An Ocean of Air

by Gabrielle Walker

  Lavoisier knew he needed to find another material, one that would soak up the mysterious ingredient from the air when it was heated but would release it afterward. Lead wouldn't do this, nor would sulfur or tin or any of the other materials Lavoisier tried. For the moment at least, he was stumped.

  Then, in October 1774, Lavoisier learned that Priestley himself was in town. Priestley was in the middle of a tour of European countries with his patron, Lord Shelburne, and Paris was their latest stop. Priestley wasn't much impressed with the city. Though its buildings were undoubtedly beautiful, parts of the city remained positively medieval. Foul-smelling open sewers ran down the centers of what, some hundred years later, would become the city's elegant boulevards, and there were none of the sidewalks that already graced London's streets. With a provincial Englishman's disregard for foreigners, Priestley also decided that many of the people he met were "too much taken up with themselves to admit of that minute and benevolent attitude to others, which is essential to politeness."

  In spite of these criticisms of the Parisians and their habits, which might have had more to do with his indifferent French than any real impoliteness, Priestley was lionized throughout the city. Though news of his experiment with mercurius calcinatus had not yet filtered out, since he had performed it just a few months earlier, his previous work on the new airs was known throughout Europe and he was already famous. By now Lavoisier was considered France's foremost natural philosopher, and a meeting between the two was inevitable. Thus one evening that autumn, the Lavoisiers invited Priestley to dine at their house along with most of the city's resident intellectuals. And naturally enough, during the course of the evening, stammering in poor French that was occasionally supplemented by Madame Lavoisier's helpful translation, Priestley told Lavoisier about his experiment.

  He told how he had made the mercurius calcinatus by burning mercury in air until the silver liquid turned into a crumbling red powder, and then how he had trapped this powder in his tube of mercury and heated it with his precious burning glass until it spewed out a mysterious new air that caused candles to burn with a dazzling, incandescent light. It was almost as if the mercurius calcinatus had trapped within it the essence of fire.

  Lavoisier was riveted. Could this finally be the material he was looking for? When Priestley left, he dropped his useless lead and tin and started work on mercurius calcinatus.

  First, Lavoisier took four ounces of very pure mercury and put it in a closed glass vessel with fifty cubic inches of common air. Then he heated it almost to its boiling point and kept it that way for twelve days. At the beginning, nothing much happened. But gradually red specks began to appear on the mercury's silver surface, and they grew larger each day. By the end of the twelve days, the reaction seemed to be at an end. Lavoisier had lost nine cubic inches of air and gained forty-five grains of red mercurius calcinatus. The air left behind in the vessel would not permit a candle to burn, but unlike fixed air it did not turn lime-water cloudy. This was some other form of air that apparently existed only to dilute the vibrant, active part.

  With the utmost care, Lavoisier collected the forty-five red grains and put them in a small glass jar whose long, thin neck twisted around itself several times and then poked up into a bell jar full of water. Now all he had to do was heat the grains of mercurius calcinatus. As he did so, out and up bubbled the very air they had trapped within them. Exactly nine cubic inches made their way into the bell jar above. As a final proof, Lavoisier took this air and recombined it with the stuff that had been left behind from the first experiment, the stuff that would not support burning but would neither turn lime-water cloudy. Immediately this mixture became indistinguishable from common air. Candles burned normally in it; animals breathed happily for exactly as long as you would expect.

  Lavoisier had found the magic ingredient, the active part of the air. He had extracted it, trapped it inside mercury, released it, and recombined it with the passive part to regenerate common air. By applying his painstaking system of accounting to science, he had looked into the heart of a flame. He now knew what fed every fire on Earth.

  But what to call it? Lavoisier had no patience with Priestley's name for this new gas, "dephlogisticated air." His experiments had clearly proved that burning had nothing to do with phlogiston and everything to do with the presence or absence of this one crucial active ingredient. Instead, since it seemed to be trapped in many different kinds of acid, he named it "oxygene," which means "acid-born."

  Lavoisier was intrigued with his new gas and began to work on it in earnest. In particular he wanted to know more about the relationship between burning and breathing, and the role that oxy-gene might play in each. Like Priestley, Lavoisier had noticed the similarity between these two processes. Place a burning candle in a closed jar of common air and eventually the flame will sputter and die. Place a living mouse in such a jar and after a while the animal will no longer be able to breathe. To Priestley, both candle and mouse were giving out phlogiston. To Lavoisier, both were using up oxy-gene. And now, he wondered how far the similarity between the two processes went. How could the same substance that fed a flame also feed life itself?

  Until now, nobody had made any truly systematic investigations into the nature of breathing. Obviously, it was necessary for life. And just as obviously, food somehow sustained life. But there was no sense that food in a person was like fuel in a machine. Aristotle had believed that the purpose of breathing was to cool the blood, and this was still a popular notion even in Lavoisier's time. Other philosophers thought that breathing in a confined space became increasingly difficult because it reduced the elasticity of the air, which prevented it from pushing back enough to inflate the lungs properly. As to what relationship this had to eating, nobody really knew.

  So, Lavoisier began his experiments. Unusually for him, he performed them with a collaborator, a young mathematical genius named Pierre-Simon Laplace. Among his other achievements, Laplace would later produce the complex equations that govern the behavior of the solar system, and it is sometimes said that his efforts in this regard were halted because his equations were so successful at accounting for the available facts that until more observations could be made, there was nothing left to explain. Laplace was already famous, the most talented mathematician in the known world, and together he and Lavoisier devised a series of experiments to understand the nature of breathing.

  For their experiments they used small hairy rodents lately returned from the jungles of South America. These "guinea pigs" were very convenient in the laboratory, wrote Lavoisier, because they were "tame, healthy creatures, easy to feed and big enough to inspire and expire air in quantities suitable for measurement." Lavoisier had designed a clever piece of apparatus to discover the relationship between the amount of oxy-gene these guinea pigs consumed and the heat they gave off. The heat was the hard part. Lavoisier had decided to measure this by the melting of ice. He made a large sealed circular chamber comprising three concentric rings. The innermost ring contained the guinea pig, the second ring was packed with a known quantity of ice, and the outermost ring was filled with snow to prevent the heat of the room from reaching the ice and melting it. Lavoisier and Laplace set out to monitor what happened first when the guinea pigs were at rest, and then when they became steadily more active.

  The result, enlivened by some complex equations from Laplace, were exactly what Lavoisier had hoped for. The more work the guinea pigs did, the more oxy-gene they used, and the more heat they gave off. Lavoisier was convinced. "Respiration is a process of combustion," he wrote, "which, though it takes place very slowly, is perfectly analogous to the combustion of coal." The same way that coal supplies the fuel for a fire, some derivative of food must also provide the raw material for the energy by which we all live. And as oxy-gene feeds the glowing flame, so, too, must it release the energy stored somewhere inside us.

  Lavoisier had discovered something truly important. Flames do indeed use oxygen to generate ener
gy from candles or wood, and he was right that when we breathe, we're using it to burn our food in much the same way. That's one reason we talk about "burning calories." If this sounds dangerous, it is. As Priestley had suspected, and Lavoisier had now begun to prove, breathing oxygen is what allows us to live such vivid, active lives. But we pay a heavy price, for oxygen is also the reason why we grow old and die.

  ***

  All living things need to breathe. That is, they have to generate energy when they need it from the food reserves stored in their bodies. In our case, we have reserves of sugar, protein, and fat, which sit around inside us like a pile of logs waiting to be burned. Every breath we take uses oxygen to convert some of those reserves into the energy we need to move, stay warm, and do everything else that makes us human.

  But oxygen isn't the only chemical that living things can use for breathing. Indeed the bacteria that constituted the first life on Earth were forced to use something much less efficient for the simple reason that, when the world first formed more than 4.5 billion years ago, the atmosphere contained no oxygen at all. Oxygen didn't appear in the atmosphere for more than two billion years, and it finally showed up only because of a dramatic but inadvertent case of planetary pollution. Without that accidental airspill, there would be no life on Earth bigger than a pinhead.

  When the planet was born, it came blanketed in an ocean of air. Like the sun and the other planets in our solar system, Earth was formed when a shapeless cloud of gases, dust, and fragments of rock began to collapse and coalesce. The rocks and dust trapped some of the gases between them like mortar between bricks, and much of the rest settled on the outside of the planets in a shroud held in place by the power of gravity.

  This early ocean of air was just as dense as today's, and it would have looked very similar. But the lack of oxygen made a big difference to Earth's surface. The rocks, for instance, were a uniform dull gray color—without oxygen, the iron they contained couldn't rust to the lovely reds and ochres that we see today. Still, the young Earth wasn't without its beauties. The skies periodically shed a gentle yellow rain of elemental sulfur, and the earliest beaches sparkled with golden iron pyrite. Also known as "fool's gold," this exists today only deep underground, safely away from the oxidizing air, where its vibrant color can still confuse naïve miners on the hunt for nuggets of the real thing.

  For animals like us, this early atmosphere would have been an impossibly suffocating place, but the world's first occupants had an alternative way of releasing their energy. Instead of oxygen, they "breathed" the gas that Priestley and his contemporaries called "inflammable air" and that we call hydrogen. In the process, they made methane—"natural gas." Since this wasn't nearly as efficient as using oxygen, the creatures breathing it couldn't grow large. Instead, they remained the way they had begun, as microscopic pinpricks in the fabric of life.

  So it was, and so it would always have been if not for a new chemical reaction invented sometime between 2.5 and 3.5 billion years ago by microbes called cyanobacteria. These creatures are so tiny that a droplet of water can contain billions of them—as many as there are people on Earth. However, they are also ubiquitous. Today you can find them in drainpipes, puddles, or anywhere water is left to stand for a while and starts to go that distinctive green color that shows they are working their magic. For they are the microbes that learned how to use the sun's energy to split water and make food, in a process we now call photosynthesis. And in doing so, they give off delicate bubbles of a certain waste product: oxygen.

  This is the reason that we can breathe today. Cyanobacteria and the green plants that later incorporated their invention are now part of a giant enterprise that acts as Earth's lungs. As fast as we animals use up oxygen by breathing, plants return it to the atmosphere. It's almost as if living plants are working to make the world habitable for us—as if the most important component of our atmosphere has been made by life, for life.

  (Oxygen didn't actually appear in the atmosphere until several hundred million years after it was created in this way as a by-product of photosynthesis. At first, it reacted with Earth's rocks and oceans as quickly as it was made. In the case of the oceans, the dissolved iron they contained turned to rust and fell to the seafloor, making vast mountains of debris that have turned into the world's biggest iron mines. Whenever you eat with a stainless steel fork, or drive a car, you're probably benefiting from this early rain of rust.)

  Oxygen is fantastically reactive. When it engages in chemistry, it can release large amounts of energy, which can in turn be used to fuel the activity of living things. So, oxygen's arrival in the air had a dramatic effect on the course of evolution. As long as there was too little oxygen in the atmosphere to be useful to the creatures below, they were forced to remain both sluggish and microscopic. For billions of years, the planet was coated with nothing more than primordial slime.

  But gradually, inexorably, more and more oxygen trickled into the sky until one point, nearly 600 million years ago, when the atmosphere tripped over its oxygen threshold. The result was the most dramatic burst of evolutionary change in Earth's history. Huge new creatures suddenly appeared, some of them more than a meter long. They weren't only big; they were inventive, and almost unbelievably varied after the dull slime that came before. These new creatures had shapes. They had eyes and teeth and legs and shells. They had learned to make their bodies out of not just one cell, but many. They were the world's first animals.

  It's hard to overemphasize the importance of this evolutionary step. Think of the transition between cottage industries and the industrial revolution. Before this point, a single cell had to do everything that life needs—eat, excrete, breathe, reproduce, all in one tiny sac. Afterward, cells could specialize and share the load. Some became arms, some hair, brains, or bones. Creatures were no longer restricted to the size of pinheads. What's more, they had muscles to drive their new bodies, and that meant that at last they could move. Imagine a life without moving, and the difference it makes when suddenly you can. The new earthlings could seek out new sources of food, including other creatures. Some could chase and others could flee. They developed armor to protect themselves and weapons to attack. They learned new skills, took on new shapes and colors, and ultimately became the vivid variable life forms we see around us on Earth today, including humans.

  Nobody knows exactly the mechanism by which that final rise in oxygen triggered the appearance of animals, but what's certain is that there could be no complex life without it. To be big and multicelled requires huge amounts of energy, and it takes oxygen to generate that sort of power. Every other way of breathing is simply too feeble. We need oxygen because we need its spectacular reactivity. Without it, humans could never have existed.

  This reactivity comes with its own dangers. As Priestley suspected when he saw how brightly candles flared up in his new gas, breathing oxygen is like playing with fire. And we are gradually, all of us, getting burned.

  That's because whenever oxygen gets involved in chemical reactions, it releases tiny negatively charged particles called electrons. All atoms and molecules contain these particles and, like people, they are most stable when they are in pairs. A chemical entity that contains one of these single, footloose electrons is called a free radical, and it is one of the most reactive, and destructive, forces on the planet. Free radicals rip through everything in their path, splitting apart stable pairs and creating yet more free radicals, which head off on destructive paths of their own. That's what happens, for instance, if you are exposed to radioactivity. The damage comes not from radiation itself, but from the free radicals it generates.

  And the trouble is that when we use oxygen to breathe, there are always some electrons that break free. Even if you're doing nothing but breathing, about 2 percent of the oxygen you consume still escapes as free radicals. If you're exercising vigorously, it's more like 10 percent. According to one calculation, the potential damage from simply breathing for one year is equivalent to the
radiation from ten thousand chest x-rays.

  When oxygen first appeared on Earth some 2.2 billion years ago, it was certainly a deadly poison for many of the earliest microbes. The methane-producers simply couldn't cope with the free radicals that were suddenly ripping through their bodies, tearing apart their vital chemicals. To survive, these organisms had to find refuge. They persist today in places that are comfortably moist and yet are hidden from the probing fingers of the atmosphere. That's why paddy fields give off methane, why swamps yield marsh gas that sometimes ignites into the ghostly dancing flames of legend, and why animals, including humans, generate natural gas in their guts. We fart because our intestines are now airless sanctuaries for those poisoned earthlings.

  Other organisms, the ones from which we're descended, developed various complex strategies to deal with the worst depredations of oxygen. In particular, our own bodies are permanently ready to deploy an army of chemicals called antioxidants. A full-scale war is taking place every second, in every cell of our bodies, to stop the free radicals from forming, mop up the ones that do, or commit cell suicide if the invading forces become overwhelming. But the powerhouses inside our cells spend their lives playing with fire, and the long, slow leakage gradually wears us down. All of the diseases of old age—dementia, cancer, heart disease—come from the accumulation of damage caused by escaped free radicals. That's one reason that eating fruits and vegetables helps protect us from these diseases—both are packed with antioxidants that help to mop up free radicals.

  It's also why smoking brings these diseases on earlier in life than we would otherwise expect. Nicotine isn't the problem, except insofar as it's addictive and so encourages you to smoke more. The real damage comes from the smoke itself, which is crammed with chemicals that react with oxygen to generate bucket-loads of free radicals—something like a million billion in every puff.

  So can we somehow stave off old age by consuming more antioxidants? It seems not. In spite of the evident benefits from fruits and vegetables, there's no clear sign that eating "antioxidant supplements" bought from the health-food section of your local grocery has the same beneficial effect. In fact too many packaged antioxidants might hurt, rather than help. It takes us so long to grow old because our bodies have evolved such careful strategies to protect us from the worst effects of free radicals. Eating extra antioxidants might interfere with these natural mechanisms, like unruly mercenaries disrupting the operation of a highly trained army.