A Bunsen burner is a burner, promoted by Robert Bunsen, used widely in laboratories and sometimes in metal heat-treatment. Through a nozzle at the base is introduced slightly pressurized fuel gas which is mixed with air (primary air) which the flow induces through adjustable inlets. The usual gas:air ratio is 1:2. The mixture proceeds up a metal tube, about 13 centimeters (5 inch) high, and is ignited at the top, giving a flame temperature of 2,000 K. Outer portions of the flame mix with further air (secondary air) to give a cooler, more luminous flame, where the gas:air ratio is about 1:4.