20 MR B. STEWART ON RADIANT HEAT. 
Philosophical Magazine for Feb. 1833) to be applicable; the intensity of radiation 
being therefore proportional to the sine of the angle which the direction makes 
with the surface. 
35. Let us now see, in conclusion, whether these investigations seem to point 
out any connection between internal radiation and conduction. 
Now, without in the least affirming that these are identical, there seem to 
be two points of similarity between them. 
1st, Since the heat which enters metals is all absorbed at a very small dep th, 
it follows that the flux of radiant heat from within upon the interior of a metallic 
surface is derived from a very small depth. 
Also, if we allow (what it has been endeavoured to prove, Art. 30) that the 
flux of heat upon the interior of a surface is proportional to the index of refrac- 
tion, this flux will be greatest in the case of metals, which may be supposed to 
have a very high refractive power; besides which, it will, as we have seen, be 
derived from a very small depth. The radiation of a metallic particle is there- 
fore very great. 
Now, if internal radiation be in any way connected with conduction, we might 
expect that good conducting substances should also be good internal radiators _ 
of heat, and we see they are so. 
2d, The second bond of similarity is this. It seems to be a law that sub- 
stances are almost invariably more diathermanous for heat of high temperature 
than for heat of low; consequently, at high temperatures, the radiation of a thin 
plate or particle of a substance will bear a smaller proportion to the total lamp- 
black radiation of that temperature than at low temperatures. The internal 
radiations of particles of bodies would therefore diminish at high temperatures 
(not absolutely, but with respect to the proportion which they would bear to the 
total radiation of these temperatures). If the same rule holds for metals, and 
conduction be connected with internal radiation, we should expect that at high 
temperatures the conducting power of metals would be less than at low tem- 
peratures. Now this has been proved to be the case by Professor Forses. 
9th March 1858. 
