He practiced as a physician and, instead of using plants, prepared his medicines in the laboratory of the day, in which the furnace, crucible, and retort were most largely employed; this made him known as the medicus per ignem. He investigated gases, notably carbon dioxide, which he discovered in various sources, and it was he who first applied the name gas (geist) to this family of substances. He applied the balance in his investigations. He discovered sulphuretted hydrogen in the human system, made hydrochloric gas, which he called gas of salt, explained the explosion of gun-powder on the theory of the expansion of gases, discovered or investigated sulphuric acid, nitric acid, and nitrogen oxide. He was one of the first to recognize the rôle played by acid in the gastric juice, attributing disease to an excess of the same. Like all other chemists of the time, he studied the transmutation of metals