144.  Gas Burners and Gas Mantles.  For a long time, the only gas flame used was that in which the luminosity resulted in heating particles of carbon to incandescence.  Recently, however, that has been widely replaced by use of a Bunsen flame upon an incandescent mantle, such as the Welsbach.  The principle of the incandescent mantle is very simple.  When certain substances, such as thorium and cerium, are heated, they do not melt or vaporize, but glow with an intense bright light.  Welsbach made use of this fact to secure a burner in which the illumination depends upon the glowing of an incandescent, solid mantle, rather than upon the blazing of a burning gas.  He made a cylindrical mantle of thin fabric, and then soaked it in a solution of thorium and cerium until it became saturated with the chemical.  The mantle thus impregnated with thorium and cerium is placed on the gas jet, but before the gas is turned on, a lighted match is held to the mantle in order to burn away the thin fabric.  After the fabric has been burned away, there remains a coarse gauze mantle of the desired chemicals.  If now the gas cock is opened, the escaping gas is ignited, the heat of the flame will raise the mantle to incandescence and will produce a brilliant light.  A very small amount of burning gas is sufficient to raise the mantle to incandescence, and hence, by the use of a mantle, intense light is secured at little cost.  The mantle saves us gas, because the cock is usually “turned on full” whether we use a plain burner or a mantle burner.  But, nevertheless, gas is saved, because when the mantle is adjusted to the gas jet, the pressure of the gas is lessened by a mechanical device and hence less gas escapes and burns.  By actual experiment, it has been found that an ordinary burner consumes about five times as much gas per candle power as the best incandescent burner, and hence is about five times as expensive.  One objection to the mantles is their tendency to break.  But if the mantles are carefully adjusted on the burner and are not roughly jarred in use, they last many months; and since the best quality cost only twenty-five cents, the expense of renewing the mantles is slight.

145.  Gas for Cooking.  If a cold object is held in the bright flame of an ordinary gas jet, it becomes covered with soot, or particles of unburned carbon.  Although the flame is surrounded by air, the central portion of it does not receive sufficient oxygen to burn up the numerous carbon particles constantly thrown off by the burning gas, and hence many carbon particles remain in the flame as glowing, incandescent masses.  That some unburned carbon is present in a flame is shown by the fact that whenever a cold object is held in the flame, it becomes “smoked” or covered with soot.  If enough air were supplied to the flame to burn up the carbon as fast as it was set free, there would be no deposition of soot on objects held over the flame or in it, because the carbon would be transformed into gaseous matter.