REPORT ON RADIANT HEAT. 29 
avoiding secondary radiation, &c., employing piles of split 
mica, and throwing parallel rays on them by means of a rock- 
salt lens, having its principal focus at the source of heat. 
He then enters upon the details of his results, in several 
series, with piles of different numbers of lamine, and at dif- 
ferent inclinations to the axis (the source of heat being a lamp), 
giving in each case the calorific transmissions in the rectangular 
positions, or proportions of heat polarized. These are com- 
prised in a series of eight tables, from which the author derives 
the following conclusions :— 
I. The proportion of heat polarized increases as the inclina- 
tion of the piles is diminished. 
II. It attains a maximum at a certain inclination. 
III. This inclination is greater as the number of lamine is 
increased. 
He points out the close agreement of these results with the 
phenomena of light according to Brewster and Biot. 
(4.) The author pursues a further series of experiments on 
polarization by reflexion, and arrives at the conclusion that the 
angle of complete polarization by refiexion is very nearly the 
same for light and for heat. 
(5.) If any diathermanous substance be interposed between 
the luminous source and the piles, the index of polarization 
does not vary with the substance employed. 
This, he contends, proves that the nature of the heat does 
not alter its polarizability. 
But also from direct experiment with the radiations from 
different sources, he makes the same inference, employing, in- 
stead of a lamp, incandescent platina, metal heated to 400°, or 
boiling water, with the same results of uniform polarizability. 
He maintains that the difference of polarizability by refrac- 
tion, arising from the different refrangibility of the rays of heat, 
is too minute to be sensible. 
And for all experiments on obscure heat he proposes to sub- 
stitute as the source a black glass heated by flame. 
(6.) On the depolarization he refers to Forbes’s experiments, in 
which he contends the difference in the rectangular position is 
very small, but nearly equal with different sources. 
He repeats the experiment, with black glass interposed, and 
finds the effects much greater, and nearly equal in the different 
cases. 
He endeavours to explain Forbes’s result of the difference 
with different sources, by secondary radiation. 
Further, by the same method (of interposing a black glass), 
he finds the equal depolarizability of every kind of heat. 
