1789

Conservation of Mass

Joseph-Louis Lagrange (1736–1813), Antoine Lavoisier (1743–1794)

French chemist Antoine Lavoisier was central to turning chemistry into a science. The discoveries being made with gases had advanced knowledge tremendously, but it was clear that the theories of the time couldn’t accommodate them. A key example was phlogiston, and Lavoisier’s work with oxygen helped to demolish that theory. But there were many more developments to come.

As he himself put it, Lavoisier wanted “to rid chemistry of every kind of impediment that delays its advance.” One of these impediments was the naming of compounds. If compounds really were formed by the combination of elements, as English chemist Robert Boyle had proposed over a hundred years before, shouldn’t the names reflect this in a systematic way? Lavoisier thought so, and he developed a system of chemical nomenclature that is still used today. For example, if iron combined with oxygen, the resulting compound was iron oxide. In Elements of Chemistry (1789)—considered the first modern chemistry textbook—Lavoisier laid out his system of nomenclature, along with a table of all the elements discovered to date, the compositions of compounds that had been determined, and his thoughts on the influence of temperature on reactions and on the formation of salts from acids and bases. He also stated a principle that no one had ever put into so few words: conservation of mass—the idea that a chemical reaction finished with the same amount of material it started with.

Unfortunately, Lavoisier’s public profile and involvement in the Ferme générale—an organization that collected taxes on behalf of the deposed French monarchy—made him a target of the French Revolution during the Reign of Terror. Along with twenty-seven codefendants, he was summarily convicted of treason and beheaded on May 8, 1794. The next day, the Italian mathematician and astronomer Joseph-Louis Lagrange reflected, “It took them only an instant to cut off that head, and a hundred years may not produce another like it.”

In this oil painting by British artist Ernest Board, Lavoisier explains the results of his experiments on air to his wife, who helped him with much of his research.