Читать книгу The Fontana History of Chemistry - William Brock J. - Страница 26

The problem of the Parisian water supply came to Lavoisier’s attention during the year of his election to the Academy when the purity of water brought to Paris by an open canal was questioned. The test for the potability of water involved evaporating it to dryness in order to determine its solid content. But the use of this technique reminded academicians, including Lavoisier, of the long tradition in the history of chemistry that water could be transmuted into earth. Obviously, if this were the case, the determination of the solid content ‘dissolved’ in water would reveal nothing about its purity.

As we have seen, the transmutation of water into earth had been a basic principle of Aristotle’s theory of the four elements, and a crucial, experimental, factor in van Helmont’s decision that water was the unique element and basis of all things. Although by the 1760s most chemists could no longer credit that such an apparently simple pure substance as water could be transmuted into an incredibly large number of complicated solid materials, it was seriously argued by a German chemist, Johann Eller, in 1746 that water could be changed into both earth and air by the action of fire or phlogiston. For Eller this was evidence that there were only two elements, fire and water. The active element of fire acted on passive water to produce all other substances.

It seems clear from the design of Lavoisier’s experiment on the distillaton of water, which he began in October 1768, that he suspected that the earth described in Eller’s experiment (which he probably read about in Venel’s article on ‘water’ in the fifth volume of the Encyclopédie in 1755) was really derived from the glass of the apparatus by a leaching effect. By weighing the apparatus before and afterwards, and also weighing the water before and after heating continuously for three months, Lavoisier showed that the weight of ‘earth’ formed was more or less equal to the weight loss of the apparatus. Intriguingly, Lavoisier did not clinch his quantitative argument by analysing the materials in the sediment and showing that they were identical to those in glass. Moreover, since the correlation of weights was not exact, some room for doubt remained until two decades later when Lavoisier showed that water was composed of hydrogen and oxygen.

Enough had been done, however, to convince Lavoisier that Eller’s contention that water could be transmuted into earth was nonsense. This was reported to the Academy in 1770. He also surmised, under the influence of Venel’s views on the chemical dissolution of air in liquids and solids, that there was a more plausible explanation of water’s apparent change into vapour or air when heated – namely, that heat, when combined with water and other fluids, might expand their parts into an aerial condition. Conversely, when air was stripped of its heat it lost its voluminous free aerial state and collapsed into, or was ‘fixed’ into, a solid or liquid condition, just as Stephen Hales had found in the 1720s when analysing the air content of minerals and vegetables.

Lavoisier recorded these ideas in an unpublished essay on the nature of air in 1772. Here was the basis for a theory of gases – though at this juncture Lavoisier knew nothing at all of the work of Priestley and others on pneumatic chemistry. He was also, not surprisingly, still interpreting his model of the gaseous state in terms of phlogiston. When air was fixed¹:

… there had to be a simultaneous release of phlogiston or the matter of fire; likewise when we want to release fixed air, we can succeed only by providing the quantity of fire matter, of phlogiston, necessary for the existence of the gaseous state [l’état de fluide en vapeurs].

Lavoisier was now clear that there were three distinct states of matter²:

All bodies in nature present themselves to us in three different states. Some are solid like stones, earth, salts, and metals. Others are fluid like water, mercury, spirits of wine; and others finally are in a third state which I shall call the state of expansion or of vapours, such as water when one heats it above the boiling point. The same body can pass successively through each of these states, and in order to make this phenomenon occur it is necessary only to combine it with a greater or lesser quantity of the matter of fire.

Moreover, it followed from the fact that metals disengaged ‘air’ when they were calcined, that metals contained fixed air:

Apparatus for the preparation, collection and study of gases was a necessary factor in the chemical revolution. It was not until 1727 that Stephen Hales hit upon a way to isolate the ‘air’ produced from a heated solid. In order to estimate as accurately as possible the amount of ‘air’ produced and to remove any impurities from it, Hales ‘washed’ his airs by passing them through water before collecting them in a suspended vessel by the downward displacement of water.

Hales, like John Mayow in the seventeenth century, still thought in terms of a unique air element, but Joseph Black’s demonstration that ‘fixed air’ (carbon dioxide) was different from ordinary air encouraged Henry Cavendish, Joseph Priestley and others to develop Hales’ apparatus to study different varieties of air – or gases, as Lavoisier was to call them. An incentive here was the invention of soda water by Priestley, which encouraged interest in the potentially health-giving properties of artificial mineral waters generally. In 1765, while investigating spa waters, the English doctor, William Brownrigg, invented a simple shelf with a central hole to support a receiving flask or gas holder. This creation of the ‘pneumatic trough’ enabled gas samples to be transferred from one container to another and for gases to join solids and liquids on the chemical balance sheet.

Joseph Priestley (1733–1804) is surely one of the most engaging figures in the history of science. The son of a Yorkshire Congregational weaver and cloth-dresser, Priestley was trained for the Nonconformist ministry at a Dissenting academy in Daventry. Like most Nonconformist academies of the period, this taught a wider curriculum than the universities that included the sciences. After serving a string of ministries, where his theological views became increasingly Unitarian, and a teaching post at the famous Warrington Academy, in 1773 Priestley became the librarian and household tutor of William Petty, the second Earl of Shelburne who, while Secretary of State in Chatham’s cabinet, had opposed George III’s aggressive policy towards American colonists. Already the author of innumerable educational works, in 1767 Priestley had published a History and Present State of Electricity, which launched him upon a part-time career in science. While minister of a Presbyterian congregation at Leeds, and living next door to a brewery, Priestley had begun investigating the preparation and properties of airs. Under Shelburne’s patronage, Priestley had the necessary leisure to prepare some five volumes containing detailed accounts of these experiments on airs, as well as a number of theological works. There was a connection here in that Priestley was attempting to explore the relationship between matter and spirit.

In 1780, retaining a life annuity from Shelburne, Priestley returned to the ministry at Birmingham’s New Meeting. Here he found convivial philosophical and scientific company in the Lunar Society composed of rising industrialists and intellectuals such as Mathew Boulton, James Watt, Josiah Wedgwood and Erasmus Darwin. Although its members were united in their support for the American War of Independence and for the initial stages of the French Revolution, it was Priestley the preacher-orator who was publicly identified with radical criticism of English politics and the discrimination against Dissenters. In 1791 a ‘Church and King’ mob destroyed Priestley’s home and chapel, forcing him to flee to London. Although he was eventually compensated for the loss of his property, in 1794 he decided to emigrate and to join two of his sons in America.

Here he was warmly welcomed in Philadelphia, where he was offered the Chair of Chemistry at the University of Pennsylvania. Instead, Priestley moved to Northumberland, in rural Pennsylvania, where he hoped to found an academy for the sons and daughters of political refugees who would join him there. It did not work out and Priestley spent his declining years cut off by distance from European, and even American, intelligence, and fighting a rearguard action against Lavoisier’s chemistry in his fascinating Considerations on the Doctrine of Phlogiston (1796). Although outmanoeuvred by Lavoisier, Priestley lived on in two ways. His young executor, Thomas Cooper (1759–1839), a fellow refugee from English politics, acquired sufficient up-to-date knowledge of chemistry from studying in Priestley’s library and laboratory to become one of America’s leading chemical educators. A century after Priestley’s discovery of oxygen, in August 1874, a national meeting of chemists, gathered at his home in Northumberland (now a Priestley Museum), decided to create the American Chemical Society.

It was Cavendish who began the collection of water-soluble gases over mercury, but Priestley who brought their study and manipulation to perfection. Curiously, believing that chemistry, like physics, required expensive and complicated instruments, Lavoisier only rarely used the pneumatic trough; instead, he developed an expensive and sophisticated gasometer. A good third of Lavoisier’s Elements of Chemistry was devoted to chemical apparatus. Until the appearance of Michael Faraday’s Chemical Manipulation in 1827, Lavoisier’s descriptions remained the bible of instrumentation and chemical manipulative techniques.

In the spring of 1772, Lavoisier read an essay on phlogiston by a Dijon lawyer and part-time chemist, Louis-Bernard Guyton de Morveau (hereafter Guyton) (1737–1816). In a brilliantly designed experimental investigation, Guyton showed that all his tested metals increased in weight when they were roasted in air; and since he still believed that their combustibility was caused by a loss of phlogiston, he saved the phenomena by supposing that phlogiston was so light a substance that it ‘buoyed’ up the bodies that contained it. Its loss during decomposition therefore caused an increase of weight. Most academicians, including Lavoisier, thought Guyton’s explanation absurd. Following his previous reflections on the role of air, Lavoisier speculated immediately that a more likely explanation was that, somehow, air was being ‘fixed’ during the combustion and that this air was the cause of the increase in weight. It followed that ‘fixed air’ should be released when calces were decomposed – just as Hales’ earlier experiments in Vegetable Staticks had suggested.

One final Encyclopédie article seems to have influenced Lavoisier decisively at this juncture. This was an essay on ‘expansibility’ published in the sixth volume in 1756 by another pupil of Rouelle’s, the philosopher and civil servant, Jacques Turgot. Like Lavoisier, Turgot combined a career of public service with spare-time research in chemistry. But he never published his reflections (or if he did so, he did it anonymously), and we only know of his interesting thoughts from his private correspondence. Turgot arrived independently at the same solution as Lavoisier, namely that Guyton’s experiments could be explained as due to the fixing of air. He had actually learned of Guyton’s work before Lavoisier in August 1771. In a private letter to Condorcet, Turgot noted³:

The air, a ponderable substance which constantly enters into the state of a vapour or expansive fluid according to the degree of heat contained, but which is also capable of uniting with all the other principles of bodies and forming in that state part of the constitution of different compounds … this air combines or separates in different chemical reactions because of a greater or lesser affinity that it has for the principles to which it was attached or with those that one presents to it.

Given that Lavoisier was party to the same intellectual influences as Turgot, it was not surprising that they should have reached the same conclusions. Whether Lavoisier was aware or not of Turgot’s thoughts, he took pains constantly to preserve priority of the idea that it was air that was fixed in calcination, rather than liberated, as he had first thought earlier in 1772. If air was an expanded fluid combined with phlogiston, as Turgot’s Encyclopédie article had suggested, then the phlogiston released during combustion (the process of ‘fixing air’) would explain the heat and light generated during the reaction. It followed that heat and light came from the air, not the metal as the Stahlians had always maintained:

Lavoisier was able to verify this in October 1772 by using a large burning lens belonging to the Academy. When litharge (an oxide of lead) was roasted with charcoal, an enormous volume of ‘air’ was, indeed, liberated. In order to investigate this phenomenon more closely, and in order to ensure priority after finding that sulphur and phosphorus also gained in weight when burned in air, Lavoisier deposited a sealed account of his findings in the archives of the Academy, which he allowed to be opened in May 1773⁴:

What is observed in the combustion of sulphur and phosphorus, may take place also with all bodies which acquire weight by combustion and calcination, and I am persuaded that the augmentation of the metallic calces is owing to the same cause. Experiment has completely confirmed my conjectures: I have carried out a reduction of litharge in a closed vessel, with the apparatus of Hales, and I have observed that there is disengaged at the moment of passage from the calx to the metal, a considerable quantity of air, and that this air forms a volume a thousand times as great as the quantity of litharge employed. This discovery seems to me one of the most interesting that has been made since Stahl and as it is difficult in conversation with friends not to drop a hint of something that would set them on the right track, I thought I ought to make the present deposition into the hand of the Secretary of the Academy until I make my experiments public.

In committing himself to the hypothesis that ordinary air was responsible for combustion and for the increased weight of burning bodies, Lavoisier demonstrated that he was ignorant of most contemporary chemical work on the many different kinds of airs that can be produced in chemical reactions. In Scotland, a decade earlier in 1756, Joseph Black had succeeded in demonstrating that what we call ‘carbonates’ (e.g. magnesium carbonate) contained a fixed air (carbon dioxide) that was fundamentally different in its properties from ordinary atmospheric air. Unlike ordinary air, for example, it turned lime water milky and it would not support combustion. Black’s work did not achieve much publicity or publication in France until March 1773. A few years later, Henry Cavendish studied the properties of a light inflammable air (hydrogen), which he prepared by adding dilute sulphuric acid to iron. These experiments were to stimulate the astonishing industry of Priestley who, between 1770 and 1800, prepared and differentiated some twenty new ‘airs’. These included (in our terminology) the oxides of sulphur and nitrogen, carbon monoxide, hydrogen chloride and oxygen. The fact that most of these were ‘acid’ airs was to be, for Lavoisier, an intriguing phenomenon.

Hence, although largely unknown to Lavoisier in 1772, there was already considerable evidence that atmospheric air was a complex body and that it would be by no means sufficient to claim that air alone was responsible for combustion. Lavoisier seems to have been aware of his chemical ignorance. He wrote in his laboratory notebook on 20 February 1773:

I have felt bound to look upon all that has been done before me as merely suggestive. I have proposed to repeat it all with new safeguards, in order to link our knowledge of the air that goes into combination or is liberated from substances, with other acquired knowledge, and to form a theory.

And, with the firm and confident intention of bringing about, in his own prescient words, ‘a revolution of physics and chemistry’, he spent the whole of 1773 studying the history of chemistry – reading everything that chemists had ever said about air or airs since the seventeenth century and repeating their experiments ‘with new safeguards’. His results were summarized in Opuscules physiques et chimiques published in January 1774.

Ironically, far from clarifying his ideas, his new-found familiarity with the work of pneumatic chemists now led him to suppose that carbon dioxide, ‘fixed air’, in the atmosphere was responsible for the burning of metals and the increase of their weight. This was not unreasonable, and the explanation for Lavoisier’s misconception will be clear. Most calces (that is, oxides) can only be reduced to the metal by burning them with the reducing agent, charcoal (C), when the gas carbon dioxide is produced:

calx + C → metal + fixed air

It was easy to suppose, therefore, that the same fixed air was responsible for combustion:

metal + fixed air → calx

As he noted plaintively in a notebook⁵:

I have sometimes created an objection against my own system of metallic reduction which consists of the following: lime [CaO] according to me is a calcareous earth deprived of air; the metallic calces, on the contrary, are metals saturated with air. However, both produce a similar effect on alkalies, they render them caustic.

Obviously, Lavoisier needed to distinguish between air and fixed air, carbon dioxide. It should be noted how this reasoning was based upon the complementarity of analysis and synthesis. If two simple substances could be combined together to form a compound, then, in principle, it ought to be possible to decompose the compound back into the same components. Lavoisier was to find a perfect example of this in the red calx of mercury, a substance that caused him to revise his original hypothesis significantly.

Two things caused Lavoisier to change his mind. First, his attention was drawn by Pierre Bayen, a Parisian pharmacist, to the fact that, when heated, the calx of mercury (HgO), a remedy used in the treatment of venereal disease, decomposed directly into the metal mercury without the addition of charcoal. No fixed air was evolved. As Bayen pointed out, this observation made it difficult to see how the phlogiston theory could be right. Here was a calx regenerating the metal without the aid of phlogiston in the form of charcoal! Secondly, the mercury calx had also come to the attention of Priestley because of a contemporary uncertainty whether the red calx produced by heating nitrated mercury was the same as that produced when mercury was heated in air. In August 1774 he heated the calx in an enclosed vessel and collected a new ‘dephlogisticated air’, which he found, after some months of confusing it with nitrous oxide, supported combustion far better than ordinary air did. Unknown to Priestley the Swedish apothecary, Scheele, had already isolated what he called ‘fire air’ from a variety of oxides and carbonates in the years 1771–2. But Scheele, working in isolation even in Sweden, did not help to shape Lavoisier’s views in the same way that Bayen and Priestley did. These experiments were reported directly to Lavoisier by Priestley when he was on a visit to Paris during October 1774, but he also published an account of the new air at the end of the same year.

Bayen’s and Priestley’s observations, together with his own experiments with mercuric oxide, caused Lavoisier to revise his hypothesis of 1774. In April 1775, Lavoisier read a paper to the Academy of Sciences ‘on the principle which combines with metals during calcination and increases their weight’ in which, still more confused, he identified the principle of combustion with ‘pure air’ and not any particular constituent of the air. This new hypothesis, which was published in May, was seen by Priestley. The latter, realizing that Lavoisier had not quite grasped that the ‘dephlogisticated air’ generated from the calx of mercury was a constituent part of ordinary air, gently put him right in another book he published at the end of 1775. This, together with further experiments of his own, finally led Lavoisier to the oxygen theory of combustion. In revising the so-called ‘Easter Memoir’ for publication in 1778, and in an essay published the year before, he wrote as follows:

The principle which unites with metals during calcination, which increases their weight and which is a constituent part of the calx is: nothing else than the healthiest and purest part of air, which after entering into combination with a metal, [can be] set free again; and emerge in an eminently respirable condition, more suited than atmospheric air to support ignition and combustion.

Because this ‘eminently respirable air’ burned carbon to form the weak acid, carbon dioxide, while non-metals generally formed acidic oxides, Lavoisier called the new substance oxygen, meaning ‘acid former’⁶:

… 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.