Eighteenth-century chemistry rested on the belief that everything which burns releases an invisible substance called "phlogiston". But there was an anomaly: when metals burned (rusted) they should have grown lighter, yet they grew heavier. The Frenchman Antoine-Laurent de Lavoisier (1743–1794) chased this anomaly with the age's most powerful instrument — the precision balance.

Weighing carefully in sealed vessels, he showed that combustion is not the release of a substance but the binding of a component of the air to the body. He isolated that component and named it "oxygen". By heating mercury oxide to separate it, then recombining the same oxygen, he closed the loop. With equal rigour he proved that water is not an element but a compound of hydrogen and oxygen — the end of the ancient four-element doctrine.

Lavoisier's method can be summed up in a sentence: in a reaction, matter is neither created from nothing nor destroyed, only rearranged. The principle of conservation of mass turned chemistry from a qualitative alchemical tradition into a quantitative science. In 1787, with colleagues, he rebuilt the chaotic language of chemistry: names like "phlogisticated air" gave way to systematic ones describing composition. His 1789 "Traité élémentaire de chimie" (Elementary Treatise on Chemistry) gathered this new language and the first modern list of elements; so the textbook of chemistry was born.

He carried out much of the work together with his wife Marie-Anne Paulze, who recorded the experiments and drew their engravings. Lavoisier was also a tax collector; for that office he was sent to the guillotine in 1794, during the Terror of the French Revolution. The mathematician Lagrange is said to have remarked: "It took but a moment to sever that head; a hundred years will not suffice to produce its like."