Thus, if the flame of a
gas which naturally gives a luminous flame is supplied with an excess of
air, its illuminating value diminishes; and this is true whether that
excess is introduced at the base of the actual flame, or is added to the
gas prior to ignition. In fact the method of adding some air to a
naturally luminous gas before it arrives at its place of combustion is
the principle of the Bunsen burner, used for incandescent lighting and
for most forms of warming and cooking stoves. A well-made modern
atmospheric burner, however, does not add an excess of air to the flame,
as might appear from what has been said; such a burner only adds part of
the air before and the remainder of the necessary quantity after the
point of first ignition--the function of the primary supply being merely
to insure thorough admixture and to avoid the production of elemental
carbon within the flame.
ILLUMINATING POWER.--It is very necessary to observe that, as the
combined losses of heat from a flame must be smaller in proportion to the
total heat produced by the flame as the flame itself becomes larger, the
more powerful and intense any single unit of artificial light is, the
more economical does it become, because economy of heat spells economy of
light.
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