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end toward the red, heat soon manifests itself, augmenting
as we approach the red. Of all the colors of the visible.
spectrum the red possesses the highest heating power. On
pushing the pile into the dark region beyond the red, the
heat, instead of vanishing, rises suddenly and enormously
in intensity, until at some distance beyond the red it
attains a maximum. Moving the pile still forward, the
thermal power falls, somewhat more rapidly than it rose.
It then gradually shades away, but for a distance beyond
the red greater than the length of the whole visible spec-
trum, signs of heat may be detected. Drawing a datum
line, and erecting along it perpendiculars, proportional in
length to the thermal intensity at the respective points, we
obtain the extraordinary curve, shown on the adjacent page,
which exhibits the distribution of heat in the spectrum of
the electric light. In the region of dark rays, beyond the
red, the curve shoots up to B, in a steep and massive peak
—-—--a kind of Matterhorn of heat, which dwarfs the portion
of the diagram C D E, representing the luminous radiation.
Indeed, the idea forced upon the mind by this diagram is
that the light-rays are a mere insignificant appendage to
the heat-rays represented by the area A B C D, thrown in
as it were by Nature for the purposes of vision.

The diagram drawn by Professor Muller to represent
the distribution of heat in the solar spectrum is not by any
means so striking as that just described, and the reason,
doubtless, is that prior to reaching the earth the solar rays
have to traverse our atmosphere. By the aqueous vapor
there difi‘used, the summit of the peak representing the
sun’s invisible radiation is cut off. A similar lowering of
the mountain of invisible heat is observed when the rays
from the electric light are permitted to pass through a film
of water, which acts upon them as the atmospheric vapor
acts upon the rays of the sun.

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