If a cold metal wire or rod is
held in a non-luminous flame, it causes an extinction of the gas for
some considerable space around itself; but as the temperature of the
rod rises, this space becomes smaller and smaller until the rod is
heated to redness, and then the flame comes in contact with the rod.
In the same way, if the burner from which the gas is issuing be heated
to redness, the space between burner and flame disappears. It has
already been shown that cooling the flame by an inert diluent reduces
the illuminating value, and finally renders it more luminous; and we
are now in a position to discuss the points which should be aimed at
in the construction of a good gas burner.
In the first place, a sensible diminution in light takes place when a
metal burner is employed, and the larger the surface and thickness of
the metal the worse will be its action on the illuminating power of
the flame; but this cooling action is only influencing the bottom of
the flame, so that with a small flame the total effect is very great,
and with a very large flame almost _nil_.
The first point, therefore, to attend to is that the burner shall be
made of a good non-conductor. In the next place, the flow of the gas
must be regulated to the burner, as, if you have a pressure higher
than that for which the burner is constructed, you at once obtain a
roaring flame and a loss of illuminating power, as the too rapid rush
of gas from the burner causes a mingling of gas and air and a
consequent cooling of the flame.
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