French chemist Antoine Lavoisier died #OTD in 1794.
He is best known for his development of the law of conservation of mass, which states that mass is neither created nor destroyed in chemical reactions. This principle helped to debunk the phlogiston theory, which was a prevailing theory at the time that suggested substances released a material called "phlogiston" when they burned. He also made significant contributions in understanding respiration as a form of combustion.