On 1 August 1774, in a laboratory in Wiltshire, an English clergyman focused sunlight through a large lens onto a sample of red mercury calx sealed in a glass vessel. A gas came off. He collected it, and found that a candle burned in it with a fierce brightness and that a mouse placed in it lived far longer than in ordinary air. He breathed some himself and reported that his chest felt peculiarly light and easy for some time afterwards.
Joseph Priestley had made oxygen. He would spend the remaining thirty years of his life explaining it wrongly, and defending the wrong explanation after everyone else had abandoned it.
The Theory Everyone Held
Eighteenth century chemistry was organised around phlogiston, a substance supposed to be released during burning. Wood was held to be rich in phlogiston, which is why it burns and leaves little ash. A metal was thought to be a compound of its calx, what we would call the oxide, and phlogiston, which is why heating a metal with charcoal restores it.
The theory was not absurd. It unified combustion, rusting and respiration into one process, which was a real achievement, and it survived because it explained a great deal.
It had one persistent embarrassment: when a metal burns, the calx weighs more than the metal did. Something supposedly leaving the substance made it heavier. The standard responses were that phlogiston had negative weight, or that weight was not the point, and both were unsatisfying.
Priestley’s new gas fitted the theory neatly. If a candle burns better in it, it must be air that is unusually empty of phlogiston and therefore able to absorb more. He called it dephlogisticated air, and the name is the entire dispute in a single word.
The Dinner in Paris
In October 1774 Priestley was in Paris as travelling companion to his patron Lord Shelburne, and dined with Antoine Lavoisier and his wife. He described the experiment and the properties of the gas.
Lavoisier had already been working for two years on the weight problem, having established that phosphorus and sulphur gain weight when burned and that the gain comes out of the air. He repeated Priestley’s experiment within weeks, and reinterpreted it completely.
Where Priestley saw ordinary air with something removed, Lavoisier saw a distinct substance, a component of ordinary air, that combines with metals and with anything that burns. Nothing leaves the burning body. Something joins it, and the weight gained is exactly the weight of the gas absorbed.
He named it oxygène, from the Greek for acid former, on the belief that it was the principle of acidity. That was wrong, and the name has stuck for two and a half centuries, which is a useful reminder that being right about the main thing does not require being right about everything.
He also, in his published account, was less than generous about where the news had come from. Priestley complained about it for the rest of his life, and he had a case.
A Third Man, Earlier
The Swedish apothecary Carl Wilhelm Scheele had prepared the same gas by several routes in 1771 or 1772, two or three years before Priestley, and called it fire air. His book was delayed by his publisher and did not appear until 1777. He wrote to Lavoisier in 1774 describing his method, and the letter, found in 1890, went unanswered as far as anyone can tell.
Scheele has a reasonable claim to priority and no claim at all to influence, since almost nobody knew what he had done until the argument was over. He also discovered chlorine, manganese, barium, molybdenum, tungsten and a long list of organic acids, tasted most of his own preparations, and died at 43.
What Counts as Discovering Something
This case is the standard example in the philosophy of science for a reason, and Thomas Kuhn used it to make the point that discovery is not a single dateable event.
Three plausible answers exist. Scheele made the gas first. Priestley made it independently and published first. Lavoisier was the first to understand what it was.
None of the three is simply right, and the question exposes what we are actually asking. If discovery means producing the stuff, Scheele wins. If it means telling the world, Priestley wins. If it means identifying it as a chemical element and knowing what it does, only Lavoisier qualifies, and he could not have done it without the other two.
Priestley’s own position is the awkward one. He isolated the gas, described its properties accurately, and used a framework that made it impossible for him to see what he had. He was an outstanding experimenter. He also discovered nitrous oxide, ammonia, hydrogen chloride, sulphur dioxide and carbon monoxide, and invented carbonated water, and he interpreted almost all of it through phlogiston.
Lavoisier’s Instrument Was a Balance
What separates Lavoisier from his contemporaries is not intelligence and not luck. It is that he weighed everything, before and after, in sealed vessels, and treated any discrepancy as a fact to be explained rather than as experimental noise.
From that habit came the conservation of mass in chemical reactions, the demonstration that water is a compound of hydrogen and oxygen rather than an element, the correct account of respiration as a slow combustion, and the first modern list of elements, published in the Traité élémentaire de chimie of 1789 with plates engraved by his wife.
Marie Anne Paulze Lavoisier was not a footnote to this. She translated the English chemical literature for him, including a critique of phlogiston that she annotated with objections of her own, kept the laboratory records, and studied drawing under Jacques Louis David in order to produce the illustrations of the apparatus.
The Traité also carried the new chemical nomenclature, devised with Guyton de Morveau, Berthollet and Fourcroy, which replaced names such as oil of vitriol and butter of antimony with names that state what a substance contains. Learning the new names meant accepting the theory behind them, which is one reason the conversion was so quick.
In 1783 Lavoisier read a paper to the Academy titled Réflexions sur le phlogistique, which is a direct demolition. Within a decade most working chemists in Europe had switched.
Two Men Destroyed by Politics
Both ends of this story are grim, and neither has anything to do with chemistry.
Priestley was a Dissenting minister, a Unitarian, and a public supporter of the American and French revolutions. On 14 July 1791, at a dinner in Birmingham commemorating the fall of the Bastille, a Church and King mob burned his house, his laboratory, his library and his papers. He moved to London, found himself unwelcome, and emigrated to Pennsylvania in 1794, where he died in 1804, still arguing for phlogiston in print.
Lavoisier had funded his science through the Ferme générale, the private consortium that collected taxes for the crown, which made him extremely rich and extremely hated. He was arrested in November 1793, tried with twenty seven other tax farmers, and guillotined on 8 May 1794. The appeal for a stay to complete scientific work was refused. Lagrange said the next day that it had taken them an instant to cut off that head, and that a hundred years might not produce another like it.
Eighteen months later the French government pronounced him wrongly convicted and returned his confiscated belongings to his widow, with a note.
What the Fight Left Behind
Modern chemistry begins here, and it begins with an argument about weight. Once combustion is a combination rather than a release, reactions can be written as equations that balance, elements become the things that cannot be broken down further, and quantities become the point.
Everything downstream depends on it. Dalton’s atomic theory of 1803 makes no sense without fixed proportions by weight, and Mendeleev’s periodic table is an ordering of the element list that Lavoisier was the first to write down.
The standard account of Lavoisier as the father of modern chemistry is fair, provided the sentence is finished properly. He built it out of a gas he did not make, described to him at dinner by a man who had no idea what it was, and prepared three years earlier by a Swedish apothecary whose letter he never answered.
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