202 Mr. Powell on Solar Light and Heat. [March, 



Two other experiments in which the same coating was thicker, 

 gave 



Exp. 1. Exp. 2. 



Green 12 — 



Orange yellow 18 12 



Red 16 11 



(61.) That the heating effect produced within the limits ot 

 the visible spectrum is of the same kind as that produced by the 

 solar light in its ordinary state; that is to say, that it is trans- 

 missible through glass, and affects a black surface more than an 

 absorptive one, is, I conceive, sufficiently established by numcr 

 rous experiments. I have frequently interposed a plate of glass, 

 but without intercepting any perceptible portion of the effect 

 on the photometer. A coating of brown or white silk also inva- 

 riably gave a much less effect than Indian ink, or a surface of 

 black glass. 



These results seem to me decisive against the hypothesis of a 

 superposition of two spectra, one of luminous, and the other of 

 calorific rays. 



(62.) It is obvious that the greater heating power displayed 

 by the rays towards the red end of the spectrum, may be owing 

 to either of the following causes, or to both jointly. 



1. A greater intrinsic power of communicating heat. 



2. A greater number of particles brought into action, or 

 absorbed. 



And this last cause may depend either upon the peculiar state 

 of division to which the rays may be reduced, or upon a greater 

 power of absorption in the surface for these than for other 

 coloured rays, or here again both causes may co-operate. 



With respect to the state of diffusion of the rays, it is obvious 

 that the red rays are more concentrated than the yellow, and 

 these more than the blue, 8cc.; so that from tliis cause alone we 

 might expect a greater heating effect ; a greater number of par- 

 ticles acting in the same space. 



With respect to a possible increase of absorptive power in 

 respect to the greater approach to the character of the extreme 

 red light, I am not aware that we at present possess any results 

 which can assist such an inquiry, unless we except the view 

 maintained by Mr. Morgan in his experiments on the light from 

 combustion, Phil. Trans. 178.3, No. 11. He considers light as 

 matter united to other bodies by attraction, blue rays having the 

 least, and red the greatest affinity. If this view of the subject 

 be admitted, cculeris paribus, more red particles would be 

 absorbed than of any other coloured ray when impinging upon a 

 surface. 



(63.) A notice has very recently appeared (see Annals of Phi- 

 losophy, Sept. 1824, p. 236) of some prismatic experiments by 



