LECTURE XV.] HISTORY OF CHEMISTRY. 317 



observations were unknown. In 1822, Sir J. Herschel ob- 

 served the bright bands 10S which the light of a flame coloured 

 by metallic salts exhibits when decomposed. This phenomenon 

 was followed up further by Talbot 109 and by Brewster. 110 Swan 

 pointed out the great delicacy of this reaction, especially in the 

 case of common salt. 111 



Fraunhofer drew attention to the coincidence of the D line 

 with the yellow sodium line. Brewster found the potassium 

 lines to correspond to others of the Fraunhofer lines, and 

 Foucault 112 also made similar observations. 



There are two points in particular which are of fundamental 

 importance with respect to spectrum analysis, and these we 

 owe to the researches of Kirchhoff and Bunsen. The first of 

 them is the fact that every element, in the state of incandescent 

 vapour, is characterised by giving a definite discontinuous 

 spectrum, a fact which even Swan did not venture to state with 

 certainty ; and the second is the law of selective absorption, 

 which Angstrom 113 and Balfour Stewart approached very 

 closely, without actually grasping it fully and clearly. 114 

 This law is now known as Kirchhoff's law, as it was proved 

 mathematically by Kirchhoff, 115 and experimentally by Kirchhoff 

 and Bunsen by means of the celebrated experiment of the 

 reversal of the lines. The law states that the ratio between 

 the emissive power and the absorptive power is the same for 

 all substances at the same temperature for rays of the same 

 wave-length. From this it follows that all opaque substances 

 begin to glow at the same temperature that is, that they give 

 out light of the same wave-length and that incandescent sub- 

 stances only absorb such rays as they themselves emit. Since, 

 however, incandescent gases possess maxima and minima of 

 light intensity, while solid and liquid substances emit light of 

 every kind when sufficiently heated, the former must also possess 



108 Treatise on Light. 109 Brewster, Edinburgh Journal of Science. 

 5, 77 ; Phil. Mag. [3] 3, 35 ; 4, 114 ; 9, 3- no Phil. Mag. [3] 8, 384; 

 Pogg. Ann. 38, 61 ; compare Comptes Rendus. 62, 17. m Trans. Roy. 

 Soc. Edin. 21, 411. 112 Ann. Chim. [3] 58, 476. 113 Pogg. Ann. 94, 

 141. m Trans. Roy. Soc. Edin. 22, i, 59. 115 Pogg. Ann. 109, 275. 



