RADIANT HEAT. 315 



In order to establish this theory, it would be necessary to show 

 that whatever maybe 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 transmitted 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 incandescence or inflammation; but at the same 

 stage it ought to be found exactly the same — only diminished in the 

 actual magnitude 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 teing 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 non-luminous hot body. 



To ascertain by experiment which of these suppositions was the 

 true one, was the object of an inquiry which I communicated to the 

 Royal Society, and which is published in the Phil. Trans., 1825, 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, No. XIX, p. 45. Some remarks 

 also on the experiments are made^n 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 authority of previous 

 experiments, that simple heat affects a thermometer 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 stopped by transparent screens of 

 ordinary thickness. It would seem, from some experiments already 

 mentioned, that from luminous hot bodies the effect is greater in ref- 

 erence to the darkness of color 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 tioo sorts of heating effect emanat- 

 ing from it at the same time ? These are the questions which my 

 experiments were directed to answer, and the mode of trying the 

 point is extremely simple; it is only to ascertain whether of the total 

 heating effect 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 relation to the surfaces on which it acts. 



The experiments were conducted simply by having two thermome- 

 ters, one coated with smooth black, the other with absorptive white, 

 observing the ratio of the effects when they were exposed together 



