DISRUPTIVE DISCHARGE — BRUSH FROM DIFFERENT CONDUCTORS. 117 



there is previous induction and charge through the dielectric, and polarity of its par- 

 ticles (1437.), the induction being, as in any other instance, alternately raised by the 

 machine and lowered by the discharge. In certain experiments the rarefaction was 

 increased to the utmost degree, and the opposed conducting surfaces brought as near 

 together as possible without producing the glow: the brushes then contracted in 

 their lateral dimensions, and recurred so rapidly as to form an apparently continuous 

 arc of light from metal to metal. Still the discharge could be observed to intermit 

 (1427.), so that even under these high conditions, induction preceded each single 

 brush, and the tense polarized condition of the contiguous particles was a necessary 

 preparation for the discharge itself. 



1452. The brush form of disruptive discharge may be obtained not only in air and 

 gases, but also in much denser media. I procured it in oil of turpentine from the 

 end of a wire going through a glass tube into the fluid contained in a metal vessel. 

 The brush was small and very difficult to obtain ; the ramifications were simple, and 

 stretched out from each other diverging very much. The light was exceedingly feeble, 

 a perfectly dark room being required for its observation. When a few solid particles, 

 as of dust or silk, were in the liquid, the brush was produced with much greater 

 facility. 



1453.' The running together or coalescence of different lines of discharge (1412.) 

 is very beautifully shown in the brush in air. This point may present a little diffi- 

 culty to those who are not accustomed to see in every discharge an equal exertion 

 of power in opposite directions, a positive brush being considered by such (perhaps 

 in consequence of the common phrase direction of a current) as indicating a breaking 

 forth in different directions of the original force, rather than a tendency to convergence 

 and union in one line of passage. But the ordinary case of the brush may be com- 

 pared, for its illustration, with that in which, by holding the knuckle opposite to 

 highly excited glass, a discharge occurs, the ramifications of a brush then leading 

 from the glass and converging into a spark on the knuckle. Though a difficult ex- 

 periment to make, it is possible to obtain discharge between highly excited shell-lac 

 and the excited glass of a machine : when the discharge passes, it is, from the nature 

 of the charged bodies, brush at each end and spark in the middle, beautifully illus- 

 trating that tendency of discharge to facilitate like action, which I have described 

 in a former page (1418.). 



1454. The brush has specific characters in different gases, indicating a relation to 

 the particles of these bodies even in a stronger degree than the spark (1422. 1423.). 

 This effect is in strong contrast with the non- variation caused by the use of different 

 substances as conductors from which the brushes are to originate. Thus, using such 

 bodies as wood, card, charcoal, nitre, citric acid, oxalic acid, oxide of lead, chloride 

 of lead, carbonate of potassa, potassa fusa, strong solution of potash, oil of vitriol, 

 sulphur, sulphuret of antimony, and hsematite, no variation in the character of the 

 brushes was obtained, except that (dependent upon their effect as better or worse 



