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. On the other
hand, experiments made with sensitive thermopiles, in order to settle
the question of emission of heat-rays from highly-heated gases, failed.

Afterward, however, Dr. Siemens was convinced, by a quite simple
experiment of a different kind, that his supposition was erroneous. An
ordinary lamp, with circular wick, and short glass cylinder, was wholly
screened with a board, and a thermopile was so placed that its axis lay
somewhat higher than the edge of the board. As the room-walls had pretty
much a uniform temperature, the deflection of the galvanometer was but
slight, when the tube-axis of the thermopile was directed anywhere
outside of the hot-air current rising from the flame. When, however, the
axis was directed to this current, a deflection occurred, which was as
great as that from the luminous flame itself. That the heat radiation
from hot gases is but very small in comparison with that from equally
hot solid bodies, was shown by the large deflection produced when a
piece of fine wire was held in the hot-air current. On the other hand,
however, it was far too considerable to admit of being attributed to
dust particles suspended in the air current.

It must be conceded to be possible (the author says) that the light
radiation of hot gases, as also the heat radiation, is only exceedingly
weak, and therefore may escape observation. It is, therefore, much to