the radiant heating effects from terrestrial sources. 189 
be explained without it ; for we may just as \vell account for 
the facts, by supposing two distinct heating influences, one 
associated in some very close way with the rays of light, 
carried as it were by them through a glass screen without 
heating it ; the other being merely simple radiant heat, 
affected by the screen exactly as the radiant heat from a non- 
luminous body. 
(5.) In order to ascertain which of these suppositions is 
true, it will not be sufficient to observe the effects produced 
by the intervention of a screen alone. We must combine this 
method with an examination of the relations of different sorts 
of heat to surfaces. These relations have been shown to dif- 
fer according as the body is luminous, or not ; in the one 
case, the direct heat affects bodies in proportion to the dark- 
ness of their colour, without regard to the texture of their 
surface : in the other, the magnitude of the effect depends 
solely on the absorptive texture without reference to colour. I 
use the term “ absorptive texture,'" to signify that peculiar 
state of division in the particles of the surface, which has 
been shown, by Professor Leslie and others to be most sus- 
ceptible of the influence of simple radiant heat, and always 
to give a proportionally greater radiating power. 
The question then is entirely one of facts ; and involves no 
hypothesis as to the nature either of light or of heat. The 
object is simply to ascertain by experiment, whether, of the 
total heating effect radiated from a luminous hot body, the 
portion intercepted by a transparent screen is of the same 
nature as, or different from the part transmitted in its rela- 
tions to the surfaces on which it acts. 
(6.) In conformity with this view of the object proposed, 
