ON ELECTRICITY IN MOTIOW^ 673* 



The principal modes, in which the electric equiUbrium is primarily de- 

 stroyed, are simple contact, friction, a change of the fofm of aggregation,, 

 and chemical combinations and decompositions. The electricity produced 

 by the simple contact of any two substances is extremely weak, and can only 

 be detected by very delicate experiments : in general it appears tlvit the substance,, 

 which conducts the more readily, acquires a slight degree of negative elec- 

 tricity, while the other substance is positively electrified in an equal degree. 

 The same disposition of the fluid is also usually produced by friction, the 

 one sutxstance always losing as much as the other gains ^ and commonly 

 although not always, the worst conductor becomes positive. At thq instant 

 in which the friction is applied, the capacities or attractions of the bodies for 

 electricity appear to be altered, and a greater or less quantity is required for 

 saturating them; and upon the cessation of the temporary cliange, this re- 

 dundancy or deficiency is rendered sensible. When two substances of the 

 same kind are rubbed together, the smaller or the rougher becomes nega- 

 tively electrified; perhaps because the smaller surface is more heated, in con- 

 se(|uence of its undergoing more friction than an equal portion of the larger^ 

 and bence becomes a better conductor; and because the rougher is in itself 

 a better conductor than the smoother, ana may possibly have its conducting 

 powers increased by the greater agitation of its parts which the friction pro- 

 duces. The back of a live cat becomes positiveh' electrified, with whatever 

 substance it is rubbed; glass is positive in most cases, but not when rubbed 

 with mercurj' in a vacuum, although sealing wax, which is generally nega- 

 tive, is rendered positive by immersion in a trough of mercury. When a 

 white and a black silk stocking are rubbed together, the white stocking 

 acquires positive electricity, and the black negative, perhaps because the blacL 

 dye renders tlie silk both rougher and a better conductor.. 



Those substances, which have very little conducting power, are some- 

 times called electrics, since they are capable of exhibiting readily the 

 electricity which friction excites on their surf^ices, where it remains 

 accumulated, so that it may be collected into a conductor; while the 

 surfaces of such substances, as have greater conducting powers, do not 

 so readily imbibe the fluid from others with which they are rubbed, since 

 they may be supplied from the internal parts of the substances themselves^ 

 when their altered capacity requires it; thus, glass, when heated to 110° of" 

 Fahrenheit, can with difficulty be excited, becoming an iniperfect conductor :; 



