THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE, 



[FOURTH SERIES.] 



JULY 1860. 



I. On the Relation between the Radiating and Absorbing Powers 

 of different Bodies for Light and Heat. By G. Kirch hoff*. 



A BODY placed within a covering whose temperature is the 

 same as its own, is unaffected by radiation, and must 

 therefore absorb as many rays as it emits. Henee it has long 

 been concluded that, at the same temperature, the ratio between 

 the radiating and absorbing powers of all bodies is the same, — 

 it being, however, assumed that bodies only emit rays of one 

 kind. This law has been verified experimentally, especially by 

 MM. de la Provostaye and Desains, in many cases in which the 

 homogeneousness of the emitted rays could at least be so far 

 assumed, inasmuch as they were all invisible. Whether the same 

 law holds good when bodies emit rays of different kinds (which, 

 strictly speaking, is always the case), has never hitherto been de- 

 termined theoretically or by experiment. I have, however, now 

 found that the law in question extends to this case also, provided 

 that by the radiating power the intensity of one species of emitted 

 rays be understood, and that the absorbing power be estimated 

 with reference to rays of the same kind. Taken in this way, the 

 ratio of the radiating and absorbing powers of all bodies at the 

 same temperature is the same. I shall first give the theoretical 

 proof of this principle, and then develope certain remarkable 

 consequences that immediately follow therefrom, which partly 

 explain phenomena already known, and partly suggest new ones. 

 All bodies emit rays, the quality and intensity of which 

 depend on the nature and temperature of the bodies themselves. 

 In addition to these, however, there may, under certain circum- 

 stances, be rays of other kinds, — as, for example, when a body 



* Translated bv Mr. F. Guthrie from Poggendorff's Annalen, vol. cix. 

 p. 275. 



Phil. Mag. S. 4, Vol. 20. No. 130. July 1860. B 



