Antoine Lavoisier, an eighteenth-century French chemist, disproved the theory of phlogiston by showing that combustion required a gas (oxygen) and that that gas has weight. Lavoisier did this by burning elements in closed containers. These solids gained mass, but the total weight of the containers did not change—what did change was the pressure inside the vessel. When Lavoisier opened the vessel up, air rushed in, and the total weight of the vessel increased. So Becher had it backward: oxygen was being used up by the candle instead of phlogiston being given off by the flame.