The
portion, D, of the annular passage, B, being made of a highly
conductive metal, the gas becomes heated in passing to the burner, so
that both gas and air are raised to the same temperature by the time
they reach the orifices of the burner. Instead of prolonging the
gas-pipe to the point of bifurcation, a chamber may be arranged
immediately below the guide, for the gas and air to become intimately
mixed by passing through several perforations or wire gauze, receiving
the excess of heat from the white porcelain guide. The admission of
gas to both the main and heating burners is regulated by a double
valve in the pipe; but this arrangement may be used without any
previous heating of the gas and air.
Fig. 4 shows a similar arrangement to that above described, but
reversed; the gas and air being previously heated by the products of
combustion. The two pipes, D, lead the gas to the burner; and the
incandescent sheet of flame is drawn over a white refractory
substance, having in its center an orifice through which the hot gases
rise to the upper portion of the burner. The luminous sheet is spread
out all the better on account of this return of the flames, which also
causes the mixture of air and gas to be more complete than when they
rise directly. The gas escapes horizontally from the orifices of the
annular burner, B, and mingles with the double current of hot air
which rushes in above the flame inside the globe, and also below
through the central portion of the burner.
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