Now, it is common to say,
merely, in explanation of this luminosity, that the gas highly heated in
combustion is self-incandescent. This explanation, however, has not been
experimentally confirmed. Dr Werner Siemens was, therefore, led recently
to investigate whether highly-heated pure gases really emit light.
The temperature employed in such experiments should, to be decisive,
be higher than those produced by luminous combustion. The author had
recourse to the regenerative furnace used by his brother, Friedrich, in
Dresden, in manufacture of hard glass. This stands in a separate room
which at night can be made perfectly dark. The furnace has, in the
middle of its longer sides, two opposite apertures, allowing free vision
through. It can be easily heated to the melting temperature of steel,
which is between 1,500 deg. and 2,000 deg. C. Before the furnace apertures were
placed a series of smoke blackened screens with central openings, which
enabled one to look through without receiving, on the eye, rays from the
furnace walls. If, now, all air exchange was prevented in the furnace,
and all light excluded from the room, it was found that not the least
light came to the eye from the highly-heated air in the furnace.
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