An Ocean of Air

by Gabrielle Walker

  Oxygen is the most active ingredient in the air, but during their experiments Priestley and Lavoisier had also inadvertently isolated the other major ingredient of our atmosphere—nitrogen. This was the gas left behind when all the oxy-gene had been used up, the diluter that makes up some four-fifths of the atmosphere. Nitrogen, it turns out, plays several important roles in the sustenance of life on Earth. It is one of the building blocks of the proteins in our bodies, which is why it is essential to eat certain vegetables that can "fix" nitrogen directly from the air. But the other function is just as important.

  Because Priestley was right that if our atmosphere contained nothing but oxygen, we would all be "living out too fast." In fact, much of the planet would burst spontaneously into flame. Dull diluting nitrogen rescues us from having too much of a good thing, and for that we should all be grateful. As Priestley put it: "A moralist ... may say, that the air which nature has provided for us is as good as we deserve."

  These two elements—oxygen and nitrogen—make up the vast majority of the air that we breathe. But another substance in air is just as important for our survival. We need oxygen to burn our fuel, but the fuel itself comes from somewhere else. The source is another gas, present in the atmosphere in such tiny quantities that for many years it was presumed to be insignificant. And yet it is responsible for every scrap of food on Earth.

  CHAPTER 3

  FOOD AND WARMTH

  THE GAS THAT FEEDS US ALL was discovered in the early eighteenth century, several decades before Priestley and Lavoisier would find its counterpart, oxygen. Its true identity was revealed by a gentle Scottish genius who created the gas twice, the first time idly, and almost by accident.

  JANUARY 1754

  EDINBURGH

  I fully intended to have wrote last post, but really I happened to be intent upon something else at the proper time, and forgot it. It was indeed an experiment that I was trying that amused me, in which I had mixed together some chalk and vitriolic acid at the bottom of a large cylindrical glass; the strong effervescence produced an air or vapour, which, flowing out at the top of the glass, extinguished a candle that stood close to it; and a piece of burning paper, immersed in it, was put out as effectually as if it had been dipped in water: yet the smell of it was not disagreeable...

  When Joseph Black wrote this letter to his former tutor, he had no idea how important his strange new "air or vapour" would turn out to be. He was tinkering, amusing himself in between the more important matters of preparing his thesis and studying how better to cure his patients of their various ailments. For Black was training to be a medical doctor, and he took his profession seriously.

  Everybody liked Black, and it was said of him that he never lost a friend. Sometimes he was too nice. He once placed all his funds in a finan cial house that then got into difficulties. Black realized there was a problem more than a year before the institution went broke but refrained from withdrawing his money for fear of causing embarrassment, and he ended up losing three-quarters of his savings.

  He was quietly confident, approachable, kind, and almost impossibly curious. He loved experimenting, not just to find new medicines, but also to see how the world really worked. Perhaps uniquely among academics then and now, Black truly had no desire for glory. Though he performed a huge number of experiments throughout his life, he hardly published any of them. He didn't want to be first, nor did he want to be famous; he simply wanted to know.

  Black also enjoyed teaching. Later in life, when he became Professor of Anatomy at Glasgow University, most of his efforts went into preparing lectures that were extraordinarily popular with the students. He wasn't flashy, but gently enthusiastic and soft spoken, and his audiences remained so respectfully silent that his low voice could be heard from the back row. Black was above all steady; he could lift a beaker of vitriolic acid high into the air and pour it safely into a thin glass tube. Indeed when he performed any of his demonstration experiments with acids and powders and colors and flames, his hand never shook.

  Black never married, though he was a favorite of the ladies of Edinburgh. He dealt out his time and attention among them in carefully considered amounts, favoring the ones with suitably active minds. His closest friends were also lifelong bachelors, and, like Boyle, Black profited greatly from his interactions with them. They were dauntingly illustrious. Such was the concentration of talented minds in Scotland at the time that one famous historian in London remarked, "I have always looked up with the most sincere respect towards the northern part of our island, whither taste and philosophy seem to have retired from the smoke and hurry of this immense capital."

  In Edinburgh with Black were philosopher David Hume; Adam Smith, the father of modern economics; and James Hutton, who created the science of geology. While natural scientists in London were still stuck on the old favored topic of the stars, those in the new industrial centers had begun on another tack. Like Galileo before them, they wanted to switch focus. Never mind the heavens, they were saying. What do we have here?

  Black's famous friends were just as agreeable as he was. The four of them set up a weekly discussion meeting, called the "Oyster Club," which was open to any inhabitants of Edinburgh, or indeed visitors, with interests in art or science. The talk was informal, and none of the founders was intimidating or aloof. As one commentator noted, the four friends were easily amused, just as ready to listen as to speak, and "the sincerity of their friendship had never been darkened by the least shade of envy."

  Black's life, however, was considerably darkened by the ill health that constantly dogged him. He was often frustrated by the slow pace at which he was forced to pursue his studies. Several days of intellectual effort at a stretch was enough to start him coughing up blood, and he sometimes excused his failure to reply to his father's letters by saying that he was "too miserable." In his later years he became steadily more frail, stretching out his life through careful exercise and an increasingly dismal diet. When he finally died, it was while eating his usual slice or two of bread and a few prunes, washed down with milk diluted with water. A servant found him already dead, still balancing the cup of milk on his knee, "as if," one friend later wrote, "an experiment had been required to show to his friends the facility with which he departed." He hadn't spilled a drop.

  When Black began his work on airs, it was almost by accident. Ever the medical man, he was trying to find a cure for an excruciatingly painful illness that plagued the people of the seventeenth century just as it continues to plague us today: bladder stones. While we now have fairly humane cures, in the seventeenth century, without the benefits of sterile instruments or anesthetics, surgery was a potentially fatal endeavor. More indirect treatments involved injecting the bladder with caustic substances that would certainly help to dissolve the stone, but would dissolve a lot more besides and often ended up being more painful and debilitating than the disease they were trying to cure.

  In an effort to avoid these alternatives, sufferers turned to increasingly bizarre concoctions. The British prime minister, Sir Robert Walpole, who made his discomfort from the stone very widely known, ensured that a cer tain Mrs. Joanna Stephens receive five thousand pounds for revealing the secret of a recipe that he believed had helped him. In the London Gazette on June 19, 1739, Mrs. Stephens reported that her brew consisted of:

  ...a Powder, a Decoction, and Pills. The powder consists of Egg-shells and Snails, both calcined. The decoction is made by boiling some Herbs (together with a Ball, which consists of Soap, Swines-Cresses, burnt to a Blackness, and Honey) in water. The Pills consist of Snails calcined, Wild Carrot seeds, Burdock seeds, Ashen Keys, Hips and Hawes, all burnt to a Blackness, Soap and Honey.

  Black had little patience with such mystic brews, and wanted to find a cure that was more scientifically based. He decided to start with a powder called magnesia alba, made from Epsom salts, which he already knew to be both mildly caustic and useful in medicine. He had prescribed it, for instance, to "an active woman of
rather full habit of body, and it purged her ten times," and concluded that "this salt, though mild to the taste, seems yet to surpass other purgatives."

  The idea was to try to induce magnesia alba to produce a new product that was caustic enough to dissolve the stone, but mild enough to cause less discomfort than the usual treatments. He decided to try heating it and then mixing the result with water, which was the standard way of making caustic medicines. He placed an ounce of magnesia alba in a crucible and blasted it with enough heat to melt copper. Much to his surprise, he discovered that this drove off every shred of causticity. The resulting white powder was milder than ever, had no effect on the water he tried to mix it with, and even refused to fizz when added to an acid. It would never be a cure for the stone.

  Black, ever careful, weighed his sample after the experiment and discovered that he had finished with "three drachms one scruple," which was just five-twelfths of the original weight. He was mystified. There may have been a little water in his sample, but not nearly enough to account for such a drastic loss in weight. Where had the rest of the magnesia alba gone?

  Putting aside his disappointment at this failure to produce a cure for the stone, Black decided to try to find out. Since the magnesia alba had clearly not lost enough water to account for its change in weight, the only other alternative was air. And this reminded Black of the work of a clergyman who, nearly thirty years earlier, had published a book about his strange experiments with vegetables.

  ***

  Stephen Hales had a straightforward, if simple-minded, approach to his profession. His style at the pulpit was all fire, brimstone, and damnation. To be sure, he often preached the Christian duties of charity and generosity toward the poor, but he was also constantly watching for any evidence of "disorderly" or "loose" behavior among his parishioners. He campaigned against swearing, and though he was partial to wine himself and had no problems with hard cider and ale for the lower orders, he warned sternly against the drinking of spirits such as gin and brandy. Though much of his detestation of spirits came from a conviction of their harmful effects on the body, he also disliked the inclination of those drinking them toward loose morals and warned rather poetically of the "bewitching of Naughtiness in these fiery liquors." His ideas for penance were also old-fashioned. Some unfortunate parishioners found guilty of fornication were made to stand barefoot outside the church, wearing a white sheet and holding a white rod, until just before the litany, at which point they were brought inside to hear the sermon and be prayed for.

  Though Hales's Sundays were spent haranguing his flock, much of the rest of the week was devoted to his other passion: science. While waiting for the job at his parish to become available, he had spent nearly thirteen years at Cambridge University, where the great Sir Isaac Newton was still in residence, and his interest had been aroused. Now, at his parish in Teddington near London, he spent large amounts of time poking things, prodding them, and cutting them up like a curious schoolboy. He saw no conflict between his two favorite activities, religion and science. Instead, like Boyle, he decided that the more he discovered about the workings of the world, the more thoroughly he believed. "What a multiplicity, variety, beauty, usefulness, and subservience to each other, may we with pleasure observe, in contemplating the Works of Creation," he declared.

  In fact, the only conflict came from his experiments on animals. For instance, in comparing the circulation of blood in the human body with the circulation of sap in trees, he performed some gruesome experiments on luckless dogs, horses, and deer until he decided that it ill-behooved a man of God to continue. He wrote to a fellow clergyman that since further experimentation along these lines would require the death of several hundred animals, "I do not think it proper for one of our profession to engage any further in it."

  Instead of cutting up God's fellow creatures, Hales decided to try heating every kind of natural, but inanimate, stuff he could find. He tried hog's blood, deer's horn, peas, tobacco, oil of cloves, beeswax, and even the stones from a human gallbladder. And this is the point where his random experiments became suddenly very important. Because Hales found that when you heated these substances, every single one gave off air.

  For scientists at the time, this was astonishing, like conjuring up a genie from Aladdin's lamp. Obviously liquids, water for instance, could turn into vapor when they boiled. But how could something as insubstantial as air be trapped inside a solid? What's more, there was so much of it.

  In his book Vegetable Staticks, published in 1727, Hales reported that:

  There arose from a piece of heart of oak, 216 times its bulk of air. Now 216 cubick inches of air, compressed into the space of one cubick inch, would, if it continued there in an elastick state, press against ... the six sides of the cube with a force equal to 19860 pounds, a force sufficient to rend the Oak with a vast explosion.

  Since Hales was no fool, and he had noticed that oak trees do not generally explode without warning, he decided that the air he had released must somehow have previously been fixed in place. Hales imagined that his "fixed air" was made of particles that repelled each other mightily. In some circumstances, he believed these particles could become bound inside his solid objects and in others, liberated again.

  However, all Hales cared about was how air became fixed, and how it subsequently recovered its bounce. He had no idea what was truly happening when gas flooded so unexpectedly out of a solid, nor did he realize that individual "airs" might have different properties.

  ***

  Steady, thoughtful Joseph Black was much better placed than the rambunctious Hales to figure this out. Inspired by Hales's work, he wondered whether his magnesia alba had been transformed by losing some quantity of fixed air. That, at least, would explain why it lost so much weight. Moreover, rather than assuming that every air was the same, just with more or less bounce, Black suspected that Hales's fixed air might have properties of its own, ones that could be quite different from those of ordinary, common air. Perhaps it even had enough individual characteristics to explain why the magnesia alba had lost its causticity and turned so mild after it vanished.

  Black didn't manage to catch any gas in the act of escaping from magnesia alba, but had more success with one of its caustic cousins: marble. He heated a cubic inch of the stuff, and—sure enough—produced a huge amount of fixed air, enough to fill a vessel holding six gallons.

  Now that he had some samples of fixed air to work with, Black determined to find out whether it was truly different in character from common air. The experiment he designed to test this was rather complicated, but also ingenious. Black knew that limewater (which is just lime, or calcium, dissolved in water) had an affinity for fixed air. He decided that the lime in limewater must be soaking up the air, the exact opposite reaction to the way that magnesia alba and marble had released it. He also knew that water always has a certain amount of common air dissolved in it. That is why fish can breathe underwater, and why tiny bubbles form long before a pot of water comes close to boiling.

  So he wondered what happened to the common air dissolved in limewater. If common air were just the same stuff as fixed air, any common air in limewater would be sucked up by the lime, leaving none behind in the water. Black realized that all he had to do was check how much common air was dissolved in equal quantities of ordinary water and limewater. If equal amounts came from both, the air the lime sucked up must be fundamentally different. And that would mean that his new air really was special.

  To put his idea into practice, Black needed an air pump. But the only one available in Edinburgh was frustratingly out of action, and its slow, surly technician was impervious to Black's pleasant requests that he speed up his attempts to fix it. Exasperated, Black wrote to his former tutor in Glasgow, begging him to use the air pump there, and explaining with great precision exactly how the limewater should be made and treated. His tutor quickly arranged for the experiment to go ahead. Word came back. Four ounces each of limewater and ordinary wa
ter had been placed under the receiver of the new Glasgow air pump. As the pump sucked, air bubbled up out of each of the two vials. Each released almost exactly the same amount.

  Black was delighted. "From this it is evident," he wrote in his thesis, "that the air which quicklime attracts, is of a different kind from that which is mixed with water.... Quicklime does not attract air when in its most ordinary form, but is capable of being joined to one particular species only, which is dispersed throughout the atmosphere." In honor of Hales, Black decided to call this extraordinary new species fixed air. We know it now as carbon dioxide.

  In the history of science, this apparently innocent moment was in fact extraordinarily profound. For this was the first time that anyone had shown there was more than one kind of gas. Because of this discovery, Black would be known as the father of modern chemistry. Lavoisier, Priestley, and their contemporaries all regarded themselves as Black's disciples. Lavoisier, usually reticent about offering credit to others, even wrote to Black saying how much he admired his work.

  But more important for our story is the nature of the gas he had found. Ever curious, Black decided to abandon his work on bladder stones for a while and find out how his new fixed air behaved. He remembered the old experiment that he had described back in January to his tutor. Sure enough, adding acid to chalk produced the same fixed air that had flooded out of marble. Black also found that he could make it by simply burning charcoal in ordinary air. And as before, though the fixed air smelled "not disagreeable," it snuffed out candles, and animals could not breathe it and live.

  Black also noticed that fixed air is a product of distillation and that it appears in our breath when we exhale. He was, however, baffled by what it could be doing in our bodies in the first place. "It is not to be doubted," he wrote, "indeed that this air, extensively united with every part of our body, serves many great uses, nor is it to be supposed that its absence could be borne without inconveniences: but we do not seem to know what its use is, or what are the inconveniences that would result from its absence."