Gas-stove burners should be adjusted so that the blue-green central part of the flame is about half the height of the entire flame. If the flame is very long and is bright yellow in parts, too little air is being admitted; if short and inclined to make a slight roaring noise, there is too much air. In the latter case the flame is liable to " strike back," under which condition much carbon monoxide is formed. In all gas burners the various openings should be kept clean. The amount of air supplied to gas burners is usually adjusted by means of a small damper or slide to be found at the base of the burner.