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For a long time the concept of "mass" had been like the concept of energy before the 19th century. There were a lot of different material substances around, but it was not clear how they related to each other, if at all.

Antoine-Laurent Lavoisier, as much as anyone else, first showed that all the seemingly diverse bits of tree and rock and iron on Earth—all the "mass" there is—really were parts of a single connected whole. Through decades of meticulous experiments, aided by his able wife Marie Anne, Lavoisier proved that the substances that fill our universe can be burned, squeezed, shredded, or hammered to bits, but they won't disappear. The different sorts floating around just combine or recombine. The total amount of mass, however, remains the same. This principle—the law of conservation of mass—was one of the great scientific achievements of the 1700s.

Antoine-Laurent Lavoisier (1743-1794)

Lavoisier was born into an aristocratic Parisian family. As a teenager, he studied botany, astronomy, and mathematics, but it was chemistry, above all, that became his passion. It was an experimental science suited to his character, for Lavoisier had the meticulous nature of an accountant with a soul that could soar.

Lavoisier demonstrated his romanticism in 1771 by rescuing the innocent 13-year-old daughter of his boss Jacques Paulze from a forced marriage to an uncouth ogre of a man. His own marriage to Marie Anne turned out to be a good one, despite the difference in age, and despite the fact that the handsome 28-year-old Lavoisier soon shifted back to the stupendously boring accountancy work he did for Paulze, collecting taxes for Louis XVI's government.

Lavoisier kept a vast tax-churning organization in operation, working long hours, six days a week on average, for the next 20 years. Only in his spare time—an hour or two in the morning, and then one full day each week—did he focus on his science. But he called that single day his "jour de bonheur"—his "day of happiness."

Proving that nature is "a closed system"

Lavoisier's devotion to detail allowed him to investigate a fundamental scientific question: Can matter ever be completely destroyed or created? Or, when it comes to the substances around us, is nature "a closed system"? Lavoisier explored, for example, what happens when objects rust. While intuition might suggest that a rusted piece of metal weighs less than a pristine one, Lavoisier took nothing on trust. He built an entirely closed apparatus and set it up in a drawing room of his house.

Aided by Marie Anne, Lavoisier put various substances in the apparatus, sealed it tight, and applied heat or started an actual burn to speed up the rusting. Once everything had cooled down, the pair took out the mangled or rusty or otherwise burned-up metal and weighed it, and also carefully measured how much air was lost. Each time they got the same, unexpected result. What they found, in modern terms, was that a rusted sample does not weigh less. It doesn't even weigh the same. It weighs more.

What was happening? There was the same amount of stuff overall, yet now the oxygen that had been in the gases floating above was no longer in the air. But it had not disappeared. It had simply stuck onto the metal. With his state-of-the-art weighing machine, Lavoisier showed that matter can move around from one form to another, yet it will not burst in and out of existence.

A life cut short, but a long legacy

With all of Lavoisier's accurate weighing and chemical analysis, other researchers were able to start tracing how that conservation happened in practice—as with his working out how oxygen molecules cascaded from the air to stick to iron. Lavoisier became known as the father of modern chemistry but only decades after his violent demise.

It was primarily his day job as a tax collector that led Lavoisier to the guillotine at the age of 51. It didn't help, however, that in the years before the French Revolution, Lavoisier had made an enemy of Jean-Paul Marat, a frustrated scientist who became a captain of the Reign of Terror.

Lavoisier was tried, convicted, and guillotined in one day. Legend has it that an appeal to spare his life was rejected by the judge with a curt "The Republic has no need of geniuses." Yet one and a half years after his death, the French government exonerated Lavoisier and sent Marie Anne his confiscated belongings along with a note: "To the widow of Lavoisier, who was falsely convicted."