ELECTRICITY IN MOTION. 29 



inre, is, however, capable of exciting a considerable degree of 

 heat ; and if it be transmitted through the hands of the operator, 

 it will produce a slight numbness, although in general some inter, 

 ruption of the current is necessary, in order to furnish an accu. 

 mulation sufficient to produce sensible effects ; and such an inter* 

 ruption may even increase the effect of a single spark or shock ; 

 thus, gunpowder is more readily fired by the discharge of a battery 

 passing through an interrupted circuit, than through a series of 

 perfect conductors. 



The most common effect of the motion of the electric fluid is the 

 production of li^ht. Light is probably never occasioned by the 

 passage of the fluid through a perfect conductor; for when the 

 dischar.e of a large battery renders a small wire luminous, the 

 fluid is not wholly confined to the wire, but overflows a little into 

 the neighbouring space. There is always an appearance of light 

 whenever the path of the fluid is interrupted by an imperfect con. 

 ductor ; nor is the apparent contact of conducting substances suffi- 

 cient to prevent it, unless they are held together by a considerable 

 force ; thus, a chain, conveying a spark or shock, appears lumi- 

 nous at each link, and the rapidity of the motion is so great, that 

 we can never observe any difference in the times of the appearance 

 of the light in its different parts ; so that a series of luminous 

 points, formed by the passage of the electric fluid, between a 

 string of conducting bodies, represents at once a brilliant delinea- 

 tion of the whole figure in which they are arranged. A. lump of 

 sugar, a piece of wood, or an egg, may easily be made luminous 

 in this manner ; and many substances, by means of their proper, 

 ties as solar phosphori, retain for some seconds the luminous 

 appearance thus acquired. Even water is so imperfect a conductor, 

 that a strong shock may be seen in its passage through it ; and when 

 the air is sufficiently moistened or rarefied to become a conductor, 

 the track of the fluid through it is indicated by streams of li^ht, 

 which are perhaps derived from a series of minute sparks passing 

 between the particles of water, or of rarefied air. When the air is 

 extremely rare, the light is greenish ; as it becomes more dense, 

 the light becomes blue, and then violet, until it no longer con- 

 ducts. The appearance of the electrical light of a point enables 

 us to distinguish the nature of the electricity with which it is 

 charged ; a pencil of light, streaming from the point, indicating 

 that its electricity is positive ; while a luminous star, with few 



