Too much air interferes with the burning of the gas; in fact, there can be so much air mixed with gas that it will not burn. The air mixer regulates the amount of air which flows into the pipe. Once this is adjusted for the kind of gas to be used, it seldom needs to be changed. The air shutter has to be changed, however, if the gas pressure varies markedly from time to time. Readjustment may be required if the stove is moved and connected with a different supply of gas. When adjusting the mixer for high pressure, artificial or natural gas, close the shutter until the flame will not blow away from the cone, but will burn with a blue, almost colorless, flame. [Illustration: FIG. 4. Part of gas stove showing air mixers.] =12. Regulating the Gas.= The amount of gas which passes into the stove is also regulated, first, by adjustment of the size of the small opening thru which the gas must flow. Once this is adjusted, it does not need to be changed so long as the gas comes from the same source. Second, the flow of gas is regulated by the lever valve. As the valve is turned, the flow of gas is restricted so that it flows less swiftly. The size of the stream of gas going into the stove always looks the same regardless of its speed. When the rate is not so fast, the fire burns lower because less gas comes to it during every unit of time. =13. Lighting the Stove.= Light the top burners by first striking a match, and then turning on the burner so that there will be an unrestricted flow of gas. Count three before applying the match. This gives time for the burner to fill with gas. If the match goes out, shut off the gas and try again. If it burns back into the air hole, also turn off the gas and begin again. Probably the match was applied too soon. Gas stoves get out of order because of carelessness in lighting them. The force of the explosions caused in burning back loosens connections and may disturb the adjustment of the mixer and valve. [Illustration: FIG. 5. Cleaning gas stove.] =14. Cleaning the Stove.= Housekeepers should keep their gas stoves clean. Dirt interferes with the passage of the gas thru the burners. Gas stoves should be cleaned thoroly once a month. Scrub the burners with a stiff brush (Fig. 5), and wash all greasy parts with soap and water. If the holes should be clogged, remove the stoppage with a wire hair-pin (Fig. 6). Clean the drip sheet every day, or as often as it becomes soiled. (Fig. 4.)