In this class, the following terms or expressions are used with the meanings indicated: "combustion" means the direct combination of oxygen gas, e.g. in air, and a burnable substance. Any other heat-producing combination of chemical substances, e.g. hydrogen peroxide and methane, iron oxide and aluminium, is covered by section C or by subclass ; "combustion chamber" means a chamber in which fuel is burned to establish a self-supporting fire or flame and which surrounds that fire or flame; "burner" means a device by which fluent fuel is passed to a combustion space where it burns to produce a self-supporting flame; "air" means a mixture of gases containing free oxygen and able to promote or support combustion. This subclass coversalso the burning of low-gradefuel of solid, liquid, or gaseous nature.