DISRUPTIVE DISCHARGE — FORMATION OF THE BRUSH. 113 



the brush, before the particles at the extremity of the ramifications have yet attained 

 their maximum intensity. 



1437. I consider brush discharge as, probably, a successive effect in this way. 

 Discharge begins at the root (1426.), and, extending itself in succession to all 

 parts of the single brush, continues to go on at the root and the previously formed 

 parts until the whole brush is complete ; then, by the fall in intensity and power at 

 the conductor, it ceases at once in all parts, to be renewed, when that power has 

 risen again to a. sufficient degree. But in a spark, the particles in the line of dis- 

 charge being, from the circumstances, nearly alike in their intensity of polarization, 

 suffer discharge so nearly at the same moment as to make the time quite insensible 

 to us. 



1438. Mr. Wheatstone has already made experiments which fully illustrate this 

 point. He found that the brush generally had a sensible duration, but that with 

 his highest capabilities he could not detect any such effect in the spark*. I re- 

 peated his experiment on the brush, though with more imperfect means, to ascertain 

 whether I could distinguish a longer duration in the stem or root of the brush than 

 in the extremities, and the appearances were such as to make me think an effect of 

 this kind was produced. 



1 439. That the discharge breaks into several ramifications, and by them passes 

 through portions of air alike, or nearly alike, as to polarization and the degree of 

 tension the particles there have acquired, is a very natural result of the previous 

 state of things, and sooner to be expected than that the discharge should con- 

 tinue to go straight out into space in a single line amongst those particles which, 

 being at a distance from the end of the rod, are in a lower state of tension than 

 those which are near : and whilst we cannot but conclude, that those parts where 

 the branches of a single brush appear, are more favourably circumstanced for dis- 

 charge than the darker parts between the ramifications, we may also conclude, that 

 in those parts where the light of concomitant discharge is equal, there the circum- 

 stances are nearly equal also. The single brushes are by no means of the same parti- 

 cular shape even when they are observed without displacement of the rod or surround- 

 ing objects (1427. 1433.), and the successive discharges may be considered as taking 

 place into the mass of air around, through different roads at each brush, according as 

 minute circumstances, as dust, &c. (1391. 1392.) may have favoured the course by 

 one set of particles rather than another. 



1440. Brush discharge does not essentially require any current of the medium in 

 which the brush appears : the current almost always occurs, but is a consequence of 

 the brush, and will be considered hereafter. On holding a blunt point positively 

 charged towards uninsulated water, a star or glow appeared on the point, a current 

 of air passed from it, and the surface of the water was depressed ; but on bringing 



* Philosophical Transactions, 1836, pp. 586, 590. 

 MDCCCXXXVIII. Q 



