Scientists once believed that all combustible objects contain an element called phlogiston that is released during burning. But in 1779, the chemist Antoine Lavoisier (1734–1794) demonstrated that when metals such as mercury are burned, they do not get lighter, as the phlogiston theory said they must. Pictured here are Lavoisier (left) and his theory-killing apparatus (right).

SEFANO BIANCHETTI/CORBIS and INTERFOTO/ALAMY