424 KNOBLAUCH ON RADIANT HEAT. 



It has moreover shown most convincingly, that, excepting char- 

 coal and metals, it cannot be said that any body reflects heat 

 better or worse than any other, because this relation varies w^ith 

 each kind of radiation. 



The differences in diffusely reflected heat which have been 

 alluded to are perfectly analogous to those which are observed 

 in the diffusely reflected luminiferous rays ; but Herschel and 

 Melloni have already pointed out that the reflexion of these lumi- 

 niferous rays is not analogous to that of the calorific rays. 



The investigations which have been detailed have shown this 

 still more distinctly, by proving that certain bodies, which ap- 

 pear of the same colour to the eye, reflect different kinds of heat ; 

 and such as are apparently of different colours, so far as experi- 

 ment has yet shown, reflect similar rays of heat. (See especially 

 Table XLIII.) 



It scarcely requires to be mentioned, that this result is not deci- 

 sive of the identity of luminiferous and calorific rays ; for since 

 it has been determined that every luminiferous source of heat 

 emits a large number of invisible rays which are susceptible of re- 

 flexion and affect the thermal jnle^, the most rigid analogy, i. e. 

 the assumption that every ray which, after penetrating the optic 

 media, excites the retina of our eye to pi'oduce vision, in propor- 

 tion to its strength acts upon a thermoscope coated M'ith lamp- 

 black, would not lead us to expect any agreement in the diffuse 

 reflexion of the luminiferous and calorific rays ; for this would be 

 to suppose that we were experimenting with a source which emit- 

 ted only one kind of luminiferous and one kind of calorific rays. 



I shall allude to one more point only in relation to this question. 



The different colours under which diffusely-reflecting surfaces 

 appear to our eyes, have usually been explained by the assump- 

 tion f, that certain colours only are reflected by them, the others 

 being absorbed. Since the differences which heat exhibits after 

 reflexion by any body have been shown experimentally to be the 

 consequence of a similar selective absorption (see p. 415-422), 

 the above assumption acquires the highest degree of probability, 

 considering the great analogy which the luminiferous and calorific 



* Compare p. 232, where it lias been shown that certain rays of heat from 

 the Argand lamp pass through black glass and hlack lac, which are therefore 

 emitted invisibly from the flame. 



f The correctness of which, from there being no accurate photometer, cannot 

 be proved with certainty. 



