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. For
success of the experiment, it was necessary to avoid any combustion in
the furnace, and to wait until the furnace-air was as free from dust as
possible. Any flame in the furnace (even when it did not reach into the
line of sight), and the least quantity of dust in it, illuminated the
field of vision.

As a result of these experiments, Dr. Siemens considers that the view
hitherto held, that highly-heated gases are self-luminous, is not
correct. In the furnace were the products of the previous combustion
and atmospheric air: consequently oxygen, nitrogen, carbonic acid, and
aqueous vapor. If even one of these gases was self-luminous, the field
of vision must have been always illuminated. The weak light given by
the flame of burning gases that separate out no solid nor liquid
constituents cannot, therefore, be explained as a phenomenon of glow of
the gaseous products.