Brush Discharge in Weak Acids and Solutions. 805 



Viewed through a low-power microscope it appears in 

 constant agitation. There is a bright luminous spot close to 

 the point. The positive brush in the strongest solutions is 

 coloured slightly according to the salt in solution. 



The change from the type of brush characteristic of 

 distilled water to that characteristic of a solution occurs 

 suddenly at a certain concentration. The same concentration 

 is not required for each brush; a slightly stronger solution 

 generally being required for the positive brush than for 

 the negative. In sodium, magnesium, and zinc sulphate 

 solutions the reverse was the case. With zinc sulphate, 

 especially, the negative brush did not change to the solution 

 type till a concentration of '1 per cent, was reached, while 

 the positive brush acquired the solution characteristic at a 

 concentration of *005 per cent., the usual concentration at 

 which the change occurs. 



In solutions, the bubbles are emitted from each point much 

 less vigorously than in distilled water. The bubbles are also 

 much fewer in number and a good deal larger. In the case 

 of the negative brush, the bubbles seem to be emitted from 

 all parts of the brush. In the case of the positive brush, the 

 bubbles are emitted only from the bottom of the stationary 

 bubble (fig. 4). The sound given out by the positive brush 

 is harsh, much harsher than that given out by the negative. 



In either brush, if the current is strong, the emission of 

 bubbles from the point may cease, while a large bubble forms 

 under the point and grows slowly, portions detaching from 

 it at intervals of several seconds. This bubble may last for 

 some time ; generally, however, it breaks away after a few 

 seconds, and is succeeded by an emission of small bubbles 

 which lasts for a similar time, after which the large bubble 

 forms once more. In dilute acids, especially of sulphuric 

 acid, this " bubble formation " occurs much more readily 

 at the positive point than at the negative, and attains its 

 greatest size (4 or 5 mm. across) at the former. This state- 

 ment holds more or less for salt solutions also. In alkaline 

 solutions (LiOH, ISTaOH, KOH) the bubble occurred only 

 at the negative point. There seems to be a concentration at 

 which the bubble forms most readily. Inversion of the 

 point prevents a large bubble from forming, but a smaller 

 bubble continues to form at the negative point. This type of 

 " bubble formation " is quite distinct from that described 

 above and represented in rig. 4; a drawing of it is shown in 

 fig. 5. 



Phil. Mag. S. 6. Vol. 27. No. 161. May 1914. 3 H 



