Colour portrait of the French chemist Antoine Laurent Lavoisier, 1743-1794. Lavoisier is considered one of the greatest experimentalists in chemistry. He showed that air is a mixture of two gases, oxygen and nitrogen and water a compound formed by hydrogen and oxygen. By studying the reactions between oxygen and metals Lavoisier was able to elaborate a theory for combustion that soon displaced the current phlogiston theory. A milestone of his work was the Law of Conservation of Matter that he formulated, which stated that in chemical reactions matter is never created or destroyed. His work was published in 1789 in Elementary Treatise on Chemistry.