REPORT ON RADIANT HEAT. St 
printed in the London and Edinburgh Journal of Science, 
vol. xiii. 
In the first section the author discusses the variable polariza- 
bility of the different kinds of heat. The establishment of this 
fact was his object in one portion of his second memoir. But 
these investigations having been objected to by some, and oppo- 
site results (as we have seen) obtained by Melloni, the author 
now repeated the inquiry with every precaution. He rendered 
‘the rays parallel by a rock-salt lens, as Melloni had done, and 
operated at a sufficient distance from the pile: still the differ- 
ences in the rectangular positions, when different sorts of heat 
were employed, were as unequal as formerly. - 
Having varied the experiments in every possible way, he still 
comes to the same conclusion as before, and gives the following 
results :— 
Source of Heat. Rays out of 100 Polarized. 
PERL LAD, . 5... gieeloe st» “marittn Mie dig 
PEO 1 eee ean Bey 5% 7 be Wy f 
Incandescent platina . Se ee Ge tO. 16 
—_—_———. with glass screen 80 to 82 
Weledhel hamelres tind, oo ¢ sale be ri7S 
RCE CU ce nw sn es eS 
Do. with mica screen mG? Al. ph. 6:SO 
Mercury in crucible at 450° . . . 48 
PROMI WERREM A Teh che) lias (fel 44 
Melloni’s opposite result of apparent uniform polarizability, 
the author then shows must necessarily arise from the use of 
mica piles, consisting of a number of distinct plates super- 
posed. Such a thickness of mica modifies heat from dark 
sources in such a way as to give the portion which it trans- 
mits the same character as to polarizability as luminous heat. 
Whereas Mr. Forbes’s results were obtained by the use of mica 
split by heat (as before described), which includes so many sur- 
faces within a very small thickness, that the polarized heat is 
comparatively unaltered in its character. He shows directly, 
that these piles transmit heat from a lamp sifted by glass, and 
from brass at 700°, in nearly equal proportions, while mica 
016 inch thick transmits five times less of the latter than of 
the former. 
The second section relates to the dipolarization of heat. 
Pursuing the methods given in the first series, the author ascer- 
tained the proportion of heat dipolarized by five different thick- 
hesses of mica. From the numerical results thus obtained, he 
deduces the value of the expression in Fresnel’s formula, for 
