MR B. STEWART ON RADIANT HEAT. 15 
' passed perpendicularly, or nearly so, through such plates; and, 2d, because the 
indexes of refraction for the substances experimented on were not very high. 
But for rays passing obliquely through such media, or for rays passing in any 
direction into substances, such as metals, we must take account of reflection from 
the surface, which will influence materially our results. 
Thus, no substance is so opaque for heat as metals, but yet only a small por- 
tion of the heat falling on them is absorbed, the rest being reflected back; conse- 
quently for such bodies the radiation (which must be equal to the absorption) 
is very small. 
It is also desirable, for another reason, to investigate the laws according to 
which the reflective nature of the surface of a body influences its radiation. For the 
question arises, Is the law of an equal and independent radiation of each particle of 
a body theoretically consistent with equilibrium of temperature? That is, suppose 
we have any irregularly-shaped inclosure walled round with a variety of substances, 
and each particle of each substance radiating into the inclosure, from the sides of 
which it is reflected many times backwards and forwards before it is finally ab- 
sorbed,—this being the case, will the law of equal and independent radiation, and 
those of reflection and refraction, so fit with one another, that every particle of the 
walls of the inclosure shall absorb precisely as much heat as it radiates? It will 
be endeavoured to show that these laws are so adapted to each other; and I shall 
select for the proof a definite form and description of inclosure, the conclusions 
arrived at rendering it highly probable (if not rigidly demonstrating) that the same 
adaptation will hold good for every inclosure, however irregular or varied. 
For those reasons, I shall now endeavour to investigate what connection the 
radiation of a substance has with the reflective power of its surface; and in doing 
so (in order to abstract entirely from the effects produced by the variable thick- 
ness of the radiating plate), I shall suppose it to be of indefinite thickness; so 
that all the heat which enters it is absorbed. Our consideration is, therefore, 
limited to the effects of one surface. 
25. Let AB be a portion of the line of section . ¥ 
of an indefinitely extended surface with the plane V/ LLB V/ 
of the paper supposed perpendicular to the sur- 
face, and let this surface belong to a body (M) 
of indefinite thickness downwards; also let \ 
there be an indefinitely extended surface of ~~ : A ie 
lamp-black parallel to this lower surface, as M \ 
in the figure. Lastly, let the whole be kept at } 
a uniform temperature. In order that the body - 
(M) may be maintained at this temperature, it is necessary that the heat which 
has left the surface AB, having come from the interior of (M), in the directions 
contained in any very small angle CAD, shall be replaced by an equal quantity of 

