A furnace is an apparatus in which heat is liberated and transferred directly or indirectly to a solid or fluid mass for the purpose of effecting a physical or chemical change. The source of heat is the energy released in the oxidation of fossil fuel (commonly known as combustion) or the flow of electric current through adjacent semiconductors or through the mass to be heated. In recent years, scientific and engineering effort has been made to utilize nuclear and solar energy for heating purposes. Therefore, according to the source of heat and method of its application, there are four categories of furnaces; combustion, electric, nuclear, and solar, in the order of their present commercial or industrial importance.