To render our series of comparisons complete, we must demonstrate the magnetization of heat.  But here a slight modification of our arrangement will be necessary.  In repeating Faraday’s experiment on the magnetization of light, we had, in the first instance, our Nicols crossed and the field rendered dark, a flash of light appearing upon the screen when the magnet was excited.  Now the quantity of light transmitted in this case is really very small, its effect being rendered striking through contrast with the preceding darkness.  When we so place the Nicols that their principal sections enclose an angle of 45 deg., the excitement of the magnet causes a far greater positive augmentation of the light, though the augmentation is not so well seen through lack of contrast, because here, at starting, the field is illuminated.

In trying to magnetize our beam of heat, we will adopt this arrangement.  Here, however, at the outset, a considerable amount of heat falls upon one face of the pile.  This it is necessary to 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.]