An Ocean of Air (2 page)

As the great Galileo rose from his knees at the end of this infamous, and forced, recantation, he is said to have muttered "
Eppur si muove!
" ("And yet it moves!"). He knew in his heart that Earth moves around the sun, in spite of what the Inquisitors had made him say. Still, devoutly religious as he was, he had no taste for defying his own church. Nor had he any desire to share the fate of the unfortunate monk Giordano Bruno, who a few decades earlier had been publicly burned for holding similar views. Galileo may have been the most famous philosopher in all Italy, but he knew that in itself wouldn't save him from the fire.

And though he was now seventy years old, frail, and steadily losing his sight, he was not yet ready to die. He had damaged his eyes by staring through a telescope at wonders he himself had discovered: blemishes that appeared periodically on the surface of the sun; craters on the moon; distant but distinct moons circling the planet Jupiter (who would have thought that other planets could have moons of their own?), and stars that nobody knew existed. Now, before the cataracts and glaucoma finally clouded his sight, in secret, if necessary, he had one last task to complete. Galileo had seen this "trial" coming; he'd known for some time that he couldn't continue his study of the heavens. So for some years he had been discreetly changing tack, turning his attention inwards to Earth itself. And, failing eyesight notwithstanding, he was about to change the way we see the most apparently ordinary substance in the world: air.

The Inquisitors knew nothing of this. They were satisfied with his recantation, and decided, graciously, to spare his life. He would be allowed to return to his villa at Arcetri in Florence, though he should understand that he was still considered dangerous and would therefore be held under house arrest. There would be no visitors, save those given prior permission by the Church. Meanwhile, Galileo himself was to spend his time reciting the holy psalms as penance, and praying for his immortal soul.

Galileo returned to his villa as instructed and performed his penance diligently. But the Inquisitors had also obliged him to swear never again to publish work that might offend the Holy Office, and he had no intention of complying. For with him to Arcetri he had taken a certain manuscript that was already nearly finished.

He had started the experiments it described while awaiting his summons to Rome. Having turned away from his telescope, Galileo had become fascinated instead by the different ways that objects move through the air. The result was to become his masterpiece. The manuscript already recounted findings that would become just as famous as the moons of Jupiter. For instance, Galileo had made the surprising discovery that Earth's gravity doesn't care in the least how much something weighs. Drop a cannonball and a pebble from a high tower, and both will reach the ground at exactly the same moment.

But within its pages was another discovery that would prove to be less famous yet no less significant. Galileo had measured the weight of air.

This might seem like a bizarre notion. How can something so insubstantial as the air weigh anything at all? In fact our planet's air is constantly pushing down on us with great force. We don't notice this because we're used to it, like lobsters sauntering along on the seafloor, unaware of the crushing weight of the ocean of water above them. We give our own overlying air-ocean so little respect that we even describe anything that's full of air as being "empty."

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.)