MR B. STEWART ON RADIANT HEAT. 11 
of heat radiated, being equal to the heat absorbed, will be very nearly as great as 
that from lamp-black. 
14. There are, therefore, two peculiarities of the radiation from plates of dia- 
thermanous substances, and which are most marked for those substances which 
are most diathermanous. 
1st, That the amount of radiation from such plates is less than that from 
lamp-black. 
2d, That the amount of radiation from such plates increases with the thick- 
ness of the plate. 
The correlation between these different properties of bodies is seen from the 
following table :— 
TABLE IV. 
Bodies ranked according to 
the proportion by which their 
Radiation is increased by in- 
creasing the thickness. 
Bodies ranked according to 
their Diathermancy (most 
diathermanous first). 
Bodies ranked according to their Radia- 
ting Capacity (least radiating first). 
° 
A stratum of heated gas (from 
Melloni’s Experiments), A stratum of gas. 
Rock-salt. Rock-salt. Rock- salt. 
Mica. Mica. Mica. 
Glass. Glass. Glass. 
Selenite. Selenite. 
Alum. Alum, 

15. The reason why radiation has hitherto been supposed to be confined to 
the surface, or to an exceedingly small distance below the surface of a body now 
becomes obvious. The effect of coating a surface of polished metal with gum, for 
instance, is to increase the radiation; but, after a very small thickness of film, 
an additional coating is powerless to increase the radiation; the reason being, 
not that radiation is incapable, in all cases, of taking place, except at the sur- 
face; but because, such films being exceedingly impervious to heat of low tempe- 
ratures, the radiation from them is very little increased by increasing their thick- 
ness. 
Since, therefore, it appears that radiation takes place from the interior as well 
as from the surface of bodies, the question arises, are we to suppose each particle 
of each substance to have, at a given temperature, an independent radiation of 
its own, equal, of course, in all directions? A priord, this is the most probable 
supposition, and it seems likewise to be conformable to experiment. 
