An Ocean of Air (4 page)

Boyle's idea was to take Torricelli's experiment and put the whole thing inside a vacuum. This had already been tried a couple of times, but without an air pump it had been very messy, involving attempts to fit one glass tube filled with quicksilver inside the vacuum created at the top of another. Hooke's adaptation of Otto von Guericke's invention was going to make the experiment much easier.

At last, the pump was ready. Hooke's design consisted of a large glass globe, with a wide opening at the neck, which could hold about fifty pints. This would be the "receiver" of the pump, where the experiments were to take place. Attached below this was a hollow brass cylinder just over a foot long, into which a plunger covered with tanned leather was tightly rammed. Through a clever system of valves, both globe and cylinder could be opened to the outside air or sealed from it. Simply pulling the plunger downwards drew air out of the globe. Adjust the appropriate valves, repeat this process several times, and you have yourself a vacuum.

The first step was to recreate Torricelli's experiment. Boyle and Hooke took a slender cylinder of glass about three feet long, closed at one end, and filled it with quicksilver. Then, as usual, they tipped it upside down into a box half filled with mercury. Just as expected, the mercury inside the tube began to fall until it reached a height of 29½ inches.

The next part was more delicate. Box, tube, and all were attached to strings and let gently down to dangle in the middle of the glass globe. (The top of the glass tube still poked up through the neck of the receiver,
but Boyle slipped a tight cover over it to prevent any leaks.) As far as the mercury in the glass tube was concerned, nothing had changed. It still rested 29½ inches above the mercury in the box below.

Now to begin sucking air out of the glass receiver. If Galileo was right, this should change nothing. According to him, the only force holding the quicksilver up was the sucking power of the vacuum in the closed space at the top of the glass tube; the presence or absence of air in the globe outside should be irrelevant. But if Torricelli and Boyle were right, removing air from the globe would take away the force holding the mercury up and it would fall.

The assistant manning the pump grasped the handle and began firmly ratcheting it downward. One cylinder's worth of air was drawn out of the great glass globe. And ... the quicksilver unmistakably fell. Turn the valve, replace the plunger, try again. Another cylinder's worth of air disappeared from the globe, and the quicksilver dropped still farther down the glass tube that protruded from the top of the pump. Soon, it had disappeared below the neck of the globe and Boyle could no longer mark its level on the paper he had attached for the purpose. Hampered by his poor eyesight, he had to peer through the walls of the glass globe to make out the shiny surface of the quicksilver as, with each crank on the pump's handle, it successively lurched its way down the inside of its tube toward the box waiting below.

This surely was the proof Boyle had been looking for, but to be extra careful he decided to try reversing the procedure. He turned the valve and allowed air to begin flooding back into the globe. Immediately the quicksilver raced back up the tube. The more air Boyle allowed inside the globe to squeeze down on the quicksilver, the higher up its tube it climbed. The more he removed air from the globe to take the pressure off, the farther the quicksilver fell. The pressure of air had to be keeping the quicksilver aloft. What could be clearer?

And yet the argument wasn't quite over. Boyle was now being baited by one of his
bêtes noires,
a Jesuit named Linus who was doggedly convinced that a vacuum could not exist. Instead, he declared, the answer lay in a bizarre invention of his that he called a "funiculus." This was some
kind of strange, invisible thread that hung in the apparently empty space above the mercury, holding it up like a puppet on a string.

The mild-mannered Boyle was polite as ever in his response to this absurd idea. But even he couldn't resist saying that it was "partly precarious, partly unintelligible, and partly insufficient, and besides..."—and this was the final blow—"needless."

For the final irrevocable proof that air really does push, and in all directions, too, was already there in the results of another of Boyle's experiments with his air pump—number 31. To do this experiment, Boyle had dispensed with the glass globe altogether. All he needed was the air pump itself.

The idea was dazzlingly simple. First, open the valve at the top of the cylinder and push the plunger all the way to the top so it fills every scrap of the cylinder's bore. Then close the valve at the top, so that no more air can rush in. Finally, attach weights to the bottom of the plunger to try to pull it back down. Ten pounds, twenty pounds, fifty ... sixty ... seventy pounds. Still the plunger wouldn't budge. Finally, with one hundred pounds dragging it downward, the plunger began to fall.

By that time Boyle had made his point. There was nothing at all inside the cylinder above the plunger, no vacuum or "funiculus" to hold it up from the inside. The force that kept the plunger in place when such a huge weight was pulling it downward had to have come from the outside. It could only be the apparently insubstantial and inconsequential stuff that surrounds us and squeezes down on us every day of our lives: our all-embracing ocean of air.

***

Boyle published his results in 1660. By then, the Oxford group of intellectuals had largely scattered. Many of them had backed Cromwell and were now fearful of the consequences of a Royalist revival. Boyle himself had remained steadily neutral, but even he left Oxford for a while to wait out the new political uncertainties at the country house of a friend. While there he prepared his book, to be called
New Experiments Physico-Mechanical Touching the Spring of Air.

Though Boyle was fluent in Latin, as in many other languages, he was unusual for philosophers of the time in that he chose to write in everyday, accessible English. Still more unusually, he eschewed the "normal" way of writing up science—philosophical discourses among fictitious persons—in favor of a straightforward description of his apparatus, what he did for each experiment, and the results he obtained. He wanted people to understand exactly what he had done, and even to be able to repeat it. In this sense, he was one of the world's first true scientists.

The book was an immediate hit, not least because it contained much more than the proof of the pressing power of air. Boyle would never have been satisfied with merely confirming what Torricelli had already discovered. Armed with his new air pump, he had always wanted to go much farther.

One of the first new things that Boyle discovered was that, unlike water, air seemed to have bounce. He'd noticed this almost as soon as he tried removing air from inside the glass globe. If you first pulled down the plunger to make a vacuum inside the brass cylinder, and only then opened the valve to the glass globe, air immediately whooshed from the globe into the cyclinder. Everyone in the room could hear it. If you closed off the valve, emptied the cylinder, and repeated the process, the whooshing still happened, but it was a bit less dramatic, with less air rushing out of the globe. And the next time you tried, even less air whooshed out.

Boyle deduced that air must contain some kind of particles that squeeze against each other. When the globe was full of air it was like an overcrowded room; as soon as the valve was opened, particles spilled out. But with each drag of the pump, the remaining particles could spread out—and were hence much more reluctant to leave.

Boyle didn't understand this quite as we do today—he imagined air to be something like a springy pile of flocks of wool. We now know that a piece of air the size of a sugar lump contains around 25 billion billion molecules all constantly darting about faster than the speed of sound. Every molecule crashes into another five billion times a second, and it is this incessant pinball barging that gives air its spring. It's why the billions of bouncing molecules inside a tire can hold up a truck, and why the weight of air doesn't only press downward but acts in every direction.

Boyle wanted to find out what role, if any, this springy air plays in the perception of sound. Nobody really knew how sound moves around, although there was a vague notion that it had something to do with the atmosphere.

He decided to try a careful experiment. Into his great glass globe he gently lowered a ticking watch, suspended from a thread. The watch was one of the latest models, which had a hand to mark out the seconds as well as the more usual minutes and hours. Using this, the experimenters would be able to assure themselves that the watch was still working as it dangled inside the globe.

At first, the sound of ticking was clear even a foot away from the globe. But when the pump began removing air, something changed: The ticking grew fainter and fainter. At last, when the pump had removed as much air as possible, Boyle and his helpers pressed their ears to the side of the globe. They could see the newfangled second hand as it continued to work its way around the watch face. But though everyone in the room strained to pick up the slightest hint of ticking, nobody could hear a thing. The air that left the globe had taken with it the power to transmit sound.

We now know that sound is made from vibrations. It can be transmitted through anything that wobbles—if your ear is touching something that's vibrating, you have no need of the air in between. But most of the sounds we care about happen at a distance, and for that our atmosphere is essential. Anything on Earth that makes a noise sets the air around it quivering, and our entire thick atmosphere acts like a giant vibrating drum. It's connected not by a skin but by those constant collisions in the pinball world of air molecules. You can send sounds across an entire room with just a little puff of effort, because your wobbling larynx passes on its vibrations to billions of barging air molecules, which then crash them on to their neighbors. Without air, a cannon could go off right next to your ear and you wouldn't hear, or feel, a thing. (Even the power of explosions comes from the air. When a bomb is detonated, it sends countless molecules of air flying in your direction to knock you off your feet.) Without air, our planet would be silent as the grave.

Next, Boyle wondered just what role air played in flight. Humans were obviously earthbound, and yet birds and insects had no trouble gliding through the air. Were they somehow floating like fish in the ocean above our heads? (And if so, why couldn't we float, too?)

To try to find out how air was necessary for flight, Boyle started with a humming bee. (He was a bit disappointed not to be able to try a butterfly, which seems to rely for flight more completely on wafts of air, but unfortunately the season was still too cold.) He put the bee inside the chamber with a bundle of flowers that hung from a thread near the top of the globe. He then prodded and teased the poor creature until she landed on the flowers and remained there. Next, he gradually began to draw out the air. At first the bee took no notice, and then suddenly the experiment was over. The bee tumbled helplessly down the wall of the globe without making any effort to use her wings. By the time he had managed to let air back into the chamber, she was already dead.

This wasn't exactly conclusive. Had the bee failed to fly because it had no air, or because it was suffocating? Boyle tried again, this time with a lark whose wing had been broken by a hunter's shot, but which was otherwise, he reported, "very lively." But once she was inside the chamber and losing air, it wasn't long before she, too, began to droop. Soon she began to writhe in convulsions, throwing herself over in frantic somersaults. Hastily, Boyle's assistant turned the stopcock and let fresh air in, but once again it was too late. "The whole tragedy," Boyle reported, "had been concluded within ten minutes of an hour."

Boyle realized that his air pump was going to tell him nothing about flight. His subjects were dying before they even had a chance to flap their wings. So he diverted his attention to trying to understand breathing. What made air so vital? He wondered if an animal more used to enclosed spaces might fare better, but a mouse "taken in such a trap as had rather affrighted than hurt him" went the way of the bird.

Observers were always welcome at Boyle's experiments, but he was now finding their presence rather awkward. One of his tests with another bird had to be abandoned when the subject was rescued by "some fair lady" who was horrified by the creature's convulsions, and insisted that Boyle immediately let air back in. After this, he did his more controversial experiments at night.

He began to wonder if his animals were dying because their exhalations somehow clogged up the globe. So he tried leaving a mouse in the closed vessel overnight, with a bed of paper to rest on and some cheese in case it was hungry, and carefully placed the vessel by the fire to make sure it didn't suffer from cold. Next morning, the mouse was not only alive but had eaten almost all of the cheese.

It was all very baffling. There were plenty of theories at the time for why breathing is necessary, but none of them was really enticing, which is perhaps not surprising, since none of them was right. Boyle himself inclined toward the idea that we breathe to cool down our lungs, which might otherwise become overheated. After all, cold-blooded animals such as fish have no lungs. On the other hand, Boyle correctly suspected that fish might somehow be making use of air dissolved in the water around them.

It wasn't only animals that needed air. Boyle also discovered that flames flickered out as soon as he drew air out of his globe. In some cases, for glowing coals for instance, readmitting air would rekindle the flame. But if he left the coals for more than four or five minutes, the spark irrevocably died. Boyle couldn't help but notice the similarity between flame and life. "The flame of a lamp," he remarked, "will last almost as little after the exsuction of the air as the life of an animal." Air was clearly vital for both processes, but Boyle had no idea why.