ON ELECTRICITY IN MOTION. 517 



affected, on account of its sudden change to an opposite state. But if 

 both ends approach bodies in opposite states of electricity, they will both 

 be strongly affected when the shock takes place, while the middle of the 

 circuit undergoes but little change. 



The manner in which the electric fluid makes its way, through a more 

 or less perfect nonconductor, is not completely understood : it is doubtful 

 whether the substance is forced away on each side, so as to leave a vacuum 

 for the passage of the fluid, or whether the newly formed surface helps to 

 guide it in its way ; and in some cases it has been supposed that the 

 gradual communication of electricity has rendered the substance more 

 capable of conducting it, either immediately, or, in the case of the air, by 

 first rarefying it. However this may be, the perforation of a jar of glass 

 by an overcharge, and that of a plate of air by a spark, appear to be 

 effects of the same kind, although the charge of the jar is principally con- 

 tained in the glass, while the plate of air is perhaps little concerned in the 

 distribution of the electricity. 



The actual direction of the electric current has not in any instance been 

 fully ascertained, although there are some appearances which seem to 

 justify the common denominations of positive and negative. Thus, the 

 fracture of a charged jar of glass, by spontaneous explosion, is well defined 

 on the positive, and splintered on the negative side, as might be expected 

 from the passage of a foreign substance from the former side to the latter ; 

 and a candle, held between a positive and a negative ball, although it 

 apparently vibrates between them, is found to heat the negative ball much 

 more than the positive. We cannot, however, place much dependence on 

 any circumstance of this kind, for it is doubtful whether any current of 

 the fluid, which we can produce, possesses sufficient momentum to carry 

 with it a body of sensible magnitude. It is in fact of little consequence to 

 the theory, whether the terms positive and negative be correctly applied, 

 provided that their sense remain determined ; and that, like positive and 

 negative quantities in mathematics, they be always understood of states 

 which neutralise each other. The original opinion of Dufay,* of the 

 existence of two distinct fluids, a vitreous and a resinous electricity, has at 

 present few advocates, although some have thought such a supposition 

 favoured by the phenomena of the galvanic decomposition of water. 



When electricity is simply accumulated without motion, it does not 

 appear to have any effect, either mechanical, chemical, or physiological, by 

 which its presence can be discovered ; the acceleration of the pulse, and 

 the advancement of the growth of plants, which have been sometimes 

 attributed to it, have not been confirmed by the most accurate experi- 

 ments.t An uninterrupted current of electricity, through a perfect 

 conductor, would perhaps be also in every respect imperceptible, since the 

 best conductors appear to be the least affected by it. Thus, if we place 



* Ph. Tr. 1733, xxxviii. 258. Hist, et Mem. 1733, p. 457. 

 t Consult Kies et Koestlin, DeEffectibus Electr. 4to, Tubing. 1775. Ingenhousz, 

 Versuche mit Pflanzen, 3 vols. Vienna, 1778-90. Bertholon, do. Leipz. 1785. 



k, De 1' Application de 1'Electr. 4to, Amst. 1788. Van Marum, Proefne- 



met Teylers Electrisir Maschine, 4to, Haarlem, 1795. 



