364 ACROPHYSICS. 



Crticles, it attracts those of ponderable substances, as of the pith 

 11s. The silk is supposed to be deprived of the electric fluid, that 

 is, to become negatively excited ; and hence to attract the pith balls 

 which contain the electric fluid, by attracting the fluid itself. The 



ith balls which are attracted by the glass, become charged with the 

 uid, on touching the glass ; and are then repelled by the self-repellent 

 power of the fluid: but two negatively excited bodies repel each other, 

 either because ponderable matter, when deprived of the electric fluid, 

 is self-repellent, or because they are attracted in opposite directions 

 by the electric fluid in the surrounding air. The electric excitement 

 is strongest when the air is cold and dry ; for moist air conducts and 

 distributes the electric fluid. 



Those substances along or through which the electric fluid passes 

 freely, are called conductors, or non-electrics ; as metals, charcoal, 

 and water : but those substances which will not permit the fluid to 

 move freely over them, are called electrics, or non-conductors ; as 

 glass, rosin, and silk ; which latter becomes negative when rubbed 

 with glass, but positive when rubbed with rosin or sealing wax ; the 

 resins being among the strongest negative electrics. When an ex- 

 cited electric is brought in contact with another in a neutral state, the 

 latter becomes similarly excited by conduction : and on this prin- 

 ciple the prime conductor of the electric machine collects the elec- 

 tric fluid from the glass plate or cylinder. This principle applies 

 also to the interior of the electric jar, commonly called the Leyden 

 jar ; when its knob is brought near to the electric machine : but the 

 exterior of the jar becomes oppositely charged, by induction; which 

 is attributed to the electric fluid on one side, repelling that on the 

 other. Electric excitement may also be produced by chemical ac- 

 tion, by heat, and probably by mere contact of bodies, though in a 

 feeble degree. 



3. To Galvanism, belongs the study of electrical currents, pro- 

 duced continuously, by chemical action ; with their direct effects, and 

 the mode of producing them. This science originated with Professor 

 Galvani, of Bologna; who, in 1790, observed that if a piece of zinc, 

 and another of silver were brought in contact, the one with a muscle, 

 and the other with a nerve of a frog recently killed, muscular convul- 

 sions would ensue, whenever the two metals were brought, at the 

 same time, in contact with each other. Galvani believed this effect 

 to be produced by a peculiar animal electricity ; while Volta main- 

 tained that it was owing to common electricity developed by mere 

 contact of the metals : but Dr. Wollaston maintained that the electric 

 excitement was produced by the chemical action of the moist animal 

 matter on the more oxidable metal ; which last is now fully proved to 

 be the chief cause. So rapid was the progress of this new science, 

 that in 1800, Nicholson and Carlisle applied it to the decomposition 

 of water; and in 1807, Davy was enabled, by its means, to discover 

 the metallic bases of the fixed alkalies. 



When a plate of zinc, and another of copper, are dipped, with- 

 out touching each other, into any acid solution, the zinc, by the 



action of the acid, is supposed to acquire an electro-positive state; 



which enables it to decompose water, and seize on the oxygen ; 



