The Fontana History of Chemistry

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.

THE CHEMICAL REVOLUTION

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.