12 MR B. STEWART ON RADIANT HEAT. 
In an experiment already described, 
A plate of crown-glass ‘05 inch in thickness being used as a screen, the quantity of heat 
radiated from crown-glass ‘05 inch thick that passed, was : 2 0:95 
While of that radiated from crown-glass, ‘10 inch thick, there passed - 2 1:45 
Another experiment gave— 
Quantity of heat from crown-glass ‘05 that passed, . ‘ La 
Quantity radiated from two plates of crown-glass, each - 05 inch thick, “the one placed 
loosely behind the other, . : : : : 4 : : : : 1°55 
From this we may infer, that the radiation from two plates of glass placed 
loosely behind each other, is the same as the radiation from a plate of double the 
thickness, and, consequently, that the radiation from a particle of a substance 
does not diminish, owing to its being placed in the interior.* 
17. Let us now refer to the radiations from rock-salt :— 
The radiation from a piece ‘18 inch thick, was. : : 3-4 
That from a piece ‘36 inch thick, was. : : 4°3 
That from a piece *77 inch thick, was. : 4 5:3 
Now, if we suppose the radiation of a particle in the interior to be as intense 
as that of a particle at the surface, why, it may be asked (since rock-salt is ex- 
tremely diathermanous), does not a piece of double the thickness give nearly a 
double radiation, and so on, the radiation increasing very nearly as the thickness? 
If we still hold the doctrine of an equal and independent radiation from every 
particle, we are shut up to the conclusion that rock-salt must be comparatively 
opaque to heat radiated by itself,—a result which is abundantly confirmed by 
experiment. 
Thus, while the radiation from rock-salt -18 inch thick, without any screen, 
is 3°4, with a screen of rock-salt -18 inch thick it becomes 1:1. 
If, therefore, we have a piece of rock-salt of double this thickness, or ‘36 inch 
thick, we should expect that the radiation from it would be =3:-4+1]:1=4°5. It 
is, in fact, 4:3. The difference (0°2) being within the limit of error of observation. 
In rock-salt, therefore, we may suppose each particle to have an independent 
radiation of its own, unaffected by its distance from the surface. 
18. We see, therefore, that the opacity of rock-salt with regard to heat radia- 
ted by itself, is a consequence of the admission, that the radiation from rock-salt 
does not increase so rapidly as the thickness increases; and this again results 
from the fact, that the absorption of heat by a plate of rock-salt does not in- 
crease so rapidly as the thickness increases. This, again, is due to the fact, that 
the first part of the plate of rock-salt sifts the heat so that it is more easily trans- 
mitted by the second part; and this confirms the results arrived at by Professor 
Forses, who, finding that rock-salt stopped heat of low temperature rather more 
* The idea of this experiment was derived from a remark of Professor Forpes, who suggested 
that several plates of rock-salt, the one behind the other, might be advantageously substituted for a 
thick plate of the same material, as giving the very same result. 
