In 1927, General Electric introduced this "Monitor Top" refrigerator, so-named by the public because of the resemblance of the exposed compressor on top of its cabinet to the cylindrical turret of the Civil War gunship, the Monitor. The product, priced at $525, was designed by Chief Engineer Christian Steenstrup (1873-1955). It had a single door, and was the first all-steel refrigerator cabinet, earlier versions having been of wood to imitate furniture cabinetry. It quickly became one of GE's most successful products and made GE the industry leader, in spite of the fact that it was perched on the top of rather traditional looking "furniture" legs. Household refrigerators had been on the market for some time. GE was the first, marketing in 1911 a wooden model invented in France by a French monk, Abbé Marcel Audiffren. Called the Audiffren, it sold for $1000, twice the cost of a car, and was produced in France. The first successful domestic refrigerator to enter full scale production in the US was the Kelvinator, in 1918. Made of wood, it looked pretty much like a small bedroom night stand, with a single door. Frigidaire, purchased by General Motors in 1919, introduced its first home refrigerator in 1921, when there were only about 5000 refrigerators in the US. Also in wood, it was larger, about the size of a bedroom armoire, or wardrobe, with three doors. By 1923, there were 56 companies making refrigerators, using sulfur dioxide, methyl chloride, or ammonia gases, all of which were dangerously toxic. Two years after the "Monitor-Top", in 1929, the Kelvinator Four refrigerator debuted with no visible "legs". Its cabinet and compressor were cleanly encased in a simple white metal box design reaching the floor. It set the design typeform for home refrigerators for the rest of the century. Freon was discovered in 1930 by Delco chemist Thomas Midgely. Non-toxic, Freon was adopted by all manufacturers, and refrigerators became safe for use in the home.