ELECTRICITY. 



surfaces, that retain their activity. When 

 the experiment is made in the dark, 

 flashes of light attend these sudden ex- 

 changes of electricity, passing between 

 the two surfaces, and accompanied with 

 a rustling noise. 



(35.) Numberless experiments have 

 been made with a view of ascertaining 

 the conditions that determine the species 

 of electricity excited in the respective 

 bodies of which the surfaces are made 

 to rub against each other, but they have 

 led to no satisfactory conclusion. The 

 mechanical configuration of the surface 

 appears to have a greater influence in 

 the result than the peculiar nature of 

 the substance itself. If a plate of glass 

 with a polished surface be rubbed 

 against one which is roughened, the 

 former always acquires the vitreous, 

 and the latter the resinous electricity. 

 No approach to an explanation of this 

 peculiarity has ever been made. Smooth 

 glass acquires vitreous electricity by 

 friction with almost every substance, 

 except the back of a cat, which gives it 

 the resinous electricity ; but roughened 

 glass, if rubbed with the same sub- 

 stances, becomes charged with resinous 

 electricity, while the rubbing bodies ac- 

 quire the vitreous. Sealing-wax, rubbed 

 with an iron chain, acquires, if polished, 

 the resinous electricity ; but if its sur- 

 face is previously rough with scratches, 

 the vitreous. Silk, rubbed by resin, 

 takes the resinous, but with polished 

 glass, the vitreous electricity. The fol- 

 lowing is a list of several substances 

 which acquire vitreous electricity when 

 rubbed with any of those which follow 

 it in the order in which they are set 

 down ; and resinous electricity if rubbed 

 with any of those which precede : 



The back of a cat. 



Polished glass. 



Woollen cloth. 



Feathers. 



Wood. 



Paper. 



Silk. 



Gum-lac. 



Roughened glass. 



In the experiment just mentioned, in 

 which a black and a white ribbon are 

 rubbed together, the former is found to 

 be resinously and the latter vitreously 

 electrified. But if two pieces of the 

 same ribbon of the same length be 

 rubbed, the one being drawn lengthwise 

 and at right angles over a part of the 



other, the one which has suffered fric- 

 tion in its whole length acquires vitreous, 

 and the other resinous electricity. In 

 like manner, when the whole length of 

 the bow of a violin is drawn over a 

 limited part of the string, the hairs of 

 the bow exhibit a vitreous, and the 

 string a resinous electricity, the body 

 whose excited portion is of the least 

 extent being generally found to be re- 

 sinously electrified. But in truth, the 

 slightest difference in the conditions of 

 these and similar experiments on the 

 species of electricity arising from fric- 

 tion, will be often sufficient to produce 

 opposite results. 



(36.) Electrical excitation may also 

 be produced by the friction of liquids 

 or of gases against solid bodies. This 

 is the case when mercury is made to 

 fall, in a fine shower, under the ex- 

 hausted receiver of an air-pump, against 

 the glass. If a current of atmospheric 

 air be directed against a pane of glass, 

 by means of a pair of bellows, the glass 

 becomes vitreously electrified. 



4. Induction. 



(37.) Another class of electrical phe- 

 nomena must here be noticed. When- 

 ever a body is charged with electricity, 

 although it be perfectly insulated, and 

 of course all escape of that electricity 

 prevented, it tends to produce an elec- 

 trical state of the opposite kind in all the 

 bodies in its vicinity. Thus the vitreous 

 electricity tends to "induce the resinous 

 electricity in a body that is situated 

 near it ; and this with greater energy, 

 as the distance is smaller. This effect 

 is termed the induction of electricity, 

 and may be ranked among the general 

 facts, or laws of the science. The fur- 

 ther development of the consequences 

 it leads to, must, for the present, be post- 

 poned, as we shall hereafter be better 

 prepared to understand them. But there 

 is one of its results which we shall now 

 point out, as it refers immediately to 

 the phenomena that have already occu- 

 pied our attention. 



(38.) If an electrified body, charged 

 with either species of electricity, be pre- 

 sented to an unelectrified or neutral 

 body, its tendency, in consequence of 

 the law of induction, is to disturb the 

 electrical condition of the different parts 

 of the neutral body. The electrified body 

 induces a state of electricity contrary 

 to its own in that part of the neutral 

 body which is nearest to it ; and conse- 

 quently a state of electricity similar to its 



