Lavoisier was an 18th-century French chemist. Toward the end of the 1700s, scientists across Europe were trying to understand combustion (how fire worked). Lavoisier, a prominent French philosopher and court administrator, had initially subscribed to the phlogiston theory, which dictated that there were special fiery substances (“phlogistons”) in the air. As he conducted more and more experiments, however, Lavoisier began to believe that combustion was less about fiery substances and more about the way different chemical compounds interacted with one another. Ultimately, Lavoisier’s experiments led him to discover oxygen as a unique compound and to develop a new understanding of chemical reactivity.