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.
It appealed to the author probable, that heated gases did not, either,
emit heat rays; and he set himself to test this idea, experimenting, in
company with Herr Froehlich, in Dresden. They first convinced themselves
in this case that the light emission of pure heated gases sunk to zero,
even when the field of vision was not always quite dark, and it was
only possible to observe this a short time; but the repeatedly observed
perfect darkness of the field of vision was demonstrative.
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