24 



ELECTRICITY. 



intensity, and will therefore pass through 

 a greater extent of air ; the spark is in 

 this case of considerable length, appear- 

 ing as a long streak of fire extending 

 from the conductor to the ball, and 

 instead of being directed towards one 

 point, being distributed to various points 

 throughout a certain extent of the sur- 

 face of the ball. 



Often, when very long, the spark is 

 seen to have an angular or zig-zag 

 course, (see fit?. 20.) exactly like that of 

 a flash of lightning. This irregularity 



Fig. 20. 



is probably occasioned by the fluid dart- 

 ing obliquely in its course to minute 

 conducting particles that are floating in 

 the air, a little removed from the direct 

 line of passage. Even particles of 

 moisture suspended in the air would be 

 sufficient to occasion these deviations. 

 The presence of such particles will ac- 

 count also for the appearance of lateral 

 scintillations, which frequently seem to 

 diverge from the principal stream of 

 electricity. The greater the number of 

 such intermediate conductors, or step- 

 ping-stones, as it were, for the electricity, 

 the more readily will the balance be- 

 tween the forces be overset, and the 

 irruption of electric fluid determined. 



When the air is either sufficiently 

 moistened, or sufficiently rarefied, the 

 electric fluid passes through it with 

 comparative facility, and its track is 

 indicated by streams of light, probably 

 occasioned by many parallel series of 

 minute sparks passing from particle to 

 particle. 



(88.) Electrical light differs in no 

 respect from the light obtained from 

 other sources. Dr. Wollaston found 

 that, when observed through a prism, 

 the ordinary colours arising from the 

 decomposition of light are obtained ; 

 but the prevailing tint of colour will 

 vary according to the different sub- 

 stances through which the sparks pass, 

 or to the nature of the surface from 

 which they emanate, or by which they 

 are received. Dr. Brewster found that 

 it is capable of undergoing polarization, 

 either by transmission through a doubly 



refracting crystal, by reflection at the 

 proper polarizing angle from a polished 

 plane surface, or by oblique refraction 

 through a series of glass plates. 



(89.) The brilliancy of the electrical 

 spark is proportional to the conducting 

 power of the bodies between which it 

 passes. When an imperfect conductor, 

 such as wood, is employed, the electric 

 light appears in the form of faint red 

 streams ; but metals afford them of 

 great brilliancy. Its colour is subject 

 to variation, from a great number of 

 different circumstances. Sparks pass- 

 ing through balls of wood or ivory, are 

 of a crimson colour ; but this depends 

 also upon their position with regard to 

 the surface. If two pointed wires be 

 inserted obliquely and in opposite di- 

 rections into a piece of soft deal, having 

 their points an inch and a half distant, 

 but penetrating to different depths below 

 the surface, and so that the line joining 

 them is in the direction of the fibres, 

 the sparks passing from the one to the 

 other, will exhibit different colours at 

 different depths ; and if one of the points 

 be inserted deeper than the other, all 

 these colours will appear at once, ac- 

 cording as the electric light is trans- 

 mitted at various depths. Electric 

 sparks passing from one polished me- 

 tallic surface to another are white ; but 

 if the finger be presented to an electri- 

 fied conductor, the sparks obtained are 

 violet. They are green when taken from 

 the surface of silvered leather ; yellow 

 when taken from finely powdered char- 

 coal ; and of a purple colour when taken 

 from the greater number of imperfect 

 conductors. If one of the bodies be- 

 tween which the spark takes place is a 

 green plant, the light is red ; and the 

 same is the case with water or ice. In 

 the vapour of ether green sparks are 

 seen when the eye is placed close to the 

 tube : but they appear reddish when 

 viewed at a considerable distance. Even 

 between the same two metallic con- 

 ductors the colour may vary from the 

 most brilliant white to the most delicate 

 violet, according to the distance through 

 which the electricity is transmitted, and 

 according to the resistance of the me- 

 dium which it is compelled to tra- 

 verse. In exceedingly rarefied air, the 

 colour of the spark is green ; in denser 

 air, it acquires a blue tint, and passes 

 to a violet and purple, in proportion as 

 the condensation of the air is increased. 

 Transmitted through other gases, the 

 colour varies according to their density. 



