518 LECTURE LIV. 



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 wire 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 interruption of the current is neces- 

 sary in order to furnish an accumulation sufficient to produce sensible 

 effects ; and such an interruption 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 produc- 

 tion of light. Light is probably never occasioned by the passage of the 

 fluid through a perfect conductor; for when the discharge 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 inter- 

 rupted by an imperfect conductor ; nor is the apparent contact of conduct- 

 ing substances sufficient to prevent it, unless they are held together by a 

 considerable force ; thus, a chain, conveying a spark or shock, appears 

 luminous 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, repre- 

 sents at once a brilliant delineation 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 

 properties 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 light, 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 conducts. 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 diverging rays, 

 shows that it is negative. The sparks, exhibited by small balls, differently 

 electrified, have also similar varieties in their forms, according to the 

 nature of their charges.* (Plate XL. Fig. 552.) 



The production of heat by electricity frequently accompanies that of 

 light, and appears to depend in some measure on the same circumstances. 

 A fine wire may be fused and dissipated by the discharge of a battery ; 

 and without being perfectly melted, it may sometimes be shortened or 



* Consult Doppelmayer, Ueber das Elektr. Licht. 1749. Nairne, Ph. Tr.JL777, 

 p. 614. Nicholson, ibid. 1789, p. 265. Davy on a Vacuum, ibid. 1822, p. 64r v 



