28 ELECTRICITY IN MOTION. 



either immediately, or, in the case of the air, by first rarefying it. 

 However this may be, the perforation of a jar of glass l>y an 

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

 effects of li<- same kind, although the charge of the jar is princi- 

 pally contained 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 in. 

 stance 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 j;ir of glass, by spon. 

 taneous 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 depend, 

 ance on any circumstance of this kind, for it is doubtful whether 

 any current of the fluid, which we can produce, possesses suffi. 

 cient 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 elec- 

 tricity, 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 < fleet, either mechanical, chemical, or 

 physiological, by which its presence can be discovered; the acce. 

 leration of the pulse, and the advancement of the growth of plants, 

 which have been sometimes attributed to it, have not been con- 

 firmed by the most accurate experiments. An uninterrupted cur- 

 rent 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 our hand 

 on the conductor of an electrical machine, the electricity will pass 

 off continually through the body, without exciting any sensation. 

 A constant stream of galvanic electricity, passing through an iron 



