Joseph Priestley is known as the "father of pneumatic chemistry," because of his pioneer work with gases. The methods he invented of collecting and testing gases enabled later chemists to carry on where he left off. His greatest discovery was the preparation of oxygen from mercuric oxide, in 1774. He is credited with having discovered a number of other gases, among them hydrogen chloride, ammonia, and carbon monoxide. Priestley's formal training was in theology, and early in life he became a Nonconformist minister. His first scientific interest was in electricity, but soon he turned to chemistry. His preparation of "soda water" won him the Royal Society of London's Copley Medal in 1773. Always sympathetic to the cause of the American Colonies, Priestley emigrated to the United States in 1794. His house in Northumberland, Pennsylvania, still stands, and is maintained as a memorial to this great pioneer chemist.