Antoine Lavoisier is credited with formulating the law of conservation of mass, establishing that in a closed system the total mass remains constant before and after a chemical reaction. He conducted quantitative experiments in the late 18th century, weighing reactants and products to show that matter is neither created nor destroyed, only transformed or rearranged. While earlier thinkers (notably Mikhail Lomonosov, and prior ideas by Rey, Black, Cavendish) anticipated the concept, Lavoisier’s rigorous weighing experiments and his summary statement—often paraphrased as “Nothing is lost, nothing is created, everything is transformed”—solidified the law in modern chemistry.