Brush Discharge in Weak Acids and Solutions. 813 



It will be seen from this that the intensity of the spectra 

 is a function both of the current and the potential. Indeed, 

 the hydrogen spectra depend upon the potential to a very 

 much greater extent than do any of the other spectra. As 

 neither the nature of the ions present nor the sign of the 

 brush has much effect upon the spectra, electrolytic action 

 can have little bearing upon their appearance. 



Solute. 



In all salt solutions the spectrum of the metal appears at 

 the negative pole only, when the solution is weak ; in the 

 strongest solutions they begin to appear at the positive pole 

 faintly (PI. XI. fig. 2,d, e, /). For constant current, the 

 spectrum of the metal increases in intensity with increasing 

 concentration, but reaches a maximum for the salts of the 

 a] kali metals at a strength of solution between "005 per cent, 

 and *01 percent. For magnesium sulphate the concentration 

 was '02 per cent. ; for zinc sulphate *1 per cent. Thereafter, 

 with slight fluctuations, it retains the same intensity for 

 further concentration, diminishing in intensity at length with 

 the rest of the spectrum as the potential begins to fall toward 

 the minimum value (figs. 1 & 2). 



An experiment was described in the previous paper which 

 showed that the solution spectrum is produced in strength 

 only in the neighbourhood of an electrode. Two vessels, 

 placed one inside the other, were filled with solution and 

 platinum wires immersed in each as electrodes. The current 

 flowed from one electrode to the other through a very fine 

 capillary in the inner vessel and rendered the capillary 

 luminous. The hydrogen lines were produced brightly, but 

 even in the strongest solutions the metallic solution lines 

 were very weak. When the wire electrode of the inner vessel 

 was made to touch the top of the capillary, its tip became 

 surrounded with a luminous glow which besides giving the 

 hydrogen lines yielded the spectrum of the metal of the salt 

 in solution very brightly. 



These phenomena receive a simple explanation from the 

 point of view of electrolysis. In the case of a capillary, the 

 metallic ions merely pass up through the capillary with 

 the current ; there is no increase in their concentration, and 

 their spectrum is consequently very weak. In the case of the 

 platinum point electrode, the metallic ions move up to the 

 negative electrode and accumulate there. Consequently a 

 greater quantity of metal is concentrated in the neighbour- 

 bourhood of the brush, and a very bright spectrum is the 

 result. As the concentration oi solution increases, the 



