in carbon more light is yielded, up to a certain
point, with a flame of a given temperature. To increase the heat of the
flame various devices were tried before the introduction of the
incandescent mantle, but they were found to be too short-lived to have
any commercial value. Inventors therefore sought for methods by which
the emission of light could be obtained from coal gas independently of
the incandescence of the carbon particles in the flame itself; and step
by step it was discovered that gas could be better employed merely as a
heating agent, to raise to incandescence substances having a higher
emissivity of light than carbon.

Dr. Auer von Welsbach found that the substances most suitable for
incandescent mantles were the oxides of certain rare metals, _thorium_,
and _cerium_. The mantle is made by dipping a cylinder of cotton net
into a solution of nitrate of thorium and cerium, containing 99 per
cent. of the former and 1 per cent. of the latter metal. When the fibres
are sufficiently soaked, the mantle is withdrawn, squeezed, and placed
on a mould to dry. It is next held over a Bunsen gas flame and the
cotton is burned away, while the nitrates are converted into oxides. The
mantle is now ready for use, but very brittle. So it has to undergo a
further dipping, in a solution of gun-cotton and alcohol, to render it
tough enough for packing. When it is required for use, it is suspended
over the burner by an asbestos thread woven across the top, a light is
applied to the bottom, and the collodion burned off, leaving nothing but
the heat-resisting oxides.

The burner used with a mantle is constructed on the Bunsen principle.
The gas is mixed, as it emerges from the jet, with sufficient air to
render its combustion perfect. All the carbon is burned, and the flame,
though almost invisible, is intensely hot. The mantle oxides convert the
heat energy of the flame into light energy. This is proved not only by
the intense whiteness of the mantle, but by the fact that the heat
issuing from the chimney of the burner is not nearly so great when the
mantle is in position as when it is absent.

The incandescent mantle is more extensively used every year. In Germany
90 per cent. of gas lighting is on the incandescent system, and in
England about 40 per cent. We may notice, as an interesting example of
the fluctuating fortunes of invention, that the once doomed gas-burner
has, thanks to Welsbach's mantle, in many instances rep