Inquire of the company who installed the wiring or electric system, how many lights and other devices can be attached and for what voltage they should be made. [Illustration: FIG. 47. Semi-indirect light.] CHAPTER XIII GAS LIGHT [Illustration: FIG. 48. Mantles.] =94. Construction of Mantles.= A mantle is a device made of thread saturated with some fireproof material like a mixture of thorium and cerium which will glow, giving off a white light when heated hot. The mantle (_A_ and _B_, Fig. 48) is placed over the burners of lamps using liquid or gaseous fuel. The gas is mixed with air so that it burns with a blue flame. The blue flame gives off little light, but it does not smoke and is much hotter than a yellow flame. When a mantle is placed over the blue flame, it is heated with less fuel consumption than is required to make a yellow illuminating flame. The light from the glow of the mantle is steadier and whiter than the light from an open flame, so that it is more hygienic. Mantles are made in different patterns so that they may be used on upright and inverted burners. The inverted mantle throws more light downward than an upright mantle. This is advantageous in lighting a room, for most of the light is wanted in the lower part of the room. Mantles can be used on lamps burning gas, kerosene, gasoline, alcohol and acetylene if the burners are made to produce a blue flame. (See Figs. 48 and 52.) [Illustration: FIG. 48-_a_. Adjusting gas light.] =95. Care of Mantles.= Strong jars and drafts will break mantles, for they are very fragile. The explosion caused by burning back when the lamp is being lighted is most destructive to mantles. To save mantles, wait until the lamp has filled with gas before touching the lighted match to it. =96. Fixtures for Burning Gas.= Gas will burn just as it escapes from a pipe. The flame of burning gas is yellow and makes considerable light. In order to secure more light for the amount of gas burned, put a tip on the end of the pipe, with a long, narrow slit in the top to spread the flame. These are usually lava tips. Natural gas gives very little light when burned in an open flame. Always burn it in mantle lamps. Its heating value is 1000 B.T.U. per cubic foot. When burned in a well-adjusted mantle lamp, natural gas will give about 15 candle hours per cubic foot. The heating value of manufactured gas is rated at 600 B.T.U. per cubic foot. It makes a fair light when used in an open flame burner. The yellow flame of burning gas makes considerable smoke, even when carefully adjusted. It gives four times as much light and no smoke when it is burned in a good mantle lamp. [Illustration: FIG. 49. Bunsen burner for gas light.]