200 Mr. Powell's experimental inquiry into the nature of 
proportion to the absorptive nature of texture of the surface, 
without respect to colour. These two characteristics are 
those which distinguish simple radiant heat at all intensities. 
Thus then when a body is heated at lower temperatures, 
it gives olF only radiant heat stopped intirely by the most 
transparent glass, and acting more on an absorptive white 
surface than on a smooth black one. 
At higher temperatures the body still continues to give 
out radiant heat, possessing exactly the same characters. 
But at a certain point it begins to give out light : precisely 
at this point it begins also to exercise another heating power 
distinct from the former ; a power which is capable of pass- 
ing directly through transparent screens, and which acts 
more on a smooth black surface than on an absorptive white 
one. 
(27.) This last sort of heat, whatever its nature may be, 
is essentially different from simple radiant heat. It appears 
to agree very closely with what the French philosophers 
term “ Calorique lumineux," and is, according to Professor 
Leslie's theory, a conversion of light into heat. These 
views of the subject are certainly gratuitous assumptions. 
We have no right whatever to identify those two agents, or 
to suppose that, because a heating effect very closely accom- 
panies the course of the rays of light, the light is therefore 
converted into heat ; but the theories above alluded to, seem 
to regard the whole heating effect of a luminous body as of 
this latter character. In this particular, the present inquiry 
has led us to an essential distinction ; and if the experiments 
are to be relied upon, this peculiar sort of heat constitutes 
only a part of the total effect. These results do not indeed 
