REPORT ON RADIANT HEAT. 
295 
those of heat and light; the media we commonly use may ab¬ 
sorb them copiously, whilst others may be found which may 
transmit them more abundantly. 
Similarly with the magnetizing rays. And thus we may ac¬ 
count for the contradictory results hitherto obtained on this 
point, by supposing that some ingredient rendered one prism 
absorptive of these rays, and another not so. 
6‘thly, Sir D. Brewster extends these views to the analogies 
between solar and terrestrial heat. 
He considers those rays of the solar spectrum just mentioned, 
which undergo little refraction, to be analogous to those thrown 
off by bodies slightly heated. The w r aves of heat are broad 
and slow in their motion ; as the temperature is raised they are 
thrown off with more velocity, and become smaller and suf¬ 
fer a greater refraction. When the velocity is such as to give 
them a refraction equal to that of the red rays, then red light 
is produced ; and successively the other colours are added, till 
at a very high temperature white light is radiated. 
He proposes to examine what transparent body transmits 
most heat, and by converting it into a lens, expects to find a 
series of foci at different distances, beginning from that of the 
violet rays to that of those corresponding to rays of very little 
refrangibility. 
7thly, He applies these views as affording an explanation of 
De la Roche’s result before mentioned, viz. that a second 
screen intercepts a much smaller proportion of the heat, after 
passing a first, than the first did of the whole effect: this De 
la Roche ascribed to something analogous to polarization. 
On the principle just stated, the explanation is very simple. 
The first plate intercepts those rays which it has a tendency to 
absorb, and transmits the rest: the second, being of the same 
kind, of course will transmit these with scarcely any further 
diminution. 
He observes, that thick masses of colourless fluid or of glass 
transmit scarcely any radiant heat in a way analogous to that 
in which thick masses of coloured glass are opake to all rays of 
light. 
He conceives that substances may be found which are opake 
to light and yet transparent to heat. These should be carefully 
sought for, as they would be of great practical value. Red glass, 
for example, which scarcely transmits any light or 1 ray in 
2000, transmits all the invisible rays of Herschel, 692 of the 
1000 red rays, 606 rays out of 1000 of solar heat, and 660 of 
“ culinary ” heat, according to Sir W. Herschel. We may expect 
therefore to find an opake metallic glass, or thin plate of metal, 
