94 Professors Kirchhoff and Bunsen on Chemical 



appearance of the spectrum can under other conditions vary 

 considerably. If the purity of the spectrum be increased, many 

 of those lines which appeared before as single ones are split up 

 into several ; thus the sodium line is divided into two separate 

 lines. If the intensity of the light, on the other hand, be in- 

 creased, new lines appear in several of the spectra represented in 

 the Plate, and the relation of the brightness of the old ones becomes 

 altered. In general an indistinct line becomes brighter upon 

 increasing the illumination, more rapidly than does a brighter 

 line, but not to such an extent that the indistinct line ever 

 overtakes in intensity the brighter one. A good example of 

 this is seen in the two lithium lines. We have only observed 

 one exception to this rule, namely in the line Ba rj, which by 

 light of small intensity is scarcely visible, whilst Ba 7 appears 

 plainly, but by light of greater intensity becomes more visible 

 than the latter. This fact appears to us of importance, and 

 we intend on a future occasion to examine this point in detail. 



We now proceed to describe the peculiarities of the several 

 spectra, the exact acquaintance with which is of practical im- 

 portance, and to point out the advantages which this new method 

 of chemical analysis possesses over the older processes. 



Sodium. 



The spectrum-reaction of sodium is the most delicate of all. 

 The yellow line Na a, the only one which appears in the sodium 

 spectrum, is coincident with Fraunhofer's dark line D, and is 

 remarkable for its exactly defined form, and for its extraordinary 

 degree of brightness. If the temperature of the flame be very 

 high, and the quantity of the substance employed very large, 

 traces of a continuous spectrum are seen in the immediate 

 neighbourhood of the line. In this case too, the weaker lines 

 produced by other bodies, when near the sodium line, are 

 discerned with difficulty, and are often first seen when the 

 sodium reaction has almost subsided. 



The reaction is most visible in the sodium salts of oxygen, 

 chlorine, iodine, bromine, sulphuric acid, and carbonic acid. 

 But even in the silicates, borates, phosphates, and other non- 

 volatile salts, the reaction is always evident. 



Swan* has already remarked upon the small quantity of 

 sodium necessary to produce the yellow line. 



The following experiment shows that the chemist possesses 

 no reaction which in the slightest degree will bear comparison, 

 as regards delicacy, with this spectrum-analytical determination 

 of sodium. In a far corner of our experiment room, the capacity 



* Trans. Roy. Soc. Edinb. vol. xxi. Part III. p. 411. 



