1450.  In reference to the theory of molecular inductive action, I may also add, the proof deducible from the long brushy ramifying spark which, may be obtained between a small ball on the positive conductor of an electrical machine, and a larger one at a distance (1448. 1504.).  What a fine illustration that spark affords of the previous condition of all the particles of the dielectric between the surfaces of discharge, and how unlike the appearances are to any which would be deduced from the theory which assumes inductive action to be action at a distance, in straight lines only; and charge, as being electricity retained upon the surface of conductors by the mere pressure of the atmosphere!

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1451.  When the brush is obtained in rarefied air, the appearances vary greatly, according to circumstances, and are exceedingly beautiful.  Sometimes a brush may be formed of only six or seven branches, these being broad and highly luminous, of a purple colour, and in some parts an inch or more apart:  by a spark discharge at the prime conductor (1455.) single brushes may be obtained at pleasure.  Discharge in the form of a brush is favoured by rarefaction of the air, in the same manner and for the same reason as discharge in the form of a spark (1375.); but in every case there is previous induction and charge through the dielectric, and polarity of its particles (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 glow (1529.):  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.