Absorbing Powers of different Bodies for Light and Heat. 15 



But this limit may be decreased by increasing the thickness of 

 the lithium-flame, and its consequent absorptive power. 



A small bead of chloride of lithium, placed in the flame of a 

 Bunsen's lamp, imparts to the latter so considerable an absorptive 

 power for waves of the particular length mentioned, that if the 

 rays of the sun be suffered to fall on the slit of the apparatus 

 that forms the spectrum through such a flame, the corresponding 

 part of the spectrum appears like a fine black line. 



The spectra produced when other salts are placed in the flame 

 are for the most part less simple than the lithium spectrum, 

 and seldom exhibit such brilliant lines. All of them, however, 

 are capable of being reversed by similar means. If flames of 

 sufficient thickness be employed and light of suitable intensity 

 be passed through them, the bright lines of the spectra may all 

 be converted into lines of shade. The only exception would be 

 in the case of a flame the light of which was partly produced by 

 some immediate chemical action, or in case of a fluorescent flame. 

 Experiment must decide whether such flames exist. 



If the source of light employed is an incandescent body, the 

 intensity of the light it emits depends on its temperature, — the 

 intensity, for the same temperature, being greatest when the body 

 is perfectly black. If this condition be fulfilled in the case of 

 two sources of light, and if their temperature be the same, the 

 spectrum of the one will be unaffected by the interposition of the 

 other. The more remote source of light can therefore only 

 reverse the spectrum of the other when it possesses a higher 

 temperature, and the reversed spectrum will be more distinct the 

 greater the excess of the temperature of the former source of light 

 over that of the latter. 



Besides the spectrum of the lithium-flame, I have succeeded in 

 ' reversing that of the common salt-flame. This spectrum consists, 

 as is well known, of two very brilliant yellow lines close together, 

 the wave-length of which corresponds to Fraunhofer's double 

 line D. If the rays of a Drummond light be passed through a 

 salt-flame of not too high a temperature, the bright lines of the 

 salt-spectrum become dark, and occupy the place of Fraunhofer's 

 lines D, presenting in every respect the same appearance as those 

 lines*. 



§ 14. The wave-lengths which correspond to maxima of the 

 radiating and absorbing powers are, as will be fully explained in 

 another place, altogether independent of the temperature ; and 



* More. recently Prof. Bunsen and I have likewise reversed the brighter 

 lines of the spectra of potassium, calcium, strontium, and barium, by explo- 

 ding before the slit of the spectral instrument mixtures of milk-sugar and 

 chlorates of the respective metals during the passage of the sun's ravs. — 

 May 9, 1860. 



