t.

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