4 PROFESSOR FORBES'S RESEARCHES ON HEAT. 



not an indispensable circumstance. Again, certain relations had been establish- 

 ed at an early period in the history of the science of heat, between the colour 

 of a surface and the quantity of heat which it absorbed, and this relation for 

 any two surfaces compared (as black and white, of similar textures), was first 

 clearly shewn 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 exe- 

 cuted an ingenious experiment, by which it is demonstrated that the interpo- 

 sition of a screen of glass, though it stops but little light, alters most materially 

 the influence of colour on the transmitted heat, thus annihilating at once the 

 principle of photometric measurement adopted by LESLIE, except in a very limited 

 class of cases.f M. MELLONI has fully confirmed the experiments of Professor 

 POWELL, \ 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 refrangibility. 



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 tempera- 

 tures 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 comes, 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 

 alcohol flame), 7. an oil lamp (LOCATELLI'S). Such is the scale of heat which has 

 often been referred to in M. MELLONI'S researches and my own ; but though our 

 apprehension of the temperature of the source ceases to be so clear above this 

 limit, and the colour and brightness of the light which accompanies the heat no 

 longer varies distinguishably, the scale may be carried upwards indefinitely by 

 interposing screens of different materials, which either may be proved directly 

 (as I have done in the Third Series of these researches) to increase the refrangi- 

 bility, or we may take Professor POWELL'S, or any similar test, which our experi- 

 ments lead us to conclude to be co-ordinate with the fact of refrangibility. Such 

 a prolongation of the scale of heat-sources would be, 



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



I Ann. de Chimie, Avril 1834. M. MELLONI finds, for instance, that the rays from an oil-lamp 

 falling on black and white surfaces, affects their temperature 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. 



