Caesar’s Last Breath


Decoding the Secrets of the Air Around Us

The fascinating science and history of the air we breathe

It’s invisible. It’s ever-present. Without it, you would die in minutes. And it has an epic story to tell.

In Caesar’s Last Breath, New York Times bestselling author Sam Kean takes us on a journey through the periodic table, around the globe, and across time to tell the story of the air we breathe, which, it turns out, is also the story of earth and our existence on it.

With every breath, you literally inhale the history of the world. On the ides of March, 44 BC, Julius Caesar died of stab wounds on the Senate floor, but the story of his last breath is still unfolding; in fact, you’re probably inhaling some of it now. Of the sextillions of molecules entering or leaving your lungs at this moment, some might well bear traces of Cleopatra’s perfumes, German mustard gas, particles exhaled by dinosaurs or emitted by atomic bombs, even remnants of stardust from the universe’s creation.

Tracing the origins and ingredients of our atmosphere, Kean reveals how the alchemy of air reshaped our continents, steered human progress, powered revolutions, and continues to influence everything we do. Along the way, we’ll swim with radioactive pigs, witness the most important chemical reactions humans have discovered, and join the crowd at the Moulin Rouge for some of the crudest performance art of all time. Lively, witty, and filled with the astounding science of ordinary life, Caesar’s Last Breath illuminates the science stories swirling around us every second.




Indulge me in a modest experiment. For the next few seconds try paying close attention to the air escaping your body, as if this were your last living breath on earth. How much do you really know about this air? Feel your lungs deflate and sag inside your chest. What’s really going on inside there? Put your hand in front of your lips and feel how the gas escaping them has transformed inside you, growing warmer and more humid, perhaps acquiring an odor. What sort of alchemy caused that? And although your sense of touch isn’t nearly discriminating enough, imagine that you can feel the individual molecules of gas pinging your fingertips, impossibly tiny dumbbells, caroming off into the air around you. How many are there, and where do these molecules go?

Some don’t get far. As soon as you take another breath, they come rushing back into your lungs, like waves that fling themselves onto the shore before being retracted by the sea. Others stray a little farther and make a break for freedom in the next room before returning as well, miniature prodigal sons. Most simply join the anonymous masses of the atmosphere and begin to spread around the globe. But even then, perhaps months later, a few weary pilgrims will stagger back to you. You might be a very different person between your first and second encounters with these molecules, but the ghosts of breaths past continue to flit around you every second of every hour, confronting you with every single yesterday.

Of course, you’re not alone in experiencing this; the same thing happens to every other person on Earth. Moreover, your ghosts are almost certainly entangled with theirs, since they almost certainly inhaled and expelled and rebreathed a few of those very same molecules after you did—or even before you did. In fact, if you’re reading this in public, you’re inhaling the exhaust from everyone around you right now—secondhand breath. Your reaction to this will probably depend on the company you keep. Sometimes we enjoy this mingling of airs, as when lovers lean in and we feel their breath on our necks; sometimes we abhor it, as when the chatterbox next to us on the plane has eaten garlic for lunch. But short of breathing from a tank, we can’t escape the air of those around us. We recycle our neighbors’ breaths all the time, even distant neighbors’. Just as light from distant stars can sparkle our irises, the remnants of a stranger’s breath from Timbuktu might come wafting in on the next breeze.

Even more startling, our breaths entangle us with the historical past. Some of the molecules in your next breath might well be emissaries from 9/11 or the fall of the Berlin Wall, witnesses to World War I or the star-spangled banner over Fort McHenry. And if we extend our imagination far enough in space and time, we can conjure up some fascinating scenarios. For instance, is it possible that your next breath—this one, right here—might include some of the same air that Julius Caesar exhaled when he died?


  • Introduction: The Last Breath
    • Chapter One: Earth’s Early Air
    • Interlude: The Exploding Lake
    • Chapter Two: The Devil in the Air
    • Interlude: Welding a Dangerous Weapon
    • Chapter Three: The Curse and Blessing of Oxygen
    • Interlude: Hotter than the Dickens
    • Chapter Four: The Wonder-Working Gas of Delight
    • Interlude: Le Petomane
    • Chapter Five: Controlled Chaos
    • Interlude: Steeling Yourself for Tragedy
    • Chapter Six: Into the Blue
    • Interlude: Night Lights
    • Chapter Seven: The Fallout of Fallout
    • Interlude: Albert Einstein and the People’s Fridge
    • Chapter Eight: Weather Wars
    • Interlude: Rumbles from Roswell
    • Chapter Nine: Putting on Alien Airs


“Sam Kean has done it again—this time clearly and entertainingly explaining the science of the air around us. He is a gifted storyteller with a knack for finding the magic hidden in the everyday.”
—Daniel H. Pink, author of Drive

“The most fun to be had from nonfiction is a good science book, with a writer of craft who can capture both the excitement and the elegance of science, the incredible fact that this is really how it works. Sam Kean is such a writer and Caesar’s Last Breath is such a book. An enormous pleasure to read.”
—Mark Kurlansky, author of Paper and Salt

“Fascinating stories, so insightful, informative, and disarmingly written. It gave this astronaut a new respect for the air around us all, and made me delightfully more aware of each breath I take.”
—Col. Chris Hadfield, author of An Astronaut’s Guide to Life on Earth

“A witty book that turns the science of stuff we breathe into a delightful romp through history. Kean, an award-winning science writer…has done it again, using his free-wheeling style to translate hard scientific facts into lively stories.”

“Riveting…Kean has a knack for distilling chemistry to its essential elements, using stories and humor…this is a dose of fresh air.”
Library Journal

“Kean pumps chemical and historical trivia into this tale about air and the gases of which it is composed…lightweight and entertaining.”
Publishers Weekly