to claim, erroneously, that combining proportions formed arithmetical and geometrical series. It was Ernst Fischer, a Berlin physicist, who, when translating Berthollet’s Recherches sur la lois de l’affinité into German in 1802, pointed out that Richter’s results could be tabulated to show equivalent weights of a series of acids and bases. If 1000 parts of sulphuric acid was taken as a standard and the base equivalents needed for neutralization arranged in one column, and the amounts of other acids needed to neutralize these bases in another, then an analyst could gather at a glance how much of a particular base would neutralize a particular acid:
Thus, 672 equivalents of ammonia neutralized 427 of fluoric, 577 of carbonic and 712 of muriatic acids. Analysts now had a definite method of controlling the accuracy of their work and of calculating beforehand the composition of salts under investigation.
Dalton’s atomic theory was to provide a rational explanation for these regularities. There has been some debate as to whether Dalton was directly influenced by Richter. He certainly knew of Richter’s investigations, but probably not until after he had derived his own explanation from other sources.
DALTON’S ‘NEW SYSTEM’ (#ulink_a690ba74-71af-5bae-ab47-60587ae95285)
What was ‘new’ in John Dalton’s A New System of Chemical Philosophy? The obvious reply seems to be the introduction of chemical atomism – the idea that each of Lavoisier’s undecompounded bodies was composed from a myriad of homogeneous atoms, each element’s atom differing slightly in mass. The surprising thing, however, is that only one chapter of barely five pages in the 916-page treatise was devoted to the epoch-making theme. These five pages, together with four explanatory plates, appeared at the end of the first part of the New System, which was published in Manchester in 1808 and dedicated to the professors and students of the Universities of Edinburgh and Glasgow, who had heard Dalton lecture on ‘Heat and the Chemical Elements’ in 1807, and to the members of Manchester’s Literary and Philosophical Society, who had ‘uniformly promoted’ Dalton’s researches. A second, continuously paginated, part of the New System, dedicated to Humphry Davy and William Henry, was published in 1810. Astonishingly, the third part, labelled as a second volume, did not appear until 1827. Even then the design was incomplete and a promised final part concerned with ‘complex compounds’ was never published.
Dalton’s apparent dilatoriness is easily explained by the fact that he earned his living as a private elementary teacher, which left him little time for the exacting experimental work and evidence upon which he based the New System. For it was a ‘new’ approach that he was taking, familiar though his scheme has become. Dalton recognized his innovation as being a ‘doctrine of heat and general principles of chemical synthesis’. A theory of mixed gases, which he developed in 1802, led him in 1803 to ‘new views’ on heat as a factor in the way elements (or, rather, atoms) combined together, a process he referred to as ‘chemical synthesis’. The fact that chemical compounds, or compound atoms (molecules), might be binary, ternary, quaternary, and so on up to a maximum of twelve atoms, gave Dalton a structure for his text: a detailed experimental examination of heat and the gaseous state, a theory of atomism and combination, which included the measure of atomic mass as a relative atomic weight, followed by a detailed account of the properties of the known elements, their binary combinations, ternary combinations and so on. Thus, although the exegesis of the atomic theory was limited to five pages, the whole of the New System was, in fact, imbued with a new stoichiometric approach to chemistry – that elements compounded together in fixed proportions by weight because of attractions and repulsions between the tiny particles of heat and elementary forms that made up laboratory chemicals. Inevitably, because Dalton was a slow worker and unable to spare time from teaching for research and writing, it was left largely to others, notably Thomas Thomson and Jacob Berzelius, to exploit the full consequences of Dalton’s insight.
DALTON’S LIFE (#ulink_0b2fc2b8-480d-5e3a-a898-591b74cad911)
John Dalton (1766–1844) was born at Eaglesfield in Cumbria, the son of a weaver, and, like most contemporary members of the Society of Friends, was a man of some learning. The highly efficient Quaker network of schooling and informal education ensured that Dalton received a good schooling; he himself began to teach village schoolchildren when he reached the age of twelve. In his teens he mastered sufficient geometry to be able to study Newton’s Principia. At the age of fifteen, Dalton and his brother moved to Kendall, in the English Lake District, where they acquired their own school, which offered Greek, Latin, French and mathematics. At Kendall, Dalton was befriended by the blind Quaker scholar, John Gough, who further encouraged Dalton’s mathematical abilities and knowledge of Newtonian natural philosophy, including the work of Boyle and Boerhaave. The constant stimulation of rapidly changing weather conditions among the mountains and lakes of Westmorland and Cumberland (present-day Cumbria) interested him in meteorology. The records he kept over a five-year period were published in Meteorological Essays in 1793. In the same year, on Gough’s recommendation, Dalton moved to Manchester as tutor in mathematics and natural philosophy at New College, a Dissenting academy that had begun its distinguished life elsewhere as the Warrington Academy. Here Priestley had taught between 1761 and 1767.
Although Manchester New College moved to York in 1803, Dalton, finding Manchester congenial, spent the remainder of his life there as a private teacher and industrial consultant. Not only was there an abundance of paid work in Manchester for private tutors because of a rising industrial middle class (Dalton’s most famous pupil was a brewer’s son, the physicist James Prescott Joule), but the presence of the Literary and Philosophical Society, whose Secretary Dalton became in 1800 and President from 1817 until his death, proved a congenial venue for the presentation and articulation of his scientific work. Dalton read his first scientific paper, on self-diagnosed colour blindness (long after known as Daltonism) to the Society in 1794. He went on reading papers and reports to the Society up to his death. From about 1815 onwards, however, Dalton failed more and more to keep pace with the chemical literature. In 1839 he suffered the ignominy of having a paper of his on phosphates and arsenates rejected by the Royal Society on the grounds that a superior account of these salts had already been published by Thomas Graham.
Despite such failings, Dalton retained the respect of the chemical and scientific communities. Together with two other Dissenters from Anglicanism, Michael Faraday and Robert Brown, the botanist, and despite the angry opposition of Oxford High Churchmen, Dalton was awarded an honorary degree by Oxford University in 1832. A year later the government awarded him a Civil List pension for life, and in 1834 Edinburgh University gave him another honorary degree. His final accolade was a public funeral in Manchester. Even if we grant that some of these honours served a secondary purpose of drawing attention to scientists and their contribution to culture in Victorian Britain, we are bound to ask: What did Dalton do to merit such public honours?
THE ATOMIC THEORY (#ulink_a640cd74-78fb-5777-8e2d-ab83b2aecd1a)
The straightforward answer is that Dalton rendered intelligible the many hundreds of quantitative analyses of substances that were recorded in the chemical literature and that he provided a model for the long-standing assumption made by chemists that compounds were formed from the combination of constant amounts of their constituents. He regarded chemical reactions as the reshuffling of atoms into new clusters (or molecules), these atoms and compound atoms being pictured in a homely way as little solid balls surrounded by a variable atmosphere of heat.
This statement, however, tells us little about Dalton’s originality; after all, the atomic theory of matter had existed for a good two-thousand years before Dalton’s birth. In Ireland, at the end of the eighteenth century, William Higgins (1762–1825) had used atomism in his A Comparative View of the Phlogiston and Antiphlogiston Theories (1789) to refute the phlogistic views of his countryman, Richard Kirwan. Higgins later claimed that Dalton had stolen his ideas – an inherently implausible notion that, nevertheless, has been supported by several historians in the past. In fact, Dalton’s originality lay in solving the problem of what philosophers of science have called transduction; he derived a way of calculating the relative weights of the ultimate particles of matter from observations and measurements that were feasible in the laboratory. Although atomic particles could never be individually weighed or seen or touched, Dalton provided a ‘calculus of chemical measurement’ that for the first time in history married the theory of atoms with tangible reality. He had transduced what had hitherto been a theoretical entity by building a bridge between experimental data and hypothetical atoms.
Dalton’s calculus involved four basic, but reasonable, assumptions. First, it was supposed that all matter was composed of solid and indivisible atoms. Unlike Newton’s and Priestley’s particles, Dalton’s atoms contained no inner spaces. They were completely incompressible. On the other hand, recognizing the plausibility of Lavoisier’s caloric model of changes of phase, Dalton supposed that atoms were surrounded by an atmosphere of heat, the quantity of which differed according to the solid, liquid or gaseous phase of the aggregate of atoms. A gas, for example, possessed a larger atmosphere of heat than the same matter in the solid state. Secondly, Dalton assumed, as generations of analysts before him had done, that substances (and hence their atoms) were indestructible and preserved their identities in all chemical reactions. If this law of conservation of mass and of the elements was not assumed, of course, transmutation would be possible and chemists would return to the dark days of alchemy. Thirdly, in view of Lavoisier’s operational definition of elements, Dalton assumed that there were as many different kinds of atoms as there were elements. Unlike Boyle and Newton, for Dalton there was not one primary, homogeneous ‘stuff’; rather, particles of hydrogen differed from particles of oxygen and all the particles that had so far been defined as elementary.
In these three assumptions Dalton moved away completely from the tradition of eighteenth-century matter theory, which had emphasized the identity of matter and of all material substances. In so doing. Dalton intimately bound his kind of atomism to the question of how elements were to be defined. In a final assumption, he proposed to do something that neither Lavoisier nor Higgins had thought of doing, namely to rid metaphysical atomism of its intangibleness by fixing a determinable property to it, that of relative atomic weight. To perform this transductive trick, Dalton had to make a number of simple assumptions about how atoms would combine to form compound atoms, the process he termed chemical synthesis. In the simplest possible case, ‘when only one combination of two bodies can be obtained, it must be presumed to be a binary one, unless some cause appear to the contrary’. In other words, although substances A and B might combine to form A
B
, it is simpler to assume that they will usually form just AB. Similarly, if ‘two combinations are observed, they must be presumed to be a binary and a ternary’:
A + 2B = AB
or 2A + B = A
B
Dalton made similar rules for cases of three and four compounds of the same elements, and pointed out that the rules of synthesis also applied to the combination of compounds:
CD + DE = CD
E etc.
These assumptions of simplicity of composition, which, as we shall see, had a theoretical justification, have long since been replaced by different criteria. Although they led to many erroneous results, the assumptions proved fruitful since they allowed relative atomic weights to be calculated. Two examples, both given by Dalton, will suffice.
Hydrogen and oxygen were known to form water. Before 1815, when hydrogen peroxide was discovered, this was the only known compound of these two gases. Dalton quite properly assumed, therefore, that they formed a binary compound; in present-day symbols:
H + O = HO
