In the special burner of the mantle lamp, the gas is mixed with air so that it will burn with a blue flame (Fig. 49). A blue flame is not good for lighting, but when a mantle is placed over the flame, it becomes heated, glowing hot. Since the mantle is made of a material which gives off a white glow, it lights the room with a steady light which is far better than the flickering light of the open flame (Fig. 48-_a_). =97. Adjustment.= See that the ports thru which air is drawn into the lamp are open as wide as needed to give a clear, smokeless flame without firing back. Some lamps are fitted with a screw beside the cocks to regulate the amount of gas flowing into the lamp. It should be adjusted so that no more gas flows into the lamp than is needed to get as bright a glow as possible from the mantle. Regulate the gas flow by closing the valve attached to this screw until the mantle decreases perceptibly in brightness, and then slowly opening it until the mantle becomes bright. Gas companies often adjust lamps for their customers. =98. Care of Lamps.= Clean the burners if they become sooted. Replace mantles if they are broken. [Illustration: FIG. 50. Open-flame acetylene burner.] =99. Lighting a Gas Light.= When lighting a lamp, turn on the gas, count three, and then light the lamp. Counting three gives time for the burner to fill with gas and prevents burning back with an explosion. Mantles are very delicate and easily broken by jars or strong drafts. Burning back may break the mantle. Burning back means that the gas ignites at the opening where it should be mixing with the air instead of at the tip of the burner. This happens when the lamp is lighted before it becomes filled with gas, or when there is too much air mixed with the gas. =100. Cold-Process Gasoline Gas.= It is more economical to use cold-process gasoline gas with a mantle lamp than an open-flame burner for lighting. Be sure to use the burners made especially for this kind of gas. The lamps are managed like all others. =101. Acetylene Lamps.= Open-flame burners are used for acetylene gas because no mantle burner has been constructed which will operate reliably with this rich gas. Acetylene gas gives about ten times as much light per cubic foot as manufactured gas burned in an open flame. The burners require little care. Sometimes the holes in burners become stopped, and they should be cleaned out with a fine pointed instrument like a needle. When they do not work well, it pays to replace the old tips with new ones. [Illustration: FIG. 50-_a_. Showing electric lighting device for acetylene burner.] Acetylene gas burners are constructed so that a very fine spray of gas strikes another fine spray, which, when ignited, makes a broad flame. This flame, which is almost white, gives off light. The burners appear as illustrated in Fig. 50.