39'i M. MELLON! ON LIGHT AND RADIANT HEAT. 



of the successive zones constantly decrease on each side with the great- 

 est regularity. Thus, notwithstanding the interposition of the coloured 

 glasses, the intensity of the heat uniformly increases from the violet to 

 the red, while the intensity of the light undergoes very irregular va- 

 riations, which render a given zone sometimes stronger and sometimes 

 more feeble than the succeeding zone. 



Let us disregard that which takes place in the obscure part, and fix 

 our attention on the alterations produced in the visible part of the 

 normal spectrum, in which each luminous band is accompanied by a 

 calorific band possessing the same refrangibility. On the one hand we 

 see uncoloured media which have no influence on the luminous rays 

 and totally alter the ratios of intensity in the accompanying calorific 

 rays ; on the other, coloured media which totally change the relative 

 energies of the luminous, without affecting the regularity of the propor- 

 tions existing between the corresponding calorific rays. 



But if heat and light were both produced by the same movement of 

 the aethereal molecules, it is evident that each reduction of force in a 

 given ray of pure light should be accompanied by an exactly propor- 

 tionate reduction in the ray of heat possessing the same refrangibility. 

 Now the variations of intensity produced in each of the two agents by 

 the interposition of uncoloured or coloured media, so far from corre- 

 sponding through the whole of the luminous part of the spectrum, ex- 

 hibit the most striking diversity. Light and radiant heat, therefore, 

 proceed from two distinct causes*. 



This being admitted, the complete separation of light from heat be- 

 comes intelligible; and such is the conclusion at which I have arrived, 

 with respect both to terrestrial fire and the solar rays. The process of 

 separation is exceedingly simple : it consists in causing the radiation 

 from the luminous sources to pass through a system of diaphanous bo- 

 dies which absorb the whole of the calorific, while they extinguish but 

 a part of the luminous rays. The only substances hitherto employed 

 by me are water, and a peculiar species of green glass coloured by 

 means of the oxide of copper. The pure liyht emerging from this 

 system contains much yellow, and possesses at the same time a tinge of 

 bluish green : it exhibits no calorific action capable of being rendered 

 jierceptihle by the most delicate thermoscopes, even when it is so concen- 

 trated by lenses as to rival the direct rays of the sun in brilliancy. 



* These two causes themselves are, perhaps, but different effects of a single 

 cause. The coiickision wliich appears to me to follow so clearly from my ex- 

 periments is therefore by no means opposed to the general theory of undula- 

 tions, according to which light and radiant heat arise from the motions commu- 

 nicated to theffither by the molecular vibrations of luminous bodies and bodies 

 possessing heat. It will only be necessary to admit that the luminous and the 

 calorific rays are 'wo essentially distinct modifications which the aetliereal fluid 

 suffers in its mode of existence. 



