472 Mr. H. Smith : Spectroscopic Study of the .Electric 



solutions varying in strength from *005 to '1 per cent. ; 

 with much stronger solutions the brush does not occur. 

 "With the condensed brush, the spark-gap being closed, the 

 spectrum of the metal of the salt generally increases in 

 brightness ; but on opening the spark-gap, the spectrum for 

 the weaker solutions grows fainter, disappearing eventually 

 as the spark-gap is further extended. In the stronger solu- 

 tions the intensity of the spectrum either remains stationary 

 or increases very slowly. 



I have chosen a few of the spectrograms for lithium salts 

 to illustrate these points (PL V. tigs. 1-5) . It will be seen that 

 the lithium red line \=6708 increases in brightness at first 

 more rapidly than the EU line of Irydrogen. Subsequently 

 the H a line greatly exceeds in brightness the lithium line. 



Another point to notice in connexion with the lithium 

 spectrum is that it is produced much more brightly with 

 the sulphate than with the carbonate. (Compare fig. 4 b with 

 fig. 3 b PL Y.) For the same percentage strength, the lithium- 

 sulphate solution has a lower resistance than the lithium- 

 carbonate solution. The current is consequently much larger, 

 though not of course comparable with the strength of the 

 higher condensed discharges which tend to destroy the spec- 

 trum. This difference in current may account for the 

 difference in brightness of the spectrum of lithium for the 

 two solutions. 



There is, however, a very great difference in the relative 

 intensities of the lines which can scarcely be accounted for by 

 this. In PL Y. fig. 4, a & b it will be seen that the lithium blue 

 line X = 4602 and the lithium orange line \=6104 are very 

 nearly of the same intensity as the red line. This is very 

 far from being the case in any of the spectra with lithium 

 carbonate. In fig. 4 it will be seen that while the red line 

 persists, though faintly, right through to the heavier con- 

 densed discharges, the blue and orange lines are scarcely 

 discernible when the discharge is heavy. The distribution of 

 intensity is now very similar to what it is in all the photo- 

 graphs with lithium carbonate. 



If the uncondensed brush is examined visually, the lithium 

 lines will be observed to fluctuate a good deal, sometimes 

 flashing up very brightly, sometimes appearing very feeble. 

 All the time the electrical conditions may remain quite 

 steady, while the hydrogen lines do not show the slightest 

 variation in brightness. After the discharge has taken place 

 for a few minutes it will be found that the spectrum has 

 brightened and become, on the whole, steadier, while the 

 potential difference has risen. If the point be now examined 



