KNOBLAUCH ON RADIANT HEAT. 423 



of the flame of alcohol in a still less degree, and that of the dark 

 cyUnder worst, we find, with regard to the above considerations 

 (p. 416 and 417), that all these phaenomena confirm the position. 

 That the changes experienced by heat on diffuse reflexion are 

 merely the result of a selective absorption of the reflecting sur- 

 faces for certain rays of heat transmitted to them"^. 



The great uniformity existing in the results which have been 

 detailed will certainly contribute to establish the view which I 

 endeavoured to found at the commencement, viz. that these re- 

 sults really depend upon diffusely reflected heat, uninterfered 

 with in any perceptible manner by foreign influences. (Compare 

 p. 385-387.) 



In the last investigation, the complementary selective absorp- 

 tion was determined to exist from the unequal intensity of the 

 reflexion of the heat. That it might have been equally well 

 proved by the surfaces exposed to the sources of heat becoming 

 heated, is evident from the observations on carmine and black 

 paper, the former of which, as shown by a previous experiment 

 (p. 206), becomes less heated when exposed to the rays of the 

 Argand lamp than those of the dark cylinder, and the latter less 

 by the heat of the cylinder than the rays of the flame ; whilst 

 the later experiments (p. 420 and 421) show that carmine reflected 

 the heat of the flame with greater intensity than that of the 

 cylinder, whilst black paper reflected the rays of the latter in a 

 greater degree than those of the lamp. 



The reason why in this case the experiments by means of 

 reflexion were preferred to those by absorption of heat, was, that 

 they were not only more quickly performed, but, as seen on 

 comparison of the indications, p. 206, and pp. 420 421, afforded 

 a more delicate test-method than the latter. 



It might be concluded, even from the phaenomena of absorp- 

 tion (p. 206 and 207), that the diffusion of heat is inde2}etident of 

 the temperature of its source. The direct investigation of the 

 latter has confirmed this, and thus removed all doubt on the point. 



* From what has heen stated, it is clear that we can judge of the degree in 

 which a body absorbs certain rays of heat, from the deportment of the heat dif- 

 fusely reflected by it under certain circumstances. It is well known (Fusinieri, 

 Annali delle Scienze del regno Lomhardo-Veitelo, 1838, Gen. et Febr., p, 38; 

 and Melloni, Coinptes Rendus, t. vi. p. 801) that «now exposed to the rays of 

 the sun melts more rapidly on trees and bushes than on a uniform surface. 



If any philosopher should compare those rays of the sun reflected by snow 

 with the direct rays, he would find the former transmitted comparatively better 

 by red glass, blue glass, alum, calcareous spar and gypsum than those unrefiected. 



