from Terrestrial Sources, 47 



The experiments wliicli I tried, were of a nature calculated to 

 give an answer to this question, whatever result they should pre- 

 sent. If they shewed tlie same relation maintained under the 

 two different conditions, the theory of one agent would be estab- 

 lished. If a different relation appeared, we must, of necessity, 

 infer two distinct radiations. 



In order to proceed with the following remarks, I mu§t, gf 

 necessity, assume tlie result of these former experiments a« esta* 

 blished ; but as it would be improper here to detail the contents pf 

 that paper, I shall n^erely state the general conqlusion) which is, 

 indeed, already before the public. 



This conclusion, in fact, simply determines the question just 

 proposed, in favour of two distinct radiant agents^ or species of 

 lieating effect, which act jointly in the emanation from luminous 

 hot bodies : of these, one is simple radiant heat, resembling in all 

 its properties that from non-luminous bodies ; that is, stopped by 

 ^lassj and having relation to the texture^ not the colour^ of surfaces 

 on which it acts : the other, a sort of effect associated in the closest 

 manner with the rays of light, passing with them through glass^ 

 and affecting bodies in proportion to their darkness of colour, 

 without respect to the texture of their surfaces. This last, for 

 the sake of distinction, I call, *' the heating power of light." 



2. I may here premise a short account of one description of ex- 

 periments, which I at first employed, in resolving this question, 

 and which is not given in the paper alluded to. 



These experiments were conducted upon the following principle, 

 which, though very simple in theory, is in practice attended by 

 eeveral inconveniences, which, if not carefully guarded against, 

 may lead to error. 



A differential thermometer, having one bulb coated with smooth 

 black, and the other with absorptive white, was exposed to the 

 radiation from luminous hot bodies, having both bulbs at an equal 

 distance from the body, first with, and then without the inter- 

 position of a glass screen. The arrangement is represented in 

 figs. 1 and 2 of the annexed sketch. (Plate II.) 



If the screen had no heating or cooling eflfect, it is evident that 



