MEMOIR I.] THE RADIATIONS OF IGNITED BODIES. 49 



immediate transmission of every variety of radiant heat 

 through rock-salt, I availed myself of that valuable prop- 

 erty to study the refraction of heat from various sources, 

 and I discovered that radiations coming from those of 

 a high temperature contain elements more refrangible 

 than those which are derived from sources that are not 

 so hot." 



M. Melloni then passes to a criticism of the methods I 

 had used in investigating the law of the increase of the 

 luminous and calorific radiations, according as the tem- 

 perature of the source of heat is elevated. 



He adds: "The method invented by Bouguer to deter- 

 mine the relative intensities of different luminous sources, 

 and employed by Draper to measure the quantities of 

 light emitted by a strip of platinum brought to different 

 degrees of incandescence, is the only one by which we 

 could hope for a successful result. The method of the 

 equality of shadows, well known under the name of 

 Rumford's method, would have furnished in the research- 

 es of the learned American uncertain data, on account of 

 the difficulty of establishing an exact comparison between 

 the accidental green tint introduced into the shadow en- 

 lightened by the yellow rays of the lamp and the red 

 light emitted by the ignited metal. As to the measures 

 of the radiant heat, they were determined by the aid of 

 the thermo-multiplier, that admirable instrument which 

 has revealed to science so many new properties of calo- 

 rific radiations, and which still is rendering eminent 

 services in the hands of able chemists far beyond the 

 Alps. 



" The numbers obtained by M. Draper show evidently 

 that the augmentations of both light and heat, though 

 feeble at first, become very rapid at last, from which it 

 results that the radiations both of light and heat follow 

 in the progression of quantity the same analogy that we 



D 



