14 MR B. STEWART ON RADIANT HEAT. 
ticles between B and C would therefore be different from that absorbed by the 
particles between A and B. But this cannot be; for, on the hypothesis of an 
equal and independent radiation of each particle, the radiation of the particles 
between B and C is equal to that of the particles between A and B, and their 
absorption equals their radiation. Hence the radiation impinging on B, in the 
direction of DB, must be equal in quality as well as quantity to that impinging 
upon A; and, consequently, the radiation of the particles between A and B must 
be equal to their absorption, as regards quality as well as quantity; that is, this 
equality between the radiation and absorption must hold for every individual de- 
scription of heat. 
21. The following experiment illustrates this law :— 
The quantity of heat radiated from crown-glass ‘05 inch thick, which passes through 
a crown-glass screen, ‘05 inch thick, = 0-95 
While that from plate-glass °3 inch thick, covered with blackened paper (the 
blackened paper being next the pile), which passes through the same screen, 1°95 
But, if the surface of crown-glass ‘05 inch thick, farthest from the pile be 
coated with paper, the polished surface being next the pile, then the amount 
of radiation which passes the screen, . = 1°85 
And if three plates, the one behind the other, ‘of ecommegliase, each -05 inch 
thick, be used as the source of heat, the surface farthest from the pile of the 
farthest off plate only being covered with paper, the amount of radiation 
which passes the screen, . : t . : ‘ : = ee 
Such a plate of glass, or series of plates, therefore, by having the farthest off 
surface coated with paper, gives out heat similar to that from paper or lamp- 
black; the reason being, that the heat from the paper on the farthest off surface 
is as much recruited as it is absorbed by its passage through the glass, both as 
regards quantity and quality; so that the radiation which falls upon the cone is 
virtually that from paper or lamp-black. 
22. There is little difficulty in explaining why heat from a thick plate of any 
substance should pass more readily through a screen of the same substance than 
that from a thin plate. The reason is, that the heat from the interior of the 
thick substance, having been sifted in its passage, is, therefore, now more easily 
able to pass through a screen of the same substance. 
23. We see also why, generally speaking, bodies at the same temperature ra- 
diate the same quality of heat; let us, for instance, take a tolerably thick plate 
of glass, and a surface of lamp-black, and compare them together. Since the 
plate of glass absorbs nearly all the rays that fall upon it, it will radiate nearly 
as much as lamp-black ; and since the quality of the radiated is the same as the 
quality of the absorbed heat, its radiated heat will very nearly have the same 
quality as that which is radiated by lamp-black. 
The Influence of the Reflective and Refractive Powers of Bodies on theirRadiation considered. 
24. Hitherto, in these investigations, no account has been taken of reflection 
at the surfaces of the plates, because—1st, those rays only were considered which 
