ON ELECTRICITY IN MOTION. 669 



density of the fluid from the velocity of a spark or shock, compared with that 

 of sound; or we might deduce its velocity from a determination of its density. 

 It has heen supposed, although perhaps somewhat hastily, that the actual ve- 

 locity is nearly equal to that of light. 



When a conducting substance approaches another, which is electrified 

 the distribution of the electric fluid within it is necessarily altered by 

 induction, before it receives a spark, so that its remoter extremity is 

 brought into a state similar to that of the first body: hence it happens that 

 when the spark passes, it produces less effect at the remoter end of the sub_ 

 stance, while the part presented to the electrified body is most 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 com- 

 munication of electricity has rendered the substance more capable of conduct- 

 ing 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 contained in the glass, while 

 the plate of air is perhaps little concerned in the distribution of the elec- 

 tricity. 



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 



VOL. I. 4 o 



