72 Professor Forbes's Researches on Heat. 



not an indispensable circumstance. Again, certain relations 

 had been established at an early period in the history of the 

 science of heat, between the colour of a surface and the quan- 

 tity of heat which it absorbed, and this relation for any two ' 

 surfaces compared (as black and white, of similar textures), 

 was first clearly shown by Sir John Leslie, to depend upon 

 the luminosity of the source of heat, to which conceiving it 

 proportional, that philosopher based upon it the principle of 

 his Photometer *. Professor Powell, of Oxford, conceived 

 and executed an ingenious experiment, by which it is demon- 

 strated that the interposition of a screen of glass, though it 

 stops but little light, alters most materially the influence of co- 

 lour on the transmitted heat, thus annihilating at once the 

 principle of photometric measurement adopted by Leslie, ex- 

 cept in a very limited class of casesf . M. Melloni has fully con- 

 firmed the experiments of Professor Powell J, which therefore 

 may be considered as establishing this conclusion, that the 

 quality of blackness or whiteness of a surface affects its power 

 of absorbing heat {not in proportion to the luminosity of that 

 heat, as was formerly supposed, but) in proportion to its re- 

 frangibility. 



4. It is both convenient and correct, therefore, to consider 

 the refrangibility of heat as the cause of most of its distinctions 

 of kind and degree of modification in our experiments, instead 

 of making vague reference to the temperature of the source 

 whence it is derived. Heat derived from the following scale 

 of temperatures corresponds to heat of progressively elevated 

 refrangibility ; as, 1 . Heat from ice has a less refrangibility 

 than that from, 2. the hand, which again is below, 3. that from 

 boiling-water : then conies, 4. that from a vessel of mercury 

 under its boiling temperature, 5. a piece of smoked metal, 

 heated by an alcohol lamp behind, but itself quite invisible in 

 the dark, 6. incandescent platinum (a coil of wire in an alco- 

 hol flame), 7. an oil lamp (Locatelli's). Such is the scale 

 of heat which has often been referred to in M. Melloni's re- 

 searches and my own ; but though our apprehension of the 

 temperature of the source ceases to be so clear above this li- 

 mit, and the colour and brightness of the light which accom- 

 panies the heat no longer varies distinguishably, the scale may 



* Essay on Heat, 1804. f Phil. Trans. 1825, p. 187. 



X Ann. de C'himic, Avril 1834. M. Melloni finds, for instance, that the 

 rays from an oil-lamp falling on black and white surfaces, affects their tem- 

 perature in the proportion of 1000 : 805. And the same proportion holds 

 if they be transmitted through a plate of rock-salt; but if a plate of alum 

 be used, though equally transparent for light with the salt, the proportion 

 is now 1000: 429. 



