neutralize, by permitting rays from another source to fall upon the
opposite face of the pile. The needle is thus brought to zero. Cutting
off the light by our ray-filter, and exciting the magnet, the needle
is instantly deflected, proving that the magnet has opened a door for
the heat, exactly as in Faraday's experiment it opened a door for the
light. Thus, in every case brought under our notice, the substantial
identity of light and radiant heat has been demonstrated.

By the refined experiments of Knoblauch, who worked long and
successfully at this question, the double refraction of heat, by
Iceland spar, was first demonstrated; but, though he employed the
luminous heat of the sun, the observed deflections were exceedingly
small. So, likewise, those eminent investigators De la Povostaye and
Desains succeeded in magnetizing a beam of heat; but though, in their
case also, the luminous solar heat was employed, the deflection
obtained did not amount to more than two or three degrees. With
_obscure_ radiant heat the effect, prior to the experiments now
brought before you, had not been obtained; but, with the arrangement
here described, we obtain deflections from purely invisible heat,
equal to 150 of the lower degrees of the galvanometer.

Sec. 11. _Distribution of Heat in the Electric Spectrum_.

We have finally to determine the position and magnitude of the
invisible radiation which produces these results. For this purpose we
employ a particular form of the thermo-pile. Its face is a rectangle,
which by movable side-pieces can be rendered as narrow as desirable.
Throwing a small and concentrated spectrum upon a screen, by means of
an endless screw we move the rectangular pile through the entire
spectrum, and determine in succession the thermal power of all its
colours.

[Illustration: SPECTRUM OF ELECTRIC LIGHT.]

When this instrument is brought to the violet end of the spectrum,
the heat is found to be almost insensible. As the pile gradually moves
from the violet towards the red, it encounters a gradually augmenting
heat. The red itself possesses the highest heating power of all the
colours of the spectrum. Pushing the pile into the dark space beyond
the red, the heat rises suddenly in intensity, and at some distance
beyond the red it attains a maximum. From this point the heat falls
somewhat more rapidly than it rose, and afterwards gradually fades
away.

Drawing a horizontal line to represent the length of