ELECTRICITY. 



27 



the effect is considerably modified by 

 the species of electricity with which the 

 bodies are charged ; or, in other words, 

 by the direction in which the fluid moves. 

 When the electric fluid is escaping out 

 of a pointed conductor, the luminous 

 appearance is that of diverging streams, 

 as represented in fig. 23 ; forming 

 what is termed a pencil of light, and 



Fig. 23. 



resembling the filaments of a brush. 

 When, on the contrary, the electric 

 fluid is entering into the pointed body, 

 the light is much more concentrated at 

 the point itself, having a resemblance 

 to a star, in which, if any streams 

 appear, they are disposed like radii, and 

 equally^o in all directions. An approach 

 to these different modifications may be 

 remarked when sparks pass between 

 balls of small diameter, especially if 

 the charge is high. Thus the direction 

 of the lateral ramifications sent out 

 from the principal line, in the branched 

 spark, fig. 20, is from the positive to 

 the negative surface. 



(98.) In describing the above ap- 

 pearances, we have, as usual, referred 

 to the hypothesis of Franklin: but if 

 we adopt that of the two electricities, 

 we have only to consider the appearance 

 of the pencil of light as arising from the 

 double current of the vitreous electricity 

 issuing from the point, and of the resi- 

 nous electricity passing into it : while 

 the star will be the effect of the irruption 

 of the resinous, and the absorption of 

 the vitreous electricities. But this re- 

 markable difference in the phenomena 

 produced, according to the particular 

 species of electricity with which the 

 point is charged, has always been urged 

 as a convincing argument in favour of 

 the Franklinian theory. They appear 

 very strongly to indicate the emanation 

 of some material fluid from the positive, 

 and its reception by the negative point. 

 The diverging lines on the one side, and 

 their inflections on the other, represent 

 exactly the paths of particles flowing 

 out as from a pipe, and urged forwards 

 by a force which gives them sueh. a 



projectile velocity as to prevent their 

 spreading out beyond a certain distance 

 from the direct line of projection. But 

 this very velocity will carry the par- 

 ticles that happen to have deviated 

 most, somewhat beyond the point to 

 which they are attracted : while the 

 attraction to this latter point will tend 

 to deflect them from the line of their 

 path, and gradually turn them back, so 

 that they will arrive at the point of 

 attraction by very different paths, and 

 some even by a retrograde motion. 

 Hence, while in the first case they form 

 a diverging cone of rays, in the latter 

 they must be distributed on all sides of 

 the point like the rays of a star. The 

 annexed diagram, fig. 24, will suffi- 

 ciently illustrate this explanation by 



Fig. 24. 





representing the supposed course of the 

 particles of electric fluid, passing through 

 the air from the positive to the nega- 

 tive point. What weight the argu- 

 ment derived from this phenomenon 

 may be allowed in deciding the question, 

 will be discussed in the sequel. 



(99.) The difference which we have 

 now described in these two appearances, 

 may be employed, on many occasions, 

 as a useful criterion of the species of 

 electricity, at least, which is passing 

 from one conductor to another, if not of 

 the absolute direction of its motion. For, 

 if a needle be presented to an electrified 

 body, the appearance of a star on the 

 needle will show that the electricity of 

 that body is positive ; while, on the con- 

 trary, a luminous brush on the needle 

 will indicate that the body is negative. 



(100.) The influence of a point pro- 

 jecting a short distance from the surface 

 of a body, is greater when that body is 

 negative than when it is positive. Hence, 

 a spark is more readily obtained in the 

 latter case than in the former. 



On this principle an instrument has 

 been invented by Mr. Nicholson for 

 distinguishing the negative from the 

 positive electricity. It consists simply 

 of two metallic balls fixed at the ends of 

 two curved rods of glass, and moveable 

 like branches on a joint, so as to admit 

 of the balls being placed at different 



