An Ocean of Air

by Gabrielle Walker

  Back in Galileo's time, notions about air were similarly hazy. Most people accepted the idea put forward by Aristotle in the fourth century B.C. that everything in the world was made up of four elements: earth, air, fire, and water. Earth and water were obviously pulled downward by gravity. Fire was obviously weightless. But air was the problem child. Was it heavy enough to be dragged to the ground, light enough to rise like flames do, or did it simply ignore Earth's gravitational tug and hover?

  Galileo believed that air is heavy and had set about testing his idea. The experiments he performed were typically ingenious. First, he took a large glass bottle with a narrow neck and a tight leather stopper. Into this stopper he inserted a syringe attached to a bellows and by working vigorously managed to squeeze two or three times more air into the bottle than it had previously contained. Next, he weighed the glass bottle most precisely, adding and subtracting the finest of sand to his scales until he was satisfied with the answer. Then, he opened a valve in the lid. Immediately, the compressed air rushed out of its confinement, and the bottle was suddenly a handful of grains lighter. The air that had escaped must account for the missing weight.

  This showed that air is not the insubstantial body we usually take it for. But now Galileo wanted to know how much air corresponded to how many grains of sand. For that he would somehow need to measure both the weight of the escaping air and its volume.

  This time, he took the same glass bottle with its long, narrow neck. However, instead of pumping it full of extra air, he forced in some water. When the bottle was three-quarters full of water, its original air was squeezed uncomfortably into a quarter of its original space. Galileo weighed the bottle accurately, opened the valve, allowed this pressurized air to escape, and then weighed the bottle again to find out how much air he had lost. As for the volume, Galileo reasoned that the portion of air that had been forced to leave the bottle had been pushed aside by the water he had squeezed in, so the volume of air that had fled must be exactly the same as the volume of water that remained. All he had to do was pour out the water and measure its volume and voilà, he had found the weight for a given volume of air.

  The value Galileo came up with was surprisingly large: Air seemed to weigh as much as one four-hundredth the weight of an equivalent amount of water. If that doesn't sound like much, consider this. Picture a particular volume of air for a moment—such as the "empty" space inside Carnegie Hall in New York. How heavy would you expect that amount of air to be? Would it weigh ten pounds? Or a hundred? Or maybe even five hundred?

  The answer is somewhere in the region of seventy thousand pounds.

  The weight of air is so extreme that even Galileo didn't see the whole story. He never considered the question of how we can shoulder such a crushing, overwhelming burden, for the simple reason that he didn't realize the air above us is still heavy. He had measured the weight of air in his bottle, but he was convinced that the moment this air was released back into its natural element, the sky, it immediately ceased to weigh anything at all.

  Galileo believed that our atmosphere as a whole is incapable of pushing. It was one of the few occasions when the great man was wrong.

  In spite of the Church's opposition Galileo finished his manuscript—and published it. After fruitless efforts to convince publishers in Florence, Rome, and Venice to defy the Inquisitors, Galileo finally smuggled the manuscript out to a printer in the Netherlands. Four years later, as he approached the end of his life, a few copies began filtering back to Italy. Each bore a disingenuous disclaimer by Galileo himself, who wrote how astonished he was that his words had somehow found their way to a printer's in spite of his obedience to the Papal diktat.

  And although Galileo was wrong about the way our air behaves aloft, the experiments his great work contained would influence two very different people to discover the truth.

  ***

  By coincidence, both of these people arrived in Florence at more or less the same time, in October 1641, just a few months before Galileo's death. One was a thirty-three-year-old Roman mathematician named Evangelista Torricelli, who had been working with Galileo in the final three months before his death.

  Torricelli had become fascinated by Galileo's experiments on air and his conviction that though air was heavy when stuffed into a bottle, it weighed nothing in its natural state. His attention was drawn most particularly to an old wrangle between Galileo and a Genoese philosopher named Giovanni Battista Baliani. The argument hinged around the use of siphons to transport water from one site to another, usually over a vertical barrier such as a hill. This works on the same principle as siphoning gasoline from a car. Fill a long tube with water, stick one end in a pond or stream, and carry the other end over your hill. Water will then conveniently spout out of the far end, and continue to do so until you've drained the original pond or you pull the tube back out again.

  Baliani had noticed that siphons seemed to have an upper height limit beyond which they didn't work. If the hill was higher than about eighteen Florentine ells (a little more than thirty feet), the siphon refused to cooperate, and no water came out.

  He believed that the force pushing water through the pipe was the weight of the Earth's atmosphere. Air, he said, was constantly squeezing down on the surface of the pond, and it was so heavy that it managed to push water up into the pipe. The siphon stopped operating, he reasoned, because even the weight of the entire atmosphere has its limits. At a height of more than thirty feet, the air pressing down on the surface of the pond was not heavy enough to overwhelm the gravity trying to pull water back down, and the siphon would lose its power.

  Galileo, however, had disagreed. Unable to believe that the atmosphere itself is heavy, he decided that the power in question wasn't pushing but sucking. On either side of the hill, he said, water was trying to fall back down out of the pipe. But as it fell, it left behind an empty space in the middle of the pipe. The complete absence of any material at all in this so-called vacuum would give it extraordinary properties, including the ability to suck. That was what drew water over the hill. If the hill was higher than thirty feet, the water inside the pipe became too heavy for the vacuum's suck.

  Torricelli thought that Galileo was wrong, and that the atmosphere really did push. He also decided to prove it.

  First he figured out how to mimic the action of the siphon, but at a rather more manageable scale. Instead of water, he used mercury—known at the time as quicksilver, not because it moved rapidly, but because it seemed almost alive. Unlike all the other cold, dead metals, liquid mercury curled itself into bright balls that darted around a tabletop and spilled onto the floor with splashes of brilliance. However, like the other metals it was also very heavy. The result from the siphons suggested that if Torricelli tried to balance the weight of the atmosphere using water, he would need a tube more than thirty feet long. But with the much heavier mercury, just three feet of tube should do the trick.

  So Torricelli took a three-foot glass tube that was closed at one end, filled it with mercury and stopped the open end with a finger. Then he tipped the tube upside down, put it into a basin of mercury, and carefully withdrew his finger. If the air had no pressing role to play, there would now be nothing to stop the mercury from succumbing to the force of gravity and spilling back down the open tube. But if Torricelli was right, the mercury should stop at exactly the point where the weight of air pressing outside balanced its own weight. By comparing the relative weights of mercury and water, he had calculated the level at which it should stop not at eighteen ells like the water in the siphons, but at a mere ell and a quarter and a finger more.

  And that's exactly what happened.

  But what force was keeping the mercury up? Was it the pressure of air, or was it, as Galileo had believed, the vacuum's powerful suck?

  To find out, Torricelli repeated his experiment with a slight twist. He put two tubes side by side. One was a straight glass tube about three feet long and the same diameter throughout. The other
was similar except that it had a large round glass globe on the closed end. Both were filled with mercury (the one with the glass globe needed somewhat more than the other) and then tipped upside down into the same basin.

  If Galileo's argument was right, the tube with the globe on the end would have more empty space to suck with, which would pull its mercury level higher. But if Torricelli was right, the mercury in both tubes should fall to exactly the same level.

  The bright silver mercury slipped down the sides of both tubes and ended up at exactly the same level, one ell and a quarter and a finger above the level of the bath. Torricelli was right. No matter how much vacuum was in the space above the mercury, the force holding it up was still the same. Vacuums don't suck; the air pushes.

  This is a truly extraordinary notion, an effect of our atmosphere that we encounter unwittingly all the time. When you sip through a straw, you may think the power of your suction pulls the drink into your mouth. But it doesn't. Your suck simply moves the air away from one side of the straw, and the drink then arrives in your mouth courtesy of the overwhelming weight of the air around you. The same thing happens when a baby drinks from its mother's breast. The baby's enthusiastic sucking just removes the air from around its mother's nipple; the force of the air above her then squeezes the mother's breast and sends milk spurting into the baby's mouth. It's the same, too, with a vacuum cleaner. The air outside pushes dust and debris up the hose because the air that had been shoving equally from the other side has now been removed. Try using a vacuum cleaner in space and you won't be picking up cosmic dust, since there isn't any air on the other side to do the pushing.

  Torricelli's experiment with the glass globe had proved the weight of the atmosphere to his own satisfaction, but it would take more than that to convince the rest of the world. Part of the problem was that this notion is so counterintuitive. The air just doesn't seem to be that heavy. We can walk through it without even noticing it's there. If it really were pushing down on us continually with such a great force, why wouldn't we be crushed? (The answer is that most parts of our bodies aren't compressible, and the few collapsible spaces contain air at exactly the same pressure as the air outside. As hard as our atmosphere pushes down on us, we push back.)

  It didn't help that news of the crucial experiments only trickled out gradually by whispers and rumors. Proud though he was of his findings, Torricelli didn't dare trumpet them to the rest of the world. The trouble was that he had been playing with vacuums. And the Church, in another of its unfortunate pronouncements on physics, had declared belief in the vacuum to be heretical.

  The Church had decided to abhor the vacuum mainly because of the teachings of various philosophers who had lived long before Christ. Aristotle, for instance, believed that a vacuum was logically impossible. For him, space was, by definition, the place where objects resided. If there were no objects there could be no space, and hence no vacuum. The materialists Democritus and later Lucretius, however, believed that all matter was composed of tiny indivisible particles called atoms, which were separated from one another by empty space.

  Not much progress was made in the following twenty-one centuries to resolve this issue, and by the sixteenth century the Catholic Church had decided to side with Aristotle. By reducing all of Creation to a collection of atoms, Democritus and Lucretius had left no room for spirit or soul, and also raised troubling questions about exactly how, scientifically speaking, the communion wafer and wine could transform into flesh and blood. Their philosophies were therefore anathema. Tarred by association, belief in the existence of a vacuum was also declared heretical. According to the religious authorities, God had decreed that a vacuum would be so unnatural that air would always rush in immediately to prevent one from being formed. To say otherwise was to risk the wrath of the Inquisition.

  Having seen the effects of Galileo's mild outspokenness, Torricelli opted for discretion. He never published his results, except in one famous letter that he wrote on June 11, 1644, to his close friend Michelangelo Ricci. Though Ricci was a Jesuit, he was also a firm advocate of Torricelli's work, and Torricelli described his experiments in careful detail, with sketches of the apparatus. Mostly, he remained matter of fact, but once in a while he let his delight in his findings shine through. "What a marvel it is!" he wrote when he contemplated the invisible air pressing his mercury up into the tube. He spoke with awe of how our blanket of air, perhaps fifty miles high, constantly presses down on the planet beneath. And he encapsulated it all in this one glorious image. "Noi viviamo sommersi nel fondo d'un pelago d'aria," he said. "We live submerged at the bottom of an ocean of air."

  ***

  With his experiments in quicksilver, Torricelli had proved the pressing power of air to his own satisfaction, but his secrecy about the results and the prevailing stubborn resistance to this extraordinary new notion meant that, for the moment at least, the old ideas continued to rule.

  Fortunately there remained the other person who had arrived in Florence just before Galileo's death and who, like Torricelli, was destined to pick up his mantle. His name was Robert Boyle, and when he reached Florence in October 1641, he was a sixteen-year-old schoolboy who had as yet no particular yen for science.

  Boyle was the son of one of Ireland's richest noblemen. He had ridden from Geneva to Florence that summer with his brother and tutor on a leg of their Grand Tour of Europe. But unlike the other privileged young gentlemen risking pox, plague, and bandits in the interest of gaining Continental polish, Boyle truly wanted to learn. He carried books everywhere; he read them walking along roads and stumbling down hillsides. He disputed philosophy and religion with fellow guests at the lodging houses and tried to make the deepest possible sense of everything he saw and heard.

  Soon after Boyle arrived in Florence, he came across a copy of Galileo's final book and was deeply struck. He was also struck with indignation by the fate of the man now dying in his villa just a few miles away. Boyle noted triumphantly in his journal how, when monks went to visit the "great star-gazer" and chided him that his blindness was a punishment sent by God, the quick-witted Galileo had replied that at least "he had the satisfaction of not being blind till he had seen in heaven what never mortal eyes beheld before."

  For Boyle, the Church was also suffering from blindness. He decided that religion was about revealing the wonders of God's nature, not hiding them behind dreary dogma. Boyle didn't want to be told what to believe about the workings of the world. He wanted to glorify God by discovering them for himself.

  Yet the seed Galileo's work had planted could easily have withered over the succeeding years. For shortly after Boyle left Florence, his home country, Ireland, erupted in rebellion, while England tumbled into its own civil war. It was more than two years before Boyle could make his way back home, and even then he got only as far as England, first to his sister's house in London and then to Stalbridge, a modest manor house that his father had bought for him in Devon.

  This would have been a good time for Robert Boyle to settle into the life of a country squire. England was by then a little less troubled. True, King Charles I had been arrested, then later arraigned and publicly beheaded, but the Protector, Oliver Cromwell, had taken control and, along with his New Model Army, had restored a large measure of political sta bility. Boyle was comfortably off. He could indulge in gentlemanly pursuits, ride, shoot, and fish.

  But there was still something missing in his life. He was full of ideas, but there were no obvious routes for gentlemen to express them. Boyle dabbled with religious writings. He wrote a series of "Occasional Reflections" addressed to his favorite sister, Katherine, Lady Ranelagh, drawing what were admittedly often mawkish morals from events such as "Upon the sighting of a fair milk-maid singing to her cow" and "Upon my spaniel's carefulness not to lose me in a strange place." This led to some mockery, which was not really fair. Boyle was pious, but never sanctimonious. He was pleasant, approachable, and almost pathologically fair-minded, and though his religious
sentiments were naïve, he was still in his early twenties.

  One of the most famous parodies of Boyle's Reflections was penned by satirist Jonathan Swift, several decades later. Swift at the time was private chaplain to a lady who was smitten by Boyle's writings and wanted them read to her constantly. Swift became so exasperated that he slipped in an extra, unauthorized and very funny piece titled "A pious meditation on a Broom Staff": "But a broomstick, perhaps you will say, is an emblem of a tree standing on its head; and pray what is man, but a topsy-turvy creature..." (In spite of his mockery, Swift may well have used Boyle's vivid imagination as inspiration for his most famous book: Gulliver's Travels.)

  Boyle even wrote a romantic, yet highly moralistic, novel, and for a while it seemed he might try expending his intellectual energies on a literary career. But his curiosity about the workings of the world tugged at him. He wanted to understand the world in a new way, the way that Galileo had shown him. He wanted, above all, to experiment.

  So in 1649, Boyle installed a laboratory at Stalbridge. He commissioned furnaces from the Continent, and he dabbled with alchemical efforts to find a way to turn lead into gold. But his attempts to experiment seemed aimless. He needed to be among people who shared his urge to understand the natural world through experiment and not through reason alone. During his visits to his sister Katherine's house in London, he had met many such men, who were already discussing the best new ways to probe nature. They met in each other's homes and called themselves the "Invisible College," though Boyle always referred to them as the "Invisibles." (This was the first glimmering of what would become London's famous "Royal Society" when the monarchy was eventually restored after the death of Cromwell.) From these men and their discussions with his thoughtful, intelligent sister, Boyle had learned much. But London had begun to seem politically too unstable for these men, and many of them had moved to take up positions behind the safe walls of Oxford's rather less invisible university. And so, in the mid-1650s, Boyle decided that he would join them. He left his stately manor house and moved into lodgings that his sister found for him in the house of an apothecary.