28 REPORT—1840. 
In the fourth section the law of polarization by reflexion is 
discussed. A number of reflecting surfaces were tried, and 
split mica was preferred. The amount of polarization by re- 
flexion at a given angle, is shown to vary with the source of 
heat ; and it is probable that the kinds of heat do not rank in 
the same order when the angle is changed. This is the case 
with light. The change of the plane of polarization by subse- 
quent reflexion, is similar to that which occurs when light is 
used. 
The circular polarization of heat by total internal reflexion, 
is discussed in the fifth section. This, as before remarked, is 
a phanomenon really produced in the experiments on dipolari- 
zation, if the mica be of a suitable thickness. The direct ex- 
periment with rhombs of rock salt, has been already mentioned 
also. The author here gives a detailed account of them, and . 
the laws of the phenomena deducible, in which the precise 
analogy with those of light is preserved. 
Equal Polarizability of Heat from different sources : 
Melloni. 
Melloni’s second memoir on the Polarization of Heat appears 
to be founded on the second part of his communication to the 
Royal Academy of Sciences in January, 1836. It is printed in 
the Ann. de Chim. lxv. May, 1837, and the translation in Tay- 
lor’s Scientific Memoirs, Part VI. 
The principal points of these extensive researches may be 
reduced to the following heads :— 
(1.) Referring to Prof. Forbes’s Researches, first series, Mel- 
loni. contends that the differences of polarizability in the heat 
from different sources there exhibited, are in fact due to differ- 
ences of secondary radiation from the heating of the mica piles, 
and subsequently appeals to Forbes’s second series, in which he 
conceives the approach to equality is much nearer, as this 
source of error was more avoided. 
At lower temperatures of the source, he observes, that mica _ 
transmits less heat in proportion, and therefore absorbs more : 
thus the secondary radiation is greater, and the apparent dif- 
ference in the two positions, or index of polarization, is less. 
(2.) He remarks, that Prof. Forbes had found the heat from a 
dark source, after transmission through glass, to become as po- 
larizable as that from incandescent platina; whereas he considers 
that the glass plate absorbed the greater part of those rays 
which otherwise would have heated the piles, and that thus the 
apparent polarization was increased. 
(3.) Melloni describes his apparatus, and the precautions for 
