‘Fixed air’ was, of course, carbon dioxide. The Frenchman Gabriel Venel had earlier combined much the same ingredients, but expected people to drink the whole sludgy concoction. Priestley’s was the first drinkable carbonated water, although he did not exploit the discovery, leaving this to Jacob Schweppe, the Swiss émigré who in 1792 established the London soda water business that still bears his name.
In France, meanwhile, a long-running national exercise was under way to gather information for a mineralogical atlas of French waters. Venel had contributed data from his analysis of the waters of Selters, or Seltz, on the French banks of the Rhine in 1755, the authentic source of seltzer water. The young Antoine Lavoisier, who would rise to become France’s greatest chemist, also played a part in this project. His experience here laid the groundwork for his journey of discovery: that ‘waters’ were simply universal water combined with different salts; that these salts were characterized in turn by different combinations of metals and acids; and that these acids generally gained their corrosive properties from their incorporation of the as yet unknown element oxygen.
Like his English competitors, Priestley and later Humphry Davy, Lavoisier was educated in the humanities but soon realized that the questions to match his intellect were to be found in science. At first, however, he followed in his father’s footsteps, studying the law and purchasing a royal concession to collect taxes. His highly profitable duties covered the prevention of smuggling of drink and tobacco and the collection of the notorious gabelle, or salt tax, that would later be one cause of the French Revolution. Meanwhile, his scientific acumen was directed towards assaying the minerals in natural water sources. The work gave ample scope for Lavoisier to refine the rigorous analytical techniques that would seal his reputation for dragging chemistry out of the era of alchemy. He invested some of the fortune he made as a ‘tax farmer’ in building the best instruments that he could. By accurately measuring the very slightly different density of various waters, he was then able to say how much salt they contained. But he did not much enjoy the routine of days spent in the sun and rain and nights billetted in cheap inns. He preferred the comforts of the laboratory and worked hard to earn them.
While Priestley was experimenting with carbon dioxide, Lavoisier, by now more happily situated in Paris and freshly elected to the French Academy of Sciences, turned his measurement skills to combustion reactions. He found that diamond, sulphur and phosphorus when burnt in air all gained weight if the gases produced were figured into the calculation. The same happened on the slower scale of metallic corrosion. In 1773, he gave an important paper to the Academy, properly recording for the first time that the transformation of copper and iron into verdigris and rust was also accompanied by a gain in weight. He explained these observations by suggesting that the substances must be absorbing something from the air.
In October the following year, Lavoisier and his fellow academicians hosted Joseph Priestley at a dinner in Paris and heard of Priestley’s latest experiment in which he had heated mercuric oxide (known as red calx of mercury) to release ‘a new sort of air’, leaving behind only pure liquid mercury. The month before, Lavoisier had received correspondence from Carl Scheele in Sweden, who had done the same experiment a little earlier. Scheele was an exceptionally modest fellow who never sought academic recognition and only ever attended one meeting of the Royal Swedish Academy of Sciences. He left behind him no reliable portrait, so that even his statue in a Stockholm park is a Grecian fancy rather than a genuine likeness, and, worst of all, he did not hasten to publish his work. Priestley, meanwhile, was in a theoretical muddle as to what he had found. This left the way free for Lavoisier, who repeated the two men’s work and performed further experiments of his own before, in 1777, naming the gas oxygen, meaning generator of acid.
Priestley’s scientific interest was chiefly in the gases of the air, whereas Lavoisier’s was in the waters. And, like most Swedish chemists, Scheele’s focus was on the minerals of the earth. Converging on this vital element from each of the three states of matter, gas, liquid and solid, it is hardly surprising that the three scientists had trouble comparing notes. However, the clouds of confusion would eventually lift to reveal the ubiquitous importance of the element in all of nature. It is fair to attribute the discovery of oxygen gas to Scheele and Priestley, but it is Lavoisier who locked the newfound element into the rest of chemistry by proving its centrality to water, acids and salts.
Eleven years earlier, in 1766, Henry Cavendish, a man of Getty-like wealth and eccentricity, had discovered hydrogen, or ‘inflammable air’, by reacting metals with acid in his private laboratory in London. He amused himself thereafter by setting off explosions by sparking mixtures of the gas with air. The liquid that condensed from these explosions was simply water. In this way, Cavendish confirmed that water was not an element because it could be created from other fundamental ingredients, namely hydrogen and something in the air.
Lavoisier was able to repeat Cavendish’s experiment on a lavish demonstration scale using pure hydrogen and what he now knew to be pure oxygen in the summer of 1783. Apparatus of the kind that he used is preserved at the Musée des Arts et Métiers in Paris. Even today the fine brass work and elegantly blown glassware suggest the precision of Lavoisier’s method. Two huge gasometers containing the gases were first weighed, before the gases were allowed to mix in a vast glass bulb. Wires running into the bulb created a spark that ignited the hydrogen. The only residue was a few grammes of water, which showed conclusively that water was comprised solely of these two gases. That same summer, the Montgolfier brothers went aloft in the first hot-air balloon. Lavoisier saw immediately that if ultra-light hydrogen gas could be made economically in bulk from water, there would be a demand for it in ballooning.
I remember performing the same demonstration at a chemical happening which I organized at my school. The event was billed as Explo ’76. The hydrogen–oxygen reaction was not the most colourful or the smelliest item on the bill, but it certainly produced the loudest bang when I set off the balloon containing the two gases using a lighted taper lashed to the end of a long stick. Indeed, the sharpness of the report was our gauge of how accurately the gases had been introduced in the correct proportions of two to one. A fraction of a second later a fine mist hung in the silent space where the balloon had been. The Explo events, I later learnt, had continued for some twenty years after I left the school until they eventually became so grandiose–I heard tell of demonstrations so bombastic that they were staged no longer in a lecture theatre but in the school’s obsolete and drained outdoor swimming pool–that they attracted the attention of the emergency services.
I have tried to reach this famous turning point in the history of chemistry without using the dread word ‘phlogiston’, a concept so tenacious during the eighteenth century and yet so mistaken and confusing that it still has the power to deter the amateur scientist. Phlogiston was the ‘principle of fire’ which Priestley and many others at the time mistakenly believed to have material existence. Phlogisticated air is therefore air where combustion has taken place, and dephlogisticated air, perversely, is air with the potential for combustion. The confusion arises because a presumed absence (of phlogiston) in fact turns out to be a presence (of the element oxygen).
The phlogiston theory explained what chemists observed very well, but provided no real understanding of the processes involved. One way to picture the confusion is to think of a moulded mask of a human face. Strongly lit from the side, you can clearly see the peak of the nose and the sockets of the eyes. But it is only by changing your perspective, or better still by reaching forward and touching the mask, that you find the light is coming not from the right as you had thought, but from the left, and you are in fact seeing the face from behind and not in front. Phlogiston was just such a reverse image, accurate to all appearances, and yet still fundamentally deceptive. It required Lavoisier’s altered perspective to see things as they really were.
Although it correctly explained nothing, phlogiston was a stubborn theoretical concept. Even Lavoisier, a notable phlogiston sceptic even before his experiments with oxygen, used terms such as air déphlogistiqué as well as air empiréal and air vital alongside his new word oxygen until at least 1784. In an amusing pre-echo of our present obsession with anti-oxidant creams, Gustave Flaubert makes reference to a ‘pommade antiphlogistique’ in Madame Bovary, which is set a good fifty years after the theory ceased to have any scientific currency.
Lavoisier’s work placed oxygen–rather than fire–at the centre of combustion and so of much of chemistry. In 1789, on the eve of the French Revolution, he published an Elementary Treatise on Chemistry. It included a comprehensive list of ‘simple substances belonging to all the kingdoms of nature, which may be considered as the elements of bodies’. These were divided into four categories. The first included the gases, hydrogen, oxygen and nitrogen, as well as light and ‘caloric’, or heat. The second comprised six non-metallic substances that formed acids–carbon, sulphur, phosphorus and the unknown bases of muriatic (hydrochloric), fluoric and boracic acids. The third category listed seventeen ‘oxydable’ metals from antimony to zinc, and the fourth added five ‘salifiable simple earthy substances’ including lime and magnesia that Lavoisier correctly intuited to be concealing further new metal elements.
Lavoisier’s textbook sold well. He had launched a chemical revolution; now came the political revolution. Lavoisier was clearly sympathetic to the ancien régime, although he rejected Louis XVI’s last-ditch invitation to become his minister of finance in 1791, claiming that to do so would jeopardize the ‘ideal of balance’ that he sought to bring to economics and politics as much as to chemistry. Across the Channel, meanwhile, Priestley threw a party to celebrate the anniversary of the fall of the Bastille, and later that day a royalist mob destroyed his home. Lavoisier was to suffer an even worse fate at the hands of the Jacobins–on 5 May 1794, he went to the guillotine, hated as a tax collector and ignored for his science.
It is possible that, if the concurrent discoveries of oxygen in air and water had not been made when they were, we would not now accord this element the importance that we do. The chemical revolution would have been postponed, perhaps not triggered until Alessandro Volta made the first battery in 1800 using electrodes of copper and zinc. Our perception of chemistry would then arise less from the doings of one ubiquitous, hyperactive element–gaseous but material nonetheless–and more from the fleeting exchange of incorporeal electrical charges between chemical bodies, and we would now be without ‘the excessive domination of oxygen in doctrine and nomenclature’.
But oxygen did move to the chemical centre, and in due course also came to acquire a far broader symbolic role in our language. This did not happen immediately, as it did, for example, with electricity. The romantic writers famously saw the dramatic and metaphoric potential of galvanism, Mary Shelley’s Frankenstein being only the most celebrated work inspired by the new understanding of electricity. But they also took inspiration from the new chemistry. Where Shakespeare had to make do with ‘sweet air’ and ‘summer’s ripening breath’, the poets of the nineteenth century could sample the concentrated essence of air and life and consider whether to add it to their lexicons. Coleridge attended Davy’s lectures–he came in order, as he said, ‘to increase my stock of metaphors’–and observed on one occasion how ether ‘burns bright indeed in the atmosphere, but o! how brightly whitely vividly beautiful in Oxygen gas’. Another time, he noted how oxygen and hydrogen could be prised from water with electricity. Though intensely aware of the discovery of oxygen and of its role in life, the romantics did not put it in their poetry, however. Poems such as Percy Shelley’s ‘Ode to the West Wind’ and ‘To a Skylark’ burst with life-giving air and water and the blues and greens they occasion in nature but do not mention oxygen by name. Perhaps they feared their readers were not so well acquainted with the latest science as they. More likely, they simply rejected the word as lyrically unfit, a polysyllable that paradoxically seemed to choke the flow of breath. Much later, Roger McGough got round the problem by using the element’s smoke-ring of a chemical symbol rather than its name in his poem, ‘Oxygen’, whose last line represents a person’s final breaths by a fading sequence of eight ‘o’s.
How then did oxygen come to gain currency as a metaphor for ‘essence’, so that we immediately understand, for example, the Victorian poet Francis Thompson’s writing of Shelley how ‘The dimmest-sparked chip of a conception blazes and scintillates in the subtile oxygen of his mind’, or the vow of Margaret Thatcher–a one-time chemist, of course–to deny terrorists ‘the oxygen of publicity’?
The answer may lie in the spread of oxygen therapy during the nineteenth century, which introduced the gaseous element to the public for the first time. Understood as necessary to support life, oxygen was now the gas of choice for use against all manner of ills. It could be readily made by heating saltpetre and was observed to produce feelings of ‘comfortable heat’ in the lungs and limbs. Oxygen treatment could relieve diseases that led to breathing difficulties such as phthisis (pulmonary tuberculosis), although the relief only lasted as long as the gas. Against many other ailments, oxygen had no obvious effect, but this of course was no barrier to those promoting the curative powers of ‘vital air’. Early enthusiasm soon waned amid accusations of quackery, but a new method of producing oxygen from the air and storing it under pressure in easily transported cylinders led to a resurgence of interest in the middle years of the century. With very little proper medical investigation of the treatment, oxygen therapy was used largely indiscriminately and continued to be challenged by sceptics. ‘A question is frequently asked, “Is Oxygen Gas Inhalation dangerous?” The reply is decidedly, not at all so; it can be used without any possible risk of harm, and always with a real hope of doing good,’ ran one defensive advertisement in 1870.
Medical respectability came to oxygen therapy after the First World War, when the distinguished physiologist John Scott Haldane showed its beneficial effect on soldiers suffering the chronic effects of poison gas. Haldane was a notorious self-experimenter. He carefully exposed himself and compliant colleagues to various unpleasant gases in a sealed chamber known as the ‘coffin’ and noted their effects on body and mind. He climbed Pikes Peak in Colorado in order to breathe for himself the thin air at 14,000 feet. His major scientific contribution was to understand the role of haemoglobin in regulating breathing, but he also made a number of helpful innovations, introducing the decompression routine for divers and the miner’s canary to warn of low oxygen levels underground.
The legacy of his work is seen in the now familiar terminology of oxygen masks and oxygen tents. Meanwhile, commercial products such as Oxydol soap also began to trade on the health-giving and cleansing properties of oxygen. Each box of Radox bath salts once explained its brand name as a contraction of the largely meaningless phrase: ‘radiates oxygen’. The restorative promise of the gas lives on in the lately fashionable oxygen bars of Tokyo and Beijing, where for a fee one can breathe a purer air.
Once it was understood that it was not an element in its own right, ozone too–comprising three atoms of oxygen bonded in a triangle rather than the hand-holding pair of atoms in the oxygen we breathe–began to be marketed as what, in essence, it was, a more intense form of oxygen. It was called ‘electric oxygen’, a reflection of the means of its manufacture as well as an exciting piece of branding, and used to purify drinking water, remove odours and generally imbue all it touched with a healthy vigour. One bottled water bore the strapline ‘ozone is life’, while long before the ‘oxygen of publicity’ there was (in John Dos Passos’s The Big Money) the ‘ozone of revolt’.
Recently, though, we have become inclined to view oxygen as the destroyer, not the supporter, of life. Following his experiments in which he observed mice thriving in oxygen and the increased rate at which candles burnt down, Priestley in his Experiments and Observations on Differen
t Kinds of Air (1776) brilliantly foresaw that any creature given too much oxygen might ‘live out too fast and the animal powers be too soon exhausted in this pure kind of air’. One of Priestley’s fellow members of the Lunar Society, Erasmus Darwin, wrote in his poem ‘The Botanic Garden’ of oxygen as ‘Air’s pure essence’ that nurtures plants and feeds the beating heart, but also as ‘soft combustion’.
This flameless fire corrupts all that it touches. It is this ubiquitous, constant and inescapable reaction that has positioned oxygen centre-stage. It is why we classify many important chemical processes either as oxidations or their reverse, reductions. Oxidation does not always require oxygen itself. It can be accomplished by other chemical oxidizing agents such as chlorine or by the application of energy such as via ultraviolet light. Photosynthesis in plants uses light from the sun to promote both oxidation and reduction. The main reactions of photosynthesis convert carbon dioxide into glucose. But in another part of the forest, as it were, light oxidizes water (using manganese as a catalyst) to release oxygen, daily repeating in each green leaf the experiments of Scheele and Lavoisier. Oxygen is merely the waste product of these processes, a corrosive gas that would destroy animal life if animals had not evolved in tandem with the increasing levels of oxygen in the earth’s atmosphere.
Periodic Tales Page 15