Antoine Lavoisier lived in 18th century Paris during the years leading up to the French Revolution. But shortly before the Reign of Terror, Antoine launched another revolution, a scientific revolution.
Lavoisier was born into affluence but multiplied his wealth many times over when he became involved in a private tax collecting agency called the Ferme Générale.
Lavoisier had two passions, finances and science. He and his wife, Marie Anne Pierrette Paulze, would spend hours every evening laboring in their extremely well equipped home laboratory. At the time, the age old alchemical concept of combustible substances possessing a substance called phlogiston was still widely believed to be true. It actually made some sense at the time because when a substance was burned, naturally it weighed less. They assumed that the less it weighed after burning, the more phlogiston (basically fire) it had contained.
Using a completely sealed system called a bell jar and taking very careful measurements on an accurate and precise scale, Lavoisier was able to show that burning some substances like phosphorous and sulfur caused them to gain weight from the air, similar to rusting iron. He also showed that lead calx (which we call something different today) lost weight when heated, despite it not catching fire and releasing any supposed phlogiston. None of this would be true if phlogiston were real.
He summarized all of his findings in his 1789 book, Traité élémentaire de Chimie (Elements of Chemistry), “… in every operation an equal quantity of matter exists both before and after the operation.”
Today, we call this idea the Law of Mass Conservation. In simple terms, it says that mass cannot be created or destroyed in a chemical reaction. A chemical reaction just involves the rearrangement and combining of elements in compounds to make new compounds. The Law of Mass Conservation was the end of phlogiston theory, the end of alchemy, and the birth of genuine quantitative scientific chemistry.
The Law of Mass Conservation means that for any given chemical reaction, the total mass of all of the reactants will be equal to the total mass of all products.
Unfortunately, because of Lavoisier’s work as a tax collector, he was on the wrong side of history during the French Revolution and thus was beheaded by guillotine in 1794. After Lavoisier’s death, the French mathematician Joseph-Louis Lagrange famously said, “it took them only an instant to cut off that head, and a hundred years may not produce another like it.”
