Guillaume Rouelle. Engraving of Guillaume Francois Rouelle (1703-1770), French chemist. Rouelle bec- ame an apothecary in 1725. 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, the most famous being Antoine Lavoisier. Rouelle int- roduced Georg Stahl's phlogiston theory of combus- tion into France. Stahl claimed that phlogiston was a flammable component of materials which was given off when they burned; it weighed less than nothing so that burnt substances became heavier. This theory was disproved by Lavoisier. Rouelle's own work included a classification of salts.