Brush Discharge in Water and Salt Solutions. 471 



In the capillary discharge, even with the finest capillaries, 

 the elementary-line spectrum occurs very feebly, if, indeed, 

 at all. This is true both as regards distilled water and 

 solutions, if we disregard the stationary bubble at either end 

 of the capillary in the latter case. In dilute sulphuric acid 

 the discharge spread over the top of the bubble is a deep 

 blue, which fluctuates in extent and intensity. If an image 

 of the discharge is focussed by means of an achromatic lens 

 on the slit of a spectrograph, the resulting photograph shows 

 that the spectrum of the bubble consists mainly of the ele- 

 mentary-line spectrum. Very little trace of this spectrum 

 is found in the capillary portion. This point is illustrated 

 in fig. 8, w T here perhaps spectrogram c shows it most 

 clearly. 



If all the photographs are considered it will be found that 

 where the elementary lines are strong the series lines are 

 weak and vice versa. Everything w^ould appear to point 

 to the fact that the elementary -line spectrum was due 

 to low current density. We should obtain this spectrum, 

 then, if we reduced the current through the fine capillaries 

 or used wider capillaries. I have tried both methods and 

 obtained negative results. Fig. 9, b, c, d represents a series 

 of photographs with a fine capillary, the current being 

 reduced by choking down the primary current of the induc- 

 tion-coil. Fig. 8 is a similar series for dilute sulphuric acid. 

 It will be noticed that in the latter case the elementary lines 

 in the spectrum of the bubble intensify a little at first, 

 though they do not do so in the spectrum of the capillary. 



As it is the elementary line-spectrum which occurs in the 

 spark in air, and in a vacuum-tube when excited by the dis- 

 charge of a condenser through a spark-gap, it is generally 

 associated with high current density. Runge and Paschen 

 have, however, observed it in the wide part of a vacuum- 

 tube wdiich was excited by the ordinary induction-coil dis- 

 charge, the capillary portion giving the series spectrum*. 

 It seems impossible, therefore, to account for this spectrum 

 from the point of view of current density alone. 



Solute. 



I have made observations on salts of lithium, sodium, 

 potassium, and calcium. The greater part of the obser- 

 vations have been confined, however, to the sulphate and 

 carbonate of lithium. The lines of the metal only appear. 

 In the uncondensed brush the lines appear equally well in 



* Ann. der Phys. lxi. p. 641 (1897). 



