Absorbing Powers of different Bodies for Light and Heat. 19 



E 



for the time being. The equation -r=e cannot generally be 



true of such a body ; but it is true if the body is enclosed in a 

 black covering of the same temperature as itself, since the same 

 considerations that led to the equation in question on the hypo- 

 thesis that the body C was not fluorescent, avail in this case even 

 if the body C be supposed to be fluorescent. To be convinced 

 of this, it is only necessary to consider that if the magnitude E 

 could have two different values in the two arrangements of the 

 system represented in fig. 3, the difference of these values could 

 only be an infinitely small quantity. 

 Heidelberg, January 1860. 



Postscript*. 



1 . Since the appearance of the above paper in PoggendorfFs 

 Annalen, I have received information of a prior communication 

 closely related to mine. The communication in question is by 

 Mr. Balfour Stewart, and appeared in the Transactions of the 

 Royal Society of Edinburgh, vol. xxii. 1858. Mr. Stewart has 

 made the interesting observation that a plate of rock-salt is 

 much less diathermic for rays emitted by a mass of the same 

 substance heated to 100° C, than for rays emitted by any other 

 black body of the same temperature. From this circumstance 

 he draws certain conclusions, and is led to a result similar to 

 that which I have established concerning the connexion between 

 the powers of absorption and emission. The principle enun- 

 ciated by Mr. Stewart is, however, less distinctly expressed, less 

 general, and not altogether so strictly proved as mine. It is as 

 follows : — " The absorption of a plate equals its radiation, and 

 that for every description of heat." 



2. The fact that the bright lines of the spectra of sodium- and 

 lithium-flames may be reversed, was first published by me in a 

 communication to the Berlin Academy, October 27, 1859. This 

 communication is noticed by M. Verdet in the February Number 

 of the Ann. de Chim. et de Phys. of the following year, and is 

 translated by Prof. Stokes in the March Number of the Philo- 

 sophical Magazine. The latter gentleman calls attention to a 

 similar observation made by M. Leon Foucault eleven years ago, 

 and which was unknown to me, as it seems to have been to 

 most physicists. This observation was to the effect that the 

 electric arch between charcoal points behaves, with respect to the 

 emission and absorption of rays of refrangibility answering to 

 Fraunhofer's line D, precisely as the sodium-flame does accord- 

 ing to my experiments. The communication made on this sub- 



* Communicated by the Author. 

 C2 



