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 ligh