Brush Discharge in Weak Acids and Solutions. 807 



bands of light, more or less patchy, the end and beginning 

 of each band being generally broken up into two or three 

 dots separated by short dark spaces. The positive brush in 

 solutions shows bands broken up throughout their whole 

 length into dots separated by short dark spaces. In 

 magnesium sulphate and zinc sulphate solutions, the negative 

 brush was harsher in sound than in other solutions, and when 

 examined in the rotating mirror, it appeared as bands broken 

 up into a series of dots in the same manner as the positive 

 brush. 



As the plate electrode may be moved close up to the brush 

 without any perceptible alteration in the readings of either 

 the voltmeter or galvanometer, the potential difference 

 measured must be very nearly equal to the potential drop 

 through the brush, and may thus be considered as the 

 potential necessary to produce the brush. The minimum 

 potential required, as thus measured, is found to vary 

 according to the sign of the brush and concentration of 

 solution. Some measurements are given in Table I. 



Table I. 



Minimum Potential Conductivity in 



in volts. tob-c mho. 



+ + - 



Distilled water 2700 2200 -0013 '0006 



NaOH-OOlp. c 1825 1250 -0023 -0024 



„ -005 „ 560 300 -041 -052 



„ -01 „ 300 200 -052 -106 



It will be seen that to produce a positive brush a larger 

 potential is always required than to produce a negative brush, 

 and that the necessary potential decreases with strength of 

 solution. The minimum current, however, varies from 1 or 

 2 milliamps. for distilled water to about 30 milliamps. for 

 the stronger solutions, e, g. 1 per cent. NaOH. The con- 

 ductivity of distilled water is greater with a positive brush 

 than with a negative. This becomes reversed in solutions. 



Evolution of Gas, 



Beyond what has already been said concerning the evolu- 

 tion of gas in the brush, there remains a further point which 

 requires mentioning as it may cast a little light on the 

 mechanism of the brush. In an experiment on the decom- 

 position of water, Wollaston used as electrodes two platinum 



3H2 



