ON ELECTRICITY IN" MOTION. 67I 



overflows a little into the neiglibouring 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 sufficient to 

 prevent it, unless they are held together by a considerable force; tluis, 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 thnes 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 properties as solar phosphor), retain for 

 some seconds the luminous appearance thus acquired Even water is so im- 

 perfect 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 con- 

 ductor, 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 accotnpanies 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 lengthened, accordingly 

 as it is loose or stretched during the experiment. The more readily a metal con- 

 ducts, the shorter is the portion of it which the same shock can destroy; and it 

 has sometimes been found that a double charge of a battery has been capable 

 of melting a quadruple length of wire oFthe same kind. 



The mechanical effects of electricity are probably in many cases the consc- 



