REPORT ON RADIANT HEAT. 
279 
creasing intensities more of it is enabled to pass in the way of 
direct radiation. 
In order to establish this theory, it would be necessary to 
show that whatever may be the particular law of relation to the 
surfaces of bodies by which the action of the “ igneous fluid” 
is determined at any stage of its evolution, the portion trans¬ 
mitted by a screen should act upon any two given surfaces in 
precisely the same ratio as the part intercepted, or as the whole. 
Such a ratio will obviously differ at different stages of incan¬ 
descence or inflammation ; but at the same stage it ought to be 
found exactly the same—only diminished in the actual magni¬ 
tude of its terms when the glass screen is interposed,—as when 
there is none. 
But no such experimental proof had been offered by any of 
the experimenters before named. It was obviously called for to 
support or refute their theory, and was capable of being easily 
supplied by experiment. That the conclusion is not a necessary 
one, will be evident by merely observing that the phenomena 
may just as well be explained 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 
stopped by the screen, exactly as in the case of a nonluminous 
hot body. 
To ascertain by experiment which of these suppositions was 
the true one, was the object of an inquiry which I communi¬ 
cated to the Royal Society, and which is published in the Phil. 
Trans . 1 825, Part I. p. 187. I also gave an abstract of the results, 
accompanied by other illustrative remarks, and some theoretical 
views in a paper in the Quarterly Journal of Science,IS o. XIX. 
p. 45. Some remarks also on the experiments are made in the 
Edinb. Journ. of Science, N.S. No. VI. p. 304. 
These experiments combine the examination of the effect of 
screens with those of surfaces. It is assumed, on the autho¬ 
rity of previous experiments, that simple heat affects a thermo¬ 
meter in proportion to the absorptive nature of its surface: for 
example, a surface washed with a paste of chalk is rather more 
absorptive than one coated with Indian ink ; and this kind of 
heat is stopjied by transparent screens of ordinary thickness. 
It would seem from some experiments already mentioned, that 
from luminous hot bodies the effect is greater in reference to 
the darkness of colour of the surface, and is transmitted through 
glass. But when a body is heated to luminosity, how does 
this change in its properties take place ? Are its relations 
gradually altered in themselves ? or are there two sorts of heating 
