ARTIFICIAL LIGHTING
141. We seldom consider what life would be without our wonderful
methods of illumination which turn night into day, and prolong the
hours of work and pleasure. Yet it was not until the nineteenth
century that the marvelous change was made from the short-lived candle
to the more enduring oil lamp. Before the coming of the lamp, even in
large cities like Paris, the only artificial light to guide the
belated traveler at night was the candle required to be kept burning
in an occasional window.
With the invention of the kerosene lamp came more efficient lighting
of home and street, and with the advent of gas and electricity came a
light so effective that the hours of business, manufacture, and
pleasure could be extended far beyond the setting of the sun.
The production of light by candle, oil, and gas will be considered in
the following paragraphs, while illumination by electricity will be
reserved for a later Chapter.
142. The Candle. Candles were originally made by dipping a wick into
melting tallow, withdrawing it, allowing the adhered tallow to harden,
and repeating the dipping until a satisfactory thickness was obtained.
The more modern method consists in pouring a fatty preparation into a
mold, at the center of which a wick has been placed.
The wick, when lighted, burns for a brief interval with a faint,
uncertain light; almost immediately, however, the intensity of the
light increases and the illumination remains good as long as the
candle lasts. The heat of the burning tallow melts more of the tallow
near it, and this liquid fat is quickly sucked up into the burning
wick. The heat of the flame is sufficient to change most of this
liquid into a gas, that is, to vaporize the liquid, and furthermore to
set fire to the gas thus formed. These heated gases burn with a bright
yellow flame.
143. The Oil Lamp. The simple candle of our ancestors was now
replaced by the oil lamp, which gave a brighter, steadier, and more
permanent illumination. The principle of the lamp is similar to that
of the candle, except that the wick is saturated with kerosene or oil
rather than with fat. The heat from the burning wick is sufficient to
change the oil into a gas and then to set fire to the gas. By placing
a chimney over the burning wick, a constant and uniform draught of air
is maintained around the blazing gases, and hence a steady,
unflickering light is obtained. Gases and carbon particles are set
free by the burning wick. In order that the gases may burn and the
solid particle glow, a plentiful supply of oxygen is necessary. If the
quantity of air is insufficient, the carbon particles remain unburned
and form soot. A lamp "smokes" when the air which reaches the wick is
insufficient to burn the rapidly formed carbon particles; this
explains the danger of turning a lamp wick too high and producing more
carbon particles than can be oxidized by the air admitted through the
lamp chimney.
One great disadvantage of oil lamps and oil stoves is that they cannot
be carried safely from place to place. It is almost impossible to
carry a lamp without spilling the oil. The flame soon spreads from the
wick to the overflowing oil and in consequence the lamp blazes and an
explosion may result. Candles, on the other hand, are safe from
explosion; the dripping grease is unpleasant but not dangerous.
The illumination from a shaded oil lamp is soft and agreeable, but the
trimming of the wicks, the refilling of bowls, and the cleaning of
chimneys require time and labor. For this reason, the introduction of
gas met with widespread success. The illumination from an ordinary gas
jet is stronger than that from an ordinary lamp, and the stronger
illumination added to the greater convenience has made gas a very
popular source of light.
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.
Unburned carbon would be objectionable in cooking stoves where
utensils are constantly in contact with the flame, and for this reason
cooking stoves are provided with an arrangement by means of which
additional air is supplied to the burning gas in quantities adequate
to insure complete combustion of the rapidly formed carbon particles.
An opening is made in the tube through which gas passes to the burner,
and as the gas moves past this opening, it carries with it a draft of
air. These openings are visible on all gas stoves, and should be kept
clean and free of clogging, in order to insure complete combustion. So
long as the supply of air is sufficient, the flame burns with a dull
blue color, but when the supply falls below that needed for complete
burning of the carbon, the blue color disappears, and a yellow flame
takes its place, and with the yellow flame the deposition of soot is
inevitable.
146. By-products of Coal Gas. Many important products besides
illuminating gas are obtained from the distillation of soft coal.
Ammonia is made from the liquids which collect in the condensers;
anilin, the source of exquisite dyes, is made from the thick, tarry
distillate, and coke is the residue left in the clay retorts. The coal
tar yields not only anilin, but also carbolic acid and naphthalene,
both of which are commercially valuable, the former as a widely used
disinfectant, and the latter as a popular moth preventive.
From a ton of good gas-producing coal can be obtained about 10,000
cubic feet of illuminating gas, and as by-products 6 pounds of
ammonia, 12 gallons of coal tar, and 1300 pounds of coke.
147. Natural Gas. Animal and vegetable matter buried in the depth of
the earth sometimes undergoes natural distillation, and as a result
gas is formed. The gas produced in this way is called natural gas. It
is a cheap source of illumination, but is found in relatively few
localities and only in limited quantity.
148. Acetylene. In 1892 it was discovered that lime and coal fused
together in the intense heat of the electric furnace formed a
crystalline, metallic-looking substance called calcium carbide. As a
result of that discovery, this substance was soon made on a large
scale and sold at a moderate price. The cheapness of calcium carbide
has made it possible for the isolated farmhouse to discard oil lamps
and to have a private gas system. When the hard, gray crystals of
calcium carbide are put in water, they give off acetylene, a colorless
gas which burns with a brilliant white flame. If bits of calcium
carbide are dropped into a test tube containing water, bubbles of gas
will be seen to form and escape into the air, and the escaping gas may
be ignited by a burning match held near the mouth of the test tube.
When chemical action between the water and carbide has ceased, and gas
bubbles have stopped forming, slaked lime is all that is left of the
dark gray crystals which were put into the water.
When calcium carbide is used as a source of illumination, the crystals
are mechanically dropped into a tank containing water, and the gas
generated is automatically collected in a small sliding tank, whence
it passes through pipes to the various rooms. The slaked lime, formed
while the gas was generated, collects at the bottom of the tanks and
is removed from time to time.
The cost of an acetylene generator is about $50 for a small house, and
the cost of maintenance is not more than that of lamps. The generator
does not require filling oftener than once a week, and the labor is
less than that required for oil lamps. In a house in which there were
twenty burners, the tanks were filled with water and carbide but once
a fortnight. Acetylene is seldom used in large cities, but it is very
widely used in small communities and is particularly convenient in
more or less remote summer residences.
Electric Lights. The most recent and the most convenient lighting is
that obtained by electricity. A fine, hairlike filament within a glass
bulb is raised to incandescence by the heat of an electric current.
This form of illumination will be considered in connection with
electricity.