An Ocean of Air

by Gabrielle Walker

  One of the first things to try to do, in testing any new gas, was to see its effect on a lighted candle. And when he did so, Priestley was astonished. The candle didn't immediately go out, as it would in fixed air. It didn't burn steadily and then gradually diminish, as it would in common air. Instead, Priestley's candle flared up and burned with a fierce, intense light, brighter than any he had ever seen before. What's more, it went on burning long after it should by rights have guttered out.

  Priestley didn't realize it, but he had stumbled across the part of our air that gives us life: the element oxygen. We now know that when anything—candles included—burns in air, it is using up the available oxygen. Priestley's "common air" is not a single element as he and his contemporaries believed, but is instead made up of many components, of which the principal ones are oxygen and nitrogen. Dull, unreactive nitrogen comprises almost four-fifths of the volume of air, but it is present mainly as a filler. It is oxygen that provides the active ingredient, the fuel that makes up about the remaining fifth. This is what permits a candle to burn. As soon as the oxygen is used up, there is nothing left in the air but unreactive nitrogen and the candle will go out. That's why common air could support a flame for only so long, and why Priestley's new air—pure oxygen—yielded a flame that was so much brighter and lasted so much longer.

  However, at the time, Priestley and most of his contemporaries had an alternative idea. They believed that a burning candle gave off a strange substance with the tongue-twisting name "phlogiston." The more phlogiston there was in the vessel, the harder the candle found it to force yet more of this substance into the air around it; it was like trying to squeeze extra people into a room that's already crowded. When you burned a candle in common air, it poured out more and more phlogiston until eventually the vessel was too crammed to admit any more and the candle went out.

  With this doctrine firmly lodged in his mind, Priestley was baffled by the behavior of his candle. After falling asleep with the conundrum in his head, and waking to find it still troubling him, he decided that his new air didn't contain a single scrap of phlogiston. If a room starts out empty, you can add people in a steady stream for some time before it finally fills up. In the same way, Priestley reasoned, if the new air was devoid of phlogiston, a burning candle could continue pouring out the stuff without choking its own flame. And so Priestley, who though prolific with words was not exactly stylish in his choices, gave his invention the unwieldy name "dephlogisticated air."

  Priestley immediately began experimenting with his new air. He tried mixing it with another of the recent discoveries of the time: inflammable air, which we now call hydrogen. Inflammable air was so called because it burned readily when mixed with common air. You could even hear a slight pop as it ignited. However, Priestley discovered that if he mixed his new air with hydrogen and inserted a flame, the resulting explosion was much more impressive. Instead of a gentle pop, it sounded more like the deafening blast from a pistol. Priestley didn't know that he had discovered a most potent mixture, the same stuff that we now use for rocket fuel, but he did realize that it made a great party trick. He carried carefully sealed vials of the raw mixture around with him in his pocket to impress friends, acquaintances, and in fact almost anyone who would stop and listen. He'd take out a vial, uncork it, expose it to a flame, and watch their faces. The results, he said, were most satisfying: "It has never failed to surprise every person before whom I have made the experiment."

  He also tested the effect his new air would have on a living creature, in this case a mouse. Partly to conserve mice—because it wasn't always easy to catch them—and partly out of consideration for a fellow being, Priestley did his best to keep the mice involved in his experiments alive. If he felt there was a good chance they wouldn't survive in the air he happened to be testing, when he pushed them through the water or mercury into the vessel, he kept a tight hold of their tails to pull them out as soon as they began to look distressed. And for cases where he thought the air was likely to be good for the mice, he built them a small shelf so they could rest in comfort above the water.

  From previous experiments, Priestley knew that a mouse could survive if the flask was full of common air for roughly a quarter of an hour before it needed to be rescued. But when he placed his mouse in the new air, the creature continued breathing for a full half hour before he had to whisk it back out. Though the mouse then seemed to be dead, Priestley realized that it was merely chilled, and a few moments beside the fire revived it fully.

  Encouraged by this, Priestley decided, with characteristic impetuousness, to try breathing the new air for himself. He wasn't particularly afraid of the consequences—instead he relished the idea of experiencing something that, until then, had only been breathed by mice. It was even better than he'd hoped: "I fancied my breast felt peculiarly light and easy for some time afterwards," he said. "Who can tell but that, in time, this pure air may become a fashionable article in luxury..."

  Once again, this was highly prescient, though perhaps even Priestley couldn't have imagined that more than two hundred years later, trendy bars across the world from Tokyo to Los Angeles to London would be offering whiffs of pure oxygen as a treatment for everything from hangovers to headaches.

  Breathing pure oxygen certainly feels good, but it is not necessarily good for the health. Priestley himself noticed the "greater strength and vivacity of the flame of a candle, in this pure air," and therein lies a warning. "As a candle burns out much faster in [oxygen] than in common air," he suggested, "so we might, as may be said, live out too fast [Priestley's italics] and the animal powers be too soon exhausted in this pure kind of air."

  He was right that breathing pure oxygen for too long can be dangerous. Half an hour in a Los Angeles bar won't do any harm, but if you breathed pure oxygen for too long your lungs would fill with blood, and after a few days you would die. That's because the very thing that makes oxygen useful to us is also its greatest hazard. The oxygen we need to breathe is an exceptional releaser of energy. We need oxygen's reactive powers to enable us to live such vigorous lives, but even the diluted amount that we breathe in ordinary air comes with its own perils.

  Priestley had other ideas for how his new air might be useful. He sug gested, for instance, that strategically placed flasks could "qualify the noxious air of a room in which much company should be confined...[to make it] sweet and wholesome." But he still persisted in thinking of it as fundamentally different from ordinary air. Even the visionary Priestley maintained that "common" breathable air was the purest possible form. That's why he was so confounded when his new air seemed even purer than the common stuff, and why he had to invent his elaborate explanation of dephlogistication to account for its properties. The doctrine of phlogiston was holding back the whole story of oxygen and its importance to us all. For Priestley, oxygen remained a curiosity, a party trick with a few potential commercial applications for right-minded entrepreneurs. To discover the vital role it plays in the life of the planet would take someone else entirely, someone who was as cool and systematic in his experiments as Priestley was chaotic, and who was prepared to think in ways that nobody had thought before.

  ***

  Antoine Lavoisier was a golden child. From infancy he was doted on, the only son of a well-to-do bourgeois French household whose father was very much on the ascent. Though he lost his mother at an early age, he was raised by a childless aunt who never doubted that he was destined for greatness.

  Ten years younger than Priestley, Lavoisier was born during the reign of Louis XV, a corrupt wielder of privilege who is said to have presciently declared, "After me, the deluge." Still, during Lavoisier's youth and early manhood there must have been little sign of the revolutionary carnage that was to come, and the tragic effect it would have on his own life.

  Instead, his family seemed fully favored by fortune. In only a few generations, his forebears had worked their way up from being postal couriers to attaining a considerable position in s
ociety. Lavoisier was always most precise in attitude and behavior, and his upbringing only served to reinforce that tendency. He was raised in a household where appearances were everything, and manners dictated by a delicate web of complex social codes.

  Even at the exclusive Parisian school that he attended from the age of eleven, precision was reckoned a precious concept. Lavoisier's math and science teacher was a renowned astronomer named Abbé La Caille who once spent four years on an astronomical expedition to the Cape of Good Hope, where he observed ten thousand new stars and named fourteen constellations. On his return, La Caille calculated his expenses with a level of accuracy that made many Parisians titter. He declared that the entire voyage had cost 9,144 livres and five sous, which is like totaling every single expense from four years of college down to the last few cents.

  And yet Lavoisier learned a great deal from La Caille and his other teachers. It didn't take them long to realize that they were dealing with an uncommon talent. To be sure, Lavoisier's grasp of the humanities was shaky—he never succeeded in mastering languages, and his understanding of art was technically appreciative rather than intuitive. But in math and science he excelled, and his teachers' encouragement fed his own natural ambition. He became determined to discover something truly remarkable. "I am young," he wrote, "and avid for glory." He dabbled in geology, astronomy, and the mysteries of the weather, as he cast around for something that would make his scientific name.

  The behavior of Lavoisier's adoring family did little to sap his supreme self-confidence. Once, as a young man, he had been accompanying a geologist named Guettard on a research trip for several weeks when his father offered to drive out and meet the travelers at a small town on their way home. Splendid, replied Lavoisier, and could he please bring along a bowl of goldfish as a gift for the lady with whom they had most recently been staying. At this, even Lavoisier's infatuated father was taken aback, protesting that he would have to carry the bowl in his own arms the whole way while the water slopped this way and that in a lurching carriage. However, he brought the fish.

  Lavoisier was undoubtedly arrogant, but he was also fair, at least when compared with the abhorrent corruptions of the time. In 1767, at the age of twenty-four, he used a family legacy to further his financial fortunes by buying a share in the notorious Ferme Générale ("General Farm"). France at the time was run under an iniquitous system of taxation. The peasants were forced on pain of deportation to the slave galleys to pay ruinous duties even for necessities such as salt, while the wealthy paid nothing. All indirect taxation on materials such as salt and tobacco was administered by a shadowy group of people called the "Farmers General," though they had little themselves to do with farming. Instead, as long as they delivered the requisite amount of money to the king, this body was free to charge the unlucky peasants as much indirect taxation as they wished. "Those who consider the blood of the people as nothing in comparison with the revenue of a prince," the great economist Adam Smith later commented, "may perhaps approve of this method of levying taxes."

  Lavoisier made a great deal of money from his involvement in the Ferme, but he abhorred its unfairness and did his best to leaven the worst injustices. Among other achievements, he managed to abolish the "tax of the cloven hoof" whereby any Jews wishing to pass through a certain region had to pay thirty pieces of silver. He also ensured that all his own dealings were as honest as the system could allow.

  But although he disliked the system of taxation in part for moral reasons, he was at least as troubled by the absurd inefficiency of overtaxing some people almost to the point of extinction while letting others off completely. Any sort of inefficiency pained him. Lavoisier dealt with his financial affairs with the same careful precision that he was to accord his scientific experiments: Unlike most of his compatriots in the Ferme, he recorded every transaction and accounted for every sou.

  Lavoisier's work with the Ferme took up almost all his time, but very little of his creative energy. He was still fired with an ambition to achieve something more remarkable than just making money, and he began to work exhaustively to find a subject worthy of his scientific attention. He conducted his research from six to nine each morning and seven to ten each evening, and in addition devoted one full day a week, his jour de bonheur (day of happiness), to his favorite activity.

  Meteorology held his attention for a while—he had been taking daily barometric measurements for several years and continued to do so for most of his life—but it didn't have quite the spark he was seeking. Then, after performing an expensive experiment to prove that diamonds are combustible, Lavoisier began to wonder why some materials burn while others don't.

  He was aware of the prevailing theory of phlogiston, but he wasn't convinced by it. To most natural philosophers of the time, Priestley included, phlogiston was a very sensible concept. If you watch something burning, it's easy to believe that the flames are releasing some substance from inside the material, and that the more of this substance—phlogiston—it contains, the more easily it burns.

  However, Lavoisier was troubled by the undeniable fact that when many substances, iron for instance, are heated in air they become not lighter, but heavier. Until then, theorists had fudged the answer to this mystery by declaring that phlogiston must have some kind of negative weight, so that losing it makes you heavier. To Lavoisier, that seemed like nonsense. If something gains weight when it burns, he reasoned, it must surely be absorbing rather than releasing something. The question was, what?

  To try to find out, Lavoisier began to study the work of every natural philosopher he could find who had worked on the problem, including that of Priestley. Lavoisier spoke no English, but his young wife was very proficient at languages and spent much of her time translating for her husband. She had every reason to be grateful to him. At the age of fourteen she had been troubled by a proposal of marriage from a wealthy and powerful man in his fifties who had seemed to her like an ogre. Lavoisier, then age twenty-eight, who knew her father and was already fond of her, had rescued her from a horrible fate by the simple expedient of marrying her himself.

  Lavoisier was impressed with the breadth of Priestley's investigation. He described it as "most painstaking and interesting work." But he was disgusted by Priestley's chaotic style of investigations, and the way that he raced from one topic to another with little thought as to what might connect the whole. Priestley's work, said Lavoisier, "consists more or less of a web of experiments, almost uninterrupted by any reasoning."

  And that was where Lavoisier saw his chance. He knew his brain was at least as brilliant as the chaotic and passionate Priestley's. But Lavoisier had something else as well—the cool head and precise habits of a financier. Put these together and he could achieve what nobody had done before. He could find out not just what happens when something burns, but why.

  So, shortly after his marriage, Lavoisier began a series of meticulous experiments. First, he confirmed what he and everybody else already knew. He carefully weighed various materials such as phosphorus and lead, burned them in common air, and measured the weight of the ash that was left. Every time he tried this, the ash was heavier than the material he had started with. This was just as he had expected.

  However, Lavoisier's next experiment was much more ingenious. He placed some lead on a balance inside a glass jar full of air, which he sealed. Then he carefully weighed the entire jar, lead, balance, and all. Next, he heated the lead from the outside and watched as the balance gradually tipped to show the lead gaining its weight. Finally—and this was the clever part—without opening the jar he weighed it again. Even though he could look through the glass walls and see from the tipped balance inside that the lead had grown significantly heavier, the weight of the entire jar remained exactly the same. Whatever had caused the lead to gain weight must have come from inside the jar.

  It seemed unlikely that the extra weight had come from the glass walls or the balance. The most obvious source was the air. But how to prove it?
Lavoisier reasoned that if some of the jar's air had disappeared into the lead, it must have left a gap, a partial vacuum waiting to be filled. So he unsealed the jar and sure enough, air from the outside rushed in to fill the gap. And then he weighed the vessel again to see how much new air had entered. The answer: precisely the same amount as had disappeared into the lead.

  It was in the very precision of his measurements that Lavoisier had started to find his answers. Many people had burned one material with another, weighed them in a desultory fashion, and surmised what might be happening. But Lavoisier of the tidy mind and precise habits was the first to put it all together into a quantitative whole. The lead increased in weight by this amount. The air above it lost this quantity. Since the two values are precisely the same, a portion of the air must have gone into the lead. And since the remainder of the air turned out to be incapable of supporting further burning, the missing air—about a fifth—must be different from the rest.

  This was momentous news. Lavoisier had discovered that common air was not a single indivisible element; instead, it was clearly a mixture of other things. And one of them, making up about one-fifth of its bulk, was the mysterious and powerful substance that allowed materials to burn, and combined with them in the process.

  But, frustratingly, Lavoisier still didn't know what this substance actually was. He could watch it disappear from common air but couldn't make it reappear. Once lead has burned and taken up its oxygen, it won't release it again no matter how much you heat it. Lavoisier managed to make the lead ash and other calxes yield fixed air, by burning them with charcoal, but he couldn't retrieve the exact gas that they had taken up from common air in the first place. He needed to get at the air trapped inside his lead in order to release it and study it and discover what it was, but it remained stubbornly locked away.