me conditions.

Sec. 9. _Double Refraction of Heat_.

I will now abandon the Nicols, and send through the piece of Iceland
spar (B, fig. 53), already employed (in Lecture III.) to illustrate
the double refraction of light, our sifted beam of invisible heat. To
determine the positions of the two images, let us first operate upon
the luminous beam. Marking the places of the light-images, we
introduce between N and L our ray-filter (not in the figure) and
quench the light. Causing the pile to approach one of the marked
places, the needle remains unmoved until the place has been attained;
here the pile at once detects the heat. Pushing the pile across the
interval separating the two marks, the needle first falls to 0 deg., and
then rises again to 90 deg. in the second position. This proves the double
refraction of the heat.

[Illustration: Fig. 53.]

I now turn the Iceland spar: the needle remains fixed; there is no
alteration of the deflection. Passing the pile rapidly across to the
other mark, the deflection is maintained. Once more I turn the spar,
but now the needle falls to 0 deg., rising, however, again to 90 deg. after a
rotation of 360 deg.. We know that in the case of light the extraordinary
beam rotates round the ordinary one; and we have here been operating
on the extraordinary heat-beam, which, as regards double refraction,
behaves exactly like a beam of light.

Sec. 10. _Magnetization of Heat_.

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