Guillaume Francois Rouelle (September 16, 1703 - August 3, 1770) was a French chemist and apothecary. In 1754 he introduced the concept of a base into chemistry, as a substance which reacts with an acid to give it solid form (as a salt). He is known as l'Ain矇 (the elder) to distinguish him from his younger brother, Hilaire Rouelle, who was also a chemist and known as the discoverer of urea. During 1742-68 he was a chemical demonstrator at the Jardin du Roi, Paris. He was an enthusiastic lecturer and taught most of the French chemists of the late 18th century; Denis Diderot, Antoine-Laurent de Lavoisier, Joseph Proust and Antoine-Augustin Parmentier. Rouelle believed in Stahl's incorrect phlogiston theory of combustion. Phlogiston was a flammable component of materials which was supposedly given off when they burned. This theory was disproved by Lavoisier. Rouelle was elected a foreign member of the Royal Swedish Academy of Sciences in 1749. He died in 1770 at the age of 66. Engraving from "Vies des savants illustres" by Louis Figuier.