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.
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