The Invention of Air

by Steven Johnson

  Pringle twisted Franklin’s rational system into a more human-centric rendition, with “every individual plant serviceable to mankind,” but even with that distortion, the scope of Priestley’s discovery comes through vividly in the language:

  I present you with this medal, the palm and laurel of this community, as a faithful and unfading testimonial of their regard, and of the just sense they have of your merit, and of the persevering industry with which you have promoted the views, and thereby the honour, of this Society. And, in their behalf, I must earnestly request you to continue those liberal and valuable inquiries, whether by further prosecuting this subject, probably not exhausted, or by investigating the nature of some other of the subtle fluids of the universe.

  There would be many “subtle fluids” to investigate in the coming years. Priestley would be the first to identify ten of them, including hydrogen chloride, ammonia, sulfur dioxide, and silicon fluoride. But his most celebrated—and contested—discovery would come nearly two years after the Copley Medal.

  Priestley’s meteoric rise to prominence as a scientist—along with his political writings—had attracted the attention of William Petty, Earl of Shelburne, former secretary of state and arguably the most intellectually nimble and inquisitive political figure in Britain. (Shelburne’s Irish roots and liberal politics also made him one of the least popular.) In late 1772, the earl had proposed an arrangement whereby Priestley would maintain Shelburne’s library, educate his two sons, and advise on subjects and materials currently being debated in Parliament. In turn, Shelburne would house the Priestleys and their children in far grander style than they had ever been accustomed to, spending the winter in a town house near Shelburne’s residence in Berkeley Square, and the rest of the year at Bowood, the family estate in Calne, Wiltshire, in the southwest. Happy with his relative freedom and extraordinary run of success in Leeds, Priestley spent months weighing the decision. He wrote to Franklin for advice, and Franklin suggested what was then a novel approach to resolving such an issue:

  My Way is, to divide half a Sheet of Paper by a Line into two Columns, writing over the one Pro, and over the other Con. Then during three or four Days Consideration I put down under the different Heads short Hints of the different Motives that at different Times occur to me for or against the Measure. When I have thus got them all together in one View, I endeavour to estimate their respective Weights; and where I find two, one on each side, that seem equal, I strike them both out: If I find a Reason pro equal to some two Reasons con, I strike out the three. If I judge some two Reasons con equal to some three Reasons pro, I strike out the five; and thus proceeding I find at length where the Ballance lies; and if after a Day or two of farther Consideration nothing new that is of Importance occurs on either side, I come to a Determination accordingly.

  Ultimately, Priestley agreed to Shelburne’s plan. Mary packed up their Leeds house, and the growing family—baby William had been born the year before—moved to Wiltshire in the summer of 1773. Priestley’s improvised kitchen laboratory was replaced by a much more opulent setting: a laboratory in the newly constructed orangery on Shelburne’s estate. The lab was next door to Bowood’s imposing library, and looked out on a verdant lawn gently sloping down toward a small lake. (The grounds had been designed by the legendary landscape architect Lancelot “Capability” Brown.) Joseph and Mary had not exactly entered English high society, but for the first time in their lives, they were down the hall from it. Mary was largely unimpressed by her firsthand view of the upper classes. One story has Shelburne arriving to welcome them at their new house in Calne, and finding Mary on a ladder, industriously papering the walls. Joseph apologized for their not providing a more gracious welcome, but Mary quickly dismissed her husband’s proprieties. “Lord Shelburne is a statesman,” she said, “and knows that people are best employed in doing their duty.” Later she would observe candidly to Shelburne, “I find the conduct of the upper so exactly like that of the lower classes that I am thankful I was born in the middle.”

  The quality of Priestley’s tools was improved under Shelburne’s patronage. Perhaps the most important was a burning glass he acquired shortly after the move, reportedly the former property of the Grand Duke Cosimo III of Tuscany. The glass was a twelve-inch convex lens, with a focal point of twenty inches that concentrated the sun’s rays with great intensity and precision. Like that eleven-year-old boy trapping spiders in jars, Priestley set about with his new gadget to burn as many substances as he could possibly imagine.

  What happened next may not be as famous as other eureka stories in the scientific canon, but as a case study in the complexities of intellectual history it may be the most analyzed “discovery” on record. In part this is because there is a dispute at its core, a question of precedence that is not easily resolved, and that rivalry makes for a rich narrative study. But it has also seen so much churn because it exemplifies the blurriness that so often accompanies paradigm shifts. Kuhn tells the story early on in his Structure of Scientific Revolutions , and published a longer version of it in the journal Science. Since then, it has become its own sort of experimental laboratory for theories about intellectual progress, a place where scientists could use competing models of innovation to test their hypotheses.

  The facts are simple enough. In early August of 1774, Priestley turned his lens on mercury calx, the ash that forms when mercury is heated in air. It produced a gas that behaved surprisingly like nitrous air (our nitric oxide), in that candles appeared to blaze with unusual intensity in its presence. This baffled Priestley, because the mercury shouldn’t have had any nitrous air in it. A friend procured purer samples and Priestley tried the experiment again. To his astonishment, the candle burned even brighter.

  Shortly thereafter, Priestley left on a long European tour with Shelburne. He ruminated over the burning flame the entire trip, and at a fateful dinner in Paris—wonderfully captured in Joe Jackson’s A World on Fire—he gave a riveting account of his experiments to an audience of philosophes, among them Antoine Lavoisier, who would soon be Priestley’s rival, and who would eventually complete the chemical revolution that Priestley had initiated in his Leeds laboratory. It was a classic case of Priestley’s inveterate openness: at a time when British and French spies actively infiltrated the industrial and scientific labs in both countries, Priestley sits down to dinner with the scientific intelligentsia of France and happily spills the beans about his most exciting new experiment. (“I never make the least secret of any thing that I observe,” he would explain later, in his description of the conversation.)

  When Priestley returned from the European excursion (earlier than planned, having tired of France before Shelburne did), he quickly launched into an in-depth study of this strange new air. He devised new experiments that generated even purer samples, and the more he probed the air, the more it seemed to differ from nitrous air. Slowly, Priestley began to think that he had produced common air, in which case it should support animal respiration. On March 8, 1775, he put a mouse in a glass with two ounces of air generated from the mercury calx. The mouse suffered no immediate discomfort, just as Priestley expected. But then something very odd happened. A mouse trapped in a vessel with ordinary atmospheric air would last fifteen minutes before collapsing. But the mouse that Priestley had trapped in the jar with his new air somehow survived for thirty minutes. It might have survived even longer: “Though it was taken out seemingly dead,” Priestley wrote, “it appeared to have been only exceedingly chilled, for, upon being held to the fire, it presently revived and appeared not to have received any harm from the experiment.”

  He ran the mouse experiment multiple times in the ensuing days, reducing the amount of air available to each mouse, and each time finding the creatures strangely unfazed by a quantity of air that should have killed them in five minutes. Priestley mulled these strange facts in his head compulsively through sleepless nights, waking early each morning to try a new variation. Eventually, he mustered up the courag
e to employ himself as a test subject and inhale the miraculous new air himself:

  The feeling of it to my lungs was not sensibly different from that of common air; but I fancied that my breast felt peculiarly light and easy for some time afterwards. Who can tell but that, in time, this pure air may become a fashionable article in luxury. Hitherto only two mice and myself had had the privilege of breathing it.

  That first breath forced Priestley, at last, to confront an astonishing truth. The ordinary atmosphere that sustained life on Earth could be improved. There was an air purer than common air. Two billion years after the cyanobacteria began pumping the Earth’s atmosphere full with the stuff, Joseph Priestley had discovered O2.

  OF ALL PRIESTLEY’S accomplishments, all the books and ideas and experiments, all the world-changing conversations that ran through his career, the discovery of oxygen conventionally ranks at the very pinnacle of his lifework. The Encyclopædia Britannica entry on Priestley devotes nearly a fifth of its text to the oxygen story. Wikipedia mentions it in the second sentence on its Priestley page.

  But the true narrative is more complicated than that easy declarative sentence—Priestley discovered oxygen—which is why so many scholars have dissected the particulars of this story. Priestley’s breakthrough illustrates the fuzzy boundaries of scientific discovery. “Discovering” oxygen is not like “discovering” the Dead Sea Scrolls or some other unique object that has a clear identity and has been undeniably hidden for ages. It is closer to, say, discovering America: the meaning of the phrase depends entirely on the perspective and values you bring to the issue.

  We know definitively from the records of their experiments that other scientists had isolated pure oxygen before Priestley took his burning glass to the mercury calx, but in each case the investigator had failed to realize the significance of what he’d done. (With one exception, to which we will turn in a moment.) Even Priestley appears to have isolated the gas in earlier experiments. What mattered was not that Priestley produced O2, but that he realized that he had done something unusual, and then convincingly proved that it was a more rarified subset of common air. (Ever the practical chemist, Priestley even managed to throw in a teaser about his discovery becoming a “fashionable article” someday. He had invented soda water five years before; now he was pointing the way toward the oxygen bar.) Priestley had stumbled on the gas in one of his classic prepared accidents, but he had possessed the good sense to notice the anomaly of the flame burning brighter than expected, and he’d had the time and tenacity to explore further variations in the subsequent months.

  The problem is that someone else had made comparable explorations before. The Swedish chemist Carl Scheele had isolated oxygen using a number of substances, including mercury calx, between 1771 and 1772. He called it “fire air” because of its combustible nature. He also demonstrated that common air was a mixture of two distinct gases, the fire air of oxygen, and what he dubbed the “foul air” of nitrogen. But Scheele failed to publish his findings until 1777, long after Priestley had been credited with the breakthrough.

  When Priestley described his discovery, in Book IV of his Experiments on Air, he introduced the section with an open admission of the role of randomness in his work—even including a subtle dig at the theoretical, synthetic mode of Newton and his followers:

  More is owing to what we call chance, that is philosophically speaking, to the observation of events arising from unknown causes, than to any proper design, or preconceived theory in this business. This does not appear in the works of those who write synthetically upon these subjects; but would, I doubt not, appear very strikingly in those who are the most celebrated for their philosophical acumen, did they write analytically and ingenuously.

  It’s a valid observation, given Priestley’s chaotic method and his general aversion to theorizing, made even more valid by the tremendous run of success he’d just enjoyed. But there was a catch lurking in those offhand dismissals of “preconceived theories.” Priestley himself was trapped in a preconceived theory, one that would prove to be almost entirely unfounded, though he clung to it for the rest of his life.

  This was not a theory hiding in the shadows; Priestley seared it directly into the name he gave his pure air: dephlogisticated air.

  That awkward name came from the closest thing to a dominant research paradigm in the nebulous field of pneumatic chemistry: the phlogiston theory, one of the all-time classics in the history of human error. Phlogiston was an attempt to explain the age-old mystery of why things burned. (The term derives from the ancient Greek word for fire.) First proposed by the German chemist Johann Joachim Becher in the late 1600s, it was refined into a working theory by Becher’s pupil, Georg Ernst Stahl, who proposed in 1716 that all substances capable of burning possessed a substance called phlogiston that was released into the air during combustion. When the flame of a burning substance goes out, the air was considered to be “phlogisticated”—having absorbed so much of the magic ingredient of combustion that nothing remained to burn.

  We now know that the phlogiston theory had things almost exactly backward, though most of the leading chemists before Priestley—including both Black and Scheele—failed to see the flaws in it and labored happily within its framework. In truth, when things burn in common air, something is being extracted from the air, not the reverse: oxygen molecules are bonding in the heat of combustion with whatever happens to be on fire. This is what we now call oxidation. When the air loses too many oxygen molecules to support the oxidation process, the flame goes out.

  Priestley, alas, was on the wrong end of the phlogiston paradigm, and so when he happened upon an air in which flames burned more brightly than common air, he interpreted his findings using the conceptual framework of the existing paradigm. Breathable air that also exacerbated combustion was, logically, air that had been entirely emptied of phlogiston. (Or, put another way, it was air primed to be filled with phlogiston.) Within the rules of that conceptual system, Priestley’s dephlogisticated air was a fitting, if ungainly, appellation. Unfortunately, the rules of that system were fundamentally flawed.

  Seeing around the flaw in the model was once again made possible by technological advances in measurement. A few chemists had noted the puzzling fact that some burned substances weighed slightly more than they did before combustion, seemingly contradicting the premise that they were releasing phlogiston into the air. But like most anomalies in the decades before a paradigm shift, those uncomfortable observations were largely swept under the rug, in part because the weight gain was so minuscule.

  To Antoine Lavoisier, however, that additional weight was a mystery that could not be dismissed. Inspired by Priestley’s dinner-table account of his inventive experiments, but equally appalled by the Englishman’s lack of theoretical rigor, Lavoisier embarked on a series of experiments that utilized his unrivaled skills with a balance. (“It can be taken as an axiom,” Lavoisier wrote, “that in every operation an equal quantity of matter exists both before and after the operation.”) His measurements led him outside the blinders of the phlogiston theory, and by 1776 he announced his finding that atmospheric air was one-fourth composed of “pure air . . . which Mr. Priestley has very wrongly called dephlogisticated air.” The historian Joe Jackson describes it well:

  Burning added weight: there was a union, shown by the most sensitive balances in Europe. There was not loss of the mysterious phlogiston. . . . All chemical changes obeyed the law of the indestructibility of matter. There were no ghosts in the process, no ether escaping notice of his scales. In the chemical change of burning, nothing was gained or lost, even in the vaporous air.

  By the next year, Lavoisier was ready to give this “pure air” its scientific name. He called it oxygen.

  Priestley’s “discovery” of oxygen turns out to be far more vexed than the standard short-form biographies suggest. He was not the first to identify the gas, and he did not give it its enduring scientific name. The name he did affix to his discovery b
etrayed a fundamental misunderstanding of the basic chemistry of oxygen. No one contests the fact that he published his findings before Scheele, of course; and there is no doubt he played an essential role in leading Lavoisier to his more nuanced understanding of the gas. But the simple fact is that he was neither first, nor the most accurate, participant in the discovery of oxygen. Kuhn makes a related point in The Structure of Scientific Revolutions when he wonders how one can responsibly date the discovery of oxygen:

  Ignoring Scheele, we can safely say that oxygen had not been discovered before 1774, and we would probably also say that it had been discovered by 1777 or shortly thereafter. But within those limits or others like them any attempt to date the discovery must inevitably be arbitrary because discovering a new sort of phenomenon is necessarily a complex event.

  What is puzzling here is not that Priestley should receive the popular acclaim for a discovery that was not entirely his; the history of exploration—whether intellectual or geographic—is ripe with false attributions and contested claims of priority. What’s strange is that Priestley should be so widely recognized for his oxygen experiments of 1774-75, and yet the mint experiments of 1771-72 are often mentioned only in passing in accounts of his scientific career. (The Britannica entry on Priestley barely mentions the mint experiment.) Both were foundational insights that led to world-changing ideas that rippled through science and society. But there is no dispute over the mint experiments; as far as we know, he was genuinely the first to discover that breathable air was a concoction of plants, and with Franklin’s help he was able to grasp and describe the far-reaching consequences that process would have on our understanding of Earth’s environment. He reached that point on the mountain before anyone else, and made no missteps in his ascent. So why is he so often celebrated for a climb where he didn’t actually make it to the peak?