Joseph Priestly was an English chemist and natural philosopher whose research lay the foundation for modern chemistry and for our understanding of chemistry. Priestley is credited with the discovery of oxygen in 1774, which he produced by focusing sunlight on mercuric oxide. He also performed important experiments with "fixed air" (carbon dioxide), "nitrous air" (nitric oxide), "marine acid air" (hydrogen chloride), "alkaline air" (ammonia), "vitriolic air" (sulfur dioxide), and"phlogisticated nitrous air" (nitrous oxide, laughing gas). Priestley believed in the phlogiston theory, and was convinced that his discovery of oxygen ("dephlogisticated air”) proved it to be correct. Phlogiston theory postulates that in all flammable materials there is present phlogiston, a substance without color, odor, taste, or weight that is given off in burning. He demonstrated that green plants can replenish stale, or oxygen-poor, air so that it is capable of supporting combustion and respiration. He also made important contributions in the fields of education, moral philosophy, theology, metaphysics, political economy, history and physical science.