given pressure depends upon and can thus be controlled by the gas permeability of the burner element 23 the gas permeability of a burner element 23 useful for combusting natural gas is a convenient reference for permeabilities useful with other gases for example it is desirable that when natural gas is to be combusted the gas permeability of the burner element 23 is between 25 and 500 cubit feet per square foot per minute 0 5 inches water column and preferably about 118 cubic feet per square foot per minute natural gas has a flame velocity of about 1 foot per second combustion gases containing molecular hydrogen in contrast have flame velocities on the order of 9 feet per second accordingly a burner element 23 useful for combusting a hydrogen containing gas mixture such as manufactured gas a commercially available mixture containing about 50 percent molecular hydrogen plus some natural gas and carbon dioxide preferably has a gas permeability of no more than 42 percent of the gas permeability of a comparable element used for combusting natural gas thus when manufactured gas is used it is desirable that the gas permeability of the burner element 23 is no more than 210 cubic feet per square foot per minute and preferably about 50 cubic feet per square foot per minute the.