on Radiant Heat. 
285 
6, The theory which we have embraced will enable us to ex- 
plain, in the most satisfactory manner, the striking difference 
between the quantities of caloric radiated from different surfaces. 
If all substances possessed the same attraction for caloric, then 
would the flow of heat from the surfaces of equal bodies, of the 
same temperature, be proportional to the number of calorific 
atoms ranged along the surface of each body. But since the 
attraction of different substances for caloric varies with the na- 
ture and density of the body, it follows that the quantity of heat 
radiated from the surfaces of different bodies will not be simply 
as their capacities for caloric, or as the specific heat of those bo- 
dies. The attraction of any particular substance for caloric will 
act as an antagonist-force to the idio-repulsive energy of the 
molecules of caloric, and, consequently, the greater this attrac- 
tion is, the less will be the heat which radiates from the surface 
of the body. From a comparison of various experiments which 
I have performed with regard to the attraction, capacity, and 
radiating powers of different substances, the following law seems 
to obtain : 64 That the quantity of radiant heat from the surface 
of a body, is directly as its capacity for caloric, and inversely as 
its attraction.” Thus, the metals, having a powerful attraction 
for caloric, whilst their capacities for that fluid are small, allow 
only a small quantity of radiant heat to escape ; whilst glass, 
lamp-black, and paper, having a feeble attraction for caloric, 
with considerable capacities, allow caloric to radiate very copi- 
ously from their surfaces. 
7. If the condensation of caloric in a given body increase the 
idio-repulsive force of the molecules (as happens to every other 
clastic body), then will caloric, emanating from a body at an 
elevated temperature, move with a greater velocity than from 
the same body at a lower temperature. Let us examine how far 
this deduction agrees with experiment. The molecules of caloric, 
in passing through glass, must suffer a considerable resistance 
from the attraction of the glass. Now, when the velocity is 
small, the resistance may be such as completely to stop the 
whole current of heat. If the velocity, however, be increased, 
by raising the temperature of the body, then many of the atoms 
of caloric will overcome the resistance, and find their way direct- 
ly through the glass ; whilst others, impinging against the solid 
