Electric Brush Discharge in Solutions. 801 



a given negative discharge depended on the dryness of the 

 air. The theory shows that the potential v required to main- 

 tain a small current I is given by the formula 



, TTN IA 2 log A/a 

 v(v-Y) = Yk , 



k being the velocity of the ions under unit electric force and 

 V the sparking potential. The potentials v required to main- 

 tain small currents were quite consistent with this expression, 

 but no very accurate determinations of k have yet been 

 deduced from the observations. 



It was found that dljdv was smaller for positive discharges 

 than for negative discharges. Also when the air was damp, 

 dl/dv for negative discharges diminished, showing that the 

 negative ions, in the space in which cx.jp is small, are affected 

 by the moisture. But in the space near the inner cylinder 

 where X/p is large the negative ions are not affected by the 

 moisture since the sparking potentials are the same for moist 

 and dry gases. 



XC. The Spectroscopy of the Electric Brush Discharge in 

 Weak Acids and Solutions. By Harold Smith, M.Sc. 

 (1851 Exhibitioner) *. 



[Plate XI.] 



Introduction. 



THE changes which occur in the spectra of the electric 

 brush discharge in solutions under varying electrical 

 conditions have already been investigated in a previous 

 paper f. In the experiments there considered the brush was 

 produced by means of a large induction-coil, the primary of 

 which was fed with alternating current. The secondary 

 terminals were connected to two electrodes immersed in a 

 small cell containing water or some dilute solution. One of 

 these electrodes was an inch square of platinum foil ; the 

 other was a platinum wire fused into a glass tube, with the 

 protruding end of the wire filed till it was flush with the 

 glass. A luminous brush occurred at the end thus exposed. 

 A condenser was arranged in parallel with the cell and an 

 adjustable spark-gap in series with it when a condensed brush 

 was required. Striking variations in the spectra were found 

 according to the nature of the solution, its concentration, and 

 the type of discharge used. Other variations occur when 



* Communicated by Prof. Sir J, J. Thomson, O.M., F.R.S. 

 t Phil. Mag-. [6] vol. xxv. p. 461 (1913). 



